Evolutionary Biology Founder Effect Speciation
by
Alan R. Templeton
  • LAST REVIEWED: 19 May 2017
  • LAST MODIFIED: 27 June 2018
  • DOI: 10.1093/obo/9780199941728-0044

Introduction

The origin of species (speciation)—the process by which two or more species evolve from a single ancestral species—is a central problem in evolutionary biology. During the evolutionary synthesis of the 20th century, the dominant theory of speciation for those working on sexually reproducing animals was allopatric speciation. Allopatric speciation posits that an ancestral species becomes subdivided into two or more geographical subpopulations by changing climates, colonization of new areas, the erection of geological barriers, etc. If these geographical subpopulations have little to no genetic interchange, they will begin to evolve separately. Speciation then arises as an incidental by-product of the independent evolution occurring within the geographical isolates. Evolution within species (microevolution) was often envisioned as being dominated by natural selection leading to adaptive divergence between the geographical isolates. However, the modern synthesis made it clear that microevolution involved many processes in addition to natural selection. One of these processes was genetic drift, the random changes in a population’s gene pool (the set of alleles or gametes collectively shared by a reproducing population) that inevitably arise from random sampling of a finite number of gametes to form the next generation. Just by chance, a particular form of a gene can decrease or increase in frequency in the population, including being completely lost or fixed. The impact of random sampling increases as the population size decreases. One special case of strong genetic drift is the founder effect, in which a population is established by a small number of founding individuals from a much larger ancestral population. Strong genetic drift in the founder population could lead to an immediate evolutionary divergence from the ancestral population. This accelerated divergence is the essence of founder effect speciation models. Founder effect speciation is a special case of allopatric speciation in which one of the geographical isolates was established from a small number of individuals. This does not mean that other microevolutionary forces, such as natural selection, are not operating, but rather that the founder effect enhances and accelerates microevolutionary divergence in concert with natural selection and other microevolutionary forces, thereby making speciation more likely.

General Overviews

A general overview of founder effect speciation can be found in the articles published in the book Genetics, Speciation and the Founder Principle (Giddings, et al. 1989). Pro and con back-to-back reviews are found in Carson and Templeton 1984 and Barton and Charlesworth 1984. Templeton 2008 provides a review of the various theories of founder effect speciation as well as a review and meta-analysis of the empirical studies on founder effect speciation. Gavrilets 2004 presents population genetic analyses of many mechanisms of speciation, including founder speciation.

  • Barton, Nicholas H., and Brian Charlesworth. 1984. Genetic revolutions, founder effects, and speciation. Annual Review of Ecology and Systematics 15:133–164.

    DOI: 10.1146/annurev.es.15.110184.001025Save Citation »Export Citation »E-mail Citation »

    Founder speciation models are equated to adaptive peak shifts in which genetic drift causes a population to evolve across a low-fitness state (an adaptive valley). The authors deem this scenario as unlikely. They also argue that the natural examples can be explained without invoking founder events.

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    • Carson, Hampton L., and Alan R. Templeton. 1984. Genetic revolutions in relation to speciation phenomena: The founding of new populations. Annual Review of Ecology and Systematics 15:97–131.

      DOI: 10.1146/annurev.es.15.110184.000525Save Citation »Export Citation »E-mail Citation »

      This review covers the three basic theories of founder speciation and provides natural examples, with a special focus on the Hawaiian Drosophila.

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      • Gavrilets, Sergey. 2004. Fitness landscapes and the origin of species. Princeton, NJ: Princeton Univ. Press.

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        This monograph covers many speciation mechanisms, including founder speciation, mainly from a theoretical population genetic perspective. Gavrilets confirms the objections of Barton, Charlesworth, and others to founder speciation by evolving through an adaptive valley, but he shows other mechanisms that make founder populations more likely to speciate.

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        • Giddings, Luther V., Kenneth Y. Kaneshiro, and Wyatt W. Anderson, eds. 1989. Genetics, speciation and the founder principle. New York: Oxford Univ. Press.

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          This book contains chapters on the theory of founder effect speciation, natural examples, and genetic studies. An interview with Hampton Carson (chapter 1) and a historical perspective by William Provine (chapter 2) are valuable general introductions to this model of speciation and how it was developed.

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          • Templeton, Alan R. 2008. The reality and importance of founder speciation in evolution. BioEssays 30.5: 470–479.

            DOI: 10.1002/bies.20745Save Citation »Export Citation »E-mail Citation »

            This is the most recent review of this area. The three major theories of founder speciation are summarized, showing that none of them are based on adaptive peak shifts. A detailed statistical analysis of the empirical literature shows that the predictions and assumptions of founder effect speciation are strongly supported.

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            Journals

            Many journals publish articles related to founder effect speciation because the relevant literature spans speciation theory, population genetics, quantitative genetics, systematics, molecular evolution, behavior, and empirical studies. The top-ranked journals that have many articles on founder effect speciation include Evolution, Genetics, and The American Naturalist. The journal Molecular Ecology has published several potential examples of founder effect speciation.

            Theory

            There is an extensive theoretical literature on founder effect speciation, ranging from verbal, qualitative models to mathematical models emphasizing population and quantitative genetics. Some are models of speciation, whereas others are models that focus on just one or a few aspects of the processes that occur during founder effect speciation.

            Basic Models

            There is not one theory of founder speciation, but rather three: Mayr 1954 proposes genetic revolution, Carson 1968 and Carson 1975 proposes founder-flush, and Templeton 1980 proposes genetic transilience. Although Ernst Mayr had articulated some of his ideas about speciation and founder events in earlier works, his 1954 article on genetic revolutions was his first fully developed treatment of this subject. He argued that there is extensive epistasis (gene-gene interaction) for fitness such that natural selection acting upon any one locus is highly dependent on the context defined by other loci (genetic background). Mayr argued that a founder event caused most genetic variation to be lost, and this, in turn, would cause natural selection to drive the founder population onto a novel evolutionary trajectory because of the new, founder-induced genetic background. The prominent population geneticist Richard Lewontin (see Lewontin 1965) criticized Mayr’s theory, pointing out that most founder events would not lead to a dramatic reduction of genetic variation. Nei, et al. 1975 expands upon Lewontin’s criticism, showing that drastic reductions in genetic variation occur only if the founder event is followed by many generations of continual small population size. Carson’s founder-flush theory envisioned one or more founder events followed by a rapid growth of population size (flush), conditions under which the founder population would retain large amounts of genetic variation. He also regarded the flush phase as occurring under open ecological conditions that would result in relaxed selection. With relaxed selection and alterations in the genetic background due to the founder event, Carson argued that the population had enhanced potential of evolving along novel trajectories after selection was reimposed. Carson also believed that sexual selection, particularly on mate recognition traits and female choice, would be important, and Carson 1997 later placed more emphasis on sexual selection. Genetic transilience was also based on a founder-flush model and hence was compatible with Carson’s model of speciation. Genetic transilience integrated the genetic revolution model of Mayr with that of Carson’s founder-flush but did not assume a major loss of genetic variation due to the founder event. Slatkin 1996 expands upon an idea in Templeton 1980, pointing out that the flush phase makes beneficial mutations more likely to persist long enough to be selected, further enhancing the responsiveness to natural selection. Recent modelling in Zhang 2014 and Khatri and Goldstein 2015 has incorporated a deeper understanding of the molecular basis of epistasis derived from genomic studies. These molecular models of epistasis reveal that founder events cause a significant increase in novel adaptive outcomes and reproductive incompatibilities with the ancestral state without the need for any adaptive peak shifts.

            • Carson, Hampton L. 1968. The population flush and its genetic consequences. In Population biology and evolution: Proceedings of the international symposium, June 7–9, 1967, Syracuse. Edited by Richard C. Lewontin, 123–137. Syracuse, NY: Syracuse Univ. Press.

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              This model did not commit the error in Mayr’s paper that founder events always cause a drastic loss of genetic variation. It emphasizes resetting the genetic background through the founder event and a period of relaxed selection during the flush phase.

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              • Carson, Hampton L. 1975. The genetics of speciation at the diploid level. American Naturalist 109.965: 83–92.

                DOI: 10.1086/282975Save Citation »Export Citation »E-mail Citation »

                An expansion of the founder-flush model, emphasizing the role of coadapted gene complexes and their reorganization due to the founder event and relaxed selection.

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                • Carson, Hampton L. 1997. Sexual selection: A driver of genetic change in Hawaiian Drosophila. Journal of Heredity 88.5: 343–352.

                  DOI: 10.1093/oxfordjournals.jhered.a023115Save Citation »Export Citation »E-mail Citation »

                  Argues that just the act of colonization and being in a low-density environment alter sexual selection in a manner that makes speciation more likely.

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                  • Khatri, Bhavin S., and Richard A. Goldstein. 2015. Simple biophysical model predicts faster accumulation of hybrid incompatibilities in small populations under stabilizing selection. Genetics 201.4: 1525–1537.

                    DOI: 10.1534/genetics.115.181685Save Citation »Export Citation »E-mail Citation »

                    This model of the epistasis induced between a protein transcription factor and a DNA binding site shows that founder events cause more rapid coevolution of the protein and its binding site in a manner that leads to reproductive incompatibilities with the ancestral state. The rapid evolution of reproductive isolation driven by this type of epistasis did not require any peak shifts.

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                    • Lewontin, Richard C. 1965. Discussion of paper by Dr. Howard. In The genetics of colonizing species. Edited by Herbert G. Baker and G. Ledyard Stebbins, 481–484. New York: Academic Press.

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                      Presents a brief but important criticism of Mayr’s genetic revolution model, pointing out that founder events do not necessarily cause a drastic reduction in genetic variation.

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                      • Mayr, Ernst. 1954. Change of genetic environment and evolution. In Evolution as a process. Edited by Julian Huxley, Alister Clavering Hardy, and E. B. Ford, 157–180. Princeton, NJ: Princeton Univ. Press.

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                        The first model of founder effect speciation based on the founder effect altering the genetic background in a system of extensive epistasis. A must-read volume because many subsequent papers have misrepresented Mayr’s model.

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                        • Nei, Masatoshi, Takeo Maruyama, and Ranajit Chakraborty. 1975. The bottleneck effect and genetic variability in populations. Evolution 29.1: 1–10.

                          DOI: 10.2307/2407137Save Citation »Export Citation »E-mail Citation »

                          A detailed theoretical examination of the impact of founder events on levels of genetic variation.

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                          • Slatkin, Montgomery. 1996. In defense of founder-flush theories of speciation. American Naturalist 147.4: 493–505.

                            DOI: 10.1086/285862Save Citation »Export Citation »E-mail Citation »

                            At first a critic of the founder-flush model, here Slatkin defends the model in several ways, among them the role of rare, beneficial mutations in founder-flush populations.

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                            • Templeton, Alan R. 1980. The theory of speciation via the founder principle. Genetics 94.4: 1011–1038.

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                              A detailed examination of the factors that would make a founder event more and less likely to yield a speciation event using population and quantitative genetic theory.

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                              • Zhang, Zhihua. 2014. The evolution of heterogeneities altered by mutational robustness, gene expression noise and bottlenecks in gene regulatory networks. Plos One 9.12: e116167.

                                DOI: 10.1371/journal.pone.0116167Save Citation »Export Citation »E-mail Citation »

                                This theoretical exploration of the evolutionary behavior of gene regulatory networks through population bottlenecks revealed a substantial increase in the evolution of novel “generator” genes that can substantially alter the course of adaptive evolution.

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                                Quantitative Genetic Theory

                                Goodnight 1988, Tachida and Cockerham 1989, and Cheverud and Routman 1996 shows that founder events can actually increase the additive variance of a trait by converting nonadditive epistatic variance into additive variance even though founder events reduce the level of genetic variation at the molecular level. Because a population responds to natural selection only through the additive variance, these models imply that a founder event can make a population more responsive to natural selection, thereby making speciation more likely when the founder population is subject to natural selection, such as after the flush phase in Carson’s founder-flush model. This counterintuitive conclusion stems from the dual meanings of the terms additive, dominance, and epistasis in Mendelian genetics versus quantitative genetics. The models of founder effect speciation all assumed epistasis in its Mendelian sense (gene-gene interactions that affect individual phenotypes), but epistatic variance in quantitative genetics refers to a component of the genetic variance of a phenotype that is left over after the additive and dominance variances have been estimated. Cheverud and Routman 1995 shows that Mendelian dominance can contribute both to additive and dominance variance, and that Mendelian epistasis can contribute to additive, dominance, and epistatic variance. Because natural selection exhausts additive variance in the process of adaptation, fitness is expected to have a low additive variance in natural populations, with any genetic variance in fitness being in the nonadditive components. Hence, any random change in allele frequencies in a system with Mendelian dominant and Mendelian epistatic genes will almost always convert some of the nonadditive variance into additive variance. Turelli and Barton 2006 develops a model in which the increase in additive variance is due mainly to dominance and not epistasis. In contrast, the quantitative genetic models of Naciri-Graven and Goudet 2003 and Hallander and Waldmann 2007 show under a wide range of conditions that epistasis, rather than dominance, plays the more important role in increasing the additive variance following a founder event. Founder events also induce linkage disequilibrium, and Wang, et al. 1998 shows that this disequilibrium can also inflate genetic and additive variance in a founder population given dominance and epistasis, with dominance playing the more important role. Ávila, et al. 2014 investigates changes in genetic variance under epistasis using theory and simulation. The authors found that population bottlenecks were followed by an initial increase in additive genetic variance and a dramatic acceleration of evolutionary divergence that was always associated with epistasis. All models therefore show that founder events should increase the additive variance, and hence the responsiveness of the founder population to natural or sexual selection. Genetic revolution, founder-flush, and genetic transilience all involve selection operating upon the founder population, and Gavrilets 2004 (cited under General Overviews) argues on pp. 407–408 that founder populations in new habitats can be more responsive to selection, which can promote explosive speciation following the founder event.

                                • Ávila, Victoria, Andrés Pérez-Figueroa, Armando Caballero, et al. 2014. The action of stabilizing selection, mutation, and drift on epistatic quantitative traits. Evolution 68.7: 1974–1987.

                                  DOI: 10.1111/evo.12413Save Citation »Export Citation »E-mail Citation »

                                  Authors use theory and simulations to show that epistasis leads to changes in the components of genetic variance and to a dramatic acceleration of divergence after population bottlenecks.

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                                  • Cheverud, James M., and Eric J. Routman. 1995. Epistasis and its contribution to genetic variance components. Genetics 139.4: 1455–1461.

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                                    Clarifies the dual use of the word epistasis in Mendelian genetics versus quantitative genetics, showing how epistasis can affect additive, dominance, and epistatic variance. A must-read article for any serious student of quantitative genetics.

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                                    • Cheverud, James M., and Eric J. Routman. 1996. Epistasis as a source of increased additive genetic variance at population bottlenecks. Evolution 50.3: 1042–1051.

                                      DOI: 10.2307/2410645Save Citation »Export Citation »E-mail Citation »

                                      The most thorough theoretical examination of why founder events can convert epistatic variance into additive variance.

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                                      • Goodnight, Charles J. 1988. Epistasis and the effect of founder events on the additive genetic variance. Evolution 42.3: 441–454.

                                        DOI: 10.2307/2409030Save Citation »Export Citation »E-mail Citation »

                                        Shows that random changes in allele frequency are sufficient to convert some epistatic variance into additive variance.

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                                        • Hallander, J., and P. Waldmann. 2007. The effect of non-additive genetic interactions on selection in multi-locus genetic models. Heredity 98.6: 349–359.

                                          DOI: 10.1038/sj.hdy.6800946Save Citation »Export Citation »E-mail Citation »

                                          Used computer simulations to show that additive variance can increase after a founder event in the presence of directional selection, dominance, and epistasis, with epistasis having the higher effect.

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                                          • Naciri-Graven, Yamama, and Jérôme Goudet. 2003. The additive genetic variance after bottlenecks is affected by the number of loci involved in epistatic interactions. Evolution 57.4: 706–716.

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                                            Develops a quantitative genetic model in which epistasis plays a significant role in increasing additive variance after a founder event under a wide range of conditions.

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                                            • Tachida, Hidenori, and C. Clark Cockerham. 1989. A building block model for quantitative genetics. Genetics 121.4: 839–844.

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                                              An alternative model showing how dominance and epistatic variance can be converted into additive variance.

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                                              • Turelli, Michael, and Nicholas H. Barton. 2006. Will population bottlenecks and multilocus epistasis increase additive genetic variance? Evolution 60.9: 1763–1776.

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                                                Develops an alternative quantitative genetic model that diminishes the role of epistasis as a contributor to additive variance after a founder event.

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                                                • Wang, Jinliang, Armando Caballero, and William G. Hill. 1998. The effect of linkage disequilibrium and deviation from Hardy-Weinberg proportions on the changes in genetic variance with bottlenecking. Heredity 81.2: 174–186.

                                                  DOI: 10.1046/j.1365-2540.1998.00390.xSave Citation »Export Citation »E-mail Citation »

                                                  Show that founder events can also increase additive variance by inducing deviations from Hardy-Weinberg genotype frequencies and by creating linkage disequilibrium. The effects of linkage disequilibrium gradually decline, but they can persist for many generations.

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                                                  Adaptive Peak Shifts

                                                  All founder speciation theories were influenced in Wright 1932, which modeled genetic drift as interacting with natural selection. Wright believed that a population could adapt to an environment in many ways, and these distinct adaptive solutions were metaphorically represented as alternative peaks in a landscape. Natural selection causes populations to climb the nearest peak. Natural selection would prevent a population from climbing a higher peak if the population had to evolve first across a low-fitness adaptive valley. Wright envisioned that genetic drift could sometimes cause a population to evolve against the gradient of selection and cross an adaptive valley. Once across the valley, the population would climb the new peak due to natural selection, resulting in an adaptive peak shift. Wright only cursorily addressed speciation (Wright 1940). Moreover, in Wright 1932 the author argued that his adaptive model was likely only in large populations that were subdivided into small local demes interconnected by limited gene flow—not founder events. Nevertheless, Charlesworth, et al. 1982 claims that founder effect speciation was the same as an adaptive peak shift, and this has been embraced by critics of founder speciation such as the authors of Coyne and Orr 2004. Gavrilets 2004 (cited under General Overviews) confirms the improbability of adaptive peak shifts as a mechanism of founder speciation. However, none of the classic models of founder speciation requires peak shifts, and neither does the newer model of Khatri and Goldstein 2015 (cited under Basic Models). Indeed, Templeton 1980 (cited under Basic Models) argues that the conditions that make peak shifts likely actually make founder speciation extremely unlikely. For all these models, the role of the founder event (and the period of relaxed selection in Carson’s model) is to alter the genetic background, which in an epistatic system will alter the adaptive landscape. Then natural and/or sexual selection drives the population to a newly created peak in this altered landscape with no need to cross an adaptive valley. Indeed, the augmented additive variance and survival probabilities of beneficial mutations that arise from founder-flush events enhance, not diminish, the effectiveness of selection. This misrepresentation of founder speciation as peak shifts has caused much confusion, and has also lead to the misleading idea that evidence for selection in speciation is evidence against founder speciation, as has been argued in Barton 1996 and Whitlock 1997. In fact, all models of founder speciation are models of enhanced natural and/or sexual selection. Wright’s adaptive landscape metaphor can be used as a tool for population genetic analysis. The authors of Gavrilets and Hastings 1996 and Gavrilets 2004 (cited under General Overviews) used such an approach to model adaptive landscapes with “ridges” and concluded that founder effect speciation can indeed allow rapid adaptive shifts that result in strong reproductive isolation even with no major adaptive valleys.

                                                  • Barton, Nicholas H. 1996. Natural selection and random genetic drift as causes of evolution on islands. Philosophical Transactions of the Royal Society B: Biological Sciences 351.1341: 785–795.

                                                    DOI: 10.1098/rstb.1996.0073Save Citation »Export Citation »E-mail Citation »

                                                    Operating in the “selection versus drift” mode and equating founder speciation to peak shifts, Barton argues that selection provides a better explanation for speciation on islands than “random” drift, and he also verbally argues against adaptive landscape models of speciation with “ridges” that allow peak shifts without adaptive valleys.

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                                                    • Charlesworth, Brian, Russell Lande, and Montgomery Slatkin. 1982. A neo-Darwinian commentary on macroevolution. Evolution 36.3: 474–498.

                                                      DOI: 10.2307/2408095Save Citation »Export Citation »E-mail Citation »

                                                      Primarily an attack on the macroevolutionary theory of punctuated equilibrium, which was inspired in part by Mayr’s genetic revolution paper, the authors equate genetic revolution to Wright’s model of adaptive peak shifts and then proceed to attack all of the models of founder speciation.

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                                                      • Coyne, Jerry A., and H. Allen Orr. 2004. Speciation. Sunderland, MA: Sinauer.

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                                                        A comprehensive textbook on speciation. Chapter 13 is devoted to founder speciation under the title of “Selection versus Drift.” The authors regard all models of founder speciation as adaptive peak shift models in which drift opposes selection. All their theoretical objections are based on this assumption, and the models of Mayr, Carson, and Templeton are not addressed.

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                                                        • Gavrilets, Sergey, and Alan Hastings. 1996. Founder effect speciation: A theoretical reassessment. American Naturalist 147.3: 466–491.

                                                          DOI: 10.1086/285861Save Citation »Export Citation »E-mail Citation »

                                                          Gavrilets has done the most rigorous modeling of adaptive landscapes, and in this paper with Hastings he concludes that founder effect speciation is plausible using models in which populations do not cross deep adaptive values, a feature consistent with all three founder speciation models.

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                                                          • Whitlock, Michael C. 1997. Founder effects and peak shifts without genetic drift: Adaptive peak shifts occur easily when environments fluctuate slightly. Evolution 51.4: 1044–1048.

                                                            DOI: 10.2307/2411033Save Citation »Export Citation »E-mail Citation »

                                                            Equating founder speciation to peak shifts, Whitlock argues that a change in the environment causing an altered landscape is more likely to result in rapid evolution. He fails to note that changes in the genetic environment in an epistatic system also create an altered landscape.

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                                                            • Wright, Sewall. 1932. The roles of mutation, inbreeding, crossbreeding, and selection in evolution. In Proceedings of the Sixth International Congress of Genetics: Ithaca, New York, 1932. Vol. 1. Edited by Donald F. Jones, 356–366. New York: Brooklyn Botanic Garden.

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                                                              A classic paper of population genetics that outlines Wright’s shifting balance theory of adaptive evolution arising from the interaction of selection, drift, and gene flow.

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                                                              • Wright, Sewall. 1940. Breeding structure of populations in relation to speciation. American Naturalist 74.752: 232–248.

                                                                DOI: 10.1086/280891Save Citation »Export Citation »E-mail Citation »

                                                                Despite the title, this is primarily a paper about Wright’s models of evolution within species. He feels that adaptive evolution, particularly under shifting balance, in two geographically isolated populations could lead to speciation. This is not a founder effect speciation model.

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                                                                Sexual Selection and the Kaneshiro Hypothesis

                                                                Founder effect speciation arises from the interaction of genetic drift with selection, both natural and sexual. Carson increasingly emphasized the role of sexual selection on mate recognition traits or mating preferences because their evolution can directly lead to pre-mating reproductive isolation and hence speciation. This position is supported by quantitative genetic models, such as Uyeda, et al. 2009, that show that genetic drift can be a powerful amplifier of speciation when sexual selection operates on mating preferences, and the idea that such sexual selection can drive rapid speciation is supported by natural examples, such as those given in Boake 2005 and Mendelson and Shaw 2005. Kaneshiro 1980 proposes a sexual selection model that was specific to founder populations with a female-choice system of mating that was inspired by the author’s studies on Hawaiian Drosophila. He proposes that in the initial generations after the founder event, densities would be low so that females showing high levels of discrimination for specific male traits would be unlikely to mate, thereby causing selection for less choosy females. This, in turn, would relax selection on male courtship displays, leading to a pattern of asymmetrical behavioral isolation in which the more choosy ancestral females would have a lower probability of mating with the derived males, but the less choosy derived females would mate with high probability with the ancestral males. This hypothesis inspired much additional work on Hawaiian Drosophila, such as Boake 2005, and on other species groups, as shown in Shaw and Lugo 2001 and Tinghitella and Zuk 2009, for example. Many of these examples support the Kaneshiro hypothesis, and Giddings and Templeton 1983 shows that those that do not tend to violate its assumptions of a founder-flush event and female choice. Hence, the Kaneshiro hypothesis is a special case of sexual selection in founder effect speciation, one that is frequently upheld, but not universally, as argued in Lambert 1984.

                                                                • Boake, Christine R. B. 2005. Sexual selection and speciation in Hawaiian Drosophila. Behavior Genetics 35.3: 297–303.

                                                                  DOI: 10.1007/s10519-005-3221-4Save Citation »Export Citation »E-mail Citation »

                                                                  A review of the theories and data from Hawaiian Drosophila related to sexual selection and speciation.

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                                                                  • Giddings, L. Val, and Alan R. Templeton. 1983. Behavioral phylogenies and the direction of evolution. Science 220.4595: 372–378.

                                                                    DOI: 10.1126/science.220.4595.372Save Citation »Export Citation »E-mail Citation »

                                                                    A review of the theories and data related to directional asymmetries in sexual isolation, showing that the data that does not support the Kaneshiro hypothesis tends to come from systems that violate one or both of Kaneshiro’s assumptions: female choice, and founder effect speciation.

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                                                                    • Kaneshiro, Kenneth Y. 1980. Sexual isolation, speciation and the direction of evolution. Evolution 34.3: 437–444.

                                                                      DOI: 10.2307/2408213Save Citation »Export Citation »E-mail Citation »

                                                                      Presents the model widely known as the “Kaneshiro hypothesis” that predicts a directional asymmetry to the evolution of sexual isolation in female choice systems during founder effect speciation.

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                                                                      • Lambert, D. M. 1984. Specific-mate recognition systems, phylogenies and asymmetrical evolution. Journal of Theoretical Biology 109.1: 147–156.

                                                                        DOI: 10.1016/S0022-5193(84)80116-2Save Citation »Export Citation »E-mail Citation »

                                                                        A theoretical analysis of many types of male-female communication systems shows that diverse patterns can evolve between ancestral and derived species.

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                                                                        • Mendelson, Tamra C., and Kerry L. Shaw. 2005. Sexual behaviour: Rapid speciation in an arthropod. Nature 433.7024: 375.

                                                                          DOI: 10.1038/433375aSave Citation »Export Citation »E-mail Citation »

                                                                          Extremely rapid speciation is found in the Hawaiian cricket genus Laupala that is driven primarily through evolution in male courtship song.

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                                                                          • Shaw, Kerry L., and Ezequiel Lugo. 2001. Mating asymmetry and the direction of evolution in the Hawaiian cricket genus Laupala. Molecular Ecology 10.3: 751–759.

                                                                            DOI: 10.1046/j.1365-294x.2001.01219.xSave Citation »Export Citation »E-mail Citation »

                                                                            Asymmetrical patterns of sexual isolation are found in the rapidly speciating sword-tailed crickets of Hawaii that are consistent with those found in the Hawaiian Drosophila.

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                                                                            • Tinghitella, Robin M., and Marlene Zuk. 2009. Asymmetric mating preferences accommodated the rapid evolutionary loss of a sexual signal. Evolution 63.8: 2087–2098.

                                                                              DOI: 10.1111/j.1558-5646.2009.00698.xSave Citation »Export Citation »E-mail Citation »

                                                                              A cricket native to Australia was recently introduced to Hawaii. Reciprocal matings with Hawaiian and ancestral crickets indicate that the colonization of Hawaii favored females with relaxed mating requirements, facilitating the rapid evolutionary loss of male courtship song in Hawaii.

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                                                                              • Uyeda, Josef C., Stevan J. Arnold, Paul A. Hohenlohe, and Louise S. Mead. 2009. Drift promotes speciation by sexual selection. Evolution 63.3: 583–594.

                                                                                DOI: 10.1111/j.1558-5646.2008.00589.xSave Citation »Export Citation »E-mail Citation »

                                                                                Computer simulations are used to show that genetic drift can be a powerful amplifier of speciation through sexual selection under a wide variety of assumptions.

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                                                                                Natural Examples

                                                                                Although the theory of founder effect speciation, particularly genetic transilience, indicates that most founder events do not lead to speciation, many natural examples of founder effect speciation have been proposed. Most prominent among these potential examples are the Hawaiian Drosophila, which served as the inspiration for Carson’s founder-flush model and the Kaneshiro hypothesis. However, many other examples have been proposed as well.

                                                                                Hawaiian Drosophila

                                                                                The Hawaiian Drosophila project was launched in 1963 and became one of the major evolutionary studies of the 20th century, with literally thousands of articles being produced from this project. The initial head of the project was Wilson Stone, and Hampton Carson became the leader of the project upon Stone’s death in 1968. The discoveries emanating from this project provided an excellent natural example of founder effect speciation and inspired the development of the founder-flush theory and Kaneshiro hypothesis. Carson, et al. 1970 provides the first review of the discoveries made in this project, which were astounding, in terms of both the number of species discovered and the extreme chromosomal, morphological, ecological, and behavioral evolution that these species displayed. Carson also published a shorter review that same year (see Carson 1970) that focuses more on founder effect speciation. Subsequent reviews of this project in Carson and Kaneshiro 1976; Magnacca, et al. 2008; and O’Grady, et al. 2011 show how new genetic technologies, behavioral studies, and ecological studies were used to uncover ever-finer evolutionary patterns. The inference of inter-island colonization events in Carson 1970 and additional ones in Carson 1983 was particularly influential in the formulation of Carson’s founder-flush model. The existence of lek behavior in the system of mating of many of the Hawaiian Drosophila (very rare in the genus outside of Hawaii, reviewed in Spieth 1982) was influential in the development of models of sexual selection and founder effect speciation, as shown for example in Carson 2003.

                                                                                • Carson, Hampton L. 1970. Chromosome tracers of the origin of species. Science 168.3938: 1414–1418.

                                                                                  DOI: 10.1126/science.168.3938.1414Save Citation »Export Citation »E-mail Citation »

                                                                                  This short but information-rich review brought much attention to the Hawaiian Drosophila model system and to founder effect speciation.

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                                                                                  • Carson, Hampton L. 1983. Chromosomal sequences and interisland colonizations in Hawaiian Drosophila. Genetics 103.3: 465–482.

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                                                                                    Long before molecular phylogenies became the norm, drosophilists could reconstruct phylogenies with chromosomal inversions. Carson used these inversion phylogenies to infer the colonization history of the Hawaiian Islands, showing that almost every inter-island colonization event resulted in a new species.

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                                                                                    • Carson, Hampton L. 2003. Mate choice theory and the mode of selection in sexual populations. Proceedings of the National Academy of Sciences of the United States of America 100.11: 6584–6587.

                                                                                      DOI: 10.1073/pnas.0732174100Save Citation »Export Citation »E-mail Citation »

                                                                                      In one of Carson’s last papers before his death in 2004 he argues that mate and/or gamete choice is a major selective force driving genetic change in sexual populations.

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                                                                                      • Carson, Hampton L., D. Elmo Hardy, Herman T. Spieth, and Wilson S. Stone. 1970. The evolutionary biology of Hawaiian Drosophilidae. In Essays in evolution and genetics in honor of Theodosius Dobzhansky. Edited by Max K. Hecht and William C. Steere, 437–543. New York: Appleton-Century-Crofts.

                                                                                        DOI: 10.1007/978-1-4615-9585-4Save Citation »Export Citation »E-mail Citation »

                                                                                        An astonishing amount of data is given in this review, which showed that Hawaiian Drosophila spp. were ideal model organisms for the study of speciation.

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                                                                                        • Carson, Hampton L., and Kenneth Y. Kaneshiro. 1976. Drosophila of Hawaii: Systematics and ecological genetics. Annual Review of Ecology and Systematics 7:311–345.

                                                                                          DOI: 10.1146/annurev.es.07.110176.001523Save Citation »Export Citation »E-mail Citation »

                                                                                          An excellent review of the geology and ecology of the Hawaiian Islands and how that promoted explosive speciation within the Hawaiian Drosophila.

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                                                                                          • Magnacca, K. N., D. Foote, and P. M. O’Grady. 2008. A review of the endemic Hawaiian Drosophilidae and their host plants. Zootaxa 1728:1–58.

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                                                                                            This review provides insights into the role ecology has played in the evolution of this large group of nearly 1,000 species. Also discussed are the threats faced by these flies and their host plants and the species loss that is already occurring.

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                                                                                            • O’Grady, Patrick M., Richard T. Lapoint, James Bonacum, et al. 2011. Phylogenetic and ecological relationships of the Hawaiian Drosophila inferred by mitochondrial DNA analysis. Molecular Phylogenetics and Evolution 58.2: 244–256.

                                                                                              DOI: 10.1016/j.ympev.2010.11.022Save Citation »Export Citation »E-mail Citation »

                                                                                              Estimates a phylogeny from mitochondrial DNA of 1,000 species, all arising from a common ancestor in ancient islands that were once part of Hawaii. The phylogeny is used to examine the evolution of plant host family and type of substrate used for oviposition and larval development, including ancestral reconstructions.

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                                                                                              • Spieth, H. T. 1982. Behavioral biology and evolution of the Hawaiian picture-winged species group of Drosophila. Evolutionary Biology 14:351–437.

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                                                                                                An excellent review of the diversity of behaviors found in the picture-winged Drosophila, including the use of leks where males congregate to attract females.

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                                                                                                Other Hawaiian Taxa

                                                                                                Because Hawaii has the appropriate geological and biogeographical conditions to make repeated founder colonization events likely, it is not surprising that many other Hawaiian groups show patterns associated with founder event speciation. Some examples are given in Cowie and Holland 2008 for several groups of terrestrial fauna, Gillespie and Roderick 2002 for arthropods, and Craddock 2000 for both plants and animals. Among the more recently studied Hawaiian groups are the Hawaiian sword-tailed crickets, as shown in Oh, et al. 2013, and the Hawaiian cave planthoppers, as shown in Wessel, et al. 2013.

                                                                                                • Cowie, Robert H., and Brenden S. Holland. 2008. Molecular biogeography and diversification of the endemic terrestrial fauna of the Hawaiian Islands. Philosophical Transactions of the Royal Society B: Biological Sciences 363.1508: 3363–3376.

                                                                                                  DOI: 10.1098/rstb.2008.0061Save Citation »Export Citation »E-mail Citation »

                                                                                                  A review of molecular studies of the endemic terrestrial fauna of the Hawaiian archipelago. Most radiations within the archipelago display a pattern of speciating as they colonize newer islands from older islands, similar to the Hawaiian drosophilids.

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                                                                                                  • Craddock, Elysse M. 2000. Speciation processes in the adaptive radiation of Hawaiian plants and animals. Evolutionary Biology 31:1–53.

                                                                                                    DOI: 10.1007/978-1-4615-4185-1_1Save Citation »Export Citation »E-mail Citation »

                                                                                                    A review of several groups of Hawaiian endemics and their likely modes of speciation.

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                                                                                                    • Gillespie, Rosemary G., and George K. Roderick. 2002. Arthropods on islands: Colonization, speciation, and conservation. Annual Review of Entomology 47:595–632.

                                                                                                      DOI: 10.1146/annurev.ento.47.091201.145244Save Citation »Export Citation »E-mail Citation »

                                                                                                      A broad review of colonization and speciation of arthropods on islands across the world, including Hawaii and other “Darwinian” islands that are formed de novo and have never been in contact with the source of colonists.

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                                                                                                      • Oh, Kevin P., Gina L. Conte, and Kerry L. Shaw. 2013. Founder effects and the evolution of asymmetrical sexual isolation in a rapidly-speciating clade. Current Zoology 59.2: 230–238.

                                                                                                        DOI: 10.1093/czoolo/59.2.230Save Citation »Export Citation »E-mail Citation »

                                                                                                        An excellent study of sexual isolation in Hawaiian crickets in the genus Laupala, supporting the Kaneshiro hypothesis and discussing the impact of founder effect speciation on chemosensory signals in mate choice in these rapidly evolving species.

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                                                                                                        • Wessel, Andreas, Hannelore Hoch, Manfred Asche, et al. 2013. Founder effects initiated rapid species radiation in Hawaiian cave planthoppers. Proceedings of the National Academy of Sciences of the United States of America 110.23: 9391–9396.

                                                                                                          DOI: 10.1073/pnas.1301657110Save Citation »Export Citation »E-mail Citation »

                                                                                                          An integrative study, using molecular genetic, behavioral, and morphometric data, on a group with one of the highest speciation rates among animals. The patterns revealed by these studies fit well to the predictions of founder effect speciation, including the quantitative genetic predictions of increased additive genetic variance after founding.

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                                                                                                          Non-Hawaiian Taxa

                                                                                                          Many other organisms outside of Hawaii have also been inferred to have undergone founder effect speciation. The authors of Funk, et al. 2016 performed genomic analyses on several populations of the island fox and found strong genetic signatures of population bottlenecks, genetic drift, and island specific natural selection. The author of Matzke 2014 used statistical biogeography on thirteen diverse island species and inferred significant founder event speciation in all of them. Several bird species across the globe show patterns indicative of founder effect speciation. For example, Baker and Moeed 1987 argues for an important role of founder populations in the diversification of populations of mynas. Balakrishnan and Edwards 2009 makes similar arguments for zebra finches. Maley and Winker 2010 does so for buntings; Rasner, et al. 2004 for juncos; and Spurgin, et al. 2014 for island populations of Berthelot’s pipit. Insects other than Drosophila also display evidence for founder effect speciation, as shown in Hard, et al. 1993 and Lair, et al. 1997 on mosquitoes; and McLeish, et al. 2011 on thrips. Other examples can be found in the book Genetics, Speciation and the Founder Principle (Giddings, et al. 1989, cited under General Overviews).

                                                                                                          • Baker, Allan J., and Abdul Moeed. 1987. Rapid genetic differentiation and founder effect in colonizing populations of common mynas (Acridotheres tristis). Evolution 41.3: 525–538.

                                                                                                            DOI: 10.2307/2409254Save Citation »Export Citation »E-mail Citation »

                                                                                                            Mynas were introduced to many parts of the world in the 19th century, often with very small founder populations. The authors conclude that in just 100 to 120 years genetic shifts have occurred in these founder populations that are equivalent to those between different bird subspecies.

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                                                                                                            • Balakrishnan, Christopher N., and Scott V. Edwards. 2009. Nucleotide variation, linkage disequilibrium and founder-facilitated speciation in wild populations of the zebra finch (Taeniopygia guttata). Genetics 181.2: 645–660.

                                                                                                              DOI: 10.1534/genetics.108.094250Save Citation »Export Citation »E-mail Citation »

                                                                                                              A contrast between Australian and island populations indicates recent founder events in island populations and much divergence in quantitative traits. The pattern of change implies that both drift and selection occurred, consistent with founder effect speciation theories.

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                                                                                                              • Funk, W. Chris, Robert E. Lovich, Paul A. Hohenlohe, et al. 2016. Adaptive divergence despite strong genetic drift: Genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis). Molecular Ecology 25.10: 2176–2194.

                                                                                                                DOI: 10.1111/mec.13605Save Citation »Export Citation »E-mail Citation »

                                                                                                                A genomic analysis reveals that population bottlenecks and genetic drift were a major force in the genetic divergence of these populations, but nevertheless there was also strong adaptive divergence. These observations undercut the widespread misconception that strong genetic drift and adaptive evolution are incompatible but supports the premise of founder-flush and genetic transilience models of strong selection being enhanced after population bottlenecks.

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                                                                                                                • Hard, Jeffrey J., William E. Bradshaw, and Christina M. Holzapfel. 1993. The genetic basis of photoperiodism and its evolutionary divergence among populations of the pitcher-plant mosquito, Wyeomyia smithii. American Naturalist 142.3: 457–473.

                                                                                                                  DOI: 10.1086/285549Save Citation »Export Citation »E-mail Citation »

                                                                                                                  Derived northern populations of this mosquito are under directional selection on critical photoperiod, yet there has been no erosion of additive genetic variance in the derived populations, consistent with founding events.

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                                                                                                                  • Lair, Kevin P., William E. Bradshaw, and Christina M. Holzapfel. 1997. Evolutionary divergence of the genetic architecture underlying photoperiodism in the pitcher-plant mosquito, Wyeomyia smithii. Genetics 147.4: 1873–1883.

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                                                                                                                    This follow-up study to the previous work demonstrates widespread epistasis in the genetic architecture of photoperiodism and a divergence pattern consistent with repeated founder-flush episodes.

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                                                                                                                    • Maley, James M., and Kevin Winker. 2010. Diversification at high latitudes: Speciation of buntings in the genus Plectrophenax inferred from mitochondrial and nuclear markers. Molecular Ecology 19.4: 785–797.

                                                                                                                      DOI: 10.1111/j.1365-294X.2009.04513.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                      Two species of bunting best fit a model of founder event speciation driven by oscillations in habitat due to climate change.

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                                                                                                                      • Matzke, Nicholas J. 2014. Model selection in historical biogeography reveals that founder-event speciation is a crucial process in island clades. Systematic Biology 63.6: 951–970.

                                                                                                                        DOI: 10.1093/sysbio/syu056Save Citation »Export Citation »E-mail Citation »

                                                                                                                        This paper verifies the importance of founder-event speciation in island populations via statistical biogeographic model choice. The results show that the inference of founder-event speciation is overwhelmingly supported over a dispersal-speciation model that does not allow founder events.

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                                                                                                                        • McLeish, Michael J., Michael P. Schwarz, and Tom W. Chapman. 2011. Gall inducers take a leap: Host-range differences explain speciation opportunity (Thysanoptera: Phlaeothripidae). Australian Journal of Entomology 50.4: 405–417.

                                                                                                                          DOI: 10.1111/j.1440-6055.2011.00831.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                          Within this species complex of thrips, restricted gene flow among populations specializing on isolated host plants implies genetic drift via allopatric mechanisms. Their results suggest that gall-thrip speciation is driven by the combined influence of ecologically based selection with genetic drift.

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                                                                                                                          • Rasner, C. A., P. Yeh, L. S. Eggert, K. E. Hunt, D. S. Woodruff, and T. D. Price. 2004. Genetic and morphological evolution following a founder event in the dark-eyed junco, Junco hyemalis thurberi. Molecular Ecology 13.3: 671–681.

                                                                                                                            DOI: 10.1046/j.1365-294X.2004.02104.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                            An example of rapid evolution after a founder event. An isolated population of juncos was naturally established in San Diego that has undergone significant molecular and morphological evolution. The analysis indicated that this evolution was due to both the founder effect and selection.

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                                                                                                                            • Spurgin, Lewis G., Juan Carlos Illera, Tove H. Jorgensen, et al. 2014. Genetic and phenotypic divergence in an island bird: Isolation by distance, by colonization or by adaptation? Molecular Ecology 23.5: 1028–1039.

                                                                                                                              DOI: 10.1111/mec.12672Save Citation »Export Citation »E-mail Citation »

                                                                                                                              A detailed genetic analysis revealed that founder effects are responsible for both genetic and phenotypic changes across colonized archipelagos and that these founder-event induced changes persist over long evolutionary timescales.

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                                                                                                                              Invasive Species

                                                                                                                              A practical application of founder effect speciation relates to the problem of invasive species. Human activities have often introduced species to new areas, frequently accompanied by a founder event. In some cases, these newly introduced species show large increases in population size or range and often can become serious pests. In some cases, the introduced species rapidly evolves new traits or adaptations after the founder event that allow it or enhance its ability to become invasive, as discussed in the review Dlugosch and Parker 2008 and the models in Kanarek and Webb 2010. This pattern is similar to founder effect speciation, although in this case the concern is not with speciation but with practical conservation issues. Auger-Rozenberg, et al. 2012 shows that the adaptive evolution that follows the founder event can involve host shifts; Firmat, et al. 2012 shows the evolution of functional morphological changes after a founder event; Piiroinen, et al. 2013 shows the evolution of insecticide resistance; Rollins, et al. 2015 shows that rapid phenotypic evolution of cane toads after their introduction to Australia is associated with dramatic shifts in gene expression and the evolution of novel dispersal-related traits; and Tsetsarkin, et al. 2011 shows the evolution of infectious ability. This rapid evolution can arise from founder events generating genetic novelty followed by rapid adaptive evolution along the invasion pathways, as shown in Konecny, et al. 2013, a pattern found with founder effect speciation. Hence, invasive species are not just an ecological problem but also an evolutionary one, and the role of founder events and their interaction with selection often constitute an important component of this evolutionary dimension.

                                                                                                                              • Auger-Rozenberg, M. A., T. Boivin, E. Magnoux, C. Courtin, A. Roques, and C. Kerdelhue. 2012. Inferences on population history of a seed chalcid wasp: Invasion success despite a severe founder effect from an unexpected source population. Molecular Ecology 21.24: 6086–6103.

                                                                                                                                DOI: 10.1111/mec.12077Save Citation »Export Citation »E-mail Citation »

                                                                                                                                A detailed phylogeographic analysis of a seed-feeding invasive wasp revealed that the introduced populations were founded by small numbers of individuals that switched to a new host, thereby demonstrating the evolutionary potential of a population founded by only a few individuals.

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                                                                                                                                • Dlugosch, K. M., and I. M. Parker. 2008. Founding events in species invasions: Genetic variation, adaptive evolution, and the role of multiple introductions. Molecular Ecology 17.1: 431–449.

                                                                                                                                  DOI: 10.1111/j.1365-294X.2007.03538.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                  This extensive review documents many patterns in invasive species, including those with founder events followed by rapid adaptive evolution in quantitative traits. The management implications of this evolutionary potential are discussed.

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                                                                                                                                  • Firmat, Cyril, Ulrich K. Schliewen, Michele Losseau, and Paul Alibert. 2012. Body shape differentiation at global and local geographic scales in the invasive cichlid Oreochromis mossambicus. Biological Journal of the Linnean Society 105.2: 369–381.

                                                                                                                                    DOI: 10.1111/j.1095-8312.2011.01802.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                    A contrast between native and invaded populations suggests that successive founder events occurred during the colonization of new areas coupled with selectively driven shape divergence. The shape differences should affect swimming abilities and relate to divergent functional requirements between the native and invaded ranges.

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                                                                                                                                    • Kanarek, Andrew R., and Colleen T. Webb. 2010. Allee effects, adaptive evolution, and invasion success. Evolutionary Applications 3.2: 122–135.

                                                                                                                                      DOI: 10.1111/j.1752-4571.2009.00112.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                      Models of the evolutionary dynamics of founder populations are used to enhance our understanding of patterns of invasiveness and management strategies. Their theoretical results indicate the importance of evolutionary potential in founder populations that are successful in biological invasions.

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                                                                                                                                      • Konecny, Adam, Arnaud Estoup, Jean-Marc Duplantier, et al. 2013. Invasion genetics of the introduced black rat (Rattus rattus) in Senegal, West Africa. Molecular Ecology 22.2: 286–300.

                                                                                                                                        DOI: 10.1111/mec.12112Save Citation »Export Citation »E-mail Citation »

                                                                                                                                        One of the most infamous invasive species, the black rat, has spread primarily by human activities. This detailed phylogeographic analysis indicates that repeated founder events have played a major role in shaping the invasive rats, probably through the generation of genetic novelty and favoring rapid evolution.

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                                                                                                                                        • Piiroinen, Saija, Leena Lindstrom, Anne Lyytinen, et al. 2013. Pre-invasion history and demography shape the genetic variation in the insecticide resistance-related acetylcholinesterase 2 gene in the invasive Colorado potato beetle. BMC Evolutionary Biology 13.1.

                                                                                                                                          DOI: 10.1186/1471-2148-13-13Save Citation »Export Citation »E-mail Citation »

                                                                                                                                          Invasive populations of this beetle have been under intense selection by insecticides. This study focuses on a gene involved in insecticide resistance. The results indicate strong founder effects followed by rapid invasion, and the utilization of a resistance allele found in the ancestral populations.

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                                                                                                                                          • Rollins, Lee A., Mark F. Richardson, and Richard Shine. 2015. A genetic perspective on rapid evolution in cane toads (Rhinella marina). Molecular Ecology 24.9: 2264–2276.

                                                                                                                                            DOI: 10.1111/mec.13184Save Citation »Export Citation »E-mail Citation »

                                                                                                                                            Invasive populations of the cane toad show dramatic heritable changes in morphology, physiology, and behavior that are accompanied by substantial shifts in gene expression. These results show rapid and continued evolution for more than eighty years following the initial founder colonizing event.

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                                                                                                                                            • Tsetsarkin, Konstantin A., Rubing Chen, Grace Leal, et al. 2011. Chikungunya virus emergence is constrained in Asia by lineage-specific adaptive landscapes. Proceedings of the National Academy of Sciences of the United States of America 108.19: 7872–7877.

                                                                                                                                              DOI: 10.1073/pnas.1018344108Save Citation »Export Citation »E-mail Citation »

                                                                                                                                              Most studies on founder effect speciation focus on higher eukaryotes, but this paper examines a particularly important class of invasive species: viruses associated with emerging diseases in humans. The emergent virus shows a signature of a founder event and a lineage-specific epistatic interaction that defines a novel adaptive landscape.

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                                                                                                                                              Empirical Approaches

                                                                                                                                              Founder effect speciation can produce significant evolutionary change within tens to hundreds of generations after the founder event. As a consequence, founder effect speciation is amenable to empirical study with organisms with reasonably short generation times. Moreover, the theory of genetic transilience (Templeton 1980, cited under Basic Models) uses population genetic models to make specific predictions of factors or traits that make founder effect speciation more or less likely. Hence, unlike most models of speciation, both positive and negative empirical results can be used to test the theory of genetic transilience. Founder effect speciation is one of the most testable and most tested models of speciation, with an extensive empirical literature.

                                                                                                                                              Founder Population Experiments

                                                                                                                                              The most common type of experiment on founder effect speciation monitors the evolution of reproductive isolation or some other adaptive change, typically with respect to the ancestral population. The experiment is judged as a success for speciation when statistically significant reproductive isolation (or adaptive change) can be detected after a founder event; otherwise, it is regarded as a failure for the initiation of speciation. A large number of experiments fall into this category, and references to most of these experiments can be found in Templeton 1996 and Templeton 2008 (as cited under General Overviews). Some experiments performed by the authors and published after these reviews are Bell and Gonzalez 2011, working on yeast; Kolbe, et al. 2012, working on lizards; Nanda and Singh 2011, working on Drosophila ananassae; Simoes, et al. 2008, working on Drosophila subobscura; Matute 2013, working on Drosophila yakuba; and Yates and Fraser 2014, performing a meta-analysis on several transplantation experiments in plants and fishes. As this list and the references in the reviews by Templeton demonstrate, founder speciation models can be empirically tested, often successfully, in a wide range of organisms, showing that founder effect speciation is not limited to a narrow range of organisms. These experiments differed greatly in the organisms used and in experimental design (and hence in many of the traits or factors predicted under genetic transilience to influence the probability of founder effect speciation). Accordingly, in Templeton 1996 and Templeton 2008 (as cited under General Overviews), Templeton performed a meta-analysis on these experiments that revealed highly significant results concordant with the predictions of genetic transilience theory. A few individual experiments had much replication and multiple treatment effects that would allow the testing of the predictions of genetic transilience theory within the confines of a single experimental design. The best-designed experiment of this nature is that of Galiana, et al. 1993. Despite their powerful design, the authors failed to make use of it to test statistically the explicit predictions of genetic transilience theory. Accordingly, in Templeton 1996 and Templeton 2008 (as cited under General Overviews), Templeton performed a statistical analysis of the data in Galiana, et al. 1993 and found statistically significant patterns, all of which were concordant with the predictions of genetic transilience theory. Genetic transilience theory significantly predicted when founder events are likely to facilitate speciation and when they are not in these empirical studies.

                                                                                                                                              • Bell, Graham, and Andrew Gonzalez. 2011. Adaptation and evolutionary rescue in metapopulations experiencing environmental deterioration. Science 332.6035: 1327–1330.

                                                                                                                                                DOI: 10.1126/science.1203105Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                The authors examined the evolution of laboratory metapopulations of yeast to salt stress, and they found that adaptation to salt stress was frequent and rapid in the peripheral populations subject to stronger founder effects.

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                                                                                                                                                • Galiana, Agustí, Andrés Moya, and Francisco J. Ayala. 1993. Founder-flush speciation in Drosophila pseudoobscura: A large-scale experiment. Evolution 47.2: 432–444.

                                                                                                                                                  DOI: 10.2307/2410062Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                  A well-designed experiment with replication and treatment effects predicted to both increase and decrease the chances for founder effect speciation. Unfortunately, the authors did not perform a statistical analysis that made use of this design, but, when performed, their results strongly supported the predictions of genetic transilience theory.

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                                                                                                                                                  • Kolbe, Jason J., Manuel Leal, Thomas W. Schoener, David A. Spiller, and Jonathan B. Losos. 2012. Founder effects persist despite adaptive differentiation: A field experiment with lizards. Science 335.6072: 1086–1089.

                                                                                                                                                    DOI: 10.1126/science.1209566Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                    A fascinating semi-natural experiment. Lizard populations were established on seven small islands from a male-female pair randomly drawn from the same large source. The founding events generated significant genetic and morphological differences that influenced the subsequent course of evolution as the populations adapted to the small islands.

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                                                                                                                                                    • Matute, D. R. 2013. The role of founder effects on the evolution of reproductive isolation. Journal of Evolutionary Biology 26.11: 2299–2311.

                                                                                                                                                      DOI: 10.1111/jeb.12246Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                      A total of 1,000 replicates of single female/male founder events were made followed by thirty generations of continued single female/male inbreeding. Little reproductive isolation was observed, which was interpreted as evidence against founder speciation. This interpretation was based on the assumption that “all the founder speciation models require genetic drift to act for a long time,” which is patently untrue for the founder-flush and genetic transilience models. Rather, these results strongly support the prediction that a population flush is needed for speciation.

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                                                                                                                                                      • Nanda, Punita, and Bashisth N. Singh. 2011. Origin of sexual isolation in Drosophila ananassae due to founder effects. Genetica 139.6: 779–787.

                                                                                                                                                        DOI: 10.1007/s10709-011-9582-0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                        A well-designed experiment with much replication showing the evolution of asymmetrical sexual isolation following laboratory founder events.

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                                                                                                                                                        • Simoes, P., J. Santos, I. Fragata, L. D. Mueller, M. R. Rose, and M. Matos. 2008. How repeatable is adaptive evolution? The role of geographical origin and founder effects in laboratory adaptation. Evolution 62.8: 1817–1829.

                                                                                                                                                          DOI: 10.1111/j.1558-5646.2008.00423.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                          A replicated investigation of founder effects in Drosophila subobscura populations that were monitored for several functional traits. The founder events cause disparate starting points for subsequent adaptive evolution, with some key traits converging across replicates after subsequent evolution, but other traits evolving in unpredictable trajectories.

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                                                                                                                                                          • Templeton, Alan R. 1996. Experimental evidence for the genetic-transilience model of speciation. Evolution 50.2: 909–915.

                                                                                                                                                            DOI: 10.2307/2410862Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                            Reviews the extensive empirical literature on founder effect speciation and performs statistical analyses on the data to test the predictions of genetic transilience theory. These predictions are all supported in a highly significant fashion.

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                                                                                                                                                            • Yates, Matthew C., and Dylan J. Fraser. 2014. Does source population size affect performance in new environments? Evolutionary Applications 7.8: 871–882.

                                                                                                                                                              DOI: 10.1111/eva.12181Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                              This meta-analysis revealed that large populations did better than small populations when transplanted back into the native environment, but small populations tended to outperform large populations when transplanted into new environments. These results show that populations that experienced a founder event had greater adaptive potential in novel environments than large populations.

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                                                                                                                                                              Inferences of Epistasis

                                                                                                                                                              Epistasis is a critical component of all the models of founder effect speciation. When the basic models of founder effect speciation were first formulated, there was little evidence for epistasis, but that was because epistasis was difficult to detect with the technologies available at the time. Among the few exceptions were the experiments reported on in Templeton 1979 using the unique parthenogenetic/sexual abilities of Drosophila mercatorum, which detected extensive epistasis throughout the genome. However, this situation has changed considerably, as shown by the many examples in the book Epistasis and the Evolutionary Process (Wolf, et al. 2000), which also gives examples showing that just a change in allele frequency (the most direct effect of founder events) can drastically change the amount of additivity assigned to a specific locus in an epistatic system. Studies such as Huang, et al. 2012 now provide direct confirmation from genome-wide scans that much quantitative genetic variability is hidden by epistasis in nonadditive genetic variance and that epistasis dominates the genetic architecture of many quantitative traits. Similarly, Bocianowski 2013 shows that much additive variance can be due to epistatic loci, and Landry, et al. 2007 shows that the evolvability of a gene (the ability to generate selectable phenotypic variation) depends upon the number of other genes with which it interacts and the number of different regulatory sites that are associated with the gene (another type of epistasis). The genome scan of Corbett-Detig, et al. 2013 not only revealed much epistasis in Drosophila populations but also showed that the type of epistasis that can drive reproductive isolation is polymorphic within species, and, hence, would be affected by founder events. The presumed genetic architectures underlying the models of founder effect speciation are now confirmed by modern experimental biology.

                                                                                                                                                              • Bocianowski, Jan. 2013. Epistasis interaction of QTL effects as a genetic parameter influencing estimation of the genetic additive effect. Genetics and Molecular Biology 36.1: 93–100.

                                                                                                                                                                DOI: 10.1590/S1415-47572013000100013Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                Extensive epistasis that influences many complex traits is detected in barley lines.

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                                                                                                                                                                • Corbett-Detig, Russell B., Jun Zhou, Andrew G. Clark, et al. 2013. Genetic incompatibilities are widespread within species. Nature 504.7478: 135–137.

                                                                                                                                                                  DOI: 10.1038/nature12678Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                  Applying a new technique for detecting epistasis to a large panel of Drosophila melanogaster lines indicated that epistasis was common. Moreover, the authors inferred that these epistatic systems were segregating contemporaneously within species and were amenable to drive reproductive isolation. They also showed that many other designs lack the power or genome-wide scope to detect epistasis.

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                                                                                                                                                                  • Huang, Wen, Stephen Richards, Mary Anna Carbone, et al. 2012. Epistasis dominates the genetic architecture of Drosophila quantitative traits. Proceedings of the National Academy of Sciences of the United States of America 109.39: 15553–15559.

                                                                                                                                                                    DOI: 10.1073/pnas.1213423109Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                    Although most traits in Drosophila had been characterized as mostly additive with classical quantitative genetic techniques, this modern SNP-based analysis underscores the importance of epistasis as a principal factor that determines variation for quantitative traits.

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                                                                                                                                                                    • Landry, Christian R., Bernardo Lemos, Scott A. Rifkin, W. J. Dickinson, and Daniel L. Hartl. 2007. Genetic properties influencing the evolvability of gene expression. Science 317.5834: 118–121.

                                                                                                                                                                      DOI: 10.1126/science.1140247Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                      Mutation accumulation experiments in yeast revealed that the sensitivity of a gene to mutations affecting gene expression increases with the number of other genes that influence the expression of the focal gene. Genes with this type of expression epistasis are the more evolvable.

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                                                                                                                                                                      • Templeton, Alan R. 1979. The unit of selection in Drosophila mercatorum. II. Genetic revolution and the origin of coadapted genomes in parthenogenetic strains. Genetics 92.4: 1265–1282.

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                                                                                                                                                                        A low-resolution precursor of a QTL experiment, the unique parthenogenetic/sexual system of D. mercatorum revealed extensive epistasis for fitness. These results inspired the development of genetic transilience theory.

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                                                                                                                                                                        • Wolf, Jason B., I. Edmund, D. Brodie III, and Michael J. Wade, eds. 2000. Epistasis and the evolutionary process. Oxford: Oxford Univ. Press.

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                                                                                                                                                                          An excellent book that explores the evolutionary significance of epistasis theoretically and empirically, and with natural examples.

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                                                                                                                                                                          Quantitative Genetics Experiments

                                                                                                                                                                          The premise that founder events can increase additive variance and hence the responsiveness to natural selection has now been empirically verified in multiple systems, as indicated by the many references given in Templeton 2008 (cited under General Overviews). The only remaining controversy is the relative importance of dominance and epistasis in the conversion of nonadditive genetic variance into additive genetic variance. Discriminating between dominance and epistasis is often difficult but can be done with some experimental designs (Meffert 2000). The authors of Bryant and Meffert 1996, working with houseflies, show that the more there is epistasis in the ancestral population, the greater the increase in additive variance after a bottleneck. The authors of Jarvis and Cheverud 2009 used a genome scan of mice strains to produce an especially detailed view of epistatic architectures, and they also found large epistatic effects contributing to the marginal genotypic values after simulated bottlenecks. Bradshaw and Holzapfel 2000 and van Heerwaarden, et al. 2008 both show that increases in additive variance in different insect species after a bottleneck were due to both dominance and epistasis. These results provide empirical confirmation of most quantitative genetic models that predict an increase in additive variance due to epistasis and dominance after a founder event, but they are incompatible with the quantitative genetic model of Turelli and Barton 2006 (cited under Quantitative Genetic Theory), which predicts little role for epistasis in this increase.

                                                                                                                                                                          • Bradshaw, William E., and Christina M. Holzapfel. 2000. The evolution of genetic architectures and the divergence of natural populations. In Epistasis and the evolutionary process. Edited by Jason B. Wolf, Edmund D. Brodie III, and Michael J. Wade, 245–263. Oxford: Oxford Univ. Press.

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                                                                                                                                                                            Experiments in bottlenecked populations of insects showed that epistasis and dominance both contributed to additive variance.

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                                                                                                                                                                            • Bryant, Edwin H., and Lisa M. Meffert. 1996. Nonadditive genetic structuring of morphometric variation in relation to a population bottleneck. Heredity 77:168–176.

                                                                                                                                                                              DOI: 10.1038/hdy.1996.121Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                              One of the first experiments that partitioned the effects of dominance and epistasis. The authors found a significant positive relationship between the initial percentage contribution of epistasis in the ancestral population and the gain in the additive component in the founder lines.

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                                                                                                                                                                              • Jarvis, J. P., and J. M. Cheverud. 2009. Epistasis and the evolutionary dynamics of measured genotypic values during simulated serial bottlenecks. Journal of Evolutionary Biology 22.8: 1658–1668.

                                                                                                                                                                                DOI: 10.1111/j.1420-9101.2009.01776.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                One of the most detailed examinations of the role of epistasis in recombinant inbred strains of mice. The authors experimentally identified epistatic genetic architectures, and then simulated founder events with this known genetic architecture, showing that additive variance was preserved or inflated by the founder event.

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                                                                                                                                                                                • Meffert, Lisa Marie. 2000. The evolutionary potential of morphology and mating behavior: The role of epistasis in bottlenecked populations. In Epistasis and the evolutionary process. Edited by Jason B. Wolf, Edmund D. Brodie III, and Michael J. Wade, 177–193. Oxford: Oxford Univ. Press.

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                                                                                                                                                                                  Discusses experimental designs in quantitative genetics, showing how some can subdivide the nonadditive variance into dominance and epistasis whereas others cannot. Some experiments are reviewed, indicating an important role for epistasis in founder populations.

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                                                                                                                                                                                  • van Heerwaarden, Belinda, Yvonne Willi, Torsten N. Kristensen, and Ary A. Hoffmann. 2008. Population bottlenecks increase additive genetic variance but do not break a selection limit in rain forest Drosophila. Genetics 179.4: 2135–2146.

                                                                                                                                                                                    DOI: 10.1534/genetics.107.082768Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                    One generation after a founding event, Drosophila bunnanda populations showed a significant increase in the additive variance for desiccation resistance. Line crosses revealed that both dominance and epistasis contributed to this increase.

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                                                                                                                                                                                    Macroevolution

                                                                                                                                                                                    Macroevolution refers to evolution at the species level and above. Eldredge and Gould 1972 invokes Mayr’s idea of genetic revolution to explain a pattern of long periods of stasis in the fossil record punctuated by brief period of rapid evolution—a pattern the authors call punctuated equilibrium. Punctuated equilibrium became controversial (see Charlesworth, et al. 1982, cited under Adaptive Peak Shifts). The coarse timescale resolution of the fossil record allows many modes of speciation to produce punctuated patterns. Nevertheless, some studies indicate a punctuated pattern related to founder event speciation, such as in the reef corals studied in Budd and Pandolfi 2010 and in the reef hermit crabs studied in Malay and Paulay 2010. Genetic transilience theory predicts that founder effect speciation should be rare, making it an unlikely choice for the widespread patterns described in Eldredge and Gould 1972. However, rare does not mean unimportant. For example, the rarest class of mutations (beneficial mutations) is responsible for adaptation, a major feature of evolution. Beneficial mutations play an important role in evolution because natural selection amplifies their frequency and impact over time. Likewise, forces can operate at the macroevolutionary level to amplify the importance of specific speciation mechanisms. Templeton 1986 argues that this amplification can be seen in the Hawaiian Drosophilidae. The conditions in Hawaii are favorable for making founder effect speciation highly probable in this family, but Hawaii represents only a minuscule fraction of the area inhabited by Drosophilidae. Nevertheless, the rapid rate of founder effect speciation has allowed this miniscule area to account for about a quarter of the Drosophilidae species in the entire world. The tendency of founder effect speciation to induce novel evolutionary trajectories is demonstrated by the many evolutionary extremes for morphology, behavior, and ecology within this family in Hawaii. Finally, the genus Scaptomyza evolved in Hawaii and then expanded to the rest of the world, a hypothesis confirmed by the molecular study O’Grady and DeSalle 2008. Hence, this rare mode of speciation that is confined to a tiny portion of the world has had a major impact on the species diversity of the family throughout the world. This pattern in which founder effect speciation is geographically restricted but is amplified to global importance has been observed in fossil marine taxa over geological time in Jablonski, et al. 1983, and in fossil terrestrial plant communities in Dimichele and Aronson 1992. In this manner, a rare form of speciation has had a major impact on the earth’s biodiversity over space and geological time.

                                                                                                                                                                                    • Budd, Ann F., and John M. Pandolfi. 2010. Evolutionary novelty is concentrated at the edge of coral species distributions. Science 328.5985: 1558–1561.

                                                                                                                                                                                      DOI: 10.1126/science.1188947Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                      As noted by Mayr, founder effect speciation tends to be concentrated on the periphery of the ancestral species range. Using morphometrics on Pleistocene corals and genetics on living corals, the authors discovered that lineages tend to be static in the central locations but the peripheral locations yield more evolutionary change.

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                                                                                                                                                                                      • Dimichele, W. A., and R. B. Aronson. 1992. The Pennsylvanian-Permian vegetational transition: A terrestrial analog to the onshore-offshore hypothesis. Evolution 46.3: 807–824.

                                                                                                                                                                                        DOI: 10.2307/2409648Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                        Another application of founder speciation theory to trends found in fossil plant evolution.

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                                                                                                                                                                                        • Eldredge, Niles, and Stephen J. Gould. 1972. Punctuated equilibria: An alternative to phyletic gradualism. In Models in paleobiology. Edited by Thomas J. M. Schopf, 82–115. San Francisco: Freeman, Cooper.

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                                                                                                                                                                                          A classic paper in macroevolution and paleontology. The authors argue that the fossil record is mostly one of evolutionary stasis punctuated by brief periods of rapid evolution, a pattern that could be explained by Mayr’s genetic revolution model.

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                                                                                                                                                                                          • Jablonski, David, J. John Sepkoski Jr., David J. Bottjer, and Peter M. Sheehan. 1983. Onshore-offshore patterns in the evolution of Phanerozoic shelf communities. Science 222.4628: 1123–1125.

                                                                                                                                                                                            DOI: 10.1126/science.222.4628.1123Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                            The first paper to apply population genetic and founder speciation theory to the interpretation of the fossil record.

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                                                                                                                                                                                            • Malay, Maria Celia D., and Gustav Paulay. 2010. Peripatric speciation drives diversification and distributional pattern of reef hermit crabs (Decapoda: Diogenidae: Calcinus). Evolution 64.3: 634–662.

                                                                                                                                                                                              DOI: 10.1111/j.1558-5646.2009.00848.xSave Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                              Speciation in this group of crabs is largely at remote, peripatric locations that are conducive to founder events. Color patterns implicated in sexual selection evolve rapidly in the peripatric populations.

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                                                                                                                                                                                              • O’Grady, Patrick, and Rob DeSalle. 2008. Out of Hawaii: The origin and biogeography of the genus Scaptomyza (Diptera: Drosophilidae). Biology Letters 4.2: 195–199.

                                                                                                                                                                                                DOI: 10.1098/rsbl.2007.0575Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                Molecular phylogenies clearly show that the global genus Scaptomyza evolved from Hawaiian Drosophila in Hawaii and later spread to other parts of the world, confirming earlier speculations.

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                                                                                                                                                                                                • Templeton, Alan R. 1986. The relation between speciation mechanisms and macroevolutionary patterns. In Evolutionary processes and theory. Edited by Samuel Karlin and Eviator Nevo, 497–512. New York: Academic Press.

                                                                                                                                                                                                  DOI: 10.1016/B978-0-12-398760-0.50025-0Save Citation »Export Citation »E-mail Citation »

                                                                                                                                                                                                  A discussion of the macroevolutionary importance of founder speciation, with the Hawaiian drosophilids being the primary example.

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