Speciation

by

Richard G. Harrison

• LAST REVIEWED: 13 January 2014
• LAST MODIFIED: 13 January 2014
• DOI: 10.1093/obo/9780199941728-0019

Introduction

The diversity of life on earth is a balance between the origination rate of new lineages or species and the rate at which such lineages go extinct. Therefore, how new species arise is a central problem in evolutionary biology, and a substantial and expanding literature focuses on the pattern and process of speciation. Multiple species concepts or definitions persist in evolutionary and systematic biology, but they all include the notion that species are independent lineages, with distinct evolutionary trajectories. Much of the speciation literature focuses on the origin of intrinsic barriers to gene exchange; these are phenotypic differences between lineages that limit or prevent interbreeding or result in the reduced fitness of hybrid offspring. Barriers to gene exchange (sometimes referred to as speciation phenotypes) are diverse; they may result from ecological or behavioral differences, differences in life cycle, or differences in physiology, biochemistry, or development. However, the central theme is always how divergence in these traits impacts the extent of genetic exchange. This focus restricts attention to organisms with sexual reproduction, and the process of “speciation” in asexual lineages does not easily fit within this framework. The speciation literature has been characterized by several long-term debates; these revolve around the importance of geographic isolation for speciation, the importance of genetic drift for divergence, and the roles of natural and sexual selection in the speciation process. A further complication is that amounts of gene exchange between diverging lineages can vary across the genome, dependent on the genomic location of targets of natural and sexual selection and on rates of recombination among genome regions. This genic view not only requires some rethinking of our definitions and concepts of species and speciation, but also provides an important framework for investigating the genetic basis of reproductive isolation.

General Overviews

Although many early naturalists and biologists (including Charles Darwin in On the Origin of Species) wrote about the nature and origin of species, the modern literature on speciation really begins with two books published in the mid-20th century, Dobzhansky 1937 by Theodosius Dobzhansky, a Drosophila geneticist, and Mayr 1942 by Ernst Mayr, an avian systematist. The ideas developed in these books provided the essential foundation for subsequent empirical and theoretical work. Later, Mayr modified and expanded his thinking (and provided additional examples) in Mayr 1963 (Animal Species and Evolution). The ideas of Mayr and Dobzhansky provide the context for much of the subsequent speciation literature (e.g., the debate about sympatric speciation, see Section on Allopatry, Parapatry, Sympatry). The 21st century has seen a number of new books on speciation and related topics. The most comprehensive is Coyne and Orr 2004, which represents an excellent summary of both concepts and data. The authors are Drosophila biologists/geneticists, and the book tends to emphasize dimensions of speciation biology that are frequently addressed in the Drosophila literature. Gavrilets 2004 provides an advanced discussion of major theoretical and mathematical aspects of speciation, mostly for the specialist. Several recent books on speciation and diversification not only focus on particular taxa, but also consider broader issues; these include Price 2007 (Speciation in Birds), Grant and Grant 2007 (How and Why Species Multiply: The Radiation of Darwin’s Finches), and Losos 2009 (Lizards in an Evolutionary Tree).

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

  A comprehensive coverage of species and speciation—definitely the place to start. Excellent review of the literature, but a lot has happened (especially in genetics and genomics) since this book was published.

• Dobzhansky, T. 1937. Genetics and the origin of species. New York: Columbia Univ. Press.

  A classic—provides an important historical perspective. Acknowledges the “fundamental importance of isolation in the evolutionary process” (p. 228) and devotes a chapter to classifying and characterizing “isolating mechanisms.” Hybrid sterility is a chapter unto itself (pp. 259–302)—foreshadowing the tendency of Drosophila evolutionary geneticists to study post-zygotic barriers.

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

  A generally very successful attempt to summarize how mathematical theory can inform us about the conditions under which speciation can proceed. An objective evaluation, without the bias that familiarity with a single system often engenders. But beware—this volume is not for the mathematically naive.

• Grant, P. R., and B. R. F. Grant. 2007. How and why species multiply: The radiation of Darwin’s finches. Princeton, NJ: Princeton Univ. Press.

  Written by a husband/wife pair who have studied Darwin’s finches for decades, this concise book summarizes the evidence that allows us to understand how and why the finches have diversified on the Galápagos Islands. A very accessible account.

• Losos, J. B. 2009. Lizards in an evolutionary tree. Berkeley: Univ. of California Press.

  Not really a book about speciation in the narrow sense, but very much about the ecology, morphology, and behavior of a group of organisms (Anolis lizards). The intent is to examine, in a phylogenetic context, how those traits influence diversification and adaptive radiation. Obviously speciation is a fundamental part of the story.

• Mayr, E. 1942. Systematics and the origin of species. New York: Columbia Univ. Press.

  Mayr’s contribution to the “Modern Synthesis”—the attempt to merge ideas and data from genetics, systematics, and paleontology. With Dobzhansky 1937, a classic contribution to the early literature. Introduces many ideas that are still current or at least still being debated.

• Mayr, E. 1963. Animal species and evolution. Cambridge, MA: Belknap.

  A more “mature” version of the 1942 book, but with many of the same themes. Perhaps a bit more genetics, and surely many more examples to buttress the arguments. Mayr had little doubt that his ideas were correct, but many of those ideas have not stood the test of population genetics theory and molecular data.

• Price, T. 2007. Speciation in birds. Greenwood Village, CO: Roberts.

  An overview of speciation in birds, with a clear focus on the role of ecology and behavior in the origin of species. The book is more than a comprehensive summary of the literature, providing interpretation of data and conclusions about patterns and pathways of speciation in birds.