Ecological Speciation

by

Timothy P. Craig

• LAST REVIEWED: 29 November 2018
• LAST MODIFIED: 29 November 2018
• DOI: 10.1093/obo/9780199941728-0116

Introduction

Speciation produces diversity of life on earth by splitting evolutionary lineages through the evolution of reproductive isolation between populations of a species. In ecological speciation, divergent natural selection to adapt to different environments results in the evolution of reproductive isolation. In contrast, non-ecological speciation occurs when reproductive isolation evolves due to genetic drift, founder events, or when natural selection fixes alternate advantageous alleles in different populations. In the Origin of Species (Darwin 1859, cited under History), Darwin recognized that new species evolved as the result of natural selection, but until recently the role of natural selection in speciation was relatively neglected. Explicit models of how speciation occurred were not developed until the new evolutionary synthesis in the 1940s. Enrst Mayr in Systematics and the Origin of Species (1942; see Mayr 1963, cited under Books) proposed the biological species concept that defined species as reproductively isolated populations. Mayr also developed the allopatric speciation model where divergence between populations arises due to natural selection and genetic drift, and reproductive isolation evolves as byproduct of these processes. In this model, geographic isolation is necessary to prevent gene flow until reproductive isolating mechanisms have evolved. Subsequent speciation work focused on the nature of reproductive isolating mechanisms and on the necessity of geographic barriers to gene flow during the initiation of speciation. Non-geographic speciation in sympatry or parapatry requires strong divergent natural selection for adaptation to different ecological niches, and thus by definition is always ecological speciation. Interest in ecological speciation was stimulated by studies of non-geographic speciation starting with Walsh’s proposal in “On Phytophagous Varieties and Phytophagous Species” (see Walsh 1864, cited under Ecological Speciation Due to Host-Associated Differentiation in the Apple Maggot Fly, Rhagoletis pomonella) that adaptation to different host plants can initiate speciation in sympatry. Guy Bush in “Sympatric Host Race Formation in Frugivorous Flies of the Genus Rhagoletis (Diptera, Tephritidae)” (1969) revived this hypothesis and stimulated studies of host race formation and sympatric speciation in phytophagous insects. Subsequent research on diversifying selection for host use in phytophagous insects supported the assumptions of newly developed sympatric and ecological speciation models. The importance of natural selection from many other sources in speciation was also demonstrated, such as the role-diversifying selection resulting from competition in Schluter and McPhail 1992 (cited under Ecological Speciation Due to Character Displacement), a study on “Ecological Character Displacement and Speciation in Sticklebacks.” The term ecological speciation was proposed in the paper “Ecological Speciation in Postglacial Fishes” by Schluter (see Schluter 1996, cited under Definitions), and since then there has been an explosion of studies exploring all aspects of ecological speciation.

History

The role of ecology speciation has been recognized since Darwin 1859, a seminal discussion of its role in speciation. The authors of the new evolutionary synthesis, including Dobzhansky and Huxley (see Dobzhansky 1941, Dobzhansky 1946, and Huxley 1942), discussed the role of ecology in the development of reproductive isolating mechanisms. Mayr 1947 discussed how ecology can influence the evolution of reproductive isolation, but it emphasized the role of physical isolation in restricting gene flow as being necessary to produce speciation. Mayr rejected the idea of sympatric speciation, and criticized some of Darwin’s ideas on speciation. Mallet 2008 rebutted Mayr’s critique of Darwin’s speciation ideas.

• Darwin, C. 1859. On the origin of species by means of natural selection, or the preservation of the favoured races in the struggle for life. London: John Murray.

  Darwin emphasized the importance of divergent selection for adaptation to different habitats in the origin of species. His concept of species has been interpreted in various ways. He emphasized that there was no clear delineation between varieties and species, and emphasized the importance of incompletely reproductively isolated populations. This is consistent with the contemporary idea of a speciation continuum envisioned in ecological speciation.

• Dobzhansky, T. 1941. Genetics and the origin of species. New York: Columbia Univ. Press.

  A pioneering work integrating genetics with the processes of natural selection and speciation. In particular, his discussion of the evolution of isolating mechanisms was important in the development of ideas about how ecological interactions influence speciation.

• Dobzhansky, T. 1946. Complete reproductive isolation between two morphologically similar species of Drosophila. Ecology 27:205–211.

  DOI: 10.2307/1932895

  Dobzhansky found that two partially sympatric and morphologically distinct species were reproductively isolated by assortative mating preferences and hybrid sterility. In an early invocation of an ecological speciation hypothesis, he proposed that speciation in Drosophila proceeds through physiological adaptation to different environments.

• Huxley, J. S. 1942. Evolution: The modern synthesis. New York and London: Harper & Brothers.

  Huxely clearly stated that selection to adapt to the environment is the primary factor driving speciation. He divided factors that produced speciation into ecological and genetic factors in a manner that foreshadows almost all of the later discussion of ecological speciation. He provided a detailed review of the data available at that time for ecological speciation.

• Mallet, J. 2008. Mayr’s view of Darwin: Was Darwin wrong about speciation? Biological Journal of the Linnean Society: 95:3–16.

  DOI: 10.1111/j.1095-8312.2008.01089.x

  Critiqued Mayr’s presentation of Darwin’s ideas about speciation and showed that he had misinterpreted them. Mallet showed that Darwin emphasized the importance of divergent selection and the presence of intermediate stages in speciation where varieties within species become adapted to different ecological niches. These ideas are consistent with the current ideas about ecological speciation.

• Mayr, E. 1947. Ecological factors in speciation. Evolution 1:263–268.

  Mayr provided a brief summary of his views on the role ecology in speciation in this paper. He argued that speciation occurred when the establishment of new geographically isolated populations was followed by the establishment of intrinsic reproductive isolation. He supported the importance of divergent ecological selection in speciation. However, he argued that sympatric speciation is implausible because any substantial gene flow between populations will overcome divergent selection and prevent speciation.

• Thorpe, W. H. 1945. The evolutionary significance of habitat selection. Journal of Animal Ecology 14:67–70.

  DOI: 10.2307/1385

  Thorpe argued that habitat selection can initiate the evolution of reproductive isolation, even in the absence of geographic isolation, and supported the primacy of strong divergent selection in speciation. He presented scenarios for ecologically driven divergence in the absence of geographic isolation due to host plant preference in insects and habitat preference in birds. These scenarios are consistent with sympatric and ecologic speciation.