Review
Ecology and the origin of species

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Abstract

The ecological hypothesis of speciation is that reproductive isolation evolves ultimately as a consequence of divergent natural selection on traits between environments. Ecological speciation is general and might occur in allopatry or sympatry, involve many agents of natural selection, and result from a combination of adaptive processes. The main difficulty of the ecological hypothesis has been the scarcity of examples from nature, but several potential cases have recently emerged. I review the mechanisms that give rise to new species by divergent selection, compare ecological speciation with its alternatives, summarize recent tests in nature, and highlight areas requiring research.

Section snippets

Mechanisms of ecological speciation

‘Ecological speciation’ is a concept that unites speciation processes in which reproductive isolation evolves ultimately as a consequence of divergent (including disruptive) selection on traits between environments. ‘Environment’ refers to biotic and abiotic elements of habitat (e.g. climate, resources and physical structure) as well as to interactions with other species (e.g. resource competition, predation, mutualism and various forms of interspecific interference). A diversity of

Tests of ecological speciation

Tests of ecological speciation must consider the alternatives that need to be distinguished (Box 1): speciation by ordinary genetic drift or genetic drift during population bottlenecks 27, 28; speciation by fixation of alternative advantageous genes in populations experiencing similar selection pressures 29; and speciation by polyploidy 30, 31. Speciation by polyploidy can be readily diagnosed genetically, but although it is more common in plants than animals, polyploidy accounts for only 2–4%

Discussion

Speciation is one of the least understood major features of evolution. The main obstacle to progress is the variety of mechanisms that might lead to the evolution of reproductive isolation (Box 1), any one of which can be difficult to rule out in a specific case. The upshot is that it is still difficult to point to even two species in nature and state with confidence the mechanism that produced them. The exceptions are speciation events resulting from polyploidy, because polyploidy leaves a

Acknowledgements

I thank N. Barton, S. Via and other anonymous reviewers for their suggestions. My research is funded by the Natural Sciences and Engineering Research Council of Canada (NSERC).

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