Ecological processes are central to the formation of new species when barriers to gene flow (reproductive isolation) evolve between populations as a result of ecologically-based divergent selection. Although laboratory and field studies provide evidence that Ôecological speciationÕ can occur, our understanding of the details of the process is incomplete. Here we review ecological speciation by considering its constituent components: an ecological source of divergent selection, a form of reproductive isolation, and a genetic mechanism linking the two. Sources of divergent selection include differences in environment or niche, certain forms of sexual selection, and the ecological interaction of populations. We explore the evidence for the contribution of each to ecological speciation. Forms of reproductive isolation are diverse and we discuss the likelihood that each may be involved in ecological speciation. Divergent selection on genes affecting ecological traits can be transmitted directly (via pleiotropy) or indirectly (via linkage disequilibrium) to genes causing reproductive isolation and we explore the consequences of both. Along with these components, we also discuss the geography and the genetic basis of ecological speciation. Throughout, we provide examples from nature, critically evaluate their quality, and highlight areas where more work is required.

Natural selection plays a fundamental role in most theories of speciation, but empirical evidence from the wild has been lacking. Here the post-Pleistocene radiation of threespine sticklebacks was used to infer natural selection in the origin of species. Populations of sticklebacks that evolved under different ecological conditions show strong reproductive isolation, whereas populations that evolved independently under similar ecological conditions lack isolation. Speciation has proceeded in this adaptive radiation in a repeatable fashion, ultimately as a consequence of adaptation to alternative environments.

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Mechanisms of speciation are not well understood, despite decades of study. Recent work has focused on how natural and sexual selection cause sexual isolation. Here, we investigate the roles of divergent natural and sexual selection in the evolution of sexual isolation between sympatric species of threespine sticklebacks. We test the importance of morphological and behavioral traits in conferring sexual isolation and examine to what extent these traits have diverged in parallel between multiple, independently evolved species pairs. We use the patterns of evolution in ecological and mating traits to infer the likely nature of selection on sexual isolation. Strong parallel evolution implicates ecologically based divergent natural and/or sexual selection, whereas arbitrary directionality implicates nonecological sexual selection or drift. In multiple pairs we find that sexual isolation arises in the same way: assortative mating on body size and asymmetric isolation due to male nuptial color. Body size and color have diverged in a strongly parallel manner, similar to ecological traits. The data implicate ecologically based divergent natural and sexual selection as engines of speciation in this group.

Hybrids may suffer a reduced fitness both because they fall between ecological niches (ecologically dependent isolation) and as a result of intrinsic genetic incompatibilities between the parental genomes (ecologically independent isolation). Whereas genetic incompatibilities are common to all theories of speciation, ecologically dependent isolation is a unique prediction of the ecological model of speciation. This prediction can be tested using reciprocal transplants in which the fitness of various genotypes is evaluated in both parental habitats. Here we expand a quantitative genetic model of Lynch (1991) to include two parental environments. We ask whether a sufficient experimental design exists for detecting ecologically dependent isolation. Analysis of the model reveals that by using both backcrosses in both parental environments, environment‐specific additive genetic effects can be estimated while correcting for any intrinsic genetic isolation. Environment‐specific dominance effects can also be estimated by including the F1 and F2 in the reciprocal transplant. In contrast, a reciprocal transplant comparing only F1s or F2s to the parental species cannot separate ecologically dependent from intrinsic genetic isolation. Thus, a reduced fitness of F1 or F2 hybrids relative to the parental species is not sufficient to demonstrate ecological speciation. The model highlights the importance of determining the contribution of genetic and ecological mechanisms to hybrid fitness if inferences concerning speciation mechanisms are to be made.