The contribution of ecological trait divergence to speciation depends on the "success" (i.e., fitness) of migrants between environments [14,35], as well as the success of any resulting hybrids (Figure 14.1). Success in both cases depends on the ability to exploit available resources, and is evident in the survival, growth, and fecundity of individuals before reproduction, i.e., natural selection, as well as on their ability to obtain mates, i.e., sexual selection. Both natural and sexual selection can be important in isolating populations from different ecological environments. With respect to natural selection, ecological and life-history performance (e.g., in resource use, growth, survival, and fecundity) is often low for migrants between environments relative to residents [14,32,33,143], or for hybrids in either parental environment relative to pure forms [23,33]. With respect to sexual selection, mating probabilities are often low for individuals from different environments relative to individuals from similar environments [26,28,144] or for hybrids relative to pure forms .
Isolating barriers manifest through either natural or sexual selection have been discussed at length [3,4], but the traits on which they act are usually, if implicitly, considered distinct entities with largely independent implications for speciation. And yet both types of barriers could be correlated if the trait(s) under natural and sexual selection share a common biomechanical basis. In such cases, adaptive divergence of a single trait can have dual fitness consequences. Beak size evolution in Darwin's finches, to illustrate, should have consequences for both feeding ability (natural selection) and mate acquisition (sexual selection). The possibility of such correlated isolating barriers is worth considering because they could increase the importance of a small subset of traits to ecological speciation.
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