This book addresses the interplay of biomechanics and ecology. Ecology has long been recognized as an important factor in evolutionary diversification and speciation. Architects of the neo-Darwinian synthesis, particularly Mayr  and Dobzhansky , argued that spatial variation in ecological parameters should facilitate divergent trajectories of adaptive evolution among populations, at least among populations that are able to maintain some degree of reproductive isolation. This insight was overshadowed for several decades by attention to genetic mechanisms of divergence and stochastic models of speciation. Empirical and conceptual advances in recent years, however, have spurred a renewed emphasis on ecological causes of evolutionary diversification and speciation [3-5].
It thus seems timely to consider how biomechanics, through its interface with ecology, might affect the processes of evolutionary diversification and speciation. The possibilities here are admittedly broad. For the purposes of this chapter, we focus on a "by-product" model of speciation. This model features two stages. In the first, adaptive divergence of phenotypic traits drives, as an incidental consequence, divergence in mechanisms that mediate the expression and production of mating
displays. Second, resulting divergent evolution of display behavior facilitates reproductive isolation, further adaptive divergence, and, ultimately, speciation. In this chapter we evaluate this model's conceptual foundations, review supporting empirical evidence, and outline some of its evolutionary implications. We begin with a more detailed explanation of the by-product model.
Consider an ecological resource that takes two discrete forms, ResourceA and ResourceB, with the frequencies of these forms varying nonrandomly in space. Now consider an animal species possessing some morphological, behavioral, or physiological trait used for exploiting this resource. Assume that different trait values are best suited for the different resource forms, say TraitA for ResourceA and TraitB for ResourceB. As long as these trait values are heritable, and as long as dispersal among sites with different resources is somewhat limited — thus restricting gene flow — natural selection should favor the evolution of TraitA in sites where ResourceA predominates, and the evolution of TraitB in sites where ResourceB predominates. Our example thus far follows the well-established logic of adaptive divergence in response to natural selection in distinct ecological environments [1-3,6-10].
Now consider the possibility that evolution of the trait in question also influences, as a by-product of selected changes in morphology, physiology, or behavior, the kind of mating displays these animals can express or produce. For example, individuals possessing TraitA might be constrained to produce a particular display variant, DisplayA, whereas those possessing TraitB might necessarily produce another, DisplayB. Possible biomechanical causes of correlated evolution among adaptive traits and mating displays are detailed later in this chapter. As male displays begin to diverge between sites, females would be expected to evolve, through sexual selection, divergent preferences that mirror the changes in display structure [11,12]. (We assume, for present purposes, that only males display and that females use displays to guide mate choice.) With sufficient time and sufficient limits on gene flow, ResourceA environments should thus evolve populations wherein males possess TraitA and produce DisplayA, and wherein females respond preferentially to DisplayA. At the same time, ResourceB environments should support populations that evolve the other suite of characteristics (TraitB and DisplayB). If individuals from these two ecological environments then come into secondary contact, the probability of mating should be diminished, and speciation thus initiated.
The above scenario for divergence is conceptualized, for the sake of argument, as occurring in allopatry (separate and isolated locations) or parapatry (separate but not isolated locations). In the remainder of this chapter, "populations" or "environments" are thus envisioned as geographically distinct, and "migrants" as individuals that move between populations or environments. It is important to point out, however, that many of the same processes could in principle occur in sympatry (populations diverging in the same physical location). Under sympatric divergence, different groups of individuals may specialize on different resources within a common geographical location. Here "populations" would refer to sympatric groups using the distinct resource "environments," and "migrants" would be individuals that switch resources. It is not our intention to distinguish between these geographical scenarios because we are more interested in general mechanisms.
Our chapter continues with a brief overview of modes of speciation, focusing in particular on a distinction between ecologically dependent and ecologically independent isolating barriers. Attention to this distinction will be helpful later as we evaluate the possible impacts of adaptive divergence and concomitant evolution in the biomechanical bases of display behavior.
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