Ecological dependence and the evolution of isolating barriers

As discussed earlier, isolating barriers in ecological speciation may be ecologically dependent or ecologically independent. The former class of barriers may evolve more quickly, but should also be more susceptible to immediate decay in the face of environmental change [35]. We suggest that isolating barriers caused by divergence in biomechanical systems, at least as conceptualized in this chapter, will be characterized substantially by ecological independence. This is because the production and evolution of mating displays usually depends more on intrinsic biomechanical properties of the organism, as compared to extrinsic ecological properties of the environment (assuming the traits involved are genetically based). In other words, the intrinsic nature of biomechanical systems suggests that the second criterion discussed in the examples above — adaptive divergence in phenotypes influencing mating displays — will be expressed partly independently of diversity in environments. Hummingbirds selected for large bodies, for example, will necessarily be constrained to produce less agile flight displays, regardless of the environments in which they display. The same would hold true for many other dynamic mating displays, such as the electric signals of fishes, visual displays in waterfowl, and calling in crickets and anurans, for which the potential for producing different kinds of displays is at least partially independent of the particular environments inhabited. For this reason we expect that isolating barriers based on biomechanical performance will initially persist in the face of environmental change. The same barriers, however, might ultimately decay if environmental changes persist long enough to cause evolutionary convergence in formerly distinct species.

We do not mean to imply that displays are not influenced by variation in environmental factors — all phenotypic traits are subject to such impacts. For example, water temperature and conductivity influence the electric organ discharges of some fishes [138], and the physical structure of vegetation in terrestrial systems influences the propagation of acoustic signals in birds [21]. Some isolating barriers may be modified by location-dependent effects, even if they are not strictly ecologically dependent. Divergence of song in oscine songbirds depends in part on learning and thus on the features of available song models [139]. Different populations of white-crowned sparrows, for example, learn local song types even though they are capable of learning and producing song types from different locales [140]. Learned female preferences for local song types might thus facilitate speciation.

Recognition that isolating barriers may include both ecologically dependent and ecologically independent components leads us to a general suggestion for future investigations of ecological speciation, as a complement to our broader suggestion of exploring mechanical and functional links between adaptation, mating displays, and reproductive isolation. Instead of showing that any given isolating barrier is ecologically independent or dependent, it might be preferable to partition relative effects between the two components. For example, one can envision a scenario in which between-type matings are found to be half as likely as within-type matings outside of the ecological context (e.g., during encounters in the laboratory), and in which between-type matings are found to be one-quarter as likely as within-type matings inside the ecological context (e.g., during encounters in nature). We could then conclude that the ecologically dependent component of isolation is one-half as strong as the ecologically independent component (0.25/0.5), and thus causes one-third of the total isolation [0.25/(0.5 + 0.25)].

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