Ecological speciation may be strengthened by positive feedback loops between adaptive divergence and barriers that reduce gene flow [14,35,141]. To elaborate, an initial reduction in gene flow between two ecological environments, such as that due to geographical separation, could allow some adaptive divergence to take place. This partial adaptive divergence could then further reduce gene flow (i.e., ecological speciation), which could allow further adaptive divergence, which could further reduce gene flow, and so on. We expect that phenotypic divergence and corresponding tradeoffs and constraints in biomechanical systems will contribute strongly to such feedback loops. Consider our example of song production and evolution in Darwin's finches. As populations begin to adapt to different resources, we have argued that corresponding changes in beak sizes lead to divergence in the types of songs different populations are able to sing. Insofar as this divergence in vocal "potential" leads to differences in realized song structure , individuals moving between environments could experience reduced mating success, particularly given that local females appear to adjust their preferences to match local song types [137, see also 142]. Reduced mating success of migrants between environments could further reduce gene flow between populations, thereby allowing further adaptive divergence and hence facilitating the positive feedback process.
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