Classical models of sexual selection posit net stabilizing selection on male signal traits due to countervailing forces of natural and sexual selection (Kirkpatrick, 1982; Lande, 1981). That is, there is often a trade-off between the cost of a mating signal trait and the benefit of that trait for increased reproduction (e.g., Heinen-Kay et al., 2014). Net stabilizing selection can be disrupted by changes to the strength of natural and/or sexual selection: Male signal traits can become more elaborated under low predation regimes (e.g.,
Divergence in mating signals typically accompanies speciation. We examine two ecologically divergent sibling species of crickets to assess the degree and timing of the evolution of prezygotic reproductive isolation. Gryllus saxatilis occurs in rocky habitats throughout western North America with long-winged individuals capable of long-distance dispersal; Gryllus navajo is endemic to red-rock sandstone areas of south-eastern Utah and north-eastern Arizona and has short-winged individuals only capable of limited dispersal. Previous genetic work suggested some degree of introgression and/or incomplete lineage sorting is likely. Here we: (1) use restriction site associated DNA sequencing (RAD-seq) genetic data to describe the degree of genetic divergence among species and populations; (2) examine the strength of prezygotic isolation by (i) quantifying differences among male mating songs, and (ii) testing whether females prefer G. saxatilis or G. navajo calling songs. Our results show that genetically distinct “pure” species populations and genetically intermediate populations exist. Male mating songs are statistically distinguishable, but the absolute differences are small. In playback experiments, females from pure populations had no preference based on song; however, females from a genetically intermediate population preferred G. navajo song. Together these results suggest that prezygotic isolation is minimal, and mediated by female behaviour in admixed populations.
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