Hybrid zone theory and empirical studies of behavior and population genetics of the field crickets Gryllus texensis and Gryllus rubens
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I investigated hybrid zone evolution empirically and theoretically. I investigated sexually selected behaviors and molecular population genetics in two species of field crickets, Gryllus texensis and G. rubens. I found that three univariate male calling song parameters (pulse rate, trill length, and intertrill interval) could not separate species in allopatry, in contrast to previous reports that pulse rate is species-specific. Using a multivariate measure of all three song parameters for each male, I was able to separate species in allopatric populations. In sympatry I found call measures were continuous and uni-modally distributed. For two kinds of preference tests on females of both species, the species differed significantly in the average strength of preference in the direction of preferring their own species’ multivariate calling song. I investigated the molecular population genetics in several populations across both species allopatry and sympatry using dominant ISSR molecular markers. I calculated a multivariate, species-specific score for each individual using all loci and found that allopatric regions of each species are well separated using this score. These two species are not strongly genetically diverged from each other by both traditional population genetic structure and by the distribution of the species-specific genotypes in sympatry. The distribution of multivariate species-specific genotypes in sympatry is unimodal. I also compared the genetic score to a similar species-specific score for male calling song measured on the same individuals and found a positive, significant covariance in sympatry. I also present a population genetic model of hybrid zone evolution that unifies two previously disparate perspectives. Clinal variation and reproductive character displacement are two extremes of a continuum of geographic patterns that a trait can show in a hybrid zone and I investigate the circumstances under which each condition evolves in the hybrid zone. In this model, two diverged allopatric populations send migrants into a common sympatric hybrid zone. Sexual selection is the primary determinant of whether clinal variation or reproductive character displacement occurs. Natural selection on hybrid incompatibility always makes displacement more likely, but cannot cause displacement by itself. Reduced gene flow into sympatry and linkage both make displacement the more likely.