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Mating trait divergence in Habronattus americanus jumping spiders and sex ratio evolution under sexual conflict Blackburn, Gwylim Seaton


I investigated two topics in evolution. The first concerns the potential role of sexual selection in population divergence of H. americanus jumping spiders. Field observations suggest males seldom feed yet travel widely, apparently seeking mates. Males displayed vigorously to females, whereas females appeared highly choosy. The apparent absence of antagonistic ehavior during male-male interactions suggests mate competition in this species is mediated by female choice. I assessed if female preferences for local males promote reproductive isolation among three H. americanus populations that are each monomorphic for a different male sexual display morph. Supporting this idea, virgin females copulated more often with local compared to foreign males during mate trials. However, the effect of other mating interaction components on mating success remains to be resolved. All crosses produced offspring, ruling out strong intrinsic reproductive barriers among morphs and suggesting divergent female mate preferences may constitute an early source of reproductive isolation. I documented low genetic divergence among several populations, further indicating selection underlies the stark display differences between them. Further, this force appears to counteract gene flow: among-morph population comparisons show “isolation by distance”, despite the fact that phenotypically similar populations, which are scattered widely across the study area, are relatively closely related. This implies a greater exchange of genes between phenotypically similar populations. Collectively, these results implicate divergent selection on, or correlated with, male sexual displays at an early stage of differentiation in this species. In a separate study, two colleagues and I use genetic models to demonstrate that sex ratio adjustment (SRA) by parents can reduce intralocus sexual conflict (IASC) by directing alleles of a sexually antagonistic trait to the sex of offspring they benefit. If the trait is autosomally inherited, this strategy evolves irrespective of which parent’s genotype SRA is based on. It can also evolve when the trait is sex-linked, provided decisions are based on the genotype of the homogametic sex—SRA based on the heterogametic sex instead promotes fixation of the allele that is detrimental to that sex. These results suggest sexual conflict might account for previously unexplained variation in the occurrence of SRA in nature.

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