Population genetics of incipient speciation in two species of jumping spiders (Salticidae: Habronattus) on the sky islands of southeast Arizona

Abstract

The population genetic forces that promote speciation, although well understood theoretically, are poorly known in nature. This dissertation focuses on the population genetics of allopatric speciation, using a system of jumping spiders (Araneae: Salticidae) whose populations are subdivided among the disjunct patches of mountain woodland habitat called "sky islands" in southeastern Arizona. I studied two species of salticids that apparently share similar histories of range fragmentation but differ greatly in their amount of intraspecific phenotypic divergence. Using sequence data from neutrally evolving mitochondrial genes, I investigated the population genetic factors influencing divergence. Analyses of gene trees for Habronattus oregonensis and H. pugillis revealed that neither gene flow, effective population size, mutation rate, nor differences in divergence time can explain the interspecific difference in phenotypic divergence. Instead, selection--in these animals, presumably sexual selection--must have acted differentially on traits encoded by nuclear loci to produce the discrepancy. A phylogeographic study of populations of H. pugillis may help clarify the influence of post-Pleistocene vegetational change on organisms dependent upon montane woodlands. Gene trees suggest limited migration between mountain ranges, but offer stronger evidence for incomplete lineage sorting. The trees provide no clear indication of the chronological sequence of woodland fragmentation, but suggest an old geographic division between northern and southern populations. Dates estimated for population divergence range from 26,000 to 291,000 years ago, but rely on molecular clock estimates from non-arachnid arthropods. Divergence estimates based on vegetation change data would require that the mutation rate be considerably faster in these spiders than in non-arachnid arthropods. Whereas there is no fossil-based molecular clock calibration for arachnids to judge whether this is likely, analyses of mitochondrial sequences from three Habronattus species do reveal other highly unusual features. For example, secondary structures that were inferred from DNA sequences of tRNA genes lack the TPsiC arm, and therefore are predicted not to form the standard tRNA cloverleaf. In addition, the 3' half of the gene encoding ribosomal 16S RNA appears to fold to a normal arthropod-like secondary structure, but the 5' half is extremely divergent and truncated with respect to other arthropods.