Undergraduate Honors Theses

Thesis Defended

Spring 2011

Document Type

Thesis

Department

Ecology and Evolutionary Biology

First Advisor

Patrik Nosil

Abstract

The theory of ecological speciation suggests that adaptation to different habitats promotes the formation of new species (speciation), but debates persist about the extent to which this can occur when populations are in geographic contact and exchange genes (i.e. gene flow). Multilocus genetic analysis (using multiple genes) can inform this debate by comparing levels of genetic divergence between taxa (groups such as species) at different points in the speciation process and with differing geographic arrangements. For example, taxa undergoing less gene flow are predicted to exhibit stronger genetic divergence than taxa undergoing more gene flow. Furthermore, multilocus data can be used to test for genealogical discordance—a scenario where different genes tell different evolutionary histories, due to being differentially affected by evolutionary processes such as selection, mutation, and gene flow. The present study uses multilocus genetic data (four nuclear genes and a mitochondrial gene) to compare the genetic structure of sic populations of herbivorous Timema walking-sticks with different geographic relationships: two diverging host-associated ecotypes (phenotypically and ecologically different groups) of T.cristinae in parapatry (geographic contact) and allopatry (geographic separation), and two closely related species, T. californicum and T. poppensis, in parapatry and allopatry. As expected, we observe that species are generally more genetically differentiated than ecotypes. Interestingly, however, allopatric populations are not always significantly more differentiated than parapatric populations. Also genealogical discordance was observed. Mitochondrial differentiation among allopatric T. cristinae population was stronger than nuclear differentiation. In contrast, mitochondrial differentiation between T. californicum and T. poppensis was weak, whereas nuclear genes were differentiated to the point of reciprocal monophyly, indicative of genetically distinct species. While these data strongly suggest differing levels of gene flow among genes and taxa, further analyses using coalescent-based models should be done to estimate gene flow specifically.

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