Publication Date

2012

Document Type

Dissertation

Committee Members

Volker Bahn (Committee Member), Kevin Omland (Committee Member), Jeffrey Peters (Advisor), Michael Raymer (Committee Member), John Stireman Iii (Committee Member)

Degree Name

Doctor of Philosophy (PhD)

Abstract

Genetic diversity and divergence at a locus are the result of interactions among the fundamental evolutionary forces of mutation, genetic drift, gene flow and natural selection. Variation in the strength of these forces can cause high heterogeneity in diversity and divergence across the genome. The overall objective of this thesis was to examine the role of population history vs. selection in generating heterogeneity in genetic diversity and differentiation. In Chapter 1, I examine the role of dispersal behavior in causing genetic differentiation and population structure within and between two morphologically distinct Australian duck species that differ in ecology and life history characteristics. A five-locus nuclear dataset revealed nearly no divergence and similar values of genetic diversity between species. However, as predicted, I found significant population structure in the sedentary chestnut teal (Anas castanea) but no structure within the vagile grey teal (A. gracilis). In Chapter 2, a more rigorous examination of differentiation among nineteen autosomal loci also failed to uncover a genetic distinction between these two species. However, DNA sequences from seven loci sampled from the Z-chromosome revealed strong differentiation between chestnut and grey teal. Furthermore, the most divergent loci are clustered on the shorter p-arm of the chromosome, close to the centromere, suggesting this region as an island of differentiation that may have been important in the speciation process. These two species of Australian teal are perhaps the most recently diverged taxa examined to date that reveal a large Z-effect.

In Chapter 3, I quantitatively tested the contribution of gene flow and introgression to the heterogeneity of genetic diversity and differentiation in two deeply divergent taxa, the falcated duck (A. falcata) and the gadwall (A. strepera). Consistent with previously published mitochondrial DNA analyses, 19 nuclear loci revealed the introgression of nuclear DNA from the falcated duck into the allopatric population of gadwall in North America, but not into the sympatric population in Eurasia. Furthermore, gene flow was insufficient to explain the high heterogeneity in genetic diversity in both species and differentiation between the taxa. Indeed, this heterogeneity failed to fit neutral models of population history, suggesting that selection may be having a pervasive effect throughout the genome.

Overall, this research reveals heterogeneous patterns of diversity and differentiation among nuclear loci in both early and later stages of divergence. Gene flow alone could not explain this heterogeneity, suggesting a prominent role of selection. Substantial divergence at some loci suggests that the strength of divergent selection overrides the homogenizing effects of gene flow and maintains species integrity.

Page Count

140

Department or Program

Department of Earth and Environmental Sciences

Year Degree Awarded

2012

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