Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Dr. Richard Crawford


The Western Grebe (Aechmophorus occidenta:lis) and Clark's Grebe (A .. clarkii) are considered to be closely related .. Until recently, they were regarded as color phases of one species--the Western Grebe. I assessed mitochondrial DNA (mtDNA) variation within and between the Aechmophorus color phases. I also evaluated the plausibility of two vicariance models that have been. proposed to explain the evolution of the color phases. Grebes were collected from seven breeding sites, representing three geographic regions of the United States-Canada breeding range. Restriction-enzyme analysis of a total sample of 109 grebes was conducted. Estimates of nucleotide divergence ( dxu dA), nucleotide diversity, and haplotypic diversity were based on restriction-site data inferred from observed restriction-fragment patterns. Population genetic structure was assessed using interdeme genetic variation ( G1n) and analysis of molecular variance (AMOVA) methods. MtDNA's of the Aechmophorus grebes showed size variation and individual heteroplasmy. No fixed fragment-pattern differences were observed between the color phases, and estimated mtDNA divergence between them was low ( dxy = 0 .. 19%; dA = zero). The level of nucleotide diversity estimated for the entire sample was 0.18%. Twelve mtDNA haplotypes were observed, with the most common haplotype being represented in 49.5% of the grebes surveyed. Haplotype frequencies were homogenous among the breeding sites and among the geographic regions. Genetic population structuring was not detected by either the G5x or the AMOVA method. A polyphyletic pattern in terms of mtDNA genealogy was observed between the color phases. The evolutionary effective population size ( Nr,a,) of Aechmophorus females was estimated to be 17,800. Relative to the current female population size ( about 60,000 to 100,000 females) , the Nft•J estimate suggests that the number of females has been relatively stable over the recent evolutionary past. Of the two vicariance models that I evaluated from a biogeographic perspective, I conclude that the model proposed by John Ratti is more unlikely than the model proposed by Jeffrey Feerer. While the mtDNA results provide only equivocal support of the proposed timing of the vicariance event hypothesized in Feerer's model, conversely the results clearly do not provide evidence for rejection of Feerer's model either.

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