Date of Award:

5-2002

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Edmund D. Brodie, Jr.

Committee

Edmund D. Brodie, Jr.

Committee

James A. MacMahon

Committee

Thomas C. Edwards, Jr.

Committee

Carol D. von Dohlen

Committee

Paul G. Wolf

Abstract

I examined both present-day and historical patterns in the herbivorous desert lizard, Sauromalus obesus (chuckwalla), within the Mojave Desert ecoregion in Nevada. My goal was to determine the process or processes that led to their present-day distribution. I expected to recover distinct haplotypes unique to each mountain range. These unique haplotypes were expected to reveal a pattern of geographic partitioning with sequence divergence among populations increasing as a function of distance. I characterized the distribution of chuckwallas in Nevada by means of a simple model using a geographic information system (GIS). This simple model was used to enhance sampling efficiency for the genetic study over a broad geographic area. The model predicted chuckwalla habitat to be patchy and within mountain ranges. The model correctly predicted habitat that contained chuckwallas with 86% accuracy. I used areas predicted by the model as my sample units for a phylogenetic analysis of mitochondrial DNA (mtDNA) haplotypes using maximum parsimony and likelihood methods. These traditional methods revealed support for two clades: one including chuckwallas largely from the Newberry Mountains and the other including all the other populations surveyed. Apart from these two clades, the analyses were unable to resolve relationships among the populations within the two major clades. In order to resolve any relationships within the clades, I used an analytical method that involves an overlay of geography on an estimated gene tree in a rigorous statistical framework that measures the strength of any geography/phylogeny associations. I inferred from the analysis two major events for chuckwalla populations surveyed in this study: a recent post-Pleistocene contiguous range expansion from southern Sonoran refugia north into the Mojave by way of the Muddy and Virgin river drainages, followed by local differentiation because of lack of present-day gene flow. This pattern is supported by paleoecological evidence from packrat middens that indicates chuckwallas have only been in the northern Mojave during the last 10,000 years.

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