Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Mark S. Hafner


The focus of this dissertation is the relative tempo of molecular evolution among organisms of varying levels of relatedness. Because this is a rapidly expanding area of research, Chapter 1 is devoted to a review of the current state of knowledge on rates of molecular evolution. In Chapter 2, I present an analysis of relative rates of evolution among several distantly related rodents. The heterogeneity in rate of evolution of the cytochrome b gene observed here cannot be explained by metabolic rate, body size, generation time, or nucleotide-generation time. Likewise, there is no evidence that these differences in rate of evolution are due to differential selection. The strongest potential correlate of rate of evolution in these data appears to be population subdivision, a factor long thought to influence heterozygosity of nuclear genes through genetic drift. However, a large number of variables, and complex interactions among variables, may influence rate of evolution of any particular gene; the relative importance of these variables probably differs from gene to gene, even within a species. Therefore, although differences in rate of molecular evolution exist, it likely will remain difficult to predict which species and which genes will show faster (or slower) rates because of the complex interactions among the numerous variables that influence rate of evolution. Cospeciating taxa offer a powerful means to compare rates of evolution between groups of organisms. Congruence in the phylogenies of two symbiotic lineages likely indicates contemporaneous cladogenic events. In Chapter 3, the phylogenies of three symbiotic groups (pocket gophers and two genera of chewing lice) are compared to evaluate their level of phylogenetic congruence, or cospeciation. Although statistical tests indicate a history of widespread cospeciation between these hosts and parasites, analysis of relative rates of evolution in these groups does not indicate the disparity in rates of evolution reported by Hafner et al. (1994).