Date of Award:

5-1997

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Paul G. Wolf

Committee

Paul G. Wolf

Committee

James W. Haefner

Committee

Frank J. Messina

Committee

Eugene W. Schupp

Committee

D. Richard Cutler

Abstract

Population genetics is the study of the mechanisms that cause genetic change in populations over time. Genetic changes may lead to both adaptive evolution and speciation. While the former process is fairly well understood, many questions remain unanswered with regard to the process of speciation. How important is population isolation in the process of speciation? How long must populations be isolated before speciation is complete? Are the genetic changes that take place during speciation caused mainly by natural selection or does genetic drift play a substantial role? Can genetic drift alone lead to reproductive isolation? These types of questions have been debated since the time of Darwin, but only for the last 30 years have scientists had the tools to examine genes directly at the population level.

This study is a first look at the population genetics of Canyon Treefrogs. Both the physiology of these frogs and the region they inhabit have contributed to a highly isolated population structure. Employing both laboratory analysis and computer modeling, I have examined the genetic changes occurring among these populations and also addressed biogeographical questions regarding how disjunct population structures arise in nature.

lsozyme and DNA sequencing results indicate an exceptional period of isolation among widely separated populations of Canyon Treefrogs. Eastern and western populations of frogs may have been isolated for as long as 1.5 million years. Divergence of the eastern-most population is at a level seen most often among congeneric amphibians based on results of previous studies. Sequencing analysis also points to the possibility of mtDNA introgression in two southern populations of Canyon Treefrogs.

Laboratory and computer modeling results suggest the importance of vicariant events in the establishment of the current disjunct population structure. Populations probably expanded their ranges along local drainage corridors during favorable periods. Subsequent shifts in drainage patterns severed avenues of gene flow, leading to genetic isolation.

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