Abstract
The role of spatial structure on the patterns and maintenance of diversity in populations is a longstanding area of research in evolutionary biology. The ešffects of spatial structure have been well documented in large eukaryotes but questions still remain about the inflžuence of specific environmental factors on structure and how historic patterns of spatial structure inžfluence modern distributions of diversity. At the level of microorganisms, research into the inflžuence of spatial structure on diversity has recently begun to develop at a rapid pace. Previous studies have shown that spatial structure prevents selective sweeps in bacterial populations, increasing diversity by limiting competition between genotypes to a local, rather than global, scale. In this dissertation I seek to address questions of the inžfluence of the environment, especially spatial structure, on the maintenance and pattern of diversity in two organisms: Ascaphus montanus, the Rocky Mountain tailed frog, and Acinetobacter baumannii, a biofilm-forming Gram-negative bacterium.
In A. montanus I addressed the inflžuence of environmental variables, incorporated through the use of Species Distribution Models, on the distribution of diversity at multiple spatial scales, from the entire species range, to within local clusters. Further, I used modeling based on estimates of past environmental conditions to investigate the role of historic separation of the species range into distinct glacial refugia ašffects current patterns of genetic diversity. I found that the inflžuence of current vs. historic conditions varied based on spatial scale, with historic factors being most important at the largest spatial scale and modern environmental conditions being increasingly important at smaller spatial scales.
In A. baumannii I utilized a large, replicated experimental evolution design to address the
role of spatial structure due to biofilm growth and the presence or absence of an environmental variable, tetracycline, on evolution of both phenotype and genotypes of A. baumannii and the pB10 plasmid it carried. The presence of tetracycline did increase improvement of plasmid persistence in biofilms but did not alter genetic diversity of the plasmid or host. Growth in the spatially structured biolm environment increased phenotypic diversity in the form of plasmid persistence, though it also limited the average strength of improvement in persistence. Biofilm growth also resulted in markedly dišfferent patterns in genetic diversity of the plasmid, with most plasmids that were isolated from the biofilm populations containing transferrable pB10. In contrast, only two plasmids isolated from the planktonic populations contained transferrable pB10. In the remaining plasmids large portions of the plasmid genome had been lost, resulting in loss of the genes involved in conjugation and making plasmid transfer impossible.is result suggests that spatial structure may dramatically modify the availability of plasmid genes in a population of bacteria compared to expectations based on studies performed with planktonic populations. Finally, I found that there were potential small dišfferences in genetic diversity of A. baumannii itself, with more unique mutations found when comparing bacteria isolated from biofilms to those isolated from planktonic populations.
As whole, these results confirm the importance of spatial structure and environmental variables on the evolution of diversity across multiple spatial and temporal scales and within widely dišffering organisms.