| Evolutionary potential is limited by the absence of genetic variability. The amount of genetic variation available to populations is affected by their genetic and evolutionary history as well as the interaction of several evolutionary forces, such as natural selection, drift, and migration. This thesis provides fossil, paleoecological, and molecular evidence in favor of an invasion by Eastern Collared Lizards (Crotaphytus collaris collaris) into the Ozark and Flint Hills regions during the warmer, drier Hypsithermal Period of the Holocene. Populations of these regions were subsequently fragmented to varying degrees by the invasion of oak-hickory forests when global climates cooled later in the Holocene. Hypervariable, neutral microsatellite loci are used to investigate the population genetic consequences of the invasion and subsequent habitat fragmentation by comparing estimates of genetic variability and subdivision between populations from "source" regions and "invaded" regions. Results indicate lower levels of genetic variation within populations and greater subdivision between populations of the invaded region relative to the source region. The relative influences of gene flow and migration on the distribution of genetic variation in the various regions (source, Kansas, Ozarks) are inferred based upon correlations between the genetic and geographic distances separating pairs of populations within each region. The results correspond closely to regional differences in duration of occupancy and forestation history. Finally, regional differences in local population subdivision are studied using data from closely-situated populations in each region. The results indicate historically higher levels of local dispersal than are inferred from regional estimates of subdivision, thus highlighting the importance of isolation by distance and deviations from assumptions of regional equilibrium. |
