Relationship between glacial refugial range and genetic diversity in eastern North American conifer species

Abstract

The aim of this thesis is to relate genetic diversity to the extent of glacial refugia in several important North American conifer species. Refugial locations were hindcasted using MaxEnt software with occurrence points retrieved from the Global Biodiversity Information Facility (GBIF 2019) and Biodiversity Information Serving Our Nation (BISON 2019), and bioclimatic variables of current conditions and conditions during the last glacial maximum according to the MPI-ESM-P global circulation model retrieved from WorldClim version 1.4 (Hijmans et al. 2005). Bioclimatic variables were removed according to correlation to the highest contributing variable in multiple iterations until all highly correlated variables were removed to arrive at a final model. The size and number of refugia was compared against heterozygosity values gathered from allozyme studies. Species distribution models performed well with and were able to adequately predict current ranges of the ten selected conifer species and predicted Pleistocene distribution that can largely be corroborated with paleoecological and phylogeographical studies. A strong relationship between expected heterozygosity measured by allozyme analysis and the number and size of modelled refugia (adjusted r2 = 0.71) suggests that population size reductions and reduced gene flow during the last glacial maximum had pronounced effect on the genetic diversity of Eastern North American conifer species. Of these two variables, the number of refugia was more closely correlated to expected heterozygosity than size of refugia (adjusted r2 = 0.67 versus adjusted r2 = 0.58) which could suggest that these multiple refugia preserved different novel alleles that resulted in higher genetic diversity when glaciers receded and population admixture occurred.