Effects of genetic drift, natural selection, and population connectivity on adaptive-linked genetic diversity of desert bighorn sheep

Graduation date: 2015The amount and distribution of neutral and adaptive genetic diversity are often assumed to follow similar patterns. However, natural selection on adaptive loci may cause divergence from patterns observed for loci which are only influenced by selectively neutral processes such as genetic drift. The interaction between selection and neutral processes has rarely been assessed for species in large, fragmented landscapes. We compared genetic diversity and structure of 20 neutral microsatellite loci with 6 microsatellite loci linked to immune system genes for 13 populations of desert bighorn sheep (Ovis canadensis nelsoni) across gradients of population connectivity and habitat quality and tested for evidence of selection. We observed high correlations in mean genetic diversity of 6 adaptive-linked and 20 neutral loci among 13 study populations. Adaptive-linked loci were more diverse, as predicted under expectations of balancing selection for loci associated with pathogen and parasite response, but overall differences in mean diversity of adaptive-linked and neutral loci were small. The strength and distribution of genetic structure for adaptive-linked and neutral loci were nearly identical, although adaptive-linked genetic structure was slightly weaker. Tests for selection suggested that neutral processes strongly influenced both sets of loci, but differences in genetic diversity and structure were consistent with a weak influence of balancing selection on the adaptive loci. Multivariate regression models indicated that population connectivity strongly predicted both adaptive-linked and neutral diversity, while habitat quality, population size, and habitat area only weakly influenced diversity. Collectively, our results suggest that neutral markers can predict adaptive diversity in complex landscapes when natural selection is weak and populations are small or fragmented, and demonstrate the importance of population connectivity in maintaining adaptive genetic diversity