2005, pp 205-246

Glacial Retreat and its Influence on Migration of Mitochondrial Genes in the Long-toed Salamander (Ambystoma macrodactylum) in Western North America

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Abstract

The long-toed salamander (Ambystoma macrodactylum) is a widespread inhabitant of the Cordilleran Region of western North America. The Cordilleran ice sheet retreated when climates changed at the end of the Pleistocene. This setting provides a natural experiment for phylogeographic tests of post-glacial migration. As migration occurs, the demographics of populations change; these changes are imprinted into the gene frequencies of descendant populations. Species ranges shifted as migrants inhabited tolerable post-glacial environments, and new genealogical mixtures formed as populations came into secondary contact. Historical climate, ecology, and geography impacted the range dynamics and consequent population genetics of the long-toed salamander. This systematic study of mitochondrial DNA tests biogeographic patterns using phylogenetic trees, nested phylogeographic clade analysis, and mismatch distributions. Phylogenetic congruence is tested first in a partitioned versus an intersected arrangement of two mitochondrial loci, including 95 cytochrome b and 103 intergenic spacer sequences. Nested phylogeographic clade analysis provides an explicit system to correlate lineages and their mismatch distributions. Although mismatch distributions operate ideally in high-migration species, and the long-toed salamander migrates little among contemporary populations, there is reason to suspect that waif dispersal increased with changes in fluvial dynamics following glacial retreat. Clade patterns support a deep vicariance across the central interior and reveal seven Pleistocene refugia. Waves in mismatch distributions indicate that population sizes increased in lineages residing in refugia near the ice margins at this time. The phylogenetic identities that spread away from refugia and their genetic patterns are placed into a historical pre- and post-glaciated context.