Quaternary climate fluctuations and topographical variation in the Pacific Northwest region of North America have interacted to affect the
historical biogeography of biota in this region. High-elevation mammals
have unique diversification patterns due to their isolation on
mountaintops and potential for population growth and range expansion in
lowland refugia that are available during glacial periods. We examined
the phylogeographic structure, dates of lineage diversification, and
historical demography of western heather vole (Phenacomys intermedius)
populations across several mountain ranges in the Pacific Northwest.
Our analysis of sequence variation in the mitochondrial control region
using both maximum-likelihood and Bayesian methods identified 3 major
geographically distinct lineages: an Oregon and California lineage,
Washington lineage, and Northern and Interior lineage. Our estimate of
divergence times using a Bayesian relaxed molecular-clock method
revealed that diversification among these major lineages began similar
to 1.8 million years ago (mya) in the early Pleistocene with the split
of the Oregon and California lineage followed by the split of the
Washington lineage and the Northern and Interior lineage similar to 1.5
mya. All 3 clades remain allopatric, suggesting that they did not share
a common refugium during cold climatic intervals of the Pleistocene.
Further diversification within each major clade occurred in the middle
Pleistocene when populations in isolated mountain ranges became
distinct. Demographic estimates from Bayesian skyline plots indicate
that each of the 3 major clades has experienced population decline
since the early Holocene, possibly due to the redistribution of
populations into higher-elevation habitats that became restricted to
mountaintops following continental and alpine deglaciation. DOI:
10.1644/09-MAMM-A-303.1.