Drainage basins serve as multiple glacial refugia for alpine habitats in
the Sierra Nevada Mountains, California
Abstract
The evolutionary histories of alpine species are often directly
associated with responses to glaciation. Deep divergence among
populations and complex patterns of genetic variation have been inferred
as consequences of persistence within glacier boundaries (i.e. on
nunataks), while shallow divergence and limited genetic variation is
assumed to result from expansion from large refugia at the edge of ice
shields (i.e. massifs de refuge). However, for some species, dependence
on specific microhabitats could profoundly influence their spatial and
demographic response to glaciation, and such a simple dichotomy may
obscure the localization of actual refugia. In this study, we use the
Nebria ingens complex (Coleoptera: Carabidae), a water-affiliated ground
beetle lineage, to test how drainage basins are linked to their observed
population structure. By analyzing mitochondrial COI gene sequences and
genome-wide single nucleotide polymorphisms, we find that the major
drainage systems of the Sierra Nevada Mountains in California best
explain the population structure of the N. ingens complex. In addition,
we find that an intermediate morphotype within the N. ingens complex is
the product of historical hybridization of N. riversi and N. ingens in
the San Joaquin basin during glaciation. This study highlights the
importance of considering ecological preferences in how species respond
to climate fluctuations and provides an explanation for discordances
that are often observed in comparative phylogeographic studies.