Abstract
The glacial cycles of the Pleistocene have been recognized as important, large-scale historical processes that strongly influenced the demographic patterns and genetic structure of many species. Here we present evidence of a postglacial expansion for the Downy Woodpecker (Picoides pubescens), a common member of the forest bird communities in North America with a continental distribution. DNA sequences from the mitochondrial tRNA-Lys, and ATPase 6 and 8 genes, and microsatellite data from seven variable loci were combined with a species distribution model (SDM) to infer possible historical scenarios for this species after the last glacial maximum. Analyses of Downy Woodpeckers from 23 geographic areas suggested little differentiation, shallow genealogical relationships, and limited population structure across the species’ range. Microsatellites, which have higher resolution and are able to detect recent differences, revealed two geographic groups where populations along the eastern edge of the Rocky Mountains (Montana, Utah, Colorado, and southern Alberta) were genetically isolated from the rest of the sampled populations. Mitochondrial DNA, an important marker to detect historical patterns, recovered only one group. However, populations in Idaho and southeast BC contained high haplotype diversity and, in general were characterized by the absence of the most common mtDNA haplotype. The SDM suggested several areas in the southern US as containing suitable Downy Woodpecker habitat during the LGM. The lack of considerable geographic structure and the starburst haplotype network, combined with several population genetic tests, suggest a scenario of demographic expansion during the last part of Pleistocene and early Holocene.
Highlights
Continental North America has been a climatically and biologically dynamic place during the last two million years, especially during the last part of the Quaternary ice ages, 21,000 years ago when the polar ice sheets were at their maximum extent [1]
We studied geographic patterns of genetic variation for the Downy Woodpecker (Picoides pubescens) using a series of population genetic statistics derived from mitochondrial DNA sequences and microsatellite data and created for the first time a species distribution model (SDM) at 21 kya
Genetic Diversity Fifty nine variable sites were found in an 850 base pair fragment of the mitochondrial genome for 263 individuals
Summary
Continental North America has been a climatically and biologically dynamic place during the last two million years, especially during the last part of the Quaternary ice ages, 21,000 years ago when the polar ice sheets were at their maximum extent [1]. Most plants and animals probably survived in areas south of the ice sheets; quite different from present-day North America, where ,20,000 species of vascular plants, more than 800 species of birds, and more than 640 species of mammals dominate a diverse and complex landscape [3]. North America has been one of the main areas for studies on post-glacial expansion, and as such is an area where the impacts of the last glacial maximum (LGM) on organisms from a geological and biological perspective are best understood [7]. In North America the ice sheets were present as far as south as 40oN with the Pacific Northwest, Northeast coast, Southwest US and Beringia serving as the major refugia for a diverse array of organisms. In contrast to Europe mountain ranges in North America run north-south and patterns of postglacial colonization from southern refugia differ between the two continents [1]
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