Of glaciers and refugia: a decade of study sheds new light on the phylogeography of northwestern North America

Abstract

Glacial cycles have played a dominant role in shaping the genetic structure and distribution of biota in northwestern North America. The two major ice age refugia of Beringia and the Pacific Northwest were connected by major mountain chains and bordered by the Pacific Ocean. As a result, numerous refugial options were available for the regions taxa during glacial advances. We reviewed the importance of glaciations and refugia in shaping northwestern North America's phylogeographic history. We also tested whether ecological variables were associated with refugial history. The recurrent phylogeographic patterns that emerged were the following: (i) additional complexity, i.e. refugia within refugia, in both Beringia and the Pacific Northwest; and (ii) strong evidence for cryptic refugia in the Alexander Archipelago and Haida Gwaii, the Canadian Arctic and within the ice-sheets. Species with contemporary ranges that covered multiple refugia, or those with high dispersal ability, were significantly more likely to have resided in multiple refugia. Most of the shared phylogeographic patterns can be attributed to multiple refugial locales during the last glacial maximum or major physiographic barriers like rivers and glaciers. However, some of the observed patterns are much older and appear connected to the orogeny of the Cascade-Sierra chain or allopatric differentiation during historic glacial advances. The emergent patterns from this review suggest we should refine the classic Beringian-southern refugial paradigm for northwestern North American biota and highlight the ecological and evolutionary consequences of colonization from multiple refugia.

Glacial refugia and modern genetic diversity of 22 western North American tree species

“…R. Soc. B 282: 20142903 that these populations were not established from repeated long-distance dispersal events [6,39,46]. Our model hindcasts suggest suitable habitat along the Pacific coast, notably in Haida Gwaii, for many temperate rainforest species including Thuja plicata, Tsuga heterophylla, Tsuga mertensiana, Cupressus nootkatensis, Abies amabilis, Picea sitchensis and Alnus rubra.…”

“…[6][7][8]32]), typically interpreting observations of a north-south genetic division centred in Oregon and inferring the origin of disjunct temperate rainforest communities of northern Idaho and the British Columbia interior. To explain these patterns, three hypotheses were proposed by Brunsfeld et al [32].…”

“…There is some fossil and genetic evidence that refugia west of the continental ice contained tree species, allowing for rapid recolonization of northern coastal areas [9,10]. Similarly, white spruce and certain other boreal tree species may have found habitat in ice-free Beringia, allowing southward post-glacial recolonization routes [5,6,14].…”

“…Modern genetic population structure can be screened comprehensively with a moderate research effort, but evolutionary events inferred from genetic markers cannot be linked to a specific time and location in the past. Nevertheless, phylogeographic studies allow formulating and testing hypotheses about refugia and migration routes, summarized in several review papers for North America [5][6][7][8].…”

When east meets west: population structure of a high-latitude resident species, the boreal chickadee (Poecile hudsonicus)

“…Temperate species were unable to survive in their pre-glaciation range and persisted instead in ice-free regions known as refugia. A combination of fossil evidence, pollen data, molecular data and sediment cores support the presence of multiple refugia during the last glacial maximum (LGM; the Wisconsin glaciation) in the mid-latitude United States and in Beringia (Pielou, 1991;Provan and Bennett, 2008;Shafer et al, 2010b). A number of smaller refugia are purported to have existed along the periphery of the ice sheets.…”