Heredity (2009) 102, 213 & 2009 Macmillan Publishers Limited All rights reserved 0018-067X/09 $32.00
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NEWS AND COMMENTARY Post-glacial phylogeography ...............................................................
New insight into an old story: the post-glacial recolonization of European biota
D Lesbarre`res ........................................................... Heredity (2009) 102, 213; doi:10.1038/hdy.2008.121; published online 12 November 2008 he study of ancient post-glacial colonization routes takes on a new significance when we consider the likely effects of climatic change on genetic diversity. In northern latitudes, glaciation and the retreat of the ice (10 000 years ago) have shaped much of the phylogeographic structuring of populations we see today (Avise, 2000; Hewitt, 2000). The work from Knopp and Merila¨ (2008) on the post-glacial history of the moor frog (Rana arvalis) improves our understanding of the biogeography and evolutionary history of such northern species and draws attention to the potential consequences of climate change. It is widely accepted that there were European glacial refugia in the Iberian and Apennine peninsulae, and in the Balkans and southwestern Russia (Hewitt, 2004). By contrast, the post-glacial colonization routes from these refugia, which have been proposed over the last decade, are not unanimously accepted (Taberlet et al., 1998; Hewitt, 1999). Northern Europe was the last region to become free of ice (Lundqvist and Mejdahl, 1995) and, consequently, was colonized more recently than southern regions. In northern populations, different phylogeographical patterns have been observed in terrestrial and aquatic organisms, presumably reflecting their different ecological requirements. Lineages of terrestrial organisms generally meet in the central Sweden although contact zones for aquatic organisms are more frequently located at higher latitudes (Hewitt, 1999). Amphibians are semiaquatic and have limited dispersal capability; hence it is not obvious that which type of colonization history they might have. Knopp and Merila¨ (2008) investigated their history using a combination of nuclear microsatellite and mitochondrial data. Although mitochondrial DNA has long been the tool of choice for phylogeography studies (Avise et al.,
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1987), nuclear markers (for example, microsatellites) may be advantageous at smaller scales, both spatially and temporally. Indeed, more and more studies are combining information from both the types of markers to help infer population history and phylogeographical patterns for post-glacial period (Hewitt, 2004). Knopp and Merila¨’s (2008) analysis completes the puzzle initiated by Babik et al. (2004) in their study at the northern tip of Fennoscandia. With minor discrepancies between the two sets of markers, revealing a recent colonization process and the existence of hybrid zones not formerly identified, they delineated a contact zone in northern Sweden, in a location comparable with other taxa (Hewitt, 1999). In fact, the northernmost population of Vuollerim comprises individuals assigned to three mtDNA lineages, suggesting that the convergence of the western and eastern R. arvalis lineages took place at this latitude. Surprisingly, the main discrepancy found by Knopp and Merila¨ (2008) is not in the genetic data from the different sets of molecular markers but between the genetic data and phenotypic data. For example, although the genetic data supports east and west lineages converging in northern Sweden, the color morphs vary according to latitude suggesting that in some cases environmental influence may override genetic divergence (Merila¨ and Crnokrak, 2001; McKay and Latta, 2002), or rapid selection on color has taken place since the post-glacial colonization. These results highlight two processes, occurring at different paces in northern populations. On one hand, global warming is more severe at high latitudes resulting in climate-induced reductions of genetic variation and further increasing the probability of population extinction (Ditto and Frey, 2007). On the other, contact zones in
these latitudes are natural locations for divergent colonization lineages to meet thus harboring extra genetic diversity, and be key sites for the study of the impacts of climate change on genetic diversity with detectable adaptive divergence occurring on a comparably rapid time scale (Davis et al., 2005). D Lesbarre`res is at the Department of Biology, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada e-mail:
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Editor’s suggested reading Hamill RM, Doyle D, Duke EJ (2006). Spatial patterns of genetic diversity across European subspecies of the mountain hare, Lepus timidus L. Heredity 97: 355–365. Muller MH, Leppa¨la¨ J, Savolainen O (2007). Genome-wide effects of postglacial colonization in Arabidopsis lyrata. Heredity 100: 47–58. Oliver C, Hollingsworth PM, Gornall RJ (2006). Chloroplast DNA phylogeography of the arctic-montane species Saxifraga hirculus (Saxifragaceae). Heredity 96: 222–231. Storfer A, Murphy MA, Evans JS, Goldberg CS, Robinson S, Spear SF et al. (2007). Putting the ‘landscape’ in landscape genetics. Heredity 98: 128–142.
