Patterns Of Postglacial Colonization

As each new glacial period took hold and the volume of ice built up in the north, the world sea level fell by various amounts, at worst by more than 120 m. Wherever rain or snow was insufficient to build up glaciers, vast ice-free plains formed on land abandoned by the falling seas, such as across Beringia, the exposed land between northeastern Asia and Alaska, and over much of central and eastern Eurasia (Kurten 1966) (Figure 1.6).

Entire ecosystems moved southward, along with the climatic zones to which they were adapted. The periglacial environment supporting the tundra vegetation and lemmings, which we now think of as typical of the far north, became established where oaks and mice had previously flourished. Weasels of all three species were often included in these displaced communities. Conard Fissure, Arkansas, has fossils of both stoats and long-tailed weasels of glacial age (Kurten & Anderson 1980) and stoats lived in southeastern New Mexico during terminal Pleistocene times (Harris 1993), even though their nearest living relatives are now found hundreds of kilometers north of these places. The broad expanses of what is now the southern United States provided ample refuge for the displaced faunas of glacial times; other populations of stoats survived in Beringia and in ice-free patches of northern Canada (mapped in Eger 1990).

Figure 1.6 The approximate extent of the last major glacial advance, north polar view. The extent of Beringia is shown by dotted lines. (Redrawn from Ehlers & Gibbard 2004.)

The story was the same in Europe, except that the east-west chain of mountain ranges stretching from the Pyrenees to the Caucasus divided the available refuges south of the permafrost zone into three distinct areas, represented by the present Iberian, Italian, and Balkan/Grecian/Turkish peninsulas (mapped in Hewitt 1999). Stoats must have lived in Britain, and on the exposed continental shelf south of the present coast, during the last interglacial, because remains dating from 35,000 years ago have been found in Castlepook Cave in Ireland (Stuart 1982). During the last glaciation, large populations of stoats and common weasels would have hunted voles and lemmings across the cold and windswept plains immediately south of the ice front (Sommer & Benecke 2004) and in the extensive southern refuges.

As the glaciers retreated, entire temperate ecosystems slowly migrated northward again. First the tundra, then the coniferous forests, then the temperate forests gradually covered the denuded landscape. Small mammals and birds, accompanied by their predators, moved north with the returning vegetation. But the population characteristics of the weasels that returned to the far north after the glaciations were not quite the same as those of their ancestors that had moved southward. When populations of any species are isolated by geographic barriers for many generations, they diverge both physically and genetically. Populations of animals that had sat out the glaciations in different places and returned via different routes had developed differences in genetic composition, which can still be detected by analysis of their descendants. Modern techniques make it possible to reconstruct the routes they took, and, in several key places around the northern hemisphere, what happened when they met.

For example, small rodents are the favorite prey of weasels, especially the various species of voles and lemmings. In North America, stoats and least weasels must have recolonized the newly reestablished forests as members of the community of small mammals and birds on which they depended. The Pacific Northwest has been a crossroads for postglacial colonists arriving from different directions. The early-succession generalist herbivores came first, along with their predators, from three main refuges—from Beringia via Alaska, and from the south along both eastern and western sides of the continent (Figure 1.7). In southwest Alaska, the postglacial histories of the long-tailed vole (Microtus longicaudus) and the stoat have been analyzed by Conroy and Cook (2000) and Cook et al. (2001), with fascinating results. For both species, there is evidence of two routes of migration from the south, one inland and one along the west coast, plus a separate migration from Beringia in the north. The differences between contemporary populations of stoats in northwestern North America do appear to reflect these different origins and routes of recolonization. By contrast, genetic differentiation among the stoats of the Palaearctic appears to be relatively low (Kurose et al. 2005).

Not only is genetic evidence strong for at least three quite separate lineages of stoats now living in the Alexander Archipelago, but also there are morphological differences visible even between adjacent populations elsewhere in the Northwest. For example, the skulls of stoats on Vancouver Island differ from those on the adjacent mainland (Eger 1990), and stoats from coastal Oregon never turn white in winter, even when translocated to Alaska, whereas those from inland Oregon always do (Feder 1990).

In northern Europe, geographical patterns of genetic relatedness among populations of voles and weasels and other species are nicely documented. Jaarola and Tegelström (1995) found a distinct difference between the field voles (Microtus agrestis) living in Finland and northern Sweden and those in southern Sweden, Denmark, and Britain. They concluded that the two groups are derived from populations that survived the glaciations in different places and

Figure 1.7 Possible routes (arrows) taken by flora and fauna recolonizing northern North America after the most recent retreat of the glaciers (stippled area). (Redrawn from Eger 1990: fig 1 and Conroy and Cook 2000: fig 4b.)

then recolonized Scandinavia from two different directions, the south and the east. The southern migration moved north via the late glacial and postglacial land bridges connecting southern Sweden with the Danish islands and Germany. The eastern migration approaching from Russia swung through Finland, around the northern Gulf of Bothnia, and south again into Sweden. Where the two advancing fronts met, they established a hybrid zone that still persists at the genetic level, although in voles the differences between them are not visible to the eye and raise no taxonomic questions.

Very similar postglacial histories have been established for other members of the same assemblage of glacial fauna, which presumably all returned northward in the same two separate streams. Examples include the brown bear, the shrew, the bank vole, and the water vole (Hewitt 1999; Jaarola et al. 1999). Naturally, weasels were part of this community, and they recolonized Europe along the same two routes (Figure 1.8). One group, which we now recognize as Mustela nivalis vulgaris, colonized all the southern and western countries of Europe; the other, now called M. n. nivalis, came from the east and north, en-



Figure 1.8 Possible routes (arrows) taken by flora and fauna recolonizing northern Europe after the most recent retreat of the glaciers (stippled area). (Redrawn from Hewitt 1999 and Jaarola et al. 1999: fig. 5.)

tering Scandinavia via Russia and eventually turning south again through Norway and Sweden. The hybrid zone marking where they met in southern Sweden is still the best place to observe the visible differences between these two subspecies that developed during their separation, including, most obviously, that one turns white in winter while the other does not (Stolt 1979). Despite this high level of intraspecific variation, the different forms of nivalis did not develop any reproductive isolating mechanisms. The two lineages are still genetically distinguishable (Kurose et al. 2005), but as taxonomic units they remain distinct only at the subspecies level. Common weasels in Poland and the former Czechoslovakia are classed as M. n. vulgaris, but with so much geographical variation that they seem to combine the characteristics of both subspecies (Schmidt 1992; Zima & Cenevova 2002).

The weasels that now live in Morocco, Algiers, and Tunisia are similar to those of southern Mediterranean Europe, so are classed as M. n. vulgaris, but they have probably not been there very long. The Straits of Gibraltar have maintained the geographic separation of Iberia from North Africa throughout the Pleistocene and ever since, so the ancestors of these weasels were probably introduced with human help (Dobson 1998). By contrast, the weasels in the lower Nile Valley of Egypt belong to a different subspecies (M. n. subpalmata). They have certainly been isolated for a long time, and are so different from other weasels classed as M. nivalis that they should perhaps be promoted from a subspecies to a separate full species (van Zyll de Jong 1992; Reig 1997; Abramov 2000; Abramov & Baryshnikov 2000). They may represent one of the refuge populations that survived the glacial periods in the south, whose descendants have stayed put rather than migrate north. As Egypt dried out, they escaped the encroaching desert by adapting to live in towns and gardens, and even in houses (Osborn & Helmy 1980).

The idea that the morphological variation we see today can be interpreted in terms of glacial history is an old one (Macpherson 1965). Others have argued that although stoats, for example, recolonized the deglaciated areas of North America from different stocks, the present differences among their descendants might just as easily be explained as adaptations to local environmental conditions. Eger (1990) used a sophisticated analysis of13 craniometric characters of stoats collected from across North America to distinguish between these possibilities. She concluded that present variation in skull size, which reflects body size, one of the key characters of life for any animal (Chapter 4), is better explained by local climatic conditions than by ancestry. Natural selection keeps a relentless scrutiny on all creatures, and those that, like weasels, maintain high levels of genetic variability and breed rapidly are able to respond quickly to changes in local conditions. That means that their most important physical characteristics, including body size, say more about their present environment than about range changes and recolonization in response to glacial advance and retreat. This process has long ago superimposed contemporary local adaptation upon any size differences that might have developed during isolation in glacial refugia.

On the other hand, the fine details of differences in skull shape have no such dire ecological consequences, so it might be expected to retain traces of differences in ancestry for much longer. Eger's analysis confirmed that this is so. Variation in skull shape of North American stoats is not closely correlated with local climate; rather, discontinuities in geographic pattern are, indeed, consistent with the refugium hypothesis. Eger identified distinct groups of samples that were probably descended from three different ancestral refugial populations in Beringia, in eastern North America, and on the west coast.

Glacial history helps explain a lot about modern populations, including puzzling observations that cannot sensibly be explained any other way. In Europe during glacial periods, the least weasel, the northern form now typical of colder climates, once occupied the continental plain north of the Alps and also the valleys between the high mountains of Switzerland and Austria. As the glaciers melted, the mountains provided a cold-climate refuge to which least weasels could retreat, and they survive into the present day, long after the surrounding lowlands had been reclaimed by the common weasel moving north from warmer climates. In Switzerland, the least weasel (often called the pygmy weasel there) is now found only at high elevation, while common weasels occupy the intervening valleys (Güttinger & Müller 1988).

The difference in appearance between the two hinges on genetically determined characters; the local details of their distributions confirm that these differences are determined by history rather than contemporary conditions. For example, in some places both whitening and nonwhitening races of weasels live in similar environments, and the conditions associated with whitening in stoats seem to be quite different from those associated with whitening in least weasels living nearby (Güttinger & Müller 1988). These differences seem to be more easily explained on the grounds of ancestry than of ecology (Chapter 3).

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