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Comment on "Buffered Tree Population Changes in a Quaternary Refugium: Evolutionary Implications"

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Science  07 Feb 2003:
Vol. 299, Issue 5608, pp. 825
DOI: 10.1126/science.1079388

Tzedakis et al. (1) reconfirmed the traditional, historic biogeographical model of temperate species in Europe. Under this model, temperate-adapted species survived the long unfavorable episodes of the Pleistocene glacials in refugial areas, and then expanded their ranges during interglacial periods. It has been assumed that refugia for taxa that recolonized northern and central Europe, were located in the Mediterranean or western Asia, and particularly in the southern European peninsulas of Iberia, Italy, and the Balkans. Although popular and well published recently (2,3), this model does not provide the only explanation of the empirical data relevant to the location of temperate refugia during the ice ages—nor does it represent the most likely areas from which postglacial recolonization took place. In fact, several reports have suggested the existence of cryptic refugia in northern Europe, citing fossil evidence from macroscopic plants and vertebrates, as well as phylogeographic studies of land snails, sedges and ferns, and freshwater fish (4–11). Although these data contradict the dominant paradigm of both palynology and phylogeography, their implications are difficult to ignore.

Tzedakis et al. (1) discussed the production of endemic species in microrefugia within the greater Balkan refugium. However, the Balkans are more generally considered to be a refugium for the pan-European temperate taxa (2, 3). It is therefore difficult to explain how the southern European peninsulas could act both as refugia for a species and as an isolated area that produced allopatric endemics (12). Furthermore, recent efforts to map mammalian species in Europe during the Late Pleistocene (13) clearly show that most, if not all, the temperate species typically associated with deciduous woodlands were found far to the north of the peninsular refugia of Iberia, Italy, and the Balkans. This includes species that are not found above 60°N in present-day Europe. Assuming that dispersal from these northern refugial areas took place by a northern leading edge mode (Leptokurtic dispersal) (14), the genetic diversity spreading north is unlikely to have emanated from peninsular Europe. Some species such as the mole Talpa europaea and wood mouse Apodemus sylvaticus have fossil records that extend as far back as the Early Middle Pleistocene (15) (about 600 ka). This suggests that their origins as pan-European taxa are ancient and that any patterns of genetic differentiation could have persisted through many glacial cycles. It is also possible that the different haplotypes originated in northern Europe and were simply pushed south into the Mediterranean peninsulas during the glacials, as opposed to having originated in southern Europe.

As mentioned by Tzedakis et al. in (1), parapatry may have played a part in the formation of the southern European endemic species, although this may have been more important for the pan-European taxa, which may have experienced a much lesser degree of isolation. There is a general over-emphasis in the literature on allopatric speciation. This has led to hypotheses like those of Coope (16) and Bennett (17), which suggest that allopatric differentiation that occurred during the glacials would have been “undone” by interbreeding between emergent refugial populations during the succeeding warm periods. Taking a more complex view of speciation that considers the existence of northern and southern refugia, the possible role of parapatry or sympatry, and the newly popularized view of ecological speciation (18) will help generate a more realistic scenario for the origins of modern European biota.


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