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A high-resolution pollen record from western Greece shows that the amplitude of millennial-scale oscillations in tree abundance during the last glacial period was subdued, with temperate tree populations surviving throughout the interval This provides evidence for the existence of an area of relative ecological stability, reflecting the influence of continued moisture availability and varied topography. Long-term buffering of populations from climatic extremes, together with genetic isolation at such refugial sites, may have allowed lineage divergence to proceed through the Quaternary. Such ecologically stable areas may be critical not only for the long-term survival of species, but also for the emergence of new ones.
The Pleistocene refugium hypothesis (1) proposed that population fragmentation during individual glacial stages of the Quaternary promoted speciation. A more recent view is that the accentuated environmental instability did not lead to increased speciation rates, with most species predating the Pleistocene (2, 3). This evolutionary stability has been attributed to orbital (2) and millennial (4) climate fluctuations, which undo microevolutionary changes by forcing repeated population crashes, range shifts, and gene flow. Central to this view is that the alternation of climate phases does not generally allow sufficient time for genetic differentiation to species rank. However, molecular genetic data reveal considerable divergence between populations of many species in southern refugial centers in Iberia, Italy, the Balkans, and Greece, which took several glacial-interglacial cycles to accumulate (5-7). Analysis of DNA divergence in animals (6, 8) shows that species have continued forming through the Pleistocene and that such divergence has proceeded apparently unhindered in some places (7, 9). DNA divergence indicates that, whereas in lowland tropical forests most species formed before the Quaternary, clusters of recently diverged lineages along with older species are found in tropical mountain regions (10, 11). It has thus been postulated (10) that these mountains are centers for speciation because they provide a relatively stable habitat through climate oscillations in which older species survive and new lineages are generated. Although little paleoecological evidence has been available, this long-term stability is thought to be a function of continned moisture availability and varied topography (7, 10). A revised view of Quaternary evolutionary trends is that, while climate variability mostly inhibited speciation, species continued to form in places where presumed ecological stability allowed...