Population genetics of ice age brown bears - PubMed (original) (raw)

Population genetics of ice age brown bears

J A Leonard et al. Proc Natl Acad Sci U S A. 2000.

Abstract

The Pleistocene was a dynamic period for Holarctic mammal species, complicated by episodes of glaciation, local extinctions, and intercontinental migration. The genetic consequences of these events are difficult to resolve from the study of present-day populations. To provide a direct view of population genetics in the late Pleistocene, we measured mitochondrial DNA sequence variation in seven permafrost-preserved brown bear (Ursus arctos) specimens, dated from 14,000 to 42,000 years ago. Approximately 36,000 years ago, the Beringian brown bear population had a higher genetic diversity than any extant North American population, but by 15,000 years ago genetic diversity appears similar to the modern day. The older, genetically diverse, Beringian population contained sequences from three clades now restricted to local regions within North America, indicating that current phylogeographic patterns may provide misleading data for evolutionary studies and conservation management. The late Pleistocene phylogeographic data also indicate possible colonization routes to areas south of the Cordilleran ice sheet.

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Figures

Figure 1

Figure 1

Maximum likelihood trees of brown bear control region and cytochrome b sequences. For simplicity, we use the clade I–IV nomenclature that was established previously (9), although in our smaller dataset several clades appear paraphyletic, particularly in the less well resolved cytochrome b tree. Extant sequences are in color [W (9), TS (10), TB (11)], and permafrost sequences (P1–P7) are in black. Reliability values (above nodes) and bootstrap percentages (below nodes) with values greater than 50% are indicated. Corrected radiometric dates (12) and reference numbers are as follows: P1, UCR3742/CAMS-51806 (15,370 ± 60 bp); P2, UCR3741/CAMS-51805 (14,980 ± 60 bp); P3, UCR3743/CAMS-54128 (13,760 ± 50 bp); P4, UCR3745/CAMS-54129 (15,680 ± 50 bp); P5, UCR3746/CAMS-54130 (42,850 ± 850); P6, UCR3744/CAMS-51808 (35,970 ± 660); and P7, Beta16162 (36,500 ± 1,150).

Figure 2

Figure 2

Current (A) and past geographic distribution of brown bear control region sequence clades 15,000 (B) and 35,000–45,000 (C) years ago. Locations of modern samples (9) are indicated by triangles. The asterisk indicates a brown bear fossil dated to greater than 40,000 years ago on Prince of Wales Island (16), and no fossils are known in the contiguous United States before 13,000 years. The approximate extent of glacial ice sheets 15,000 ago is superimposed on current continental boundaries in hatching (17).

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