Smurfit Institute of Genetics, Trinity College, University of Dublin, Dublin, Ireland.
Curr Biol. 2011 Aug 9;21(15):1251-8. doi: 10.1016/j.cub.2011.05.058. Epub 2011 Jul 7.
Polar bears (Ursus maritimus) are among those species most susceptible to the rapidly changing arctic climate, and their survival is of global concern. Despite this, little is known about polar bear species history. Future conservation strategies would significantly benefit from an understanding of basic evolutionary information, such as the timing and conditions of their initial divergence from brown bears (U. arctos) or their response to previous environmental change.
We used a spatially explicit phylogeographic model to estimate the dynamics of 242 brown bear and polar bear matrilines sampled throughout the last 120,000 years and across their present and past geographic ranges. Our results show that the present distribution of these matrilines was shaped by a combination of regional stability and rapid, long-distance dispersal from ice-age refugia. In addition, hybridization between polar bears and brown bears may have occurred multiple times throughout the Late Pleistocene.
The reconstructed matrilineal history of brown and polar bears has two striking features. First, it is punctuated by dramatic and discrete climate-driven dispersal events. Second, opportunistic mating between these two species as their ranges overlapped has left a strong genetic imprint. In particular, a likely genetic exchange with extinct Irish brown bears forms the origin of the modern polar bear matriline. This suggests that interspecific hybridization not only may be more common than previously considered but may be a mechanism by which species deal with marginal habitats during periods of environmental deterioration.
北极熊（Ursus maritimus）是对快速变化的北极气候最敏感的物种之一，它们的生存受到全球关注。尽管如此，人们对北极熊物种的历史知之甚少。未来的保护策略将从了解基本进化信息中受益匪浅，例如它们与棕熊（U. arctos）最初分化的时间和条件，或它们对以前环境变化的反应。
我们使用了一个具有空间显式的系统地理学模型，来估计在过去 12 万年中，在其目前和过去的地理范围内，对 242 只棕熊和北极熊母系样本的动态进行估计。我们的研究结果表明，这些母系的当前分布是由区域稳定性和冰期避难所的快速长距离扩散相结合形成的。此外，在更新世晚期，北极熊和棕熊之间可能发生了多次杂交。
重建的棕熊和北极熊的母系历史有两个显著特征。首先，它以剧烈和离散的气候驱动扩散事件为特征。其次，这两个物种在其分布范围重叠时的机会交配在遗传上留下了深刻的印记。特别是，与已灭绝的爱尔兰棕熊的可能遗传交换构成了现代北极熊母系的起源。这表明种间杂交可能比以前认为的更为普遍，并且可能是物种在环境恶化时期应对边缘栖息地的一种机制。
