Department of Environmental Science and Policy, One Shields Avenue, University of California, Davis, California 95616, USA.
Evolution. 2011 Feb;65(2):365-76. doi: 10.1111/j.1558-5646.2010.01114.x. Epub 2010 Sep 29.
Habitat specialization plays an important role in the creation and loss of biodiversity over ecological and evolutionary time scales. In California, serpentine soils have a distinctive flora, with 246 serpentine habitat specialists (i.e., endemics). Using molecular phylogenies for 23 genera containing 784 taxa and 51 endemics, we infer few transitions out of the endemic state, which is shown by an analysis of transition rates to simply reflect the low frequency of endemics (i.e., reversal rates were high). The finding of high reversal rates, but a low number of reversals, is consistent with the widely hypothesized trade-off between serpentine tolerance and competitive ability, under which serpentine endemics are physiologically capable of growing in less-stressful habitats but competitors lead to their extirpation. Endemism is also characterized by a decrease in speciation and extinction rates and a decrease in the overall diversification rate. We also find that tolerators (species with nonserpentine and serpentine populations) undergo speciation in serpentine habitats to give rise to new serpentine endemics but are several times more likely to lose serpentine populations to produce serpentine-intolerant taxa. Finally, endemics were younger on average than nonendemics, but this alone does not explain their low diversification.
生境特化在生态和进化时间尺度上对生物多样性的创造和丧失起着重要作用。在加利福尼亚，蛇纹岩土有独特的植物群，其中有 246 种蛇纹岩土特化种（即特有种）。我们使用包含 784 个分类群和 51 个特有种的 23 个属的分子系统发育，推断出很少有从特有状态转变的情况，这可以通过对转变率的分析来反映，即简单地反映出特有种的低频率（即反转率高）。反转率高但反转次数少的发现与蛇纹石耐受性和竞争力之间广泛假设的权衡一致，根据这种权衡，蛇纹石特有种在生理上能够在压力较小的生境中生长，但竞争者会导致它们灭绝。特有现象还表现为物种形成和灭绝率降低，以及整体多样化率降低。我们还发现，耐受力者（具有非蛇纹石和蛇纹石种群的物种）在蛇纹石生境中经历物种形成，从而产生新的蛇纹石特有种，但更有可能失去蛇纹石种群，从而产生不耐蛇纹石的分类群。最后，特有种的平均年龄比非特有种小，但这本身并不能解释它们多样化程度低的原因。
