Limits to adaptation along environmental gradients
Limits to adaptation along environmental gradients
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DOI：
10.1073/pnas.1421515112
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发表时间：
2015-05-19
影响因子：
11.1
通讯作者：
Barton, Nicholas H.
中科院分区：
文献类型：
--
作者：
Polechova, Jitka;Barton, Nicholas H.
为什么物种不能适应越来越广泛的条件，逐渐扩大它们的生态位和地理范围？跨环境的基因流动有两个相互冲突的影响：虽然它增加了遗传变异，这是适应的先决条件，但基因流动可能会淹没对当地条件的适应。1956年，霍尔丹提出，当环境在不同的空间发生变化时，基因流动的“淹没”在低种群规模和适应不良之间产生了正反馈，导致了急剧的范围差距。然而，当前的确定性理论表明，当方差可以演变时，没有这样的极限。使用简单的分析工具和模拟，我们表明，通过将遗传变异降低到适应空间可变条件所需的水平以下，遗传漂移可以产生一个物种范围的急剧边际。在生态和进化时间尺度分离的帮助下，识别的有效无量纲参数揭示了一个简单的阈值，该阈值预测在范围边缘的适应何时失败。有两个可观察到的参数决定了阈值：(I)有效环境梯度，可通过分散到不同环境造成的适应度损失来衡量；(Ii)相对于遗传漂移的选择效率。该理论预测，即使在环境没有突然变化的情况下，利润率也会大幅上升。此外，它还意味着，当不可能适应单一物种内的大范围条件时，整个物种栖息地条件的逐渐恶化可能会导致范围突然碎片化。
Why do species not adapt to ever-wider ranges of conditions, gradually expanding their ecological niche and geographic range? Gene flow across environments has two conflicting effects: although it increases genetic variation, which is a prerequisite for adaptation, gene flow may swamp adaptation to local conditions. In 1956, Haldane proposed that, when the environment varies across space, "swamping" by gene flow creates a positive feedback between low population size and maladaptation, leading to a sharp range margin. However, current deterministic theory shows that, when variance can evolve, there is no such limit. Using simple analytical tools and simulations, we show that genetic drift can generate a sharp margin to a species' range, by reducing genetic variance below the level needed for adaptation to spatially variable conditions. Aided by separation of ecological and evolutionary timescales, the identified effective dimensionless parameters reveal a simple threshold that predicts when adaptation at the range margin fails. Two observable parameters determine the threshold: (i) the effective environmental gradient, which can be measured by the loss of fitness due to dispersal to a different environment; and (ii) the efficacy of selection relative to genetic drift. The theory predicts sharp range margins even in the absence of abrupt changes in the environment. Furthermore, it implies that gradual worsening of conditions across a species' habitat may lead to a sudden range fragmentation, when adaptation to a wide span of conditions within a single species becomes impossible.