The phoenix hypothesis of speciation

AbstractGenetic divergence among allopatric populations builds reproductive isolation over time. This process is accelerated when populations face a changing environment, but abrupt change also places populations at risk of extinction. Here we use simulations of Fisher’s geometric model with explicit population dynamics to explore the genetic changes that occur in the face of environmental changes. We show that evolutionary rescue leads to the fixation of mutations whose effects are larger on average and that these mutations are more likely to lead to reproductive isolation, compared with populations not at risk of extinction. We refer to the formation of new species from the ashes of populations in decline as the phoenix hypothesis of speciation. The phoenix hypothesis predicts more substantial hybrid fitness breakdown among populations surviving a higher extinction risk. The hypothesis was supported when many loci underlie adaptation. When, however, there was only a small number of potential rescue mutations, we found that mutations fixed in different populations were more likely to be identical, with parallel changes reducing isolation. With a limited genomic potential for adaptation, we find support for a modified version of the phoenix hypothesis where reproductive isolation builds fastest in populations subject to an intermediate extinction risk.摘要：异地种群间的遗传分化随时间推移逐渐形成生殖隔离。当种群面临环境变化时，这一过程会加速，但环境的骤然变化也会使种群陷入灭绝风险。本研究采用结合了明确种群动态的费希尔几何模型（Fisher's geometric model）模拟，探究环境变化背景下发生的遗传变化。研究结果表明，相较于未面临灭绝风险的种群，进化拯救会固定平均效应更大的突变，且这类突变更易引发生殖隔离。我们将从衰退种群的“灰烬”中诞生新物种的过程，命名为物种形成的“凤凰假说”（phoenix hypothesis of speciation）。该凤凰假说预测，在经历更高灭绝风险的存活种群之间，杂种适合度崩溃的程度更为显著。当适应性演化由众多基因座（loci）驱动时，该假说得到了验证。然而，当潜在的拯救突变数量较少时，我们发现不同种群中固定的突变更有可能完全一致，这类平行演化的改变会降低生殖隔离的程度。当基因组的适应性演化潜力有限时，我们的结果支持修正版的凤凰假说：即生殖隔离在承受中等灭绝风险的种群中构建速度最快。