Am Nat. 2020 Feb;195(2):349-360. doi: 10.1086/705993. Epub 2019 Dec 6.
Understanding the causes and consequences of range expansions or range shifts has a long history in evolutionary biology. Recent theoretical, experimental, and empirical work has identified two particularly interesting phenomena in the context of species range expansions: (i) gene surfing and the relaxation of natural selection and (ii) spatial sorting. The former can lead to an accumulation of deleterious mutations at range edges, causing an expansion load and slowing down expansion. The latter can create gradients in dispersal-related traits along the expansion axis and cause an acceleration of expansion. We present a theoretical framework that treats spatial sorting and gene surfing as spatial versions of natural selection and genetic drift, respectively. This model allows us to analytically study how gene surfing and spatial sorting interact and derive the probability of fixation of pleiotropic mutations at the expansion front. We use our results to predict the coevolution of mean fitness and dispersal rates, taking into account the effects of random genetic drift, natural selection, and spatial sorting, as well as correlations between fitness- and dispersal-related traits. We identify a "rescue effect" of spatial sorting, where the evolution of higher dispersal rates at the leading edge rescues the population from incurring expansion load.
理解范围扩展或范围转移的原因和后果在进化生物学中有很长的历史。最近的理论、实验和实证工作在物种范围扩展的背景下确定了两个特别有趣的现象：（i）基因冲浪和自然选择的放松，以及（ii）空间排序。前者可能导致在范围边缘积累有害突变，从而导致扩展负荷并减缓扩展。后者可以在扩展轴上沿扩散相关性状形成梯度，并导致扩展加速。我们提出了一个理论框架，将空间排序和基因冲浪分别视为自然选择和遗传漂变的空间版本。该模型使我们能够分析研究基因冲浪和空间排序如何相互作用，并得出扩展前沿多效突变固定的概率。我们利用我们的结果来预测平均适应度和扩散率的共同进化，同时考虑随机遗传漂变、自然选择和空间排序的影响，以及适应度和扩散相关性状之间的相关性。我们确定了空间排序的“拯救效应”，其中前缘更高的扩散率的进化使种群免于遭受扩展负荷。
