Pleistocene glaciation drove shared population coexpansion in eastern North American snakes

Glacial cycles during the Pleistocene had profound impacts on local environments and climatic conditions. In North America, some regions that currently support diverse biomes were entirely covered by ice sheets, while other regions were environmentally unsuitable for the organisms that live there now. Organisms that occupy these regions in the present day must have expanded or dispersed into these regions since the last glacial maximum, leading to the possibility that species with similar geographic distributions may show temporally concordant population size changes associated with these warming trends. We examined 17 lineages from 9 eastern North American snake species and species complexes to test for a signal of temporally concordant coexpansion using a machine learning approach. We found that the majority of lineages show population size increases towards the present, with evidence for coexpansion in five out of fourteen lineages, while expansion in others was idiosyncratic. We also examined relationships between genetic distance and current environmental predictors and showed that genomic responses to environmental predictors are not consistent among species. We therefore conclude that Pleistocene warming resulted in population size increases in most eastern North American snake species, but variation in environmental preferences and other species-specific traits results in variance in the exact timing of expansion.

更新世冰期循环对区域环境与气候状况产生了深远影响。在北美地区，当前拥有多样生物群系（biome）的部分区域曾完全被冰盖覆盖，而其余区域彼时的环境并不适宜如今栖息于此的生物生存。现今栖息于这些区域的生物必然是自末次冰盛期（last glacial maximum）以来才扩张或扩散至此，因此地理分布相似的物种可能会出现与气候变暖趋势同步的种群规模变化，这一可能性由此产生。本研究针对北美东部9种蛇类及物种复合群（species complex）的17个谱系（lineage）展开分析，采用机器学习方法检验同步协同扩张（coexpansion）的信号。研究结果显示，多数谱系的种群规模随时间推移至当代呈现增长趋势；14个谱系中有5个存在协同扩张的证据，其余谱系的扩张模式则较为独特。此外，本研究还分析了遗传距离与当前环境预测因子（environmental predictor）之间的关联，结果表明不同物种对环境预测因子的基因组响应并不一致。综上，本研究认为更新世气候变暖导致北美东部多数蛇类的种群规模有所增长，但由于物种间环境偏好及其他物种特异性（species-specific）性状存在差异，其种群扩张的精确时间也存在差异。