Patterns of morphological and ecological similarities of small-eared shrews (Soricidae, Cryptotis) in tropical montane cloud forests from Mesoamerica

It has been proposed that high morphological similarity between closely related species of small-eared shrews resulted from a recent divergence and intermittent population connectivity, presumably due to Pleistocene climatic fluctuations and associated changes in forest habitat distribution. Here we examined the morphological variation of two sister species of small-eared shrews inhabiting cloud forests from Mexico, Cryptotis obscurus and C. mexicanus. We then used ecological niche modelling to provide compelling evidence for current environmental barriers for population connectivity, and for detecting divergent ecological niches between candidate species. Our results indicated that the species boundaries in this clade should be subject to change. High morphological similarity suggested that populations of C. obscurus and C. mexicanus located west of the Isthmus of Tehuantepec, a major geographic barrier for montane species, are conspecific. Niche divergence between these two putative species was not supported indicating niche conservatism across the evolutionary history of these small-eared shrews. In addition, several barriers seem to play a main role for current lineage divergence between populations within this clade. The population located east of the Isthmus, previously referred to C. mexicanus, might prove to represent a new species based on morphological distinction and current geographic isolation. We have highlighted that estimating species’ potential distributions provides insights to evaluating the effect of geographic barriers on lineage divergence and making stronger inferences when delimiting species.

有研究指出，亲缘关系紧密的小耳鼩类群间存在高度形态相似性，该现象被认为源于近期的物种分化与间歇性种群连通，推测其成因与更新世（Pleistocene）气候波动及伴随的森林栖息地分布变化相关。本研究针对栖息于墨西哥云雾林的两种姊妹种小耳鼩——隐尾鼩属（Cryptotis）的C. obscurus与C. mexicanus，开展了形态变异分析。随后，本研究借助生态位模型（ecological niche modelling），为当前阻碍种群连通的环境屏障提供了有力证据，并验证了候选物种间存在分化的生态位。研究结果显示，该演化支（clade）内的物种界定亟需重新考量。高度的形态相似性表明，分布于特万特佩克地峡（Isthmus of Tehuantepec，山地物种的主要地理屏障）西侧的C. obscurus与C. mexicanus种群实为同一物种。这两种假定物种间并未检测到显著的生态位分化，说明这类小耳鼩在演化历程中存在生态位保守性（niche conservatism）。此外，多重地理屏障似乎是当前该演化支内不同种群发生谱系分化的主要驱动因素。而分布于该峡地东侧、此前被归为C. mexicanus的种群，凭借其形态差异与当前的地理隔离特征，极有可能代表一个全新的物种。本研究强调，通过估算物种的潜在分布范围，能够为评估地理屏障对谱系分化的影响提供新思路，同时也能在物种界定过程中得出更具说服力的推论。