Idiosyncratic responses to drivers of genetic differentiation in the complex landscapes of Isthmian Central America

Isthmian Central America (ICA) is one of the most biodiverse regions in the world, hosting an exceptionally high number of species per unit area. ICA was formed < 25 million years ago and, consequently, its biotic assemblage is relatively young and derived from both colonization and in situ diversification. Despite intensive taxonomic work on the local fauna, the potential forces driving genetic divergences and ultimately speciation in ICA remain poorly studied. Here, we used a landscape genetics approach to test whether isolation by distance, topography, habitat suitability, or environment drive the genetic diversity of the regional frog assemblage. To this end, we combined data on landscape features and mitochondrial DNA sequence variation for nine co-distributed amphibian species with disparate life histories. In five species, we found that at least one of the factors tested explained patterns of genetic divergence. However, rather than finding a general pattern, our results revealed idiosyncratic responses to historical and ecological processes, indicating that intrinsic life-history characteristics may determine the effect of different drivers of isolation on genetic divergence in ICA. Our work also suggests that the convergence of several factors promoting isolation among populations over a heterogeneous landscape might maximize genetic differentiation despite short geographical distances. In conclusion, abiotic factors and geographical features have differentially affected the genetic diversity across the regional frog assemblage. Much more complex models (i.e. considering multiple drivers), beyond simple vicariance of Caribbean and Pacific lineages are needed to better understand the evolutionary history of ICA's diverse biotas.

中美洲地峡区（Isthmian Central America，ICA）是全球生物多样性最丰富的区域之一，单位面积内物种丰度极高。该区域形成于2500万年前以内，因此其生物群落相对年轻，且类群来源兼具外来迁入与原地分化两种途径。尽管针对当地动物类群已开展了大量分类学研究，但驱动中美洲地峡区物种遗传分化乃至最终物种形成的潜在驱动力，迄今仍未得到充分解析。本研究采用景观遗传学（landscape genetics）方法，旨在检验距离隔离（isolation by distance）、地形、生境适宜性以及环境因子是否对区域蛙类群落的遗传多样性具有驱动作用。为此，我们整合了9种分布范围重叠但生活史特征迥异的两栖类的景观特征数据与线粒体DNA（mitochondrial DNA）序列变异数据。在其中5个物种种群中，本研究发现至少一种待测因子能够解释其遗传分化模式。但研究结果并未发现普适性的统一规律，反而揭示出不同类群对历史与生态过程存在特异性响应，这表明内在的生活史特征或许决定了各类隔离驱动因子对中美洲地峡区物种遗传分化的影响强度。本研究还表明，在异质化景观中，多种促进种群隔离的因子共同作用，即便地理距离较短，也可能最大化种群的遗传分化程度。综上，非生物因子与地理特征对区域蛙类群落的遗传多样性存在差异化影响。若要更深入理解中美洲地峡区多样生物类群的演化历史，亟需构建更为复杂的模型——即纳入多重驱动因子，而非仅考虑加勒比与太平洋支系的简单异域分化事件。