Data from: Evolutionary processes driving spatial patterns of intra-specific genetic diversity in river ecosystems

Describing, understanding and predicting the spatial distribution of genetic diversity is a central issue in biological sciences. In river landscapes, it is generally predicted that neutral genetic diversity should increase downstream, but there have been few attempts to test and validate this assumption across taxonomic groups. Moreover, it is still unclear what are the evolutionary processes that may generate this apparent spatial pattern of diversity. Here, we quantitatively synthesized published results from diverse taxa living in river ecosystems, and we performed a meta-analysis to show that a downstream increase in intraspecific genetic diversity (DIGD) actually constitutes a general spatial pattern of biodiversity that is repeatable across taxa. We further demonstrated that DIGD was stronger for strictly waterborne dispersing than for overland dispersing species. However, for a restricted data set focusing on fishes, there was no evidence that DIGD was related to particular species traits. We then searched for general processes underlying DIGD by simulating genetic data in dendritic-like river systems. Simulations revealed that the three processes we considered (downstream-biased dispersal, increase in habitat availability downstream and upstream-directed colonization) might generate DIGD. Using random forest models, we identified from simulations a set of highly informative summary statistics allowing discriminating among the processes causing DIGD. Finally, combining these discriminant statistics and approximate Bayesian computations on a set of twelve empirical case studies, we hypothesized that DIGD were most likely due to the interaction of two of these three processes and that contrary to expectation, they were not solely caused by downstream-biased dispersal.

描述、理解并预测遗传多样性的空间分布，是生物科学的核心议题之一。在河流景观中，学界普遍推测中性遗传多样性（neutral genetic diversity）应随河流下游递增，但目前鲜有研究尝试在不同分类群中验证这一假设。此外，究竟是何种进化过程催生了这一表观的多样性空间格局，目前仍不明确。本研究对河流生态系统中不同分类群的已发表研究结果进行了定量综合，并通过元分析（meta-analysis）证实：种内遗传多样性下游递增（downstream increase in intraspecific genetic diversity，以下简称DIGD）实为一种可在不同分类群间重复出现的生物多样性通用空间格局。我们进一步发现，相较于依靠陆地扩散的物种，严格依赖水流扩散的物种其DIGD效应更为显著。不过，针对一组聚焦于鱼类的受限数据集，并未发现DIGD与特定物种性状存在关联的证据。随后，我们通过在类树枝状水系系统中模拟遗传数据，探究了驱动DIGD的通用过程。模拟结果显示，本研究考量的三种过程（下游偏向扩散、下游生境可用性提升以及上游定向定植）均可能催生该格局。借助随机森林模型（random forest models），我们从模拟结果中筛选出一组高信息量的汇总统计量，可用于区分不同的DIGD驱动过程。最终，结合这些判别统计量与针对12个实证案例的近似贝叶斯计算（approximate Bayesian computations），我们提出推测：DIGD大概率由上述三种过程中的两种交互作用所致，且与学界此前的预期相悖，该格局并非仅由下游偏向扩散单一过程引发。