A legacy of geo-climatic complexity and genetic divergence along the lower Colorado River: Insights from the geological record and 33 desert-adapted animals

Aim: To review the histories of the Colorado River and North American monsoon system to ascertain their effects on the genetic divergence of desert-adapted animals. Location: Lower Colorado River region, including Mojave and Sonoran deserts, United States. Methods: We synthesized recent geological literature to summarize initiation phases of lower Colorado River evolution, their discrepancies, and potential for post-vicariance dispersal of animals across the river. We simulated data under geological models and performed a meta-analysis of published and unpublished genetic data including population diversity metrics, relatedness and historical migration rates to assess alternative divergence hypotheses. Results: The two models for arrival of the Colorado River into the Gulf of California impose east-west divergence ages of 5.3 and 4.8 Ma, respectively. We found quantifiable river-associated differentiation in the lower Colorado River region in reptiles, arachnids and mammals relative to flying insects. However, topological statistics, historical migration rates and cross-river extralimital populations suggest that the river should be considered a leaky barrier that filters, rather than prevents, gene flow. Most markers violated neutrality tests. Differential adaptation to monsoon-based precipitation differences may contribute to divergence between Mojave and Sonoran populations and should be tested. Main Conclusions: Rivers are dynamic features that can both limit and facilitate gene flow through time, the impacts of which are mitigated by species-specific life history and dispersal traits. The Southwest is a geo-climatically complex region with the potential to produce pseudocongruent patterns of genetic divergence, offering a good setting to evaluate intermediate levels of geological-biological (geobiological) complexity.