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

Abstract

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 We simulated data under geological models and performed a meta‐analysis of published and unpublished genetic data including population diversity metrics, relatedness and historical migration rates to assess alternative divergence hypotheses.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.

Montane regions shape patterns of diversification in small mammals and reptiles from Madagascar’s moist evergreen forest

“…Briefly, the trait decomposition method assesses whether or not a trait is phylogenetically structured and can be used in cases where geography is the trait of interest (e.g. Dolby, Dorsey, & Graham, 2019). In this study the Northern, Central, or Southern highlands were coded as three separate discrete traits.…”

Refining genetic boundaries for Agassiz’s desert tortoise (Gopherus agassizii) in the western Sonoran Desert: the influence of the Coachella Valley on gene flow among populations in southern California

“…For example, prehistoric aquatic barriers in the lower Colorado River basin drove divergence between Mojave and Sonoran desert biota (Bell et al 2010, Dolby et al 2015. The first barrier, a marine incursion of the Gulf of California, extended northwestward into what is now the Salton Trough as early as 6.3 Ma (Dolby et al 2019). Fossil-rich marine sediments show that the Gulf extended as far north as San Gorgonio Pass near Palm Springs, California, and possibly east to Parker, Arizona along what is now the Colorado River (Dolby et al 2015(Dolby et al , 2019.…”

“…The first barrier, a marine incursion of the Gulf of California, extended northwestward into what is now the Salton Trough as early as 6.3 Ma (Dolby et al 2019). Fossil-rich marine sediments show that the Gulf extended as far north as San Gorgonio Pass near Palm Springs, California, and possibly east to Parker, Arizona along what is now the Colorado River (Dolby et al 2015(Dolby et al , 2019. The Colorado River extensional corridor of the Miocene left a series of basins that were flooded in the Pliocene by the precursor to the lower river.…”

“…Colorado River arrived in the lower basin after 5.24 Ma, forming a second barrier (Howard et al 2019) or filter (Dolby et al 2019) to dispersal and gene flow in what is now the Salton Trough. Over the last several thousand years, the Salton Trough has also been inundated periodically with freshwater from overflow of the Colorado River, forming a large waterbody known as Lake Cahuilla (Waters 1983), and it is now partially filled by the hypersaline Salton Sea.…”

“…Situated at the tectonic and zoologically important interface of the North American and Pacific Plates (Atwater 1970, Gottscho 2016, the Desert Southwest has been profoundly shaped and influenced by two major geological processes in the last 6 million years: the formation of the Gulf of California (Dolby et al 2015) and the evolution of the lower Colorado River (Howard et al 2019), both of which presented aquatic barriers to some plants and animals. Disentangling the effects of those and other geological processes on the evolution of the biota of the region is an ongoing topic of interest to researchers (Dolby et al 2015(Dolby et al , 2019.…”