BIOGEOGRAPHIC STRUCTURE IN NORTH AMERICAN ARIDLANDS: HOW HAS TECTONIC AND CLIMATE CHANGE INFLUENCED THE GEOGRAPHY OF DIVERSIFICATION ACROSS CO-DISTRIBUTED TAXA?

Increasing landscape complexity has been linked with increases in biological diversity through generation of barriers to dispersal, providing opportunities for the evolution of independent lineages. Molecular genetic datasets bringing insight into the timing and geography of divergence are now available for multiple taxa, including rodents, distributed across the North American deserts. We can use these data to address hypotheses regarding the process of diversification during the profound geological and climatic transformations of these landscapes within the past several million years.

Within the southwestern warm deserts, multiple vertebrate, invertebrate, and plant taxa demonstrate similar patterns of transition from one evolutionary lineage to a related but divergent lineage across three different transition zones: between the Chihuahuan and Sonoran deserts; across the Gulf of California; and between the southern and northern parts of the Baja California Peninsula. The simplest explanation for these 3 highly replicated patterns is that in each case a barrier arose and isolated populations on either side at a single point in time (simple vicariance hypothesis). However, two alternative hypotheses are available: barriers arose progressively over prolonged periods of time as landscapes changed through geological and climatic dynamics, isolating some taxa early and others later according to taxon-specific ecological traits (layered vicariance hypothesis); and isolation occurred at localities other than locations a present-day transition zone, which nevertheless represents the limits to subsequent dispersal and therefore acts as a biogeographic barrier but not as the driver of a diversification event (pseudo-vicariance hypothesis).

Here, we test a prediction of the simple vicariance hypothesis – that molecular divergence between independent taxa will demonstrate similar times of divergence across a specific transition zone – through use of hierarchical approximate Bayesian computation. Doing so across 3 different transition zones, and among multiple taxa will provide a framework for further exploration of the process of diversification on the geologically and climatically turbulent landscape of western North America.