Natural Versus Anthropogenic Dispersal of Freshwater Turtles Between the Baja California Peninsula and Mainland Mexico

Trachemys nebulosa is a freshwater turtle found on the Baja California Peninsula (BCP) and mainland northwestern Mexico. Two subspecies are recognized: Trachemys nebulosa nebulosa from the BCP and Trachemys nebulosa hiltoni from the mainland. Previous genetic studies found small differences between these taxa, but limited sampling left questions about their distinctiveness and origins. Most authors who have studied this species have invoked human-mediated transport to explain its distribution. To test this hypothesis, we combined previously published genetic data with expanded sampling. Mainland and peninsular populations did not share mitochondrial or nuclear sequences, and data from both genomes separated them into distinct groups. These results support retaining T. n. hiltoni and T. n. nebulosa as separate taxa and favor a natural dispersal from the BCP to the mainland, followed by isolation of mainland populations. Because T. n. hiltoni has a limited distribution and is harvested for food, recognizing it as a distinct lineage has conservation implications. Additional sampling, particularly from the Cape Region of the BCP, will help further test historical connections between mainland and peninsular populations.