Distinguishing intraspecific from interspecific variation in the leopard frog species complex

In an era of unprecedented biodiversity loss, the need for standardized
practices to describe biological variation is becoming increasingly
important. As with all scientific endeavors, species delimitation needs to
be explicit, testable, and refutable. A fundamental task in species
delimitation is distinguishing within-species variation from among-species
variation. Many species that are distributed across large geographic areas
exhibit levels of genetic variation that are as great or greater than
those that exist between well-defined sympatric species. Here, we provide
a workflow to distinguish between intra- and interspecific genetic
variation and apply the workflow to a taxonomically problematic group of
frogs (the Rana pipiens complex, or leopard frogs) that are widely
distributed across Mexico and Central America. Our workflow makes use of
recent advancements that pair genome-scale datasets with model-based
species delimitation methods, while emphasizing the need for positive
evidence of reproductive isolation to confirm the validity of
geographically contiguous species boundaries. We find that intraspecific
geographic variation in widespread leopard frog species has resulted in
considerable taxonomic inflation of species. Ten currently recognized
species are not supported in our analyses, and we here synonymize them
with previously named taxa. Furthermore, we find positive evidence for the
presence of three undescribed species. In addition to proposing these
taxonomic changes, we provide descriptions of the data or analyses that
would be needed to refute and overturn our recommendations. We recommend
that all species delimitation studies (especially of geographically
variable groups) clarify what new evidence would be sufficient to change
the taxonomic recommendations.