Population Genetics and the Eastern Ribbon Snake
Tara Morgan
University of Central Florida
In the southeastern United States, recent environmental changes pose significant challenges to native reptile and amphibian populations. These challenges include ecological change and habitat fragmentation caused by rapid urban development, novel pressures from emerging diseases such as snake fungal disease (Ophidiomyces ophiodiicola), and environmental shifts caused by a changing climate. In these conditions, maintaining phenotypic and genetic variation in populations is critical for long-term survival. Adaptation to new conditions is contingent on natural selection, which in turn can only act on existing variation in populations. Sufficient genetic variation allows populations to adapt to novel conditions, which in turn reduces risk of decline. Conversely, populations with insufficient genetic diversity may be less capable of responding to ecological changes. These vulnerabilities are frequently compounded by inbreeding, which often occurs in the same small, isolated populations where genetic diversity is already low.
Given these pressures, it is important to identify and conserve species and populations that 1) represent unique genetic groups whose loss would reduce overall species diversity and local survival and 2) are vulnerable to decline due to limited genetic diversity and gene flow. Techniques in population genetics provide effective and efficient methods for identifying these populations by evaluating levels of variation both within populations and between them. In such a way, genetically unique groups can be recognized, and populations with low diversity can be identified for potential conservation action.
As someone raised in Florida, a state well-known for both its biodiversity and increasing urban development, I have witnessed how rapidly ecosystems can change. These experiences shaped my interest in native herpetofauna and ultimately guided me toward my study species: the Eastern Ribbon Snake (Thamnophis saurita). In the eastern United States, the Eastern Ribbon Snake is a widespread semiaquatic species found near bodies of water such as lakes, streams, and wetlands. Its range extends as far south as the Florida Keys and upward through southeastern Canada. In Florida, there are three subspecies of T. saurita. This includes the Common Ribbon Snake (Thamnophis saurita saurita) in the Panhandle west of the Apalachicola River, the Blue-Striped Ribbon Snake (Thamnophis saurita nitae) on the Gulf coast, and the Peninsular Ribbon Snake (Thamnophis saurita sackenii) found throughout the remaining peninsula. Other than their geographic locale, these subspecies can be distinguished from one another by differences in coloration and body size.
In addition to these recognized subspecies, a population of T. s. sackenii exists in the Lower Florida Keys. The taxonomy of the Lower Keys population of T. s. sackeni has been the subject of some debate. The Lower Florida Keys are home to several unique subspecies, including the Lower Keys Marsh Rabbit and the Key Deer, prompting the question of if the ribbon snake population could also be its own distinct subspecies. When previously evaluated, it was noted that the Lower Keys population had differences in scale counts when compared to other populations (Christman 1980). This observation followed a general trend observed down the Florida peninsula and was not considered sufficient evidence of subspecies status (FWC 2011). However, no molecular evaluation of this population has been completed. Although the population is listed in the Florida Fish and Wildlife Conservation Commission’s Imperiled Species Management Plan, it is not formally recognized as a separate subspecies.
Despite the widespread distribution of T. saurita, little genetic data for the species currently exists. This leaves several gaps in knowledge, including questions regarding population structure. Are the three subspecies of T. saurita in Florida truly genetically distinct? Should the Lower Keys population be considered a separate subspecies? Regardless of taxonomic status, are any populations seeing a reduction in genetic diversity that warrants conservation concern? Addressing these questions will not only clarify taxonomic relationships, but also provide a baseline for monitoring populations as Florida continues to undergo rapid ecological change.
These questions shaped the foundation of my master’s research. Using techniques in population genetics, I am investigating both the genetic differentiation between populations of T. saurita in Florida and the genetic diversity within these populations. To do so, I have been collecting T. saurita blood and tissue samples from across Florida via collaborators, museums, and my own fieldwork. Samples from living snakes included small (< 5 mm) tail clips, scale clips, or blood. The remaining samples originated from T. saurita individuals found as roadkill. Roadkill samples were either donated as small tissue samples, or in the case of the entire snake being provided, I was able to both collect tissue and document the morphology of the individual. This has been particularly helpful for verifying if the color morphology of the collected snake matched that of T. s. nitae, which, as its common name suggests, has a lateral blue stripe on either side. These specimens, along with living specimens sampled during my own fieldwork, have allowed me to record phenotypic variation within T. s. sackenii that may complement genetic analyses.
So far, DNA has been extracted from around 100 T. saurita individuals from across Florida. Extraction success has varied, particularly for older stored tissues or roadkill samples where conditions have degraded DNA. This has resulted in poor-quality DNA in some samples. For samples with sufficiently high‑quality DNA, I am now beginning library preparation for sequencing. These initial sequences will provide a preliminary view of genetic variation across Florida. At the same time, I am finishing re‑extracting DNA from lower‑quality samples to determine whether high-quality DNA can still be recovered. This will help identify geographic gaps in sampling and guide future collection efforts to supplement areas where samples are lacking.
The coming months promise to offer the first genetic insights into Eastern Ribbon Snake diversity in Florida. Ultimately, this work will contribute to informed conservation decision‑making in an environment where resources are limited, and ecological change is accelerating. By establishing a genetic baseline and identifying populations that may require additional protection, this project represents an important step toward understanding and conserving species such as Thamnophis saurita in a rapidly changing world.
Literature Cited
Christman, S.P. 1980. Pattern of Geographic Variation in Florida Snakes. In (Vol. 25, pp. 170-173): Florida State Museum of the University of Florida.
Florida Fish and Wildlife Conservation Commission (FWC). 2011. Biological Status Review for the Peninsula Ribbon Snake (Lower Keys Population).