The slimy salamander complex: A window into genetic divergence and the definition of a species



Travel to almost any deciduous forest in the eastern United States and you can potentially encounter one of the members of the slimy salamander (Plethodon glutinosus) species complex.  Slimy salamanders are large members of the family Plethodontidae or lungless salamanders, which rely completely on cutaneous respiration for gas exchange (i.e., they breathe through their skin).  Despite the large range and divergence into many species, most slimy salamanders look fairly similar.  A black or dark background color is highlighted with varying degrees of white to gold spots or blotches that can occasionally fuse together into a nearly continuous stripe.  Slimy salamanders get their name from the sticky secretions that they emit when handled by humans or threatened by a predator.  In many locations, slimy salamanders are among the most widespread and abundant salamander species, and they can often be found under rotting logs in backyards or small wooded areas in otherwise urban landscapes.  Encounters like these are often the only experience people have with native salamanders, especially in the Coastal Plain where most salamanders spend a majority of their time underground.

Originally described in the early 1800s as a single, wide-ranging species, slimy salamanders have since been divided into approximately 13 distinct species.  Georgia’s diverse set of landscapes actually create somewhat of a hotspot for the slimy salamander diversity, packing 6–7 species into a single state.  This includes the Savannah Slimy Salamander (P. savannah) and the Ocmulgee Slimy Salamander (P. ocmulgee), which are both Georgia endemics with relatively restricted distributions.  But how do so many different, yet closely related, species form?

Species form when populations of a single species are isolated from one other, creating genetic divergence that ultimately results in two genetically distinct populations or groups of populations.  Ideally, these newly formed species would also have a reproductive barrier but for recently diverged groups this is not always the case.  Isolation creating genetic divergence can occur in several ways: 1) by a barrier in the environment that prevents movement (e.g., a large river), 2) by the distances between populations (i.e., populations are less likely to interact and share genetic material the farther away they are from one another), 3) by the amount of environmental heterogeneity (i.e., increased environmental differences can lead to genetic divergence regardless of the geographic distance involved).  Species complexes containing many closely related species present an ideal scenario to test how these different isolating mechanisms work in nature.  Recent research by Smith et al. (2018) examined the drivers of genetic divergence in six species of slimy salamanders found in Georgia.  The authors found that the genetic data indicate that isolation by environment has had a large effect on slimy salamander speciation in Georgia.  Factors like temperature and precipitation better explained the current distribution of genetic variation than things like the current distribution of rivers.  Smith et al. (2018) also point out genetic similarities between P. ocmulgee and P. savannah, indicating that these species (and others in the slimy salamander complex) need a thorough genetic analysis to examine the validity of current species designations.

If you ask a group of biologists what defines a species, you are likely to get several slightly different answers.  In theory, a species is defined as a group or groups of individuals that can or could potentially interbreed and exchange genetic information.  However, in the real world this definition can often be challenging to apply.  What about two visually distinct animals that occasionally hybridize or single celled organisms that produce asexually?  Furthermore, as our ability to describe differences in an organism’s or a population’s genetic code increases, it can blur the line on what to describe as a species.  Speciation is literally being viewed in real time as it happens.  At what point does one conclude that two groups have diverged sufficiently to warrant separating into species?  These discussions of what defines a species are unlikely to be resolved anytime soon, but the definition of what constitutes a species or even a genetically distinct population has important conservation implications.

Identifying and classifying species has become increasingly fluid in recent years as new and faster methods allowing scientists to examine species’ DNA have become the norm.  Scientists can now observe a snapshot of evolutionary processes that happen over long time scales, often making absolute definitions of what is and is not a species difficult.  Despite these difficulties (it could be argued that these are just advances in our ability to understand the natural world), defining what is a species or is a genetically distinct population within a species has important conservation decisions.  Most wildlife regulations and laws function at the species or distinct population level, necessitated by the need for clear definitions about what organism falls under those laws.  New data that suggests a species of conservation concern should be split into multiple species can have immediate consequences for existing conservation programs.  Shrinking ranges could immediately warrant increasing a new species’ listing status.  Cryptic species that are difficult to distinguish can lead to the later realization that conservation efforts thought appropriate for a single species are actually negatively impacting a species complex (Yan et al. 2018).  Ultimately, scientists and natural resource professionals must work together to conserve species using the best available data and most efficient methods, while being prepared to shift strategies when new data suggest that current species descriptions need updating.

  • Smith, W., J. Wooten, C. Camp, D. Stevenson, J. Jensen, M. Turner, and N. Alexander 2018. Genetic divergence correlates with the contemporary landscape in populations of Slimy Salamander (Plethodon glutinosus) species complex across the lower Piedmont and Coastal Plain of the southeastern United States. Canadian Journal of Zoology 96:1244–1254.
  • Yan, F., J. Lü, B. Zhang, Z. Yuan, H. Zhao, S. Huang, G. Wei, X. Mi, D. Zou, W. Xu, S. Chen, J. Wang, F. Xie, M. Wu, H. Xiao, Z. Liang, J. Jin, S. Wu, C. Xu, B. Tapley, S. Turvey, T. Papenfuss, A. Cunningham, R. Murphy, Y. Zhang, and J. Che. 2018. The Chinese giant salamander exemplifies the hidden extinction of cryptic species. Current Biology 28:R581–R598.