So, anyone familiar with evolutionary biology has probably heard about the Ensatina superspecies complex of lungless salamanders. This wouldn’t be interesting were it not for some relatively new research (and continuous production of research) indicating that this is much more complex than previously thought. Populations seem to have been geographically split and reunited in several locations, maybe. This is evident by the genetic discontinuities and divergence responsible for the color patterns observed. Mitochondrial DNA (mtDNA) points to reproductive isolation and returning to low level gene flow between populations (termed “gene leakage” in the paper). Secondary contact seems to be the reason this superspecies did not completely diverge into multiple species. Indeed, at the far southern end of one sympatric region, it could be said that two subspecies have diverged enough to be considered distinct species were it not for the northern portions having low frequency hybridization (<20% hydbridization).
Previous mitochondrial evidence indicated that while reproductive isolation was historically important for these salamanders, new research suggests only one small area has achieved reproductive isolation. However, the authors then go on to explain something a bit interesting about the misleading nature of mitochondrial evidence in this case.
Furthermore, if migration is mainly led by males, as suggested by field estimates in Ensatina (Staub et al. 1995), the admixture rate would also be higher in markers carried by both sexes, which could further enhance the differential effect of genetic drift. Therefore, these results call for caution in the interpretation of species boundaries based solely on mitochondrial markers, which could be misleading especially in cases of organisms that are philopatric and that have sex-biased dispersal and/or low vagility.
What the authors are trying to say here can be summarized as “males do not pass mitochondria to offspring; since males are the primary cause of migration in this species, there may be more nuclear gene flow than indicated by mitochondrial evidence.”
If you thought that was the interesting part, you’re still missing quite a bit. Evolutionarily, fertile hybrids alone do not mean the two semi-isolated populations will not diverge. If, for example, the individuals rarely backcross (for any reason; untimely demise, mate preference, etc) with the parental population, gene flow between the two populations is still prevented. This would result in a stable hybrid zone where the two populations overlap, but allow genetic drift and selection to still result in the populations diverging. This seems to be the case in some of the regions, but also, it seems the differentiation and divergence has been completely stopped in some regions, and partially reversed with high levels of gene flow between populations due to stable and successful hybrids. Some areas are even DOMINATED by hybrids, and not only were most of them hybrids in these areas, but they were backcrossed hybrids! The two populations had merged in this area!
So it would seem that, not only is Ensatina a superspecies and still a ring species, but gives us amazing insight into new possibilities for speciation to occur. We basically see sympatric and allopatric divergence occurring as well as divergence REVERSAL. I leave you with the final sentence from this research:
Although prized as an example of evolutionary clarity, Ensatina presents a pattern of taxonomic irresolution (different systematists might recognize from one to many species, depending on what criteria they choose) that one expects from Darwinian species formation.
Genetic Leakage After Adaptive and Nonadaptive Divergence in the Ensatina eschscholtzii Ring Species. Pereira, Ricardo and David B. Wake. Evolution.