How Salamander Species Can Hybridize Extensively Yet Remain Distinct: Insights from Habitat Data, Molecules, and Behavior

Abstract

This project explores a situation where two distinctive species hybridize extensively at zones of contact yet appear not to be collapsing into a single species. Lack of impending merger is inferred when regions made up of organisms of mixed ancestry (‘hybrid zones’) are not increasing in extent but instead are stable. There is growing recognition that this phenomenon is common nature. Several forces have been proposed to function in preventing lineage merger in these situations. Selection for different ecological conditions (‘exogenous selection’) is widely acknowledged to maintain species limits, especially when the contacting lineages are associated with different habitats. Genome incompatibilities (when hybridization leads to the breakdown of co-adapted gene complexes, or ‘endogenous selection’) have also been cited as contributing to hybrid zone stability. Positive assortative mating, where individuals preferentially mate with similar individuals, has also been put forward as a process that could prevent the merger of species. Two species of lungless salamanders, Plethodon shermani and P. teyahalee, have parapatric distributions in the southern Appalachian Mountains and form hybrid zones where their distributions meet. I investigated hybrid zone stability and potential factors maintaining species limits between these salamanders. In Chapter 1, I use coalescent simulations to identify a lower bound on the timing of secondary contact, which in turn leads to an expected hybrid zone extent under a model of neutral diffusion. By comparing observed and expected hybrid zone extent, I show that the two species are not in the process of freely merging despite extensive introgression of molecular markers. In Chapter 2, I evaluate predictions of ex- and endogenous selection using ground-level temperature data and character clines fitted to molecular and morphological data. I find evidence for exogenous selection but not for endogenous selection. In Chapter 3 I present data from courtship trials performed in the laboratory that reveal that despite frequent interspecific hybridization, these two species show a preference for conspecifics. These findings shed light on Plethodon hybrid zone dynamics and the maintenance of species limits for hybridizing species.