Paul A. Verrell

The evolution of asymmetry in sexual isolation : a model and a test case

We constructed a model for the evolution of sexual isolation by extending Landes (1981) model of sexual selection. The model predicts that asymmetric sexual isolation is a transient phenomenon, characteristic of intermediate stages of divergence in sexually selected traits. Unlike the Kaneshiro (1976, 1980) proposal, our model does not depend upon drift and the loss of courtship elements to produce asymmetries in sexual isolation. According to our model, the direction of evolution cannot be predicted from asymmetry in sexual isolation. We tested some features of the model using data from an experimental study of sexual isolation in the salamander Desmognathus ochrophaeus. We tested for sexual isolation between 12 allopatric populations and found significant asymmetry in sexual isolation in about a quarter of the test cases. The highest degrees of asymmetry were associated with intermediate levels of divergence. A curvilinear relationship between isolation asymmetry and divergence was predicted by our model and was supported by statistical analysis of the salamander data.

Male mate choice for fecund females in a plethodontid salamander

Etude, chez Desmognathus ochrophaeus, du choix du partenaire par les mâles en fonction de la fecondite et de la taille des femelles

Illegitimate exploitation of sexual signalling systems and the origin of species

The sexual signals passed between conspecific males and females in the context of mate attraction and stimulation also are available for exploitation by predators and parasites. These «illegitimate» receivers have profoundly influenced the evolution of sexual signalling systems within populations, as briefly reviewed in this paper. I then extend this concept to groups of conspecific populations exposed to different communities of exploiters. I hypothesize that selection in the form of arms-races among «exploited» and «exploiter» species may cause divergence of sexual signalling systems in the former (in both traits and preferences). This divergence may be sufficient to generate sexual isolation among populations, permitting further population differentiation and, perhaps, the origin of new species. Results of recent research on the ethology and ecology of male sexual signals and female preferences in the guppy Poecilia reticulata are used to test this hypothesis.

Tests for sexual isolation among sympatric salamanders of the genus Desmognathus

Populations of the dusky salamanders Desmognathus ochrophaeus (Cope), D. imitator (Dunn) and D. santeetlah (Tilley) (Amphibia: Caudata: Plethodontidae) are sympatric at the locality of Indian Gap in the Great Smoky Mountains of eastern North America. In order to determine the extent of sexual isolation ( = sexual behavioural incompatibility) among these species, individuals of both sexes of all three species were collected and subjected to both intra- and interspecific courtship trials in the laboratory. Sexual isolation was very strong between D. ochrophaeus and D. imitator. Sexual isolation was complete between D. ochrophaeus and D. santeetlah, and between D. santeetlah and D. imitator. Mechanisms which prevent interspecific mating are highly developed in these salamanders, and likely are responsible (at least in part) for the apparent absence of hybridization and introgression among these species.