Quantitative Genetics and Evolution of Head Shape in Plethodon Salamanders

The evolution of morphological diversity via selection requires that morphological traits display significant heritable genetic variation. In Plethodon salamanders, considerable evidence suggests that head shape evolves in response to selection from interspecific competition, yet the genetic underpinnings of head shape have not been quantitatively examined. Here I used geometric morphometrics and quantitative genetics to assess heritable patterns of head shape variation from hatchling salamanders in two Plethodon species (P. cinereus and P. nettingi). Head shape differed significantly between species and among clutches within species, suggesting that a sizeable proportion of head shape variation was the result of clutch effects. Further, using a full-sib animal model and restricted maximum likelihood (REML), I identified large values of maximal additive heritability for all study localities (h(max)(2) > 0.65), revealing that Plethodon exhibit considerable heritable genetic variation for head shape. Comparisons of the components of heritable shape variation showed that the magnitude of shape heritability (h(max)(2)) did not differ among localities or species. Therefore, the potential microevolutionary shape change displayed by the two species would be similar if they were exposed to comparable selective forces. However, the direction of maximal shape heritability in morphospace differed between P. cinereus and P. nettingi, indicating that potential evolutionary shape change along these heritability trajectories would diverge between the two species. This finding implies that distinct head shapes could evolve in the two species, even if subjected to the same selection pressure. When combined with previous knowledge of patterns of head shape variation among species and ecological selection on head shape, these findings suggest that microevolutionary and macroevolutionary trends of morphological diversification in Plethodon may be explained as a result of the interaction between ecological selection and underlying patterns of genetic covariance for this multi-dimensional trait.