Selection in a cycling population: differential response among skeletal traits

Population density cycles influence phenotypic evolution through both density-dependent selection during periods of high density and through enhanced genetic drift during periods of low density. We investigated the response of different phenotypic traits to the same density cycles in a population of the yellow-necked mouse, Apodemus flavicollis, from Białowieza National Park in Poland. We examined nonmetric skull traits, skull and mandible size, skull and mandible shape, and transferrin allele frequencies. We found that all of the traits changed significantly over the seven-year study period. The greatest changes in nonmetric traits and mandible size occurred during periods of increasing density, and the magnitude of changes in skull and mandible shape was correlated with the magnitude of density changes. Frequencies of transferrin alleles changed the most when population density was in decline. Changes among the five phenotypic traits were generally uncorrelated with one another, except for skull and mandible shape. Nonmetric traits were selectively neutral when assessed with Q(ST)/F(ST) analysis, whereas mandible size, mandible shape, and skull shape showed evidence of fairly strong selection. Selection on skull size was weak or nonexistent. We discuss how different assumptions about the genetic components of variance affect Q(ST) estimates when phenotypic variances are substituted for genetic ones. We also found that change in mandible size, mandible shape, skull size, and skull shape were greater than expected under a neutral model given reasonable assumptions about heritability and effective population size.