Geographic Variation in Genital Morphology among Insular and Mainland Populations of Peromyscus maniculatus and Peromyscus oreas

Abstract

Geographic relationships among insular and mainland populations of deer mice ( Peromyscus maniculatus and P. oreas ) from the Pacific Northwest were assessed by multivariate analyses of morphometric data from bacula and phalli. Individuals examined represent two distinct forms based on number of autosomal arms in standard karyotypes (fundamental number). Results indicate that the two karyotypic groups have significantly different genitalia, with the form with high fundamental numbers (FN = 85–92) being larger than the form with low fundamental numbers (FN = 74–78) in all characters except depth of the dorsal-lappet cleft. This pattern of morphometric differentiation also characterized allopatric and sympatric pairs of the two groups, thus supporting the hypothesis that individuals are referable to two species (high fundamental numbers = P. oreas; low fundamental numbers = P. maniculatus ). The magnitude of differences in genitalia between allopatric and sympatric samples of the two species failed to corroborate the hypothesis that morphologic differences in genitalia may become amplified in sympatry as a mechanism facilitating reproductive isolation.