RAPID MORPHOLOGICAL CHANGE IN CHANNEL ISLAND DEER MICE.

Deer mice, Peromyscus maniculatus, collected over 90 years from three California Channel Islands, were examined for evidence of morphological change. Rapid morphological change has occurred in the endemic subspecies from Santa Barbara (P. m. elusus), Anacapa (P. m. anacapae), and Santa Cruz Island (P. m. santacruzae). Data were divided into two temporal classes, 1897-1941 and 1955-1988. Of the 16 morphological characters measured, between five and 10 measures changed significantly (P ⩽ 0.05) with temporal class in each subspecies, and multivariate test statistics were significant (P ⩽ 0.05) for all three subspecies. For each subspecies, depth of braincase, total length, tail length, and hind foot length became smaller over time, except depth of braincase, which became larger in P. m. elusus. The rates of change dramatically exceed those estimated from paleontological records and are even higher than those reported in some experimental selection studies. Temporal change in two characters exceeds differentiation between subspecies. Although changing, each subspecies remained well differentiated, and incorporation of temporal change allowed correct classification of most specimens. Unlike nearly all previous reports of rapid evolution, the changes in these populations were not associated with a founder events or recent introductions. This study demonstrates that rapid phenotypic change can occur in long-established natural populations and temporal stability of morphological characters in such populations, even over short evolutionary time periods, cannot be assumed.