Stabilizing Selection for Odontometric Traits in Peromyscus leucopus

Significant changes attributable to natural selection were found in the reproductive performance and in the widths of the second and third mandibular molars in a laboratory colony of Peromyscus leu- copus. Over 1000 individuals were generated from 58 wild-trapped founders reared over three generations. The mean and variance of litter size, the efficiency of producing progeny, and the incidence of cannibal- ism increased in each generation, although no differences were observed in overall fertility. An increase in the mean molar width among the fertile individuals of each generation represented a correlated response to in- creased litter size and was most probably brought about by superior nutrition in the laboratory. Infertile mice exhibited a greater phenotypic variance for molar widths and for litter size than did fertile mice, the differences being most pronounced in the wild mice. It was postulated that stabilizing selection, although noticeably more relaxed in the labora- tory, acted against individuals from peripheral litter sizes, and favored (heterozygous) mice of intermediate tooth dimensions.