ZELDITCH, M.L.*; MEZEY, J.; GARLAND, T.; LUNDRIGAN, B.L.; Univ. of Michigan, Ann Arbor; Univ. of California, Davis; Univ. of California, Riverside; Michigan State Univ.: Canalization Restructures Variance

The structure of phenotypic variation is determined by the balance of processes generating, forestalling and reducing variance. Because canalization both forestalls and reduces variance, it potentially plays an important role in molding its structure. Postnatal ontogeny of rodent skull shape offers an attractive model for exploring that role because variance reduces dramatically prior to weaning and thereafter is nearly constant. We examine the ontogeny of variation patterns in two species, cotton rats (Sigmodon fulviventer) and house mice (Mus musculus domesticus) to determine if either the reduction of variation or its later constancy is marked by restructuring of variance. We compare patterns of variation within age-classes of both species using matrix correlations and Common Subspace Analysis (a method related to Common PCA). We find substantial changes in variation patterns even when the level of variance is constant, indicating that variation is continually being generated and removed. The rate of change eventually decreases in house mice, to the point that 30-day-olds do not differ from 40-day-olds, nor do 40- from 50-day-olds; but 30-day-olds differ significantly from 50-day-olds. When comparisons are restricted to the dimensions in which variance is most concentrated, the temporal dynamics are very different. Under these conditions, the two youngest samples do not differ significantly from each other in either species. In cotton rats, variation is restructured only after weaning and in house mice, that restructuring occurs over the five days surrounding weaning. However, the magnitude of the change in structure is no greater (or less) in early postnatal growth than later.