Directional selection and developmental stability: evidence from fluctuating asymmetry of morphometric characters in rats.

Developmental stability was assessed among lines of rats selected for increased (up lines), decreased (down lines), and no body weight gain (controls) by the measurement of fluctuating asymmetry (FA) in 11 bilateral osteometric characters. This was done primarily to test the hypothesis that developmental stability should be lower (and thus FA higher) in both selection lines compared to the control line. FA values were obtained for all characters in each sex, line, and replicate (two each) within lines. Expressed as a proportion of the total variance, the FA values ranged from about 2% to over 50%, the average being 23%. FA for 3 of the 11 characters showed statistically significant differences among the lines, and in a multivariate analysis of variance, lines (but not sex or replicates) also showed overall significance (P less than 0.05). For the pooled sexes and replicates, seven of 11 FA values in the high line were greater than the comparable values in the control line (especially that for femur length), this conforming with theoretical expectations. Only one FA value in the down line was greater than that of the controls, however, and it was hypothesized that this was a consequence of the action of natural selection opposing artificial selection for decreased body weight gain.