Directional selection and developmental stability: evidence from fluctuating asymmetry of dental characters in mice.

Developmental stability was assessed among lines of mice subjected to 11 generations of selection for increased (up line) and decreased (down line) widths of the first maxillary molar (M1), primarily to test the hypothesis that this stability would decline over time as a result of selection. Fluctuating asymmetry (FA) was used as an inverse measure of developmental stability, and was calculated for the M1 and a correlated character, the second mandibular molar (M2), in each generation. As measured by the regression of FA on generations, there was a statistically significant decline in stability only in the down selection line for the M2, and this trend did not differ significantly from that for the control line. The combined regressions obtained from pooling over all three lines were significantly different from zero in both molars, however, and this was taken to be evidence of a decline in stability due to increased homozygosity presumably generated by inbreeding during the selection experiment. The variation in FA among generations was greater for the selection lines than the control line for both molars, especially for the M2. The heritability of the M1 (and M2) was high, but there was no significant additive genetic variance for FA in either molar.