The tooth crowns of most mammals do not change in size except by attrition. Therefore, by comparing age groups within a population and finding differences in size or variation, the effects of such agencies as directional or stabilizing selection can be demonstrated. Mesiodistal and buccolingual measurements were taken on the maxillary and mandibular permanent dentition of protohistoric Arikara Indian population from South Dakota. The tooth sizes of juveniles ages 6 through 15 were compared to those of adults ages 16 through 20. The two age groups were also compared by the cross-sectional areas of the posterior teeth. When a difference between age groups was demonstrated in either size or variation, a selection intensity was estimated using published graphs.
Results from the comparisons show that adults, in general, have larger and less variable teeth than the juveniles. The data suggest the possibility that both directional and stabilizing selection were operating on the Arikara dentition. Furthermore, the selection intensities suggest that selection was operating not on the individual teeth but on functional complexes such as the posterior teeth. It appears that in populations like the Arikara where crown attrition is severe, selection will in the direction of large teeth.