Evolution and mammalian dental morphology.

Evidence for the action of natural selection on the mammalian dentition is reviewed. Dental variation largely reflects genetical differences, but environmental factors have a significant effect, especially on the later developing teeth. Reduction of dental variance with age provides direct evidence of selection, but most evidence consists of demonstrations of adaptiveness. Dental adaptation is partly external, mainly to the food, and partly internal, resulting in the functional integration of teeth with each other, as in occlusal relations, and with the neuromuscular system responsible for jaw movements. Ontogeny is also adapted to produce a functional end-product. Most interspecific differences have not yet been explained in terms of function, and many of them may be due to drift.