A MODEL RELATING DEVELOPMENTAL INTERACTION AND DIFFERENTIAL EVOLUTIONARY REDUCTION OF TOOTH SIZE

Reduction in size of the jaws during hominid evolution has been accompanied by an overall reduction of tooth size, but the degree of reduction varies from one tooth to another in a definite pattern. Within each morphological class (incisors, premolars and molars) the later a tooth develops the more it has been reduced. Similar relationships between developmental timing and degree of reduction may have been commonplace during the evolution of mammalian dentitions in general (Ziegler, 1971). However, later developing teeth tend to have lower heritabilities for tooth size than their earlier developing neighbours (Lundstrom, 1948; Hunter, 1959; Bader, 1965; Bader and Lehmann, 1965; Sofaer et al., 1971a). Thus there appears to be an association between lower heritability and more rapid evolutionary change. It is possible that this is the result of selection that has acted directly on the later developing teeth of each class causing a reduction of genetic variation. It is perhaps more logical to consider that the bulk of any selection acts on each morphological class as a whole rather than differentially on the individual teeth of which it is composed. In the absence of direct differential selection on members of the same class the association between lower heritability and more rapid evolutionary change can be explained if it is assumed that some proportion of the reduction that has taken place in the dentition has been secondary to skeletal reduction, through selection for harmony between size of teeth and size of jaws. In such a system the primary reduction of jaw size might be expected to have resulted in restriction of the developing teeth, with consequent compensatory interaction between adjacent tooth germs due to competition for requirements necessary for growth. That is, if for some reason a tooth which developed early was larger than normal, then its later developing neighbour would have tended to be smaller than normal, and vice versa. There is evidence to suggest that this kind of compensatory interaction occurs in contemporary populations of both man and experimental animals (Griineberg, 1951; Grewal, 1962; Van Valen, 1962; Gould and Garwood, 1969; Sofaer, 1969a; Sofaer et al., 1971a; Sofaer et al., 1971b). In order to explain differential tooth reduction in terms of selection for harmony between tooth size and jaw size, it is necessary to show that genotypes with the potential to produce relatively large early developing teeth and small late developing teeth are favoured over those with the potential to produce relatively small early and large late teeth. A simple example of how compensatory interaction under conditions of local restriction can produce this result has already been proposed (Sofaer et al., 1971a). The purpose of the present paper is to put forward a generalised model and to discuss the patterns of variation that might be expected in contemporary populations if the model were valid.