Variation

Teeth may vary in number, size, shape and position within the same species. Variation in number, size and shape tends to be of largely genetic origin or at least to involve genetic mechanisms; it used to be thought that this variation, which involves quantity of tooth material, was beyond the influence of environmental factors such as nutrition, but new evidence suggests that this is not so. Variation in position on the other hand is much influenced by environmental factors and, because tooth position depends on bone development and bone growth, is frequently secondary to conditions that affect growth.

Before considering the evidence that bears on the mechanisms that bring about these types of variation, which is one of the main purposes of this introductory chapter, it is necessary to discuss the term variation itself.

Variation in the biological sense comprises differences of every kind, morphological and other, that exist between individuals of the same species. Variation in the observable characteristics of an organism (phenotype) is the product of individual differences in genetic constitution (genotype) and of various environmental influences. Heritable variation is ‘the material on which natural selection acts to bring about the evolution of species’ (Bateson, 1894).

Variation implies deviation from a mean or norm; when the observed difference is slight, the term normal variation is sometimes applied. When the deviations are more gross and uncommon and amount to abnormalities, they may be said to constitute abnormal variation. The distinction between normal variation and abnormality is quite arbitrary, especially in con tinuous variation, that is variation in characters, like stature and tooth size, that can be measured against a continuous scale, even an ordinal one.