Heterochrony—literally, "different timing"—describes the occurrence of a change in the timing of the development of different body parts between an ancestor and its descendants. The concept of heterochrony is intimately associated with allometry, which describes the relationship between the size of different structures or organs of an organism throughout its life; both concepts involve the study of growth patterns.

Describing Heterochrony

Heterochronic phenomena may be described with respect to somatic (body) and gonadal (reproductive) maturation and may be global (effecting the entire individual) or local (affecting only one structure, organ, or system). Further, the growth of a structure or organ may be isometric with respect to other structures (shape does not change with growth) or it may follow either a positive or negative allometric path (shape changes with growth). Finally, different kinds of heterochronies can occur in different parts of the body, producing ontogenies (courses of...