Any change in the relative rates or timing of development of different cell lines in the body. Such changes can lead to significant alterations in form; for example, if a particular organ develops earlier or faster it may be larger. Hence, mutations causing such changes play an important role in evolution. Heterochronic changes affecting the relative rates of development of germ (reproductive) cells and somatic (body) cells are of particular interest. They fall into four categories—acceleration, progenesis, neoteny, and hypermorphosis—depending on whether the change in rate (speeded up or slowed down) affects the somatic tissues or the reproductive tissues (see table). However, they have only two possible outcomes: one is paedomorphosis, in which reproduction occurs in an ancestrally juvenile form; the other is peramorphosis, in which development is extended by the addition of stages to the sequence shown in the ancestral form. Compare heterometry; heterotony; heterotypy.