What is "heterochrony"

Heterochronism \Het`er*och"ro*nism\, Heterochrony \Het`er*och"ro*ny\, n. [Gr. ? of different times; ? other + ? time.] (Biol.) In evolution, a deviation from the typical sequence in the formation of organs or parts.

n. (context biology English) the variation in ontogeny of individuals of a species that gives rise to evolution

In biology, heterochrony is defined as a developmental change in the timing or rate of events, leading to changes in size and shape. There are two main components, namely (i) the onset and offset of a particular process, and (ii) the rate at which the process operates. A developmental process in one species can only be described as heterochronic in relation to the same process in another species, considered the basal or ancestral state, which operates with different onset and/or offset times, and/or at different rates. The concept was first introduced by Ernst Haeckel in 1875.

An example can best illustrate the three dimensions of heterochrony.

• Predisplacement and postdisplacement: If a developmental process, such as the growth of a tail in the embryo of "species A", starts earlier and ends earlier than that of "species B", but the rate of growth is the same for both, the final result may basically be the same, although the tail of species A develops earlier than the one of species B. The earlier exhibits predisplacement, and the later species exhibits postdisplacement.
• Neoteny: if the rate of growth is increased, and the time between the start and end of development is decreased proportionally, the tail will end up the same size. The species with faster growth exhibits acceleration, and the species with slower exhibits neoteny.
• Hypermorphosis: if the end of development is delayed and the rate is unaffected, development progresses further, and the tail will be also larger. The species that develops further exhibits hypermorphosis, and the species that does not develop as far exhibits progenesis.