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An evolutionary phenomenon that involves changes in the rate and timing of development. As animals and plants grow from their earliest embryonic stages to the adult, they undergo changes in shape and size. This life history of an individual organism is known as its ontogeny. The amount of growth that an organism experiences during its ontogeny can be more or less than its ancestor. This can apply to the organism as a whole or to specific parts.
Evolution can be viewed as a branching tree of modified ontogenies. Heterochrony that produces these changes in size and shape may be the link between genetics at one extreme and natural selection at the other.
If a character of one species in an evolutionary sequence undergoes less growth than its ancestor, the process is known as pedomorphosis. If it undergoes more growth, the process is known as peramorphosis. Each state can be achieved by varying the timing of onset, offset, or rate of development.
If development is stopped at an earlier growth stage in the descendant than in the ancestor (for example, by earlier onset of sexual maturity), ancestral juvenile features will be retained by the descendant adult (progenesis). If the onset of development of a particular structure is delayed in a descendant, the structure will develop less than in the ancestor (postdisplacement). The third process that produces pedomorphosis is neoteny, whereby the rate of growth is reduced.
For peramorphosis, development can start earlier in the descendant than in the ancestor (predisplacement); or the rate of development can be increased, thus increasing the allometric coefficient (acceleration); or development can be extended by a delay in the onset of sexual maturity (hypermorphosis).
As an organism grows, the number of cells that it produces increases. Ultimately, changes to rate and timing of growth are reflections of changes to the timing of onset and rate of cell development, and the balance between cell growth and cell death. Morphogens and growth hormones play a major role in controlling development in terms of initiation, rate of division, and migration. Therefore, changes to the timing of their expression affect the shape and size of the final adult structure. Inception of hormonal activity is under the control of genes that regulate the timing of its production.