Heterochrony

Heterochrony (from the Greek heteros ἕτερος "deviant " and χρόνος chronos "time" ) refers to an evolutionary change in the time course of individual development of an organism that causes

• The beginning or the end of a development process - for example, the development of the dentition - moves or
• The speed of such a process changes. An example is the acceleration of the growth in size endothermic vertebrates ( " warm-blooded " ) compared to their ektothermen ( " cold-blooded " ) ancestors.

Pädomorphose and Peramorphose

Contact characteristics of the early stages of development ( juveniles ) of an ancestor in the adult stage ( adult stage ) a descendant on, we call this as Pädomorphose. Conversely, if characteristics of the later stages ( adult stages ) of an ancestral species are already undergo during adolescence a Nachfolgeart, which is called Peramorphose.

Forms of Pädomorphose are the neoteny, in which delays the formation of an adult feature, and the Progenese, wherein the characteristic value is terminated at an earlier stage of development. Such phenomena are both tart and fossil evidence in many groups. Thus, for example, the dwarfing in Agnostid trilobites, brachiopods thecideiden and many other groups a form of Pädomorphose dar. also maintaining of external gills until the adult stage, which is characteristic of some species of amphibians, caused by Pädomorphose.

A lineage of species in the adult stage more and more resembles the early stages of the ancestors, that is, always "childish " is may be referred to as Pädomorphokline. As a consequence the developmental series in which there are former adult stages of their ancestors to an ever earlier time, the name Peramorphokline.

Peramorphose exists, for example, if by prolonging the growth phase ( Hypermorphose ) or by growth acceleration certain institutions to which the ancestors are enlarged. Also, a general body enlargement within developmental series can be interpreted as Peramorphose.

Mechanisms and significance of heterochronic evolution

Heterochrony can already be caused by one or a few mutations, for example, of genes that regulate the pouring of growth hormones (see gigantism ) or otherwise affect the timing during ontogeny (see also Hox gene).

For this reason, react populations to specific selection pressures often with heterochronia: In scarcity of resources on an island, the reduction of body size ( " Inselverzwergung " ) solution is usually a " more economical " (that is, it requires less mutations and evolutionary steps ) as an example, the formation of wings or fins.

In this way, the ability to heterochronia often prevents the realization of other, more elaborate solutions and acts as an evolutionary barrier ( evolutionary constraint ), which directs the course of evolutionary history of a group in certain paths ( channels ).