Heterochrony

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Heterochrony (from Greek ἕτερος heteros "deviating" and χρόνος chrónos "time") denotes an evolutionary change in the temporal course of the individual development of a living being, which causes itself

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

Pedomorphosis and Peramorphosis

If features of the early developmental stages ( juvenile stages ) of an ancestor occur in the adult stage (adult stage) of a descendant, this is called pedomorphosis . Conversely, if characteristics of the later stages (adult stages) of an ancestor species are already passed through during the youth of a successor species, this is called peramorphosis .

Forms of pedomorphosis are neoteny , in which the development of an adult trait is delayed, and progenesis , in which the trait expression is terminated at an earlier point in development. Such phenomena have been proven both recently and in fossil form in many groups. For example, the dwarfing of agnostid trilobites , thecidid brachiopods and many other groups is a form of pedomorphosis. The retention of outer gills into the adult stage, which is characteristic of some amphibian species , is also caused by pedomorphosis.

An ancestral series of species in which the adult stage increasingly resembles the early stages of the ancestors, that is, becomes more and more "childlike", can be described as pedomorphocline . Correspondingly, development series in which the former adult stages of the ancestors occur at an ever earlier point in time receive the designation peramorphocline .

Peramorphosis occurs, for example, when certain organs are enlarged compared to those of the ancestors due to the prolongation of the growth phase ( hypermorphosis ) or due to growth acceleration . A general increase in body size within a developmental series can also be interpreted as peramorphosis.

Mechanisms and importance of heterochronous evolution

Heterochrony can be caused by one or a few mutations , for example in genes that regulate the release of growth hormones (see giant stature ) or otherwise influence the timing during ontogenesis (see also Hox gene ).

For this reason, populations often react to certain selection pressures with heterochrony: When resources are scarce on an island, reducing body size (" island dwarfing ") is usually a "more economical" solution (that is, it requires fewer mutations and evolutionary steps) than, for example, the development of wings or fins .

In this way, the ability to heterochrony often prevented the realization of other, more elaborate solutions and acts as evolutionary barrier ( evolutionary constraint ) that the course of the phylogeny directs a group in certain tracks (channels).

literature

  • Glenn S. Jaecks, Sandra J. Carlson: How phylogenetic inference can shape our view of heterochrony: examples from thecideide brachipods. In: Paleobiology. Volume 27, No. 2, 2001, pp. 205-225.
  • Michael L. McKinny (Ed.): Heterochrony in evolution: a multidisciplinary approach. Plenary, New York 1988.
  • Kenneth J. McNamara: Importance of Heterochrony. In: Derek. EG Briggs, Peter R. Crowther (Eds.): Palaeobiology II. Blackwell, Oxford 2001, ISBN 0-632-05147-7 , pp. 180-187.
  • Kenneth J. McNamara, Michael L. McKinney: Heterochrony, disparity, and macroevolution. In: Paleobiology. Volume 31, No. 2, 2005, pp. 17-26.
  • Douglas S. Jones, Stephen Jay Gould: Direct Measurement of Age in Fossil Gryphaea: The Solution to a Classic Problem in Heterochrony. In: Paleobiology. Volume 25, No. 2, 1999, pp. 158-187.

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