Neoteny is apparently the best known heterochrony. Neoteny consists in delay of the development of some body organs compared to development of the sex organs (Wakahara 1996). Most authors now distinguish progenesis as special type of heterochrony, during which premature development occurs of the sexual organs without delay in the development of the somatic organs. An adult organism capable of reproduction thus frequently bears a number of traits characteristic for younger developmental stages, juveniles or larva of a related species of organisms. The best known example of a neotenic animal is the Mexican axolotl, a tailed amphibian, whose body structure differs drastically from related species and is remarkably similar to their larvae. Experimental intervention, specifically administration of a hormone, can induce metamorphosis in this species and the resultant organisms do not differ much from related species in which neoteny does not occur. Neoteny most probably played an important role in the anagenesis of humans (Gould 1978). A number of our body and other traits, e.g. the size of the cerebral cavity, shape of the facial part of the skull and, of nonanatomical traits, e.g. playfulness, are remarkably reminiscent of the traits of immature individuals of related species of anthropoids.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more