Mechanism for the origin and the evolutionary significance of heterochronia

Using material on the evolution of the fundus of the oral cavity in amphibians, four principles were established for the origin of heterochronia: efficiency of ontogenesis; adaptability of an earlier (than in ancestors) beginning for functioning of traits; loss of the adaptive significance of a trait at a previous stage, but a preservation of it at a subsequent stage; and, a prolongation of the adaptive significance of a trait at later stages. The origin of heterochronia is associated with changes in the succession of the functioning of genes, with reorganization of systems of morphogenetic correlations, and with change in the traits themselves that undergo heterochronia. The uniqueness of heterochronia as an adaptation consists of an increase in the evolutionary flexibility of specific stages of ontogenesis.