Peramorphosis

In evolutionary developmental biology, heterochrony is a developmental change in the timing or rate of events, leading to changes in size and shape of organs and features over evolutionary time scales. It is contrasted with heterotopy, a change in spatial positioning of some process in the embryo, which can also create morphological innovation. Heterochrony can be divided into intraspecific heterochrony that explains variation within a species, and interspecific heterochrony that explains developmental variation phylogenetically, in the timing or rate of events of a descendant species with respect to an ancestral species. In evolutionary developmental biology, heterochrony is a developmental change in the timing or rate of events, leading to changes in size and shape of organs and features over evolutionary time scales. It is contrasted with heterotopy, a change in spatial positioning of some process in the embryo, which can also create morphological innovation. Heterochrony can be divided into intraspecific heterochrony that explains variation within a species, and interspecific heterochrony that explains developmental variation phylogenetically, in the timing or rate of events of a descendant species with respect to an ancestral species. These changes all affect the start, end, rate, or timespan of a particular developmental process. The concept of heterochrony was introduced by Ernst Haeckel in 1875, and given its modern sense by Gavin de Beer in 1930. The concept of heterochrony was introduced by the German zoologist Ernst Haeckel in 1875, where he used it to define deviations from recapitulation theory, which held that 'ontogeny recapitulates phylogeny'. As Stephen Jay Gould pointed out, Haeckel's term is now used in a sense contrary to his coinage. He assumed that embryonic development (ontogeny) of 'higher' animals recapitulated their ancestral development (phylogeny). This, in his view, necessarily compressed the earlier developmental stages, representing the ancestors, into a shorter time, meaning accelerated development. The ideal for Haeckel would be when the development of every part of an organism was thus accelerated, but he recognised that some organs could develop with displacements in position (heterotopy, another concept he originated) or time (heterochrony), as exceptions to his rule. He thus intended the term to mean a change in the timing of the embryonic development of one organ with respect to the rest of the same animal, whereas it is now used, following the work of the British evolutionary embryologist Gavin de Beer in 1930, to mean a change with respect to the development of the same organ in the animal's ancestors. In 1928, the English embryologist Walter Garstang showed that tunicate larvae shared structures such as the notochord with adult vertebrates, and suggested that the vertebrates arose by paedomorphosis (neoteny) from such a larva. De Beer anticipated evolutionary developmental biology in his 1930 book Embryos and Ancestors, showing that evolution could occur by heterochrony, such as in paedomorphosis, the retention of juvenile features in the adult. De Beer argued that this enabled rapid evolutionary change, too brief to be recorded in the fossil record, and in effect explaining why apparent gaps were likely.