What is Heterochrony?
'''Heterochrony''' (= ''different time'') is any change in the timing of development in an animal or plant. It is a key concept in [[developmental biology]] and [[evolution]], introduced by [[Ernst Haeckel]] in 1875. It applies to the time a [[Trait (biology)|trait]] appears in the [[growth]] of an organism, or to the timing of [[gene expression]] in [[morphogenesis|development]].<ref>King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics'', 7th ed. Oxford.</ref><sup>p203</sup><ref>Horder, Tim 2006. Heterochrony. In ''Encyclopedia of Life Sciences''. Wiley.</ref> Its importance in biology is to explain the way closely related animals may come to have quite large differences in shape, and size, whilst nevetheless having a rather similar [[genome]]. The main components are the time a process starts and stops in development, and the rate at which the process works. In his book ''Embryos and evolution'' (1930) [[Gavin de Beer]] stressed the importance of heterochrony, and especially '''paedomorphosis''', in evolution.<ref>Brigandt I. 2006. Homology and heterochrony: the evolutionary embryologist Gavin Rylands de Beer (1899-1972). ''Journal of Experimental Zoology (Molecular and Developmental Evolution)'' '''306B''':317-328. [https://www.ualberta.ca/~brigandt/de_Beer.pdf preprint]</ref> Changes to the [[gene]]s which control timing in development can produce quite striking changes in the adult. de Beer also conceived the idea of ''clandestine evolution'', which helped to explain the sudden changes in the [[fossil]] record which were apparently at odds with Darwin's gradual theory of evolution. If a novelty were to evolve gradually in an animal's juvenile form, then its development might not appear in the fossil record at all. Then, if the species were to undergo [[neoteny]], the feature would appear suddenly in the fossil record, despite having evolved gradually.<ref>Jones S, Martin R. & Pilbeam D. 1992. ''The Cambridge Encyclopedia of Human Evolution'', p104 ''Neoteny''.</ref> [[Stephen Jay Gou