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 appears in the growth of an organism, or to the timing of gene expression in development.p203
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. Changes to the genes 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. Stephen Jay Gould commented on de Beer's book: