Heterochrony has often been proposed as a leading mechanism for macroevolutionary change in morphology of organisms, based simply on a relative shift in the timing of events during embryonic development in the evolving organisms. Its most recent formulation was in Gould's 'clock model'. Such timing shifts were seen as being controlled genetically (by 'rate genes') or by hormones (such as thyroxine). Heterochrony has much in common with the concept of allometry. The concept originated in the context of the idea of recapitulation where embryonic development was interpreted as a repetition of the organism's evolutionary history but in accelerated form; deviations from this recapitulation were identified as instances of heterochrony. In a later version used to explain macroevolutionary change (paedomorphosis), a new adult morphology is derived from ancestral larval forms by truncation of ancestral adult stages. However, it is difficult to see how genuine structural novelty can be explained, given that timing changes during development can in themselves do no more than redeploy preexisting characters. The explanatory value of the concept of heterochrony is increasingly questioned; at best it only indirectly relates to, or identifies, specific underlying mechanisms. It is in effect a form of redescription.
Heterochrony is a supposedly important mechanism for macroevolutionary change based on shifts in the timing of developmental events in the embryo.
Heterochrony is a supposedly important mechanism for macroevolutionary change based on shifts in the timing of developmental events in the embryo.
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