Heterochrony

AbstractHeterochrony 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.Key Concepts:The extension across time of stages and component events in embryonic development is an inherent feature because development involves a cumulative succession of cell multiplication and differentiative processes, all starting from the single cell stage of the egg.The controversial concept of heterochrony attempts to characterise a mechanism controlling variations in developmental timing of the embryo or its component parts, which could potentially mediate and explain the evolution of new morphologies.Neoteny is a paradigm example of heterochrony, in which the adult morphology of a species is thought to be derived from the larval form of an ancestral species whose adult form is lost, perhaps by abandonment of the preexisting developmental events of metamorphosis.It has been proposed that a heterochronic process such as neoteny could be the basis for major innovations of new taxa (macroevolution); in a form of heterochrony termed paedomorphosis, a new body plan can be established by elaboration of a larval morphology rather than being restricted to modifications to the preexisting adult body plan.