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.

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.

Keywords: neoteny; paedomorphosis; recapitulation; rate genes; clock model; larval forms; macroevolution

Figure 1.

The classic example of neoteny. The neotenous Mexican axolotl, unlike other urodeles, becomes adult and achieves sexual maturity, although still retaining typical larval features including external gills and tail fins (a). However, thyroxine can induce metamorphosis artificially to a typical urodele adult form as shown in (b). After Huxley ().

Figure 2.

A classic example of the role of paedomorphosis in the origin of new phyla and novel Bauplans. The larval ‘tadpole’ stage of an invertebrate ascidian (a) is here seen as the possible starting point for the origin of the chordates. According to this theory, maturity would be reached in the larval stage and the normal invertebrate adult stage (b) would be abandoned.

Figure 3.

Human evolution has been interpreted as a case of paedomorphic modification and as the retention of the younger developmental stages of an ancestral ape. The young chimpanzee (a) has obviously human‐like features compared with the adult ape (b). Reproduced from Naef () © Springer.



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Further Reading

Minush‐Purvis N and McNamara KJ (2002) Human Evolution through Developmental Change. Baltimore: Johns Hopkins University Press.

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Zelditch ML (2001) Beyond Heterochrony: The Evolution of Development. New York: Wiley‐Liss.

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Horder, Tim(Feb 2013) Heterochrony. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001778.pub3]