Paedomorphosis and Peramorphosis

The six types of heterochrony fall into two general patterns: paedomorphosis and peramorphosis. In these patterns, ancestral and descendant onto-genetic trajectories are compared. If the descendant morphology exceeds or surpasses the ancestral morphology, this is called peramorphosis. There are three types of peramorphosis: acceleration, hypermorphosis, and predisplacement. In acceleration, the beginning and end of development occurs at the same time in both the ancestral and descendant ontogenetic trajectories. The rate of development is faster in the descendant, however, so its morphology transcends that of the ancestor. In hypermorphosis, development begins at the same time and ensues at the same rate in both the ancestor and descendant; however, the descendant continues development for a longer interval—that is, it stops growing later. In predisplacement, the descendant begins development earlier than the ancestor, and the rate of development and the time at which growth ceases remain the same. The result of both predisplacement and hypermorphosis is a longer interval of growth. In acceleration, however, the interval of growth is the same in both the ancestor and its descendant; only the rate of growth has changed.

If a descendant morphology is a truncated or an abbreviated version of ancestral morphology, this is called paedomorphosis. There are also three types of paedomorphosis: neoteny, progenesis, and postdisplacement. In progenesis, descendant development begins and proceeds at the ancestral rate but stops sooner. In postdisplacement, the ancestral rate of development and the time of cessation are the same in the descendant; however, development begins later. Thus, for post-displacement and progenesis, the time interval over which the descendant morphology develops is truncated when compared to the ancestral ontogenetic trajectory. In contrast, neoteny is characterized by a slower descendant rate of development but an unchanged time interval for growth and development.

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  • leah trommler
    What is paedomorphosis n peramorphosis?
    2 years ago

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