retention by an organism of juvenile or even larval traits into later life. There are two aspects of paedomorphosis: acceleration of sexual maturation relative to the rest of development (progenesis) and retardation of bodily development with respect to the onset of reproductive activity (neoteny).

Classic examples include certain amphibian species in which development is arrested so that the larval form and aquatic habit persist as the organism attains sexual maturity and becomes capable of reproduction. In some species only a few morphological features are retarded, but the number of features retarded may differ from species to species. Adult humans, for example, display various neotenic body features that other adult primates do not.

In other species all morphological development is retarded; the organism is juvenilized but sexually mature. Such shifts of reproductive capability would appear to have adaptive significance to organisms that exhibit it. In terms of evolutionary (evolution) theory, the process of paedomorphosis suggests that larval stages and developmental phases of existing organisms may give rise, under certain circumstances, to wholly new organisms.