In developmental biology, paedomorphosis or juvenification is a phylogenetic change in which the adults of a species retain traits previously seen only in juveniles. Peramorphosis is change in the reverse direction.
Paedomorphosis is common in many animal species domesticated by humans, including dogs, chickens, pigs and cattle. It is believed to be a side-effect of the selective pressure of human-directed breeding.
Natural paedomorphosis occurs in many species of Amphibians, especially Ambystomatid and Protean salamanders. Paedomorphosis in amphibians can be obligate or facultative.
An example of this would be some salamanders which retain the gills which, in most amphibians, are lost upon reaching adulthood. It's assumed that at some point in the past, their gills were lost just like all others, but some genetic change caused them to be retained, at a point where it was evolutionarily advantageous or neutral.
There are several kinds of paedomorphosis which may appear independently or in combination:
- Neoteny, in which somatic (or physical) development is slowed.
- Progenesis, in which development is halted before full maturity.
- Postdisplacement, in which the start of development is delayed.
Paedomorphosis is believed by some to be what caused the beginning of the phylum Chordata.
Humans are actually thought to be paedomorphic, due to their flattened face compared to other adult primates.