Pedomorphosis

In developmental biology, pedomorphosis (also spelled paedomorphosis) or juvenification is a phenotypic and/or genotypic change in which the adults of a species retain traits previously seen only in juveniles. Peramorphosis is change in the reverse direction. Pedomorphosis was first proposed by Walter Garstang in 1922. The underlying mechanisms for this include heterochrony.

Pedomorphosis 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 for juvenile behavioral characteristics such as docility.

Natural pedomorphosis occurs in many species of Amphibians, especially Ambystomatid and Protean salamanders. Pedomorphosis in amphibians can be obligate or facultative. It also occurs in termites and several species of cockroaches.

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.

Pedomorphosis is also seen in the plant kingdom, where species can change rapidly in response to environmental shifts. One such example is the single species in the genus Oreostylidium, where a founder population of a more specialized plant from Australia reached New Zealand. The flowers were typically associated with a single pollinator species, but pollinators were unspecialized in its new home. It's hypothesized that in response to this new lack of pollinator pressure, the plants underwent a rapid evolution that allowed flowers to reach sexual maturity earlier as immature plants, thus fueling the pedomorphic change.

There are several kinds of pedomorphism which may appear independently or in combination:
 * Neoteny, in which somatic (or physical) development is slowed, resulting in a sexually mature juvenile or larval form.
 * Progenesis, in which development is halted before full maturity.
 * Postdisplacement, in which the start of development is delayed.

Humans are considered by some scientists to be pedomorphic, due to their flattened face, short jaw, and bulbous forehead compared to other adult primates.