paedomorphosis

Paedomorphosis is a form of heterochrony in evolutionary biology in which the adult of a species retains traits that were juvenile or larval in its ancestors. In other words, changes in the timing or rate of development produce adults that resemble juvenile stages of related species. This concept helps explain why closely related lineages can look different even when they share a recent common ancestry.

Two classic routes to paedomorphosis are neoteny and progenesis. Neoteny occurs when the somatic development of an organism slows down while reproductive development continues, resulting in adults that retain juvenile features. Progenesis, by contrast, truncates development early so that sexual maturity is reached before the organism fully develops its typical adult morphology, yielding an adult with juvenile characteristics.

Examples and significance. The axolotl (Ambystoma mexicanum) is the best-known paedomorphic vertebrate, retaining aquatic larval traits such as external gills and an aquatic lifestyle as an adult. Many salamanders exhibit facultative paedomorphosis, with individuals capable of metamorphosing into a terrestrial adult or remaining paedomorphic and aquatic depending on environmental conditions. Paedomorphosis can arise through natural selection in stable environments, or through shifts in developmental timing, and it is a central concept in evo-devo studies that explore how changes in development shape evolutionary diversity. It is contrasted with peramorphosis, another form of heterochrony in which development extends beyond the ancestral adult form.