heterochroni

Heterochrony is a concept in evolutionary biology that refers to changes in the timing or rate of developmental events between ancestral and descendant organisms. It is a significant mechanism by which evolutionary novelties can arise. The term was coined by evolutionary biologist Stephen Jay Gould.

There are two main types of heterochrony: paedomorphosis and peramorphosis. Paedomorphosis occurs when the rate of somatic (body) development slows down relative to the rate of germ line (reproductive) development, or when reproductive development is accelerated. This results in the retention of juvenile features into adulthood. An example is the axolotl, which retains its larval gills throughout its life.

Peramorphosis, on the other hand, involves an acceleration of somatic development relative to germ line development, or a delay in reproductive maturation. This leads to the exaggeration of ancestral adult features or the development of entirely new traits. For instance, the evolution of the large antlers in some deer species might be considered a form of peramorphosis.

Heterochrony can involve changes in the timing of specific developmental events or alterations in the overall rate of development. It is considered a fundamental process in macroevolution, as it can lead to substantial changes in morphology and the establishment of new species. The study of heterochrony helps us understand how diverse life forms have evolved over time.