ochronie

Ochronie, often called heterochrony in English, is an evo-devo concept describing evolutionary changes in the timing or rate of developmental events, which can alter an organism’s morphology without changing the basic sequence of developmental steps. The term derives from the Greek chrono, meaning time. In practice, ochrondie encompasses two broad outcomes: paedomorphosis and peramorphosis. Paedomorphosis results when adults resemble the juvenile form of their ancestors, often through progenesis (earlier reproductive maturity) or neoteny (slower development). Peramorphosis occurs when development is extended or accelerated, producing adult traits that exceed the ancestral condition; this can arise via acceleration, hypermorphosis, or displacement (changes in the onset or termination of development).

A classic example is the axolotl, a salamander that retains larval characteristics, including gills, into sexual maturity, illustrating paedomorphosis through neoteny. Heterochronic shifts have also been invoked to explain a wide range of morphological variation across vertebrates and invertebrates, including limb proportions, skull shapes, and organ size, by altering the timing of growth and differentiation rather than the underlying genetic pathways per se.

Methods to study ochrondie include comparative ontogeny, fossil records, and quantitative modeling of developmental timing. The concept remains a central tool in evo-devo for understanding how small shifts in developmental timing can generate significant evolutionary diversity. See also: heterochrony, paedomorphosis, peramorphosis, evo-devo.