Heterochrony

In evolutionary developmental biology, heterochrony is defined as a developmental change in the timing or rate of events, leading to changes in size and shape. There are two main components, namely (i) the onset and offset of a particular process, and (ii) the rate at which the process operates. A developmental process in one species can only be described as heterochronic in relation to the same process in another species, considered the basal or ancestral state, which operates with different onset and offset times, and at different rates. The concept was introduced by Ernst Haeckel in 1875.

There are three dimensions of heterochrony:

Heterochrony can be identified by comparing phylogenetically close species, for example a group of different bird species whose legs differ in their average length. These comparisons are complex because there are no universal ontogenetic time markers. The method of event pairing attempts to overcome this by comparing the relative timing of two events at a time. This method detects event heterochronies, as opposed to allometric changes. It is cumbersome to use because the number of event pair characters increases with the square of the number of events compared. Event pairing can however be automated, for instance with the PARSIMOV script. A more recent method, continuous analysis, rests on a simple standardization of ontogenetic time or sequences, on squared change parsimony and phylogenetic independent contrasts.

Paedomorphosis can be observed following two general methods: neoteny, which is the retention of juvenile traits into the adult form as a result of retardation of somatic development; and progenesis, which is the acceleration of developmental processes such that the juvenile form becomes a sexually mature adult.

Neoteny retards the development of the organism into an adult, and has been described as “eternal childhood”. In this form of heterochrony, the developmental stage of childhood is itself extended, and certain developmental processes that normally take place only during childhood (such as accelerated brain growth in humans), is also extended throughout this period. Species that display neoteny do eventually reach an adult morphology, but have an extended childhood compared to their close evolutionary relatives. Neoteny also has been implicated as a developmental cause for a number of behavioral changes, as a result of increased brain plasticity and extended childhood.

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