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.
- Predisplacement and postdisplacement: If a developmental process, such as the growth of a tail in the embryo of "species A", starts earlier and ends earlier than that of "species B", but the rate of growth is the same for both, the final result may basically be the same, although the tail of species A develops earlier than the one of species B. The earlier exhibits predisplacement, and the later species exhibits postdisplacement.
- Neoteny: if the rate of growth is increased, and the time between the start and end of development is decreased proportionally, the tail will end up the same size. The species with faster growth exhibits acceleration, and the species with slower exhibits neoteny.
- Hypermorphosis: if the end of development is delayed and the rate is unaffected, development progresses further, and the tail will be also larger. The species that develops further exhibits hypermorphosis, and the species that does not develop as far exhibits progenesis.
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.
Progenesis (or paedogenesis) can be observed in the Axolotl (Ambystoma mexicanum). Axolotls reach full sexual maturity while retaining their fins and gills, and never enter the adult form at all, instead developing adult characteristics while still in the juvenile morphological form. They also remain in aquatic environments, rather than moving onto land as other sexually mature salamander species.
Peramorphosis is delayed maturation with extended periods of growth. The extinct Irish elk is an example of peramorphosis. From the fossil record, its antlers spanned up to 12 feet wide, which is about a third larger than the antlers of its close relative, the moose. The Irish elk had larger antlers due to extended development during their period of growth.
Another example of peramorphosis is seen in insular (island) rodents. Their characteristics include gigantism, wider cheek and teeth, reduced litter size, and longer life span. Their relatives that inhabit continental environments are much smaller. Insular rodents have evolved these features to accommodate the abundance of larger food and resources they have on their islands. These factors are part of a complex phenomenon termed Island syndrome. With less predation and competition for resources, selection favored overdevelopment of these species. Reduced litter sizes enable overdevelopment of their bodies into larger ones. In some species of frogs, such as the Puerto Rican tree frog, they skip their entire larval stage. These non-aquatic frogs hatch out of their eggs into froglets with limbs, severely reduced gills (or no gills), and gill slits.
Paedomorphic species are mainly aquatic, while peramorphic species are mainly terrestrial. The mole salamander, a close relative to the Axolotl, displays both paedomorphosis and paramorphosis. The larva can develop in either direction, but not backwards. Population density, food, and the amount of water may have an effect on the expression of heterochrony. A study conducted on the mole salamander in 1987 found it evident that a higher percentage of individuals became paedomorphic when there was a low larval population density in a constant water level as opposed to a high larval population density in drying water. This had an implication that led to hypotheses claiming that selective pressures imposed by the environment, such as predation and loss of resources, were instrumental to the cause of these trends. These ideas were reinforced by other studies, such as peramorphosis in the Puerto Rican Tree frog. Another reason could be generation time, or the lifespan of the species in question. When a species has a relatively short lifespan, natural selection favors evolution of paedomorphosis (e.g. Axolotl: 7–10 years). Conversely, in long lifespans natural selection favors evolution of peramorphosis (e.g. Irish Elk: 20–22 years).
Several heterochronies have been described in humans, relative to the chimpanzee. For instance, in chimpanzee fetuses brain and head growth starts at about the same developmental stage and present a growth rate similar to that of humans, but end soon after birth. Humans, on the contrary, continue their brain and head growth several years after birth. This particular type of heterochrony is named hypermorphosis and involves a delay in the offset of a developmental process, or what is the same, the presence of an early developmental process in later stages of development. In addition, humans are known for presenting about 30 different neotenies in comparison to the chimpanzee.
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