Heterochrony

[3][2] As Stephen Jay Gould pointed out, Haeckel's term is now used in a sense contrary to his coinage; Haeckel had assumed that embryonic development (ontogeny) of "higher" animals recapitulated their ancestral development (phylogeny), as when mammal embryos have structures on the neck that resemble fish gills at one stage.

This, in his view, necessarily compressed the earlier developmental stages, representing the ancestors, into a shorter time, meaning accelerated development.

The proposal implied (if it were correct) a shared phylogeny of tunicates and vertebrates, and that heterochrony was a principal mechanism of evolutionary change.

It seeks to explain each step in the creation of an adult organism from an undifferentiated zygote in terms of the control of expression of one gene after another.

De Beer to some extent anticipated such late 20th-century science in his 1930 book Embryos and Ancestors,[7] showing that evolution could occur by heterochrony, such as in paedomorphosis, the retention of juvenile features in the adult.

[16] These ultimately result in extended, shifted, or truncated development of a particular process, such as the action of a single toolkit gene,[17] relative to the ancestral condition or to other conspecifics, depending on whether inter- or intraspecific heterochrony is the focus.

Where a typical vertebrate like a mouse has only around 60 vertebrae, snakes have between around 150 to 400, giving them extremely long spinal columns and enabling their sinuous locomotion.

The oscillator clock runs some four times faster in snake than in mouse embryos, initially creating very thin somites.

[21] A recent method, continuous analysis, rests on a simple standardization of ontogenetic time or sequences, on squared change parsimony and phylogenetic independent contrasts.

Neoteny has been implicated as a developmental cause for a number of behavior changes, as a result of increased brain plasticity and extended childhood.

Axolotls reach full sexual maturity while retaining their fins and gills (in other words, still in the juvenile form of their ancestors).

They will remain in aquatic environments in this truncated developmental form, rather than moving onto land as other sexually mature salamander species.

As an organism such as this aged, they would change greatly in their cranial morphology to develop a robust skull with larger, overlapping bones.

[36] Evidence from molecular experiments suggests both fibroblast growth factor 8 (FGF8) and members of the WNT signalling pathway have facilitated paedomorphosis in birds.

[38] This retention of the juvenile ancestral state has driven other changes in the anatomy that result in a light, highly kinetic (moveable) skull composed of many small, non-overlapping bones.

[43] This had an implication that led to hypotheses that selective pressures imposed by the environment, such as predation and loss of resources, were instrumental to the cause of these trends.

[42] Heterochrony is responsible for a wide variety of effects[45] such as the lengthening of the fingers by adding extra phalanges in dolphins to form their flippers,[46] sexual dimorphism,[6] and the polymorphism seen between insect castes.

If the larvaceans constitute a recent re-enactment of an ancient Garstang scenario, they should find closer kinship with some modern sea squirts than with others.

Giraffes acquired their long necks through heterochrony, extending the development period of the seven neck vertebrae 's growth in the embryo to add length to the bones, not by adding more bones. [ 1 ]
Ernst Haeckel supposed that embryonic development recapitulated an animal's phylogeny, and introduced heterochrony as an exception for individual organs. Modern biology agrees instead with Karl Ernst von Baer 's view that development itself varies, such as by changing the timing of different processes, to cause a branching phylogeny. [ 2 ]
Diagram of the six types of shift in heterochrony, a change in the timing or rate of any process in embryonic development. Predisplacement, hypermorphosis, and acceleration extend development (peramorphosis, in red) ; postdisplacement, hypomorphosis, and deceleration all truncate it (paedomorphosis, in blue) . These may be combined, e.g. to shift some aspect of development earlier. [ 10 ]
Despite greatly differing neck lengths, giraffes (right) have no more cervical vertebrae , just 7, than their fellow giraffids , okapi (left). With the number constrained, the development of the vertebrae is extended, allowing them to grow longer.
Axolotls retain gills and fins as adults; these are juvenile features in most amphibians .
Irish elk skeleton with antlers spanning 2.7 metres (8.9 ft) and a mass of 40 kg (88 lb)
A 1901 comparison of a frog tadpole (a vertebrate) and a tunicate larva; in 1928 Walter Garstang proposed that vertebrates derived from such a larva by neoteny.
Neoteny in human development