In biology, heterochrony is defined as a developmental change in the timing of events, leading to changes in size and shape. Foundations of modern biology There are five unifying principles 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/or offset times, and/or at different rates. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. An ancestor is a Parent or ( recursively) the parent of an ancestor (i The concept was first introduced by Haeckel in 1875^[1]. Ernst Heinrich Philipp August Haeckel ( February 16, 1834 — August 9, 1919)also written von Haeckel, was an eminent German

An example can best illustrate the concept. 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.
If the onset and offset are unaffected, but the rate of growth is increased, the tail will be larger. If the offset is delayed and the rate is unaffected, the tail will be also larger. All possible combinations have been identified in living animals.

Heterochronies are easily identifiable when comparing phylogenetically close species, for example a group of different bird species whose legs differ in their average length.

Several heterochronies have been described in humans, relative to the chimpanzee. Chimpanzee (often shortened to chimp) is the common name for the two extant Species of Apes in the Genus Pan. For instance, brain and head growth in the chimpanzee fetus starts at about the same developmental stage and present a growth rate similar to that of humans, but end soon after birth. A fetus (or foetus or fœtus) is a developing Mammal or other Viviparous Vertebrate, after the Embryonic stage and 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. Humans are known for presenting about 30 different neotenies in comparison to the chimpanzee^[2].

References

1. ^  Horder, Tim (April 2006) Heterochrony. In: ENCYCLOPEDIA OF LIFE SCIENCES. John Wiley & Sons, Ltd: Chichester.
2. ^  See "Comparison of cranial ontogenetic trajectories among great apes and humans" by P. Mitteroecker et al in Human Evolution (2004) Volume 46, pages 679-697 Entrez PubMed 15183670 and also "Ontogenetic study of the skull in modern humans and the common chimpanzees: neotenic hypothesis reconsidered with a tridimensional Procrustes analysis" by X. The Entrez Global Query Cross-Database Search System is a powerful Federated search engine or Web portal that allows users to search many discrete Health sciences Penin, C. Berge and M. Baylac in American Journal of Physical Anthropology (2002) Volume 118, pages 50-62. Entrez PubMed 11953945. The Entrez Global Query Cross-Database Search System is a powerful Federated search engine or Web portal that allows users to search many discrete Health sciences

See also

• Developmental biology
• Embryogenesis
• Evolutionary biology
• Neoteny
• Pedomorphosis
• Progenesis
• Phylogeny

Developmental Biology is the official journal of the Society for Developmental Biology. Embryogenesis is the process by which the Embryo is formed and develops Evolutionary biology is a sub-field of Biology concerned with the origin of Species from a Common descent, and Descent of species Neoteny (niːˈɒtɨniː also called juvenilization, is the retention by adults in a species of traits previously seen only in juveniles ( pedomorphosis/paedomorphosis In Developmental biology, pedomorphosis (also spelled paedomorphosis) or juvenification is a phenotypic and/or genotypic change in Progenesis is a mechanism in Developmental biology that is associated with Paedomorphosis.

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