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HeterochronyIn biology, heterochrony is defined as a developmental change in the timing 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/or offset times, and/or at different rates. The concept was first introduced by Haeckel in 1875 . Additional recommended knowledgeAn 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. 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. 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. 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 . References
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Heterochrony". A list of authors is available in Wikipedia. |