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In 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.
An example can best illustrate the concept. If a developmental process such as the growth of a tail in the embryo of "species A" of a salamander starts earlier and ends earlier, at the same rate, than that of "species B", although the tail of the animal will be developed earlier in development the final result may be basically the same. 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 unafected, the tail will be also larger. All possible combinations have been identified in living animals.
Heterochronies are easily identifyable 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 neoteny 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.
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