Heterochrony
Encyclopedia
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. The concept was first introduced by Ernst Haeckel
in 1875.
An example can best illustrate the three dimensions of heterochrony.
Combinations thereof 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.
. For instance, in chimpanzee fetus
es 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.
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, 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
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
can only be described as heterochronic in relation to the same process in another species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, 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 Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
in 1875.
An example can best illustrate the three dimensions of heterochrony.
- 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.
- 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.
- 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.
Combinations thereof have been identified in living animals:
- If the onset and completion times of growth are unaffected, but the rate of growth is increased, the tail will be larger. This is a combination of hypermorphosis (because more growth occurs) and acceleration (because the growth is faster).
Heterochronies are easily identifiable when comparing phylogenetically close species, for example a group of different bird species whose legs differ in their average length.
Humans
Several heterochronies have been described in humans, relative to the chimpanzeeChimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
. For instance, in chimpanzee fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
es 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
Neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...
in comparison to the chimpanzee.
Races
Chinese and Japanese reach full skeletal maturity earlier than Africans and much earlier than Europeans, resulting in a shorter final height and a different pattern of skeletal maturation.See also
- Developmental biologyDevelopmental biologyDevelopmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
- EmbryogenesisEmbryogenesisEmbryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
- Evolutionary biology
- NeotenyNeotenyNeoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...
- Phylogeny