Antagonistic pleiotropy hypothesis
Encyclopedia
The antagonistic pleiotropy hypothesis was first proposed by George C. Williams
in 1957 as an explanation for senescence
. Pleiotropy
is the phenomenon where one gene
controls for more than one phenotypic trait in an organism. Antagonistic Pleiotropy is when one gene
controls for more than one trait where at least one of these traits is beneficial to the organism's fitness and at least one is detrimental to the organism's fitness. The theme of G.C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence
would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.
. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic and control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene
, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and mis-allocating their resources.
, an altered path of evolution
, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, Antagonistic Pleiotropy has some evolutionary benefits. In fact, The conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context.
. In one experiment with mice and the morphology of the mandible
, 1/5 of the loci had effects of pleiotropy for the entire mandible
. One other example was in the Russian biologist Dmitry K. Belyaev's study on the domestication of the fox. In Dmitry K. Belyaev's farm-fox experiment, wild foxes were bred for docile behavior alone. After 40 generations, other physiological changes had surfaced including shortened tails, floppy ears, a white star in the forehead, rolled tails, shorter legs. Since the only thing being selected for was behavior, this leads scientists to believe that these secondary characteristics were controlled by the same gene or genes as docile behavior.
levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence
. Senescence
is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However there is more than one theory out there for aging. The competing model to explain senescence is Medawar's "mutation accumulation" hypothesis, saying that "over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability
and/or fertility as an organism ages."
George C. Williams
Professor George Christopher Williams was an American evolutionary biologist.Williams was a professor emeritus of biology at the State University of New York at Stony Brook. He was best known for his vigorous critique of group selection. The work of Williams in this area, along with W. D...
in 1957 as an explanation for senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
. Pleiotropy
Pleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
is the phenomenon where one gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
controls for more than one phenotypic trait in an organism. Antagonistic Pleiotropy is when one gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
controls for more than one trait where at least one of these traits is beneficial to the organism's fitness and at least one is detrimental to the organism's fitness. The theme of G.C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.
As a constraint on perfection
Antagonistic pleiotropy is one of the several reasons evolutionary biologists give for organisms never being able to reach perfection through natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic and control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and mis-allocating their resources.
Benefits and costs
Antagonistic Pleiotropy has several negative consequences. It results in delayed adaptationAdaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
, an altered path of evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, Antagonistic Pleiotropy has some evolutionary benefits. In fact, The conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context.
Ubiquity
Although there are so many negative effects related to genes that are antagonistically pleiotropic, it is still present among most forms of life. Indeed, pleiotropy is one of the most common traits possessed by genes overall. In addition to that, pleiotropy is under strong stabilizing selectionStabilizing selection
-Description:Stabilizing or ambidirectional selection, , is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection...
. In one experiment with mice and the morphology of the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
, 1/5 of the loci had effects of pleiotropy for the entire mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
. One other example was in the Russian biologist Dmitry K. Belyaev's study on the domestication of the fox. In Dmitry K. Belyaev's farm-fox experiment, wild foxes were bred for docile behavior alone. After 40 generations, other physiological changes had surfaced including shortened tails, floppy ears, a white star in the forehead, rolled tails, shorter legs. Since the only thing being selected for was behavior, this leads scientists to believe that these secondary characteristics were controlled by the same gene or genes as docile behavior.
Adaptivity and senescence
An antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age. An example of this is testosteroneTestosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
. Senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However there is more than one theory out there for aging. The competing model to explain senescence is Medawar's "mutation accumulation" hypothesis, saying that "over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability
Viability
Viable or viability is the ability of a thing to maintain itself or recover its potentialities.Viable or viability may also refer to:...
and/or fertility as an organism ages."