Description

Stabilizing or ambidirectional selection, (not the same thing as negative or purifying selection ), is a type of natural selection

Natural 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....

 in which genetic diversity

Genetic diversity

Genetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....

 decreases as the population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection. Stabilizing selection commonly uses negative selection (a.k.a. purifying selection) to select against extreme values of the character.

Stabilizing Selection is the opposite of disruptive selection, instead of favoring individuals with extreme phenotypes, it favors the intermediate variants. It reduces phenotypic variation and maintains the status quo. Natural selection tends to remove the more severe phenotypes, resulting in the reproductive success of the norm or average phenotypes.

A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious disease more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.

Stabilizing selection operates most of the time in most populations. This type of selection acts to prevent divergence of form and function. In this way, the anatomy of some organisms, such as sharks and ferns, has remained largely unchanged for millions of years.

Stabilizing selection can sometimes be detected by measuring the fitness of the range of different phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

s by various direct measures, but it can also be detected by a variety of tests of molecular sequence data, such as Ka/Ks ratio

Ka/Ks ratio

In genetics, the Ka/Ks ratio , is the ratio of the number of non-synonymous substitutions per non-synonymous site to the number of synonymous substitutions per synonymous site , which can be used as an indicator of selective pressure acting on a protein-coding gene...

s, changes in allele frequency

Allele frequency

Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...

 distributions, and the McDonald-Kreitman test

McDonald-Kreitman test

The McDonald–Kreitman Test looks for ancient selection over long periods, as opposed to the steady accumulation of mutations that confer no selective advantage predicted by the Neutral theory. It was first devised by John McDonald and Martin Kreitman in 1991, based on an investigation of...

.

In human populations, 5% of the genome DNA is under purified selection for common mammalian genes. More loci may rapidly evolve including regulatory sequences and miRNA

Mirna

Mirna may refer to:geographical entities* Mirna , a river in Istria, Croatia* Mirna , a river in Slovenia, tributary of the river Sava* Mirna , a settlement in the municipality of Mirna in Southeastern Sloveniapeople...

s.

See also

• Balancing selection
  Balancing selection
  Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value...
  
  
• Directional selection
  Directional selection
  In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
  
  
• Disruptive selection
  Disruptive selection
  Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups...
  
  
• Negative selection (natural selection)
  Negative selection (natural selection)
  In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilizing selection through the purging of deleterious variations that arise....
  
  
• Assortative mating
  Assortative mating
  Assortative mating , and the related concept Disassortative mating, is the phenomenon where a sexually reproducing organism chooses to mate with individuals that are similar or dissimilar to itself in some specific manner...
  
  
• Hypergamy
  Hypergamy
  Hypergamy is the act or practice of seeking a spouse of higher socioeconomic status, or caste status than oneself....
  
  
• Koinophilia
  Koinophilia
  Koinophilia is a term used by biologist Johan Koeslag, meaning that when sexual creatures seek a mate, they prefer that mate not to have any unusual, peculiar or deviant features....