Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time. Stabilizing selection commonly uses negative selection (a.k.a. purifying selection) to select against extreme values of the character, leading to a decrease in genetic diversity. 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. Because most traits change little over time, stabilizing selection is thought to be the most common type of selection in most populations. However, a meta-analysis of studies that measured selection in the wild failed to find an overall trend for stabilizing selection. The reason can be that methods for detecting stabilizing selection are complex. They can involve studying the changes that causes natural selection in the mean and variance of the trait, or measuring fitness for a range of different phenotypes under natural conditions and examining the relationship between these fitness measurements and the trait value, but analysis and interpretation of the results is not straightforward.
The Russian evolutionary biologist Ivan Schmalhausen founded the theory of stabilizing selection, publishing a paper in Russian titled "Stabilizing selection and its place among factors of evolution" in 1941 and a monograph "Factors of Evolution: The Theory of Stabilizing Selection" in 1945.
Phenotypes under selection
Human birth weight
A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases 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. The bell curve of the human population peaks at a birth weight that the newly born babies exhibit the minimum death rate.
Another example of a trait, that might be acted on by stabilizing selection, is plant height. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.
Butterfly's wing eyespots
The African butterfly Bicyclus anynana exhibits stabilizing selection with its wing eyespots. It has been suggested that the circular eyespots positioned on the wings are favoured functionally compared to other shapes and sizes.
Genotypes under selection
Human immune system genes
Genotypes too show balancing selection, in loci where single mutations can be harmful to the organism. A large-scale study of the human genome determined that the BTNL2 gene, which binds to the major histocompatibility complex and is implicated in numerous human diseases, was under the strongest balancing selection of any gene in more than half of the populations and geographic regions analyzed.
- Assortative mating
- Balancing selection
- Directional selection
- Frequency-dependent foraging by pollinators
- Fluctuating selection
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