Stabilizing Selection

Stabilizing or ambidirectional selection, (not the same thing as negative or purifying 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. 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 phenotypes by various direct measures, but it can also be detected by a variety of tests of molecular sequence data, such as Ka/Ks ratios, changes in allele frequency distributions, and the McDonald-Kreitman test.

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 miRNAs.