Stabilizing Selection Definition n Stabilizing selection is a

Stabilizing Selection

Definition: n Stabilizing selection is a type of natural selection that favours the average individuals in a population. This process selects against the extreme phenotypes and instead favours the majority of the population that is well adapted to the environment. Variation is constantly occurring in a genetic population. If it is well adapted and its “average” (most frequent) traits are time-tested and proven selection will eliminate such less common variation as they arise.

§ It is the opposite of disruptive selection, instead of favoring individuals with extreme phenotypes, it favours the intermediate variants. Natural selection tends to remove the more severe phenotypes, resulting in the reproductive success of the norm or average phenotypes. This is probably the most common mechanism of action for natural selection. 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. It works mostly on traits that are polygenic. This means that more than one gene controls the phenotype and there is a wide range of possible outcomes. Over time, some of the genes that control the characteristic can be turned off or masked by other genes, depending on where the favorable adaptations are coded. Since stabilizing selection favours the middle of the road, a blend of the genes is often what is seen.

Graph for stabilizing selection: n Stabilizing selection is often shown on a graph as a modified bell curve that is narrower and taller than the norm. The narrow tails on both sides will be sliced-off, while the curving centre will be enhanced. The average or intermediate trait is selected.

Importance of stabilizing selection n n Diversity in a population is decreased due to stabilizing selection. However, this does not mean that all individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are actually a bit statistically higher than those in other types of populations. This and other kinds of microevolution keep the population from becoming too homogeneous. Acts against extremes and favours intermediate phenotypes: This refers often to heterozygous advantage, in which natural selection favours the heterozygote because it possesses benefits that homozygotes lack. For example, people homozygous recessive for sickle cell anaemia, meaning they have the disease, are also immune to malaria, a disease common in Africa, where sickle cell originated. Those heterozygous for the trait show no symptoms of sickle cell, but they are immune to malaria, greatly benefitting the heterozygote.

Example: n 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 Figure: stabilizing selection on human birth weight. The graph is based on infants born in London from 1935 to 1946. From Karn and Penrose (1951).