Stabilizing selection is a type of natural selection that favors the average individuals in a population. This process selects against the extreme phenotypes and instead favors the majority of the population that is well adapted to the environment. Stabilizing selection is often shown on a graph as a modified bell curve that is narrower and taller than the norm.
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.
Stabilizing selection 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 favors the middle of the road, a blend of the genes is often what is seen.
Many human characteristics are a result of stabilizing selection. Human birth weight is not only a polygenic trait, but it is also controlled by environmental factors. Infants with average birth weight are more likely to survive than a baby that is too small or too large. The bell curve peaks at a birth weight that has the minimum death rate.