Stabilizing selection

Stabilizing selection, also known as normalizing selection or balancing selection, is a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value. This evolutionary process favors the intermediate variants over extreme forms in a population's trait distribution, leading to a decrease in a population's genetic variance around the optimal value of the selected trait.

Stabilizing selection is critical in maintaining the stability and consistency of certain traits within a population over time. It acts against the extremes of the trait variations, thus reducing the chances of significant evolutionary changes. This form of selection is often contrasted with directional selection, where one extreme of a trait is favored over the other, and disruptive selection, where both extremes are favored over the intermediate.

An example of stabilizing selection can be seen in human birth weights. Babies of average weight have a higher survival rate than those who are very small or very large. The average weight is thus favored, and over generations, the variation in birth weights tends to decrease, centering around an optimal average weight.

Another example is the coloration of certain species in a specific habitat. For instance, peppered moths (Biston betularia) during the industrial revolution in Britain showed a shift in population from light to dark coloration due to pollution darkening the trees where the moths rested. However, once pollution controls were implemented and the trees returned to their lighter color, the dark moths were again selected against, stabilizing the population back to its original, lighter coloration. This is an example of how stabilizing selection can act to maintain a specific phenotype in a population over time, despite environmental changes.

Stabilizing selection plays a significant role in the conservation of species and the maintenance of ecosystems. By ensuring that populations do not deviate too far from the optimal trait values, stabilizing selection helps to maintain the balance within ecosystems, allowing species to adapt to gradual changes in their environment without significant shifts in their genetic makeup.