Question

Newborn human babies with lesser body weight are prone to infections. Heavier babies are difficult to deliver through the narrow pelvic passage of human mothers and may not survive. Babies with normal body weight have a better chance of survival and can be delivered normally. What kind of selection is exhibited in this case?

Stabilizing selection

Directional selection

Disruptive selection

Answer 1

A, stabilizing selection.
Stabilizing selection (not the same thing as negative selection[1][2]) is a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular 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. [3] 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.[4] For example, the Bicyclus anynana exhibits stabilizing selection with its eyespots.[5]

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.[citation needed]