Stabilizing natural selection
Step-by-step explanation:
Stabilizing selection is a natural selective force which stabilizes the population mean based on favorable well adapted phenotypes and will discard non-extreme phenotypes to develop within a population. This results in over-representation of a specific trait which is the intermediate trait and under-representation or avoidance of extreme traits.
The human birthweight curve is bell-shaped due to this stabilizing selection. Infant's survival rate is based on its birthweight.
A low birthweight baby will have lesser surface to volume ratio will lose more body heat and are more prone to infections.
Similarly babies with large bodyweight will have difficulties passing through the birth canal and pelvis during delivery face difficulties to survive and may require incubator conditions to survive or Caesarian section to deliver, all of which affect the survival rate of the infant.
A baby with moderate or medium weight (peak in the bell curve) will have the highest survival rate rather than the low or high (base extremes of bell curve) birthweight babies.