Stabilizing Selection
Definition:

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:

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


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:

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).