MODES OF NATURAL SELECTION

For certain traits, many populations have a range of phenotypes,
characterized by a bell-shaped curve that shows that phenotypic
extremes are less common than the intermediate phenotypes.
Natural selection may affect a range of phenotypes in three ways.
Directional selection occurs when individuals at one phenotypic
extreme are at a disadvantage compared to all other individuals
in the population. In response to this selection,
the deleterious gene(s) decreases in frequency, and all other
genes increase in frequency. Directional selection may occur when
a mutation gives rise to a new gene, or when the environment
changes to select against an existing phenotype.
Industrial melanism, a classic example of directional selection,
occurred in England during the Industrial Revolution. Museum
records and experiments document how environmental
changes affected selection against one phenotype of the peppered
moth, Biston betularia.
In the early 1800s, a gray form made up about 99% of the
peppered moth population. That form still predominates in nonindustrial
northern England and Scotland. In industrial areas of
England, a black form replaced the gray form over a period of about
50 years. In these areas, the gray form made up only about 5% of
the population, and 95% of the population was black. The gray
phenotype, previously advantageous, had become deleterious.

The nature of the selection pressure was understood when
investigators discovered that birds prey more effectively on moths
resting on a contrasting background. Prior to the Industrial Revolution,
gray moths were favored because they blended with the
bark of trees on which they rested. The black moth contrasted
with the lighter, lichen-covered bark and was easily spotted by
birds. Early in the Industrial Revolution, however,
factories used soft coal, and spewed soot and other pollutants into
the air. Soot covered the tree trunks and killed the lichens where
the moths rested. Bird predators now could easily pick out gray
moths against the black background of the tree trunk, while the
black form was effectively camouflaged.
In the 1950s, the British Parliament enacted air pollution
standards that have reduced soot in the atmosphere. As expected,
the gray form of the moth has experienced a small but significant
increase in frequency.
Another form of natural selection involves circumstances
selecting against individuals of an intermediate phenotype.
Disruptive selection produces distinct subpopulations.
Consider, for example, what could happen in a population of
snails with a range of shell colors between white and dark brown
and living in a marine tidepool habitat with two background colors.
The sand, made up of pulverized mollusc shells, is white, and
rock outcroppings are brown. In the face of shorebird predation,
what phenotypes are going to be most common? Although white
snails may not actively select a white background, those present
on the sand are less likely to be preyed on than intermediate phenotypes
on either sand or rocks. Similarly, brown snails on rocks
are less likely to be preyed on than intermediate phenotypes on either
substrate. Thus, disruptive selection could produce two distinct
subpopulations, one white and one brown.
When both phenotypic extremes are deleterious, a third
form of natural selection—stabilizing selection—narrows the
phenotypic range. During long periods of environmental
constancy, new variations that arise, or new combinations
of genes that occur, are unlikely to result in more fit phenotypes
than the genes that have allowed a population to survive for thousands
of years, especially when the new variations are at the extremes
of the phenotypic range.
A good example of stabilizing selection is the horseshoe
crab (Limulus), which lives along the Atlantic coast of the United
States. Comparison of the fossil record with living
forms indicates that this body form has changed little over 200
million years. Apparently, the combination of characteristics present
in this group of animals is adaptive for the horseshoe crab’s
environment.