Contrast stabilizing selection, directional selection, and diversifying selection.

KEY POINTS[ EDIT ]

o Stabilizing selection results in a decrease of
a population's genetic variance when natural selection favors an
average phenotype and selects against extreme variations.
o In directional selection, a population's genetic variance shifts toward a
new phenotype when exposed to environmental changes.
o Diversifying or disruptive selection increases genetic variance when
natural selection selects for two or more extreme phenotypes that each have specific
advantages.
o In diversifying or disruptive selection, average or intermediate phenotypes
are often less fit than either extreme phenotype and are unlikely to feature
prominently in a population.

TERMS[ EDIT ]
directional selection
a mode of natural selection in which a single phenotype is favored, causing the allele
frequency to continuously shift in one direction
stabilizing selection
a type of natural selection in which genetic diversity decreases as the population
stabilizes on a particular trait value
disruptive selection
(or diversifying selection) a mode of natural selection in which extreme values for a
trait are favored over intermediate values
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FULL TEXT[ EDIT ]

Stabilizing Selection
If natural selection favors an average phenotype by selecting against extreme

variation, the population will undergo stabilizing selection. For example, in a

population of mice that live in the woods, natural selection will tend to favor
individuals that best blend in with the forest floor and are less likely to be

spotted by predators. Assuming the ground is a fairly consistent shade of

brown, those mice whose fur is most-closely matched to that color will most

probably survive and reproduce, passing on their genesfor their brown coat.

Mice that carry alleles that make them slightly lighter or slightly darker will

stand out against the ground and will more probably die from predation. As a

result of this stabilizing selection, the population's genetic variance will

decrease.

Stabilizing selection

Stabilizing selection occurs when the population stabilizes on a particular trait value and genetic
diversity decreases.

Directional Selection
When the environment changes, populations will often undergo directional

selection, which selects for phenotypes at one end of the spectrum of existing

variation.

A classic example of this type of selection is the evolution of the peppered

moth in eighteenth- and nineteenth-century England. Prior to the Industrial

Revolution, the moths were predominately light in color, which allowed them

to blend in with the light-colored trees and lichens in their environment. As

soot began spewing from factories, the trees darkened and the light-colored

moths became easier for predatory birds to spot.

Directional selection

Directional selection occurs when a single phenotype is favored, causing the allele frequency to
continuously shift in one direction.

Over time, the frequency of the melanic form of the moth increased because

their darker coloration provided camouflage against the sooty tree; they had
a higher survival rate in habitats affected by air pollution. Similarly, the

hypothetical mouse population may evolve to take on a different coloration if

their forest floor habitat changed. The result of this type of selection is a shift

in the population's genetic variance toward the new, fit phenotype.

The Evolution of the Peppered Moth

Typica and carbonaria morphs resting on the same tree.The light-colored typica (below the bark's
scar) is nearly invisible on this pollution-free tree, camouflaging it from predators.

Diversifying (or Disruptive) Selection

Sometimes natural selection can select for two or more distinct phenotypes

that each have their advantages. In these cases, the intermediate phenotypes

are often less fit than their extreme counterparts. Known as diversifying or

disruptive selection, this is seen in many populations of animals that have

multiple male mating strategies, such as lobsters. Large, dominant alpha

males obtain mates by brute force, while small males can sneak in for furtive

copulations with the females in an alpha male's territory. In this case, both the

alpha males and the "sneaking" males will be selected for, but medium-sized

males, which cannot overtake the alpha males and are too big to sneak

copulations, are selected against.

Diversifying (or disruptive) selection

Diversifying selection occurs when extreme values for a trait are favored over the intermediate
values.This type of selection often drives speciation.
Diversifying selection can also occur when environmental changes favor

individuals on either end of the phenotypic spectrum. Imagine a population of

mice living at the beach where there is light-colored sand interspersed with

patches of tall grass. In this scenario, light-colored mice that blend in with the

sand would be favored, as well as dark-colored mice that can hide in the grass.

Medium-colored mice, on the other hand, would not blend in with either the

grass or the sand and, thus, would more probably be eaten by predators. The

result of this type of selection is increased genetic variance as the population

becomes more diverse.

Comparing Types of Natural Selection

Types of natural selection
Different types of natural selection can impact the distribution of phenotypes within a
population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection,
a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying
selection, two or more extreme phenotypes are selected for, while the average phenotype is selected
against.