Directional Selection
Directional selection shifts allele frequencies in a
consistent direction in response to environmental pressures. Forms of traits at one end of the
range of variation become more common than midrange forms.
The story of the peppered moths in England is the classic example of directional selection:
- At first, the light-gray form of the peppered moth enjoyed a survivorship advantage on
light-gray tree trunks.
- When industrial pollution (created by the industrial revolution) darkened the tree trunks, the
numbers of dark-gray moths increased because they were more camouflaged and thus escaped notice by
bird predators.
- In recent years pollution controls have led to decreased amounts of soot on the trees and the
light colored moths are increasing in numbers.
This animation (Audio - Important) will tell you more about the
peppered moths.
Pesticide resistance is a more recent example:
- When insecticides are first applied, susceptible insects (most of the population) die.
- Those few individuals that have the adaptation that affords survival will live and pass the
heritable character on.
- Eventually most of the population will become resistant.
Antibiotic resistance is a third example:
Antibiotics first came into use in the 1940s. Overuse has led to increase in resistant forms of
bacteria. The most susceptible cells died out and were replaced by resistant forms.
An antibiotic is a metabolic by-product of certain microorganisms that kills or inhibits the growth
of other microorganisms. Examples include penicillin and streptomycin. Overuse of antibiotics has
selected for resistant strains, which now threaten to become more predominant than the susceptible
ones.
This animation (Audio - Important) reviews
directional selection.
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