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: This animation (Audio - Important) will tell you more about the peppered moths.

Pesticide resistance is a more recent example:

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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