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Directional Selection AP Biology Spring 2011
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Directional Selection: shifts allele frequencies in a consistent direction (selection in the direction of a particular range of phenotypes) May be in response to environmental pressures or occur as a new mutation appears and is adaptive
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Read in the textbook of a few examples of directional selection
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The peppered moth Populations of peppered moth show classic directional selection where the moth colouration is in direct response to their environment Light moths are camouflaged on lichen-covered tree trunks, and dark moths are camouflaged on soot- covered tree trunks
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The peppered moth H. B. Kettlewell demonstrated experimentally that moths with a certain colour phenotype would be most abundant depending upon the pollution accumulating on the tree trunks When population controls decreased the soot on trees, selection shifted to favour light-coloured moths
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Pocket Mice The larger population of rock pocket mice in the Arizona desert survives well because of its genetically- determined lighter fur colour that blends in with the granite Predator birds cannot spot them easily
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Pocket Mice A smaller population of mice has darker coats, which allow them to blend in with the dark basalt (from lava) and avoid being seen by predators Night-flying predatory birds are the selecting agents for the dark fur
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Resistance to Antibiotics Antibiotics are drugs that have proven very effective in treating bacterial-induced diseases Some antibiotics, like streptomycin, affect the formation of bacterial cell walls
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Resistance to Antibiotics Although bacteria do not reproduce sexually, some strains reproduce rapidly Each generation has the potential to contain a beneficial mutation Overuse of antibiotics has led to the directional selection of resistant strains that are no longer susceptible to the drug
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