directional selection ap biology spring 2011. directional selection: shifts allele frequencies in a...
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Directional Selection
AP BiologySpring 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 mothPopulations 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 mothH. 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 MiceThe 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 MiceA 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 AntibioticsAntibiotics are drugs that have proven very
effective in treating bacterial-induced diseasesSome antibiotics, like streptomycin, affect the
formation of bacterial cell walls
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Resistance to AntibioticsAlthough bacteria do not reproduce sexually,
some strains reproduce rapidlyEach 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