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Chapter 18.4. Directional Selection. AP Biology Spring 2011. Directional Selection. Directional Selection : shifts allele frequencies in a consistent direction (selection in the direction of a particular range of phenotypes)

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chapter 18 4

Chapter 18.4

Directional Selection

AP Biology

Spring 2011

directional selection
Directional Selection
  • 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
directional selection examples
Directional Selection Examples
  • Read in the textbook of a few examples of directional selection
response to predation
Response to Predation
  • 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
response to predation1
Response to Predation
  • 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
response to predation2
Response to Predation
  • 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
response to predation3
Response to Predation
  • 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
response to predation4
Response to Predation
  • 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
response to predation5
Response to Predation
  • 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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