Directional Selection

AP BiologySpring 2011

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

Read in the textbook of a few examples of directional selection

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

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

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

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

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

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