Directional Selection AP Biology Spring 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 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

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

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

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

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

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