Chapter 18.4

1. Chapter 18.4 Directional Selection AP Biology Spring 2011

2. 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

3. Directional Selection Examples • Read in the textbook of a few examples of directional selection

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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