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Unformatted text preview: pulation becomes adapted to environment Stabilising selection Natural selection favours "average" organisms best adapted to that environment
Organisms with extreme forms of characteristics/mutations are selected against
Heaviest and lightest babies have highest mortality
o Less likely to survive, reproduce, pass on their alleles
[Graph] Normal distribution curve with thinner bell-shaped curve Directional selection Natural selection favours organisms with one extreme form of a characteristic
Pesticide resistance (warfarin - poison used to kill rats)
o Resistant rats / need a lot of vitamin K / stabilising selection
o New environmental effect: warfarin / kills normal rats
o Resistant rats survived, reproduced, pass on resistance gene
o New population forms by directional selection
Antibiotic resistance (penicillin resistance) Resistant bacteria / unnecessary enzymes / selected against
New environmental factor: penicillin / kills normal bacteria
Resistant bacteria survived, reproduced, passed...
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- Summer '10