17.2 Natural selection
Definitions
Fitness - the ability of an organism to survive and reproduce
Selection pressure - an environmental factor (biotic or abiotic) that affects the survival chances of an organism
Natural selection - the process by which individuals with a particular set of alleles are more likely to survive and reproduce, resulting in a shift in allele frequencies over time
Stabilising selection
Natural selection that maintains allele frequencies over time
Normal bell curve
Directional selection
Natural selection that causes a gradual shift in allele frequencies
Bell curve but shifted to one side
Disruptive selection
Natural selection that favours both extremes of a population, resulting in two main phenotypes as opposed to one
Bell curve with dent in middle
Intermediate features are not selected for
Usually occurs when selection pressures change
Organisms with a new allele that is advantageous in dealing with the new selection pressures are more likely to survive and reproduce
Examples
Antibiotic resistance in bacteria
Alleles for antibiotic resistance often occur on plasmids
The more antibiotics are used, the greater the selection pressure we exert on bacteria to evolve resistance
Antibiotics causing directional selection, with bacteria with alleles for antibiotic resistance surviving and multiplying
Industrial melanism
Originally, peppered moths were white with dark speckles
A random mutation resulting in a new dominant allele gave rise to the black moth
Main selection pressure for moths is birds
In industrial areas, lichen could not grow on trees so dark tree bark was exposed beneath
Moths with white allele were more easily spotted by birds
White moths were less likely to survive and reproduce
White allele frequency decreased
In other areas, lichen could still grow as normal
Moths with white allele were more camouflaged than moths with black allele
Black moths were spotted less and therefore had a higher survival rate
Black allele frequency increased
Directional selection
No changes in allele frequency
Stabilising selection