Natural selection

As enviromental conditions change, certain characteristics within a randomly varying population are favoured, and natural selection occurs. This has become known as the 'survival of the fittest'
Fitness is defined as the ability of an organism to pass on its alleles to subsequent generations compared with pother individuals of the same species
Natural selection affects a gene pool by increasing the frequency of alleles that give an advantage, and reducing the frequency of alleles that give a disadvantage
Two types of natural selection; there are two different types of selection; stabalising and directional selection. These reflect the two different ways in which natural selection acts on the phenotypes in a population.
1. Stabalising selection - usually takes place in an unchanging enviroment. Extremes of the phenotype range are selected against, leading to a reduction in variation
2. Dircetional selection - favours on extreme of the phenotype range and results ina shift of the mean either to the right or to the left. This type of selection usually follows some kind of enviromental change

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