Genetic drift is a change in allele frequency that occurs by chance, because only some of the organisms in each generation reproduce. Genetic drift in small populations will alter the frequencies further (founder effect)
Genetic and phenotypic variation means organisms can have a better chance of survival than others. This is natural selection.
- increases chance of advnatageous adaptive alleles being passed to the next generation.
- decreases the chance of non-adaptive alleles being passed to the next generation.
An individual's fitness is a measure of it's ability to transmit it's alleles to the next generation as a result of natural selection.
For unchanging conditions, stabilising selection maintains existing adaptations and so maintains existing allele frequencies.
In changing conditions, directional selection alters allele frequencies.