Individuals with alleles best adapted to the environment will survive and reproduce, and pass these favourable alleles onto their offspring and therefore the gene pool.

Better suited alleles increase in frequency, and less suited alleles decrease in frequency in the gene pool. Non-favourable alleles will be lost from the population as it is selected against due to its lower chance of survival and reproduction.

Selection pressures may reduce the frequencies of certain alleles.

For example. the phenotype produced by the allele combination aa is more vulnerable to frost.

Founder Effect

The founder effect describes the gene pool of a small population that was established from a subset of a larger population. As the founder population is small, it is likely subject to genetic drift.

The allele frequency of the founder population may not be representative of the allele frequency of the original population. Therefore, there could be less genetic diversity in founder effect populations.

Bottleneck Effect

It is possible for populations to suddenly be reduced to very small numbers due to catastrophic events (e.g. floods, volcanic eruptions, landslide) and heavy predation or disease.

The bottleneck effect is where a species is reduced to a few individuals and then recovers with low genetic diversity. The small population will be subject to genetic drift.