Evolution is a change in allele frequency in a population of organisms over time.

Natural selection

Many organisms are unable to move to new habitats and will only survive if they can adapt to new conditions – there is a struggle for existence. Organisms experience selection pressures due to climate change. Individuals within a population vary because they have different alleles. The more adapted to the environment, the more likely they will survive and reproduce, passing on their genes (survival of the fittest). So a greater proportion of the proportion will have the beneficial allele, increasing the allele frequency.

Gene mutations

The gene pool is the complete range of alleles present in a population. New alleles are generated through mutations of the gene during DNA replication. How often an allele occurs in a population is the allele frequency. The frequency of an allele changes over time.