How does natural selection affect gene frequency?

Natural selection decreases the frequency in a population of genes that decrease fitness and increases the frequency of genes that increase fitness. **Note that fitness in ecology refers to an individual's ability to survive and produce viable offspring.

For example, say we have a population of ants that live on the jungle floor. Half of the ants are dark brown and half are a lighter brown (50% dark brown, 50% light brown).

A pollutant enters the soil and the color of the soil changes to a very dark brown. Before, both dark brown and light brown ants survived on the forest floor, but now the light brown ants are more visible. They're consumed by insects and birds and other predators at a higher rate. Thus, their numbers decline.

Before the pollutant, four out of eight ants in this population are dark brown, or 50%. After the soil has been polluted, four out of five ants are dark brown, or 80%.

Genes that are beneficial will be selected for and genes that are harmful will be selected against.

Note: natural selection operates on phenotypes, the expression of the gene, not the genotype. The example above assumes that the light brown ants are all of one genotype and the dark brown ants are all of another genotype. Learn about genotypes and phenotypes here.