What happens to deleterious alleles in a population?

The persistence of deleterious alleles in a population can often be balanced by a process known as mutation-selection balance. This concept suggests that while harmful alleles can have negative effects on an individual's fitness and are subject to selection pressures that favor the survival of individuals without those alleles, new mutations can continuously introduce these alleles into the gene pool.

In a stable population, the rate at which these deleterious alleles are introduced by mutations can offset the rate at which they are removed through natural selection when they confer a considerable fitness disadvantage. Therefore, even if they reduce the fitness of individuals, a balance can be achieved where these alleles remain present at low frequencies within the population.

This mechanism helps explain why not all deleterious alleles are completely eliminated and can coexist with beneficial alleles in a population. Over time, various factors such as genetic drift, heterozygote advantage, or fluctuating environmental conditions can also contribute to the persistence of these alleles despite their harmful effects.