Although selection operates against deleterious alleles in populations, they are rarely lost from these populations. For example, most deleterious recessive alleles in population have equilibrium q values slightly above 0. Why might that be?

Deleterious alleles appear sporadically in a population

Explanation:

A population with a deleterious allele will have no or few individuals that have the ability to pass along these traits. These alleles appear less in a population because of selective pressure but they are not always absent. The alleles appear less often but are are not always passed on and the others that are genetically fit are able to pass along their genes. The reason the population equilibrium is not zero is because these alleles do appear but they are not necessarily passed along. These individuals may not be able to reproduce or reach the age of reproduction.

Deleterious alleles appear more often, making individuals less fit genetically, i.e. they pass fewer copies of their genes to future generations. Put another way, natural selection purges the deleterious alleles.

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