What condition allows for both alleles to be maintained even if one is deleterious when in homozygous form?

Balancing selection is a condition in evolutionary biology that allows for the maintenance of multiple alleles within a population, even when one of those alleles may have a deleterious effect in homozygous individuals. This phenomenon often arises when heterozygous individuals (those carrying two different alleles) confer a selective advantage.

For instance, in cases such as sickle cell anemia, individuals who are heterozygous (carrying one normal and one sickle cell allele) are more resilient to malaria compared to those who are homozygous for either allele. While the homozygous condition for the sickle cell allele is harmful and can lead to serious health issues, the presence of the allele is preserved due to the survival advantage it offers in certain environments, such as areas where malaria is prevalent.

This selective pressure maintaining the alleles in balance fosters genetic diversity within the population, supporting adaptability to changing environmental conditions. The other options, while related to selection and genetic variation, do not typically maintain alleles in the same manner as balancing selection does.