What type of natural selection maintains two or more alleles in a population?

The type of natural selection that maintains two or more alleles in a population is balancing selection.

Instead of driving one variant to fixation, balancing selection preserves genetic diversity. It can operate when heterozygotes have an advantage, when rare forms are favored, or when different environments favor different variants. These mechanisms keep multiple alleles in the gene pool at frequencies higher than mutation and genetic drift alone would usually produce.

A classic example is the sickle-cell allele in regions where malaria is common. People with two copies can develop sickle-cell disease, while people with one copy often gain increased resistance to severe malaria. This opposing pattern of selection helps maintain both the sickle-cell and normal hemoglobin alleles.

Balancing selection is easy to confuse with disruptive selection. Disruptive selection favors two extreme phenotypes, but it does not necessarily maintain alleles over generations. Balancing selection specifically refers to the long-term preservation of genetic polymorphism.