Any mechanism of natural selection that maintains genetic diversity in a population. Such diversity better equips the population to adapt to changes in its environment, by producing individuals of varying fitness in respect of, e.g., seasonal changes in climate or spatial variations in habitat. Balancing selection thus maintains genetic and phenotypic polymorphism in the population and prevents loss of rare alleles by genetic drift. One mechanism of balancing selection is frequency-dependent selection, in which the fitness of a particular phenotype, and thus its corresponding alleles, depends on its frequency in the population. In negative frequency-dependent selection, rare phenotypes are favoured over more common variants of the same characteristic. A classic example is self-incompatibility in plants, which prevents male pollen from fertilizing female flower parts (pistils) on the same or a closely related plant, thereby preventing inbreeding. In essence, compatibility is controlled by alleles at a single S locus, and if these are the same or similar in male and female tissues, fertilization is prevented. Hence pollen carrying a rare S allele is more likely to find a compatible plant and breed successfully than one carrying a common S allele; consequently the frequency of the rare allele in the population is likely to increase over time. This type of balancing selection accounts for the often large number of S alleles that are maintained in a plant species over the long term. Another mechanism of balancing selection is heterozygote advantage, in which individuals carrying different alleles for a particular gene (i.e. heterozygotes) have greater reproductive success than either of the corresponding homozygotes. An example is sickle-cell disease in humans, which is due to a mutant form of haemoglobin causing abnormally shaped red blood cells. Individuals carrying both the mutant sickle-cell allele and the normal allele are more likely to survive in areas where malaria is endemic, compared to individuals who are homozygous for either allele. This results in a balanced polymorphism. See polymorphism. Compare positive selection.