Natural selection
By effect on genetic diversity

Alternatively, selection can be divided according to its effect on genetic diversity. Purifying or negative selection acts to remove genetic variation from the population (and is opposed by de novo mutation, which introduces new variation. In contrast, balancing selection acts to maintain genetic variation in a population, even in the absence of de novo mutation, by negative frequency-dependent selection. One mechanism for this is heterozygote advantage, where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human ABO blood group locus has been explained in this way.
keywords:existence, detectable_content:postscript