Rank-order selection is capable of maintaining all genetic polymorphisms.
The fitness of organisms may be due chiefly to a fitness curve imposed on their ranking in the population with respect to heterozygosity. If this is so, then the number of polymorphisms that can be retained at a particular selective equilibrium increases as the square of the population size. All of the genetic variation that we currently observe and infer to exist can probably be maintained by selection in a population of about 10( 5) individuals. Selection acting in this way is so strong that these polymorphisms can be expected to behave very differently from neutral ones.
Wills
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Dept. of Biology, University of California at San Diego, La Jolla, California 92093.