 (The asterisk indicates that this is an equilibrium condition.) Since \(p\) must be non-negative, this condition can only be satisfied if there is heterozygote superiority or inferiority; it represents an equilibrium state of the population in which both alleles are present. This equilibrium is known as polymorphic, by contrast with the monomorphic equilibria that arise when either of the alleles has gone to fixation. The possibility of polymorphic equilibrium is quite significant. It teaches us that natural selection will not always lead to homogeneity; in some cases, selection preserves the genetic variation found in a population.