 Intrinsic, or endogenous, selection can give rise to clines in characters through a variety of mechanisms.One way it may act is through heterozygote disadvantage, in which intermediate genotypes have a lower relative fitness than either homozygote genotypes.Because of this disadvantage, one allele will tend to become fixed in a given population, such that populations will consist largely of either "AA" (homozygous dominant) or "aa" (homozygous recessive) individuals.