The most intimate environment in which a gene is selected is the other genes at the same locus. Gene a may be favorably selected in a population in which the normal allele at the a-locus is A, but unfavorably in a population in which it is mainly A'. It might be favorably selected initially, when it is so rare as to be judged only in the combination Aa, but if the homozygote is less fit than the heterozygote, the selection coefficient of a will drop to that of A as it increases in frequency.
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