There are known cases where a cell subpopulation bearing a selective advantage tends to or eventually replaces another cell subpopulation within an organism. This process is supposed to underlie the purging of the most frequent mitochondrial DNA pathogenic variant from leukocytes and other tissues. It is also likely to explain the normal increase of sex chromosome aneuploidy in human leukocytes with age or under pathological conditions and the expansion of a cell subpopulation after the spontaneous in vivo reversion to normal of an inherited pathogenic variant. Furthermore, it is the rationale behind gene therapy for certain diseases. This (total or partial) population replacement process can be described by a logistic function. Here, I also explore the commonalities between somatic selection and the classical case of directional selection in population genetics.