As noted earlier, Darwin was befuddled because blending inheritance would make genetic variation disappear so rapidly that evolution by means of natural selection could not occur. Although Darwin was completely unaware of Mendel’s findings, segregation was the answer to his problem.
An important consequence of segregation is that it demonstrates that the alleles encoding a trait do not alter one another when they are present together in a heterozygous genotype (except in very rare instances). The recessive trait, masked in one generation, can appear in the next, looking exactly as it did in the true-breeding strains. Mendel fully appreciated the significance of this discovery. In one of his letters, he emphasized that “the two parental traits appear, separated and unchanged, and there is nothing to indicate that one of them has either inherited or taken over anything from the other.” In other words, no hint of any sort of blending between the parental genetic material takes place and therefore no homogenization of the trait in the population. Because the individual genes maintain their identity down through the generations (except for rare mutations), genetic variation in a population also tends to be maintained through time. The maintenance of genetic variation is discussed further in Chapter 21.