A simple example will help to clarify the key differences between Mendelian and blending heredity, in terms of their effects on natural selection. Even though blending inheritance clearly never used the concept of genes, we can still employ this language while preserving the essence of the process of blending. Imagine that organisms that carry a particular gene A are black, while the bearers of gene a are white. We will start with two individuals, one black and one white, each one having two copies of the respective gene (as in figure 4). If no gene dominates over the other, then in both blending heredity and Mendelian heredity, the offspring from such a couple would be gray, since they would have the gene combination (or genotype) Aa. Now, however, comes the key difference. In the blending theory, the A and the a would physically blend to create a new type of gene that gives its carrier the color gray. We can call this new gene A⁽¹⁾. Such blending would not occur in Mendelian heredity, where each gene would keep its identity. As figure 4 shows, in the grandchildren’s generation, all the offspring would be gray under blending heredity, while they could be black (AA), white (aa), or gray (Aa) under Mendelian heredity. In other words, Mendelian genetics pass down extreme genetic types from one generation to the next, thereby efficiently maintaining genetic variation. In blending heredity, on the other hand, variation is inevitably lost, as all the extreme types vanish rapidly into some intermediate mean. As Jenkin observed correctly, and the following (highly simplified) example will clearly demonstrate, this feature of blending heredity was catastrophic for Darwin’s ideas on natural selection.

Figure 4

Imagine that we start with a population of ten individuals. Nine have the gene combination aa (and are therefore white), and one has the combination Aa (say, by some mutation), which renders it gray. Suppose further that being black is advantageous in terms of survival and reproduction, and that even having a somewhat darker color is better than being entirely white (although the advantage decreases with decreasing darkness). Figure 5 attempts to follow schematically the evolution of such a population under blending heredity. In the first generation, the blending of A with a will produce the new “gene” A⁽¹⁾, which, when mating with aa will yield A⁽¹⁾a, which will blend again to produce the gene A⁽²⁾, corresponding to an even lighter and less advantageous color. You can easily see that after a large number (n) of generations, the most that can happen is that the population will be transformed into one with the combinations A⁽ⁿ⁾A⁽ⁿ⁾, which will be only slightly darker than the original white population. In particular, the color black will become extinct even after the first generation, since its gene will be blended out of existence.