We may still wonder what might have happened had Darwin actually read Mendel’s paper before November 21, 1866, when he completed his chapter on the ill-conceived pangenesis. Of course, we shall never know, but my guess is that nothing would have changed. Darwin was neither ready to think in terms of variation affecting just one part of an organism and not the others, nor was his mathematical ability sufficient to follow and fully appreciate Mendel’s probabilistic approach. To develop a specific, universal mechanism from a few isolated cases of a 3:1 ratio in the transmission of some properties of a particular plant was not Darwin’s forte. Moreover, Darwin’s stubborn defense of his theory of pangenesis demonstrates that at that point in his life, he may have been afflicted with what modern psychologists refer to as the illusion of confidence: a common state in which people overestimate their abilities. While this principle generally applies to people who are unskilled but unaware of it, it can affect everybody at some level. For instance, studies show that most chess players think that they can play much better than their formal ranking implies. If Darwin indeed had the illusion of confidence, it would be quite ironic, since it was Darwin himself who once observed insightfully that “ignorance more frequently begets confidence than does knowledge.”

The complexities of developing a quantitative approach to the phenomena of variation and rate of survival, and the complete integration of Darwinian selection and Mendelian genetics, took about seventy years to resolve. Initially, in the years following the 1900 rediscovery of Mendel’s pioneering 1865 paper, Mendel’s laws of heredity were even thought to be opposed to Darwinism. Geneticists argued that mutations—the only acceptable form of heritable variation—were abrupt and ready-made rather than gradually selectionist. This opposition subsided by the 1920s, following a number of seminal research projects. First, breeding experiments with the Drosophila fruit fly by biologist Thomas Hunt Morgan and his group demonstrated unambiguously that Mendel’s principles were universal. Second, the geneticist William Ernest Castle was able to show that he could produce inherited change by the action of selection on small variations in traits in a population of rats. Finally, the English geneticist Cyril Dean Darlington discovered the actual mechanics of chromosomal exchange of genetic material. All of these, and similar studies, showed that mutations occurred infrequently and most of the time were disadvantageous. On those rare occasions in which advantageous mutations arose, natural selection was identified as the only mechanism that could enable their propagation through the population. Biologists further came to understand that a large number of separately acting genes could affect a continuous variation in a characteristic. Darwin’s gradualism won the day, with natural selection acting on tiny differences to cause adaptation.

Darwin’s blunder and Jenkin’s criticism had another unexpected consequence: They essentially paved the way for the mathematical population genetics theory developed by Ronald Fisher, J. B. S. Haldane, and Sewall Wright. This was the work that provided the ultimate proof that Mendelian genetics and Darwinian selection were complementary and indispensable to each other. Given that Darwin got the fundamental fact of genetics wrong, it is absolutely amazing how much he got right.

The story of evolution is therefore not a simple narrative leading from myth to knowledge but a collection of diversions, blunders, and winding paths. Eventually, all of these intertwined threads converged into one conclusion: Understanding life requires understanding very intricate chemical processes that involve some very complex molecules. We shall pick up this important thread again in chapters 6 and 7, when we’ll discuss the discovery of the molecular structure of proteins and of DNA.

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