small, green, and divided into three parts
Saint No. 8: James Watson
Saint No. 7: Francis Crick
Saint No. 6: Rosalind Franklin
It is amazing that one of the most vital elements of evolutionary theory was not understood until almost a century after Darwin published The Origin of Species: the mechanism by which traits are inherited and in which variations can occur. Remember that he did not come up with the basic idea that organisms changed over time; Lamarck's theory had been quite popular. Darwin's leap of understanding was that organisms did not change adaptively; but that children differ slightly from their parents, the "invisible hand" of natural selection allows those with beneficial differences to survive and destroys those with harmful ones, and the differences are somehow inherited by the children of the surviving organisms. But one of the legitimate criticisms of this theory when it was proposed, was that no one could explain how these differences either arose or were passed on. Darwin himself proposed a theory called pangenesis, in which particles called "gemmules" were produced by all parts of the body and then passed on to offspring through the reproductive organs; but there was no experimental evidence to support it.
This is a fascinating overview of the way scientists slowly pieced together the puzzle of the mechanism of inheritance, finally narrowing in on the molecule DNA. Any one of the people mentioned on that page - Gregor Mendel, Thomas Hunt Morgan, Alfred Hershey, Martha Chase & many others - could have been on this page. But I am choosing (and I am nothing if not arbitrary) to focus on the three major figures in the discovery of the structure (and self-reproduction) of the DNA molecule.
James Watson and Francis Crick were working together at Cambridge, trying to beat Linus Pauling to the discovery of what the structure of DNA really was. They were using giant 3D models to try to explain the observed physical properties of the molecule; meanwhile, at King's College, London, Rosalind Franklin was taking X-ray crystallography photographs of DNA. She had good evidence not only that one of the forms of DNA was helical, but that the phosphate backbone was on the outside of the molecule and the nucleotide bases were on the inside. However, she did not want to publish without more data. Before she managed to do that, her colleague Maurice Wilkins, with whom she had a professional rivalry and a personal antipathy, showed her results to Watson without her knowledge. "'The instant I saw the picture my mouth fell open,' Watson recalled. The sneak preview 'gave several of the vital helical parameters.'" When that data was combined with Crick's understanding of Chargaff's rules - that DNA contains equal amounts of adenine and thymine, and of guanine and cytosine - and with Watson's models that showed that "an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair" - Crick and Watson realized that the base pairs made the rung of the twisted ladder, that the two strands of DNA are complementary in opposite directions, and that "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
Understanding of the mechanism of heredity and the way DNA works has made it possible to combine Darwin's theories of natural selection and Mendel's theory of genetic inheritance into the modern synthesis, a much more complete way of studying evolutionary biology. It also opened up genetics, especially molecular genetics, as a truly experimental field of biology.
And a side of stigmata for the readherring.
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