Wonderful Life

Article excerpt

Debating Evolution in the Age of DNA

Francis Crick winged into the Eagle, a pub popular with researchers at Cambridge University's nearby Cavendish Laboratory, boasting to one and all, "We have found the secret of life." It was early in 1953, and the "we" referred to thirty-six-year-old British biophysicist Crick and twenty-four-year-old American biochemist James D. Watson, then working at the Cavendish on a postdoctoral fellowship. Watson reported feeling "slightly queasy" at Crick's boast, but over the next few decades, many biologists came to see it as fully justified. In one of the great "eureka" experiences of modern science, Watson and Crick had discovered the gene's double-helical structure by brilliantly and rapidly combining the findings of others with insights of their own. This surprisingly simple, highly elegant structure shed new light on the mechanics of evolution by suggesting how genetic reproduction, inheritance, and variation operated at the molecular level.

Although the gene stood at the heart of the modern synthesis, it was a black box prior to 1950. Until then, many scientists envisioned the gene as a complex assemblage of proteins that would take decades to decipher. Yet a growing body of evidence suggested that a much simpler macromolecule, deoxyribonucleic acid (or DNA), carries hereditary information. Watson and Crick followed the latter trail, and it led them to glory. They found that DNA is structured somewhat like a twisted railroad track with sturdy rails along its outer edges and a sequence of connecting ties, each composed of one of two different pairings of four base molecules commonly identified by their initials: A, T, G, and C. If DNA splits lengthwise, then each half replicates the whole by attracting new pairs for its remaining bases from the cell's organic soup, A to T and G to C. The macromolecule carries genetic information in the sequence of its base molecules, which serve as a template for forming ribonucleic acid (or RNA) and, in turn, proteins. Information flows only one way in this mechanism-from the DNA to the proteins that construct the organism, never from the organism back to the DNA. The result nicely matches the neo-Darwinian principle that inborn hereditary information guides individual development without any gene-altering feedback from the environment. In these and other respects, DNA structure provides a serviceable molecular foundation for evolution to proceed in a manner fitting the modern synthesis. Both concepts are starkly materialistic and functionally reductionist. Still, tensions developed between molecular biologists and neo-Darwinian evolutionists.

Watson and Crick did not work out all the implications of DNA structure themselves. In their initial 1953 papers, they simply noted that it "immediately suggests a possible copying mechanism for the genetic material," which they spelled out in some detail, and that "spontaneous mutations may be due to a base occasionally occurring in one of its less likely tautomeric forms." These and other implications inspired a generation of scientists to pursue molecular biology. Traditional ways of studying evolution suddenly seemed terribly old-fashioned. "For those not studying biology at the time in the early 19505, it is hard to imagine the impact the discovery of the structure of DNA had on our perception of how the world works," zoologist Edward O. Wilson later recalled. "If heredity can be reduced to a chain of four molecular letters-granted, billions of such letters to prescribe a whole organism-would it not also be possible to reduce and accelerate the analysis of ecosystems and complex animal behavior?"

Watson and Wilson, who became two of the most influential scientists of the late 2oth century, both joined Harvard's biology department as assistant professors in 1956. Watson led the shock troops for the newer forms of molecular biology. Wilson, who studied ants, upheld the older naturalist tradition associated at Harvard with zoologist Ernst Mayr, an early proponent of the modern synthesis. …