Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [39]
Within a few hours almost all the bacteria the Lederbergs had put in the dishes were dead from infections. A few mutants survived, however, and began to produce colonies visible to the eye. The Lederbergs photographed each dish and then looked at the pictures side by side. The constellation of mutant colonies was the same in each dish.
The Lederbergs concluded that the bacteria must have acquired mutations in the original dish. When they stamped it, the Lederbergs picked up mutants from the same spots. They transferred the bacteria to the same spot in the dishes laden with viruses. If E. coli had obeyed Lamarck, it would have acquired resistance only after the Lederbergs had exposed it to the viruses. There would be no reason to expect resistant bacteria to emerge in precisely the same spots in different dishes.
The Lederbergs recognized that they were seeing resistant bacteria only after they had been exposed to the viruses, so they took the experiment one step further to prove that the mutants were resistant before they encountered viruses. They pressed a velvet stamp into a dish that contained just a few colonies and then pressed it into a dish full of viruses. They waited for resistant bacteria to produce new colonies in the virus-laden dish. Each new colony corresponded to a colony in the original dish. The Lederbergs took some bacteria from the original colonies and put them in flasks, where they could breed into huge numbers. They then repeated the experiment on the new bacteria, growing a few colonies in a dish and pressing them with the velvet stamp. Now all the colonies were resistant. The Lederbergs seeded a second flask of bacteria from the dish and repeated the experiment yet again.
No matter how many times they repeated the procedure, the bacteria remained resistant to viruses even though none of them had been exposed to viruses over the course of the experiment. In 1952, the Lederbergs published their results, arguing that a few resistant bacteria had acquired mutations before the experiment began. Those bacteria had passed down the resistance gene to their descendants. To cling to Lamarck now became absurd.
These experiments on E. coli helped fuse evolution and genetics into a new synthesis. And as scientists continued to learn more about genes and proteins, the workings of natural selection became more clear. A mutation may change the sequence of a gene and thus the structure of its protein. In some cases, a lethal mutation might disable an essential protein. Others make no difference. And a few actually increase reproductive success. The advantage or disadvantage of a mutation depends on the environment. A mutation that confers resistance to viruses will give E. coli a reproductive advantage if viruses are menacing it. If not, the mutation makes no difference. It may even be a burden.
Over the past fifty years, evolutionary biologists have heaped up a mountain of evidence demonstrating that evolution does indeed take place this way. In most cases, though, they have had to study evolution indirectly, by comparing the genes of different organisms to see how natural selection has driven them apart from a common ancestor. But in a few species scientists have observed evolution as it happens, generation by generation, mutation by mutation. Among the most generous of these species is E. coli.
EVOLUTION UNFOLDING
When Salvador Luria ran his slot machine experiment, he captured a single round of evolution. A population of E. coli faced a challenge—an attack of viruses—and natural selection favored resistant mutants. But in every generation, natural selection can shape a species. New mutations arise, genes mix to form new combinations as they pass from parent to offspring, and the shifting environment creates new challenges. On this grander scale, evolution