Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [42]
Short of time travel, Gould thought the best way a scientist could answer that question was by examining the fossil record, documenting the emergence and extinction of species. But experiments on E. coli can also address the question, at least on a scale of years. What makes experiments such as Lenski’s particularly powerful is that evolution unfolds many times over, not just once. From an identical ancestor, Lenski produced twelve lines, each of which went through its own natural selection. Lenski and his colleagues may not be able to rewind the tape of E. coli’s evolution, but they can create twelve identical copies of the same tape and watch what happens when they all play at the same time.
It turns out that the tapes are not identical, nor are they entirely different. In Lenski’s experiments all twelve lines grew faster than their ancestors, but some lines grew far faster than others. They all grew larger, but some became round while others remained rod-shaped. When scientists have taken a close look at the genomes of evolved bacteria, they have found many differences in their DNA. One reason evolution can take different paths is that mutations are not simple. A mutation may be beneficial in one microbe but downright harmful in another. That’s because a mutated gene’s effects depend in part on how it cooperates with other genes. In some cases the genes may work together well, but in other cases they may clash.
Despite those differences, natural selection can override many of the quirky details of history. While Lenski’s lines may not be identical, they have tended to evolve in the same direction. They have also converged on a molecular level. Lenski and his colleagues have found several cases in which the same gene has mutated in all their lines. Even genes that have not evolved a new sequence have changed in a similar way. Some genes now make more proteins, and some make fewer. Lenski and his colleagues took a close look at how the expression of genes changed in two lines of E. coli. They found fifty-nine genes, and in all fifty-nine cases, the genes had changed in the same direction in both lines. The evolutionary song remains the same.
THE TANGLED BANK
“It is interesting to contemplate a tangled bank,” Darwin wrote in The Origin of Species, “clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us.”
Darwin did not believe that he could see the production of life’s tangled bank as it happened. Life evolves into new species over vast stretches of time, he argued, changing as slowly as mountains rise and islands sink. He could only look at the results of evolution around him, such as the distribution of related species around the world, to reconstruct the tangled bank’s history. Today most scientists who study the diversity of plants and animals still follow Darwin’s lead. The evidence they’ve amassed indicates that new species generally take thousands of years or more to branch off from other species. For the most part, it’s a waste of time to sit around hoping to watch a new species emerge.
It turns out, however, that some of the same forces that drive the origin of species can be observed in a dish of E. coli. In the early 1990s, Julian Adams, a microbiologist at the University of Michigan, used a single microbe to found an E. coli population. Adams and his colleagues supplied the bacteria with a little glucose. Unlike Lenski, Adams replenished their sugar so that they never faced outright starvation. The bacteria began to evolve, adapting to the new conditions. But to Adams’s surprise, natural selection did not favor a single strategy. When he put the bacteria on petri