Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [41]
Lenski’s students continue to nurture his dynasty of E. coli from one generation to the next, and other scientists have used similar methods to run experiments of their own. Some have watched E. coli adapt to life at the feverish temperature of 107 degrees Fahrenheit. Others have unleashed viruses on the bacteria and observed them become resistant, only to have the viruses evolve ways to overcome their resistance, starting the cycle all over again. While Lenski’s experiment remains the longest running by far, much shorter experiments have been able to yield striking results. Bernhard Palsson and his colleagues, for example, fed five populations of E. coli glycerol, a carbon compound used in soaps and face creams. Ordinary E. coli does a lousy job of feeding on glycerol, but Palsson drove the evolution of glycerol gourmets. After only forty-four days (660 generations of E. coli), the bacteria could grow twice as fast as the founders of the population.
Whether it battles viruses, adapts to a diet of glycerol, or copes with heat, E. coli unmistakably evolves. Its swift pace of evolution in these experiments may reflect rapid evolution in the wild. After all, each time the microbe finds itself in a new environment, its evolutionary pressures suddenly shift. Genes that allow E. coli to thrive in a gut may mutate into forms better suited to life in the soil.
These experiments have allowed scientists to put natural selection under a microscope, teasing apart the individual mutations that benefit E. coli. Each time the microbe divides, it has a roughly 1-in-100,000 chance of mutating in a way that lets its descendants grow faster. The boost is often small, but it can allow a mutant’s descendants to outbreed their cousins. And those mutants in turn have a small chance of picking up a second mutation that makes them even faster growers. In Palsson’s 660-generation experiment, he and his colleagues confirmed two or three mutations in each population. Lenski estimated that over the course of 40,000 generations his lines have picked up as many as 100 beneficial mutations.
Beneficial mutations can take several forms. Some involve the change of a single base in a gene, something equivalent to changing LIFT to LIFE. These mutations can change the structure of a protein and thus change the way it works. It may slice a molecule more effectively than before, or start responding to a new signal. Other mutations accidentally create an extra copy of a stretch of DNA. In Palsson’s experiment these duplicated segments ranged from 9 bases long to 1.3 million. Accidental duplications can create new copies of old genes. Natural selection may favor them because they produce extra proteins, which E. coli can use to grow and reproduce. But over time one of the copies may acquire new mutations, allowing it to take on a new function. Mutations can also snip out chunks of DNA, and microbes that lose genetic material are sometimes favored by natural selection. It’s possible that proteins that were originally useful become a burden to E. coli.
Experiments such as these show that mutations arise randomly. And the effects of the mutations depend on how the mutations allow an organism to thrive in its own peculiar set of conditions. But does that mean evolution plays out purely by chance? The late paleontologist Stephen Jay Gould dreamed of an experiment to answer the question, which he called replaying life’s tape. “You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time