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By Root 1261 0

can be far harder to observe. Life has had millions of years to change, whereas scientists are on this earth for only a few decades. Darwin had resigned himself to studying evolution from a distance, and a century later most evolutionary biologists were following suit. They would compare genes in different species to learn how they diverged or search for new versions of genes that had arisen in response to new challenges. They would look for the effects of natural selection in the past. But in the 1980s a number of scientists decided to watch evolution in the present. They set out to observe E. coli and other bacteria undergo natural selection in their laboratories.

One of those scientists was Richard Lenski. Lenski started his scientific career hiking the Blue Ridge Mountains in search of beetles. He wanted to learn how beetles help hold together the Southern Appalachian food web. Lenski focused his work on a handful of species of Carabus ground beetles. He hoped to determine what controlled their population—cold snaps and heat waves perhaps, or maybe the competition for prey. The question was not just academic. The ground beetles might well be protecting the forests by keeping tree-destroying pests in check. Understanding the ecology of ground beetles might make it possible to predict outbreaks of pests and perhaps even prevent them.

Each spring, Lenski climbed the slopes and dug holes. He put plastic cups in them, covering the cups with funnels. Beetles tumbled down the funnels into the cups, and Lenski returned each day to count them. He marked the beetles and set them free. He tracked how much weight they gained each summer. He compared how many Carabus sylvosus he caught with how many Carabus limbatus. He compared how many beetles lived in dense forests with how many inhabited clear-cuts.

Lenski looked for patterns. In science, patterns become stronger the more times an experiment can be repeated. Doctors put thousands of people on an experimental drug. Physicists fire a laser millions of times to discover the ways of the photon. Ecologists also replicate their experiments when they can, but each datum demands far more labor. For his clear-cutting study, Lenski built a grand total of four enclosures, two in the clear-cut and two in the forest, each holding sixteen traps. With so few trials he could catch sight of only fleeting shadows, hazy signs of the forces governing the beetles.

Lenski came down from the mountains. He decided he would have to find another creature he could study to get some answers to the big questions on his mind. He found E. coli. When Lenski looked at a flask of E. coli, he saw a mountain. It was an ecosystem filled with billions of individual organisms. Like his beetles, E. coli searched for food and reproduced. They were preyed upon by viruses rather than by salamanders. E. coli’s ecosystem might be simpler than the Blue Ridge Mountains, but simplicity can be a virtue in science. A researcher can precisely control every variable in an experiment to see the effect of each one.

Best of all, E. coli is the sort of creature that can, in theory, evolve very fast. Mutations may occur only rarely, but with millions of microbes in a single flask a few mutations will arise in every generation. And because E. coli can reproduce in as little as twenty minutes, a beneficial mutation may let a mutant overtake a colony in a matter of days.

Lenski set up an experiment that was simple yet powerful. He gave his bacteria a limited supply of glucose and thus created a huge evolutionary pressure. Their ancestors had been fed endless meals of sugar, and they had adapted to that diet. The microbes that could convert the food to offspring fastest took over the population. In Lenski’s experiment, genes that sped up breeding were no longer beneficial. His bacteria grew slowly if at all. Any new mutation that allowed the microbes to survive the conditions better, Lenski reasoned, would be strongly favored by natural selection.

As E. coli passed through thousands of generations in his laboratory, evolution’s