Evolution In The Test Tube

We can go a step further. Instead of breeders picking out favored variants, we can let this happen "naturally" in the laboratory, by exposing a captive population to new environmental challenges. This is easiest to do in microbes like bacteria, which can divide as often as once every twenty minutes, allowing us to observe evolutionary change over thousands of generations in real time. And this is genuine evolutionary change, demonstrating all three requirements of evolution via selection: variation, heritability, and the differential survival and reproduction of variants. Although the environmental challenge is created by humans, these sorts of experiments are more natural than artificial selection because humans don't choose which individuals get to reproduce.

Let's start with simple adaptations. Microbes can adapt to virtually anything that scientists throw at them in the lab: high or low temperature, antibiotics, toxins, starvation, new nutrients, and their natural enemies, viruses. Probably the longest-running study of this type has been carried out by Richard Lenski at Michigan State University. In 1988, Lenski put genetically identical strains of the common gut bacterium E. coli under conditions in which their food, the sugar glucose, was depleted each day and then renewed the next. This experiment was thus a test of the microbe's ability to adapt to a feast-and-famine environment. Over the next eighteen years (40,000 bacterial generations), the bacteria continued to accumulate new mutations adapting them to this new environment. Under the varying-food conditions, they now grow 70 percent faster than the original unse-lected strain. The bacteria continue to evolve, and Lenski and his colleagues have identified at least nine genes whose mutations result in adaptation.

But "laboratory" adaptations can also be more complex, involving the evolution of whole new biochemical systems. Perhaps the ultimate challenge is simply to take away a gene that a microbe needs to survive in a particular environment, and see how it responds. Can it evolve a way around this problem? The answer is usually yes. In a dramatic experiment, Barry Hall and his colleagues at the University of Rochester began a study by deleting a gene from E. coli. This gene produces an enzyme that allows the bacteria to break down the sugar lactose into subunits that can be used as food. The geneless bacteria were then put in an environment containing lactose as the only food source. Initially, of course, they lacked the enzyme and couldn't grow. But after only a short time, the function of the missing gene was taken over by another enzyme that, while previously unable to break down lactose, could now do so weakly because of a new mutation. Eventually, yet another adaptive mutation occurred: one that increased the amount of the new enzyme so that even more lactose could be used. Finally, a third mutation at a different gene allowed the bacteria to take up lactose from the environment more easily. All together, this experiment showed the evolution of a complex biochemical pathway that enabled bacteria to grow on a previously unusable food. Beyond demonstrating evolution, this experiment has two important lessons. First, natural selection can promote the evolution of complex, interconnected biochemical systems in which all the parts are codependent, despite the claims of creationists that this is impossible. Second, as we've seen repeatedly, selection does not create new traits out of thin air: it produces "new" adaptations by modifying preexisting features.

We can even see the origin of new, ecologically diverse bacterial species, all within a single laboratory flask. Paul Rainey and his colleagues at Oxford University placed a strain of the bacteria Pseudomonas fluo-rescens in a small vessel containing nutrient broth, and simply watched it. (It's surprising but true that such a vessel actually contains diverse environments. Oxygen concentration, for example, is highest on the top and lowest on the bottom.) Within ten days—no more than a few hundred generations—the ancestral free-floating "smooth" bacterium had evolved into two additional forms occupying different parts of the beaker. One, called "wrinkly spreader," formed a mat on top of the broth. The other, called "fuzzy spreader," formed a carpet on the bottom. The smooth ancestral type persisted in the liquid environment in the middle. Each of the two new forms was genetically different from the ancestor, having evolved through mutation and natural selection to reproduce best in their respective environments. Here, then, is not only evolution but speciation occurring the lab: the ancestral form produced, and coexisted with, two ecologically different descendants, and in bacteria such forms are considered distinct species. Over a very short time, natural selection on Pseudomonas yielded a small-scale "adaptive radiation," the equivalent of how animals or plants form species when they encounter new environments on an oceanic island.

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