Jul 21 2008

Seeing evolution in real time

Evolution opponents tend to try and dismiss the evidence in its favor, as a last resort often resorting to the argument that no one has actually seen evolution occurring and a new species emerging, with all the intermediate stages clearly identified. One reason for this is, of course, that evolutionary change occurs very slowly, not visible in the transition from one generation to another. The emergence of a new species is almost always a retrospective judgment, made long after the fact, of a process that often takes thousands, or tens of thousands, of generations. By that time, most of the intermediate forms have become extinct and left no trace, since fossilization is such a rare event.

This is why researchers are finding that bacteria and other microbes, organisms that can go through multiple generations in a single day, to be valuable targets for study, allowing them to see evolutionary change and speciation within the span of a human lifetime.

In a truly remarkable piece of work, Richard Lenski of Michigan State University, starting from a single E. coli bacterium in 1989, kept breeding them in environments with a limited supply of food to see how they would adapt to their situation.

The experiment ran as follows:

He created 12 identical lines of E. coli and then fed them a meager diet of glucose. The bacteria would run out of sugar by the afternoon, and the following morning Dr. Lenski would transfer a few of the survivors to a freshly supplied flask.

From time to time Dr. Lenski also froze some of the bacteria from each of the 12 lines. It became what he likes to call a “frozen fossil record.” By thawing them out later, Dr. Lenski could directly compare them with younger bacteria.

Within a few hundred generations, Dr. Lenski was seeing changes, and the bacteria have been changing ever since. The microbes have adapted to their environment, reproducing faster and faster over the years. One striking lesson of the experiment is that evolution often follows the same path. “We’ve found a lot of parallel changes,” Dr. Lenski said.

The clever part of this experiment was that by freezing samples every 500 generations or so along the way, Lenski could go back in time if necessary and identify when specific changes occurred. He now has over 40,000 generations of bacteria and has thus been able to track closely the way that random mutations and natural selection, the fundamental basis of evolution, works. What these and other similar experiments do is show evolution occurring in real time.

One result of his experiments is that the bacteria are now twice as big as their common ancestor and reproduce 75 percent faster.

But the more dramatic result that Lenski observed was that after 33,127 generations, suddenly one of the colonies of the E. coli bacteria evolved the ability to absorb citrate, a nutrient found in abundance in the broth in which the bacteria are cultured. One of the signature marks of standard or ‘wild’ E. coli is their inability, unlike many other microbes, to absorb citrate.

Science reporter Carl Zimmer, who has been following these experiments, reports on the analysis they did of what happened.

[Lenski's graduate student Zachary] Blount took on the job of figuring out what happened. He first tried to figure out when it happened. He went back through the ancestral stocks to see if they included any citrate-eaters. For the first 31,000 generations, he could find none. Then, in generation 31,500, they made up 0.5% of the population. Their population rose to 19% in the next 1000 generations, but then they nearly vanished at generation 33,000. But in the next 120 generations or so, the citrate-eaters went berserk, coming to dominate the population.

This rise and fall and rise suggests that the evolution of citrate-eating was not a one-mutation affair. The first mutation (or mutations) allowed the bacteria to eat citrate, but they were outcompeted by some glucose-eating mutants that still had the upper hand. Only after they mutated further did their citrate-eating become a recipe for success.

So we see the clear emergence of a new form of E. coli, able to live on citrate in a way that ‘wild’ E. coli are not found to be able to do. The fact that these bacteria developed the ability to switch their diet from the meager glucose to the abundantly available citrate is a significant evolutionary step, showing how an organism can adapt to its environment in ways that make it better able to survive.

This really is a beautiful experiment, illustrating once again how much of science depends on painstaking, long-term, careful study.

Next: Religious anti-evolutionists attack Lenski’s work.

POST SCRIPT: Comedian Dave Allen on the story of Genesis


  1. 1

    Wow. What a fantastic experiment. Looking forward to reading more.

  2. 2

    That dave allen video is hilarious!

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