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A Brand-New Test Tube World

29 August 1997 8:00 pm
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ARNHEM, THE NETHERLANDS--A test tube teeming with strangely shaped bacteria suggests that diversity rapidly blooms in a world of untapped resources. Experts say the experiment, described here this week at the biennial meeting of the European Society for Evolutionary Biology, is also one of the first laboratory demonstrations that newly evolved life quickly invades distinct habitats.

Ecologists have shown that organisms in the wild can evolve rapidly--just 4 years for male guppies to become 15% heavier--when placed into new environments (Science, 28 March, p. 1934). Microbial geneticists Paul Rainey and Michael Travisano of Oxford University wanted to examine diversification in simpler organisms within the confines of the lab. They placed a common aerobic bacterium, Pseudomonas fluorescens, in a vial filled with nutrient-rich broth--a novel environment, because the bacteria normally live in soil or on plants. "It was something like the world after a major extinction event," says Rainey. "The bacteria had lots of opportunity to diversify."

The bacteria snapped up the chance. Within 5 days, the original ancestor, which the researchers dubbed the "smooth morph" (SM), had diversified into numerous new forms. To test if these forms had adapted to specific environments, the researchers put the original stock and the two most common morphs into a fresh vial. Several days later, each morph filled a different part of the vial. A morph called "Wrinkly Spreader" had colonized the broth's surface, forming a thick mat. Colonies of another morph, nicknamed "Fuzzy Spreader", settled in at the vial's bottom, and the ancestral SM morph thrived in the middle--suggesting to Rainey that competition and natural selection must have honed these preferences and driven the diversification.

"It's a very exciting experiment," says Richard Lenski, an evolutionary geneticist from Michigan State University in East Lansing. "Other people have shown the complexity and rapidity of evolution, but [Rainey's team] is demonstrating it together in one experiment." Adds Peg Riley, a molecular evolutionist at Yale University: "He's reestablishing the ties between ecology and evolution. For those of us who work with microbes, most of the time we never think about the ecological underpinning of what we're growing in our lab."

Genetic studies by Oxford team member Sophie Kahn, not yet published, indicate that only minor mutations underlie the variations in these three morphs, says Rainey. Together, the experiment and genetics show that adaptive radiation, at least in the vial, is due "to the most basic evolutionary processes: simple mutations and natural selection."

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