Intelligent Design

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Evolutionary biologists since Darwin have thought a lot about speciation, but by 2005, when Kohn began her research, there was not a lot of experimental evidence—just theory, she says. Watching speciation in action is difficult because it takes many generations and the right mix of environmental factors to occur. Popular model organisms like mice, zebrafish, and fruit flies are too complex and live too long for one scientist to conduct a revealing experiment within her lifetime.

Kohn, with her mastery of mycology, knew yeast would be the perfect fungus for the job. The yeast used for brewing and baking produces six generations per day in laboratory flasks, which makes it ideal for experimental evolution. Plus, unlike most other model organisms, yeast can be frozen, which means ancestors and descendants can be easily stored and revived at will.

“Yeast is a terrific system,” Kohn says. “We can do evolutionary drag races to compare different lineages.”

Kohn designed an experiment in which she watched the descendants of a single yeast cell in three different environments: normal, high salt, and low glucose. Would yeast offspring maintain the genetics of their parents, or adapt to the stressful environmental conditions? Kohn found that after 500 generations, yeast grown in high salt tolerated that environment much better than the others did; yeast grown in low glucose was similarly successful in a low-glucose environment.

But what was really interesting was that when a salt-adapted yeast was mated with a low-glucose-adapted yeast, their offspring were disasters: They were much less able to reproduce than their parents. There was clearly a penalty in bringing the two adaptations together in one individual—a good explanation for why species remain largely distinct from each other once they have evolved.

“This is at last, hard, empirical evidence of ecological speciation” and of the reproductive isolation that keeps species separate and cohesive, Kohn says about her laboratory model. In 2007, the results were published in Nature, and her protocol was recognized by the Faculty of 1000, a group that highlights the most interesting papers in science.