Under Pressure, Does Evolution Evolve?

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Under Pressure, Does Evolution Evolve?

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In 1996, Susan Rosenberg, then a young professor at the University of Alberta, undertook a risky and laborious experiment. Her team painstakingly screened hundreds of thousands of bacterial colonies grown under different conditions, filling the halls outside her lab with tens of thousands of plates of bacteria. It stank, Rosenberg recalled with a laugh. My colleagues hated me.

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The biologist, now at Baylor College of Medicine in Houston, hoped to resolve a major debate that had rocked biology in different incarnations for more than 100 years. Were organisms capable of altering themselves to meet the needs of their environment, as Jean Baptiste Lamarck had proposed in the early 1800s? Or did mutations occur randomly, creating a mixture of harmful, harmless or beneficial outcomes, which in turn fueled the trial-and-error process of natural selection, as Charles Darwin proposed in On the Origin of Species?

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Although Darwin’s ideas have clearly triumphed in modern biology, hints of a more Lamarckian style of inheritance have continued to surface. Rosenberg’s experiments were inspired by a controversial study, published in the late 1980s, that suggested that bacteria could somehow direct their evolution, choosing which mutations will occur, the authors wrote a modern molecular biologist’s version of Lamarckian theory.

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Rosenberg’s results, published in 1997, disputed those findings, as other’s had before, but with a twist. Rather than targeting specific traits, as Lamarck’s theory would have predicted, the mutations struck random genes, with some good outcomes and some bad. However, the process wasn’t completely random. Rosenberg’s findings suggested that bacteria were capable of increasing their mutation rates, which might in turn produce strains capable of surviving new conditions.

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Cells are able to adapt to stress not by knowing exactly what they need to do, but by throwing the dice as a population and making random changes to the genome, said James Broach, a biologist at Pennsylvania State University’s College of Medicine in Hershey who studies a similar phenomenon in yeast. That will allow stressed progeny to find an escape route.

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Rosenberg expected the biology community to be relieved. Darwin, after all, had prevailed. But some scientists questioned the findings. Indeed, the research triggered debates that played out in the pages of scientific journals for several years. Accurately measuring mutation rates can be tricky, and given that most mutations are harmful to the cell, boosting their frequency seemed like a risky evolutionary move.

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Over the past decade, however, labs around the world have found similar patterns in bacteria, human cancer cells and plants. And Rosenberg and others have pinpointed the molecular mechanisms underlying the stress-induced mutations, which vary from organism to organism.

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... the rest of the story

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