Evolving E. coli

25-year experiment sees real-time natural selection

2012 SCIENCE NEWS TOP 25: 23 

A simple flask of bacteria has given scientists a glimpse of evolution in action.

Brian Baer and Neerja Hajela

Escherichia coli bacteria that were part of a 25-year evolution experiment acquired a new ability at some point during the study. They can now eat a chemical called citrate in the presence of oxygen. E. coli lost that trait more than 13 million years ago, so when bacteria in one flask — designated Ara-3 (shown) — started gorging on the chemical, scientists were fascinated. Because they had saved samples of multiple generations from the flask, the researchers were able to trace the genetic changes that led the microbes to redevelop citrate-eating capabilities.

The feat required several genetic changes that took place in three phases over thousands of generations, Zachary Blount and Richard Lenski of Michigan State University in East Lansing and their colleagues reported this year (SN: 10/20/12, p. 8).

Untangling the bacteria’s genetic contortions may help scientists explain gaps in the fossil record, says Paul Rainey of Massey University in Auckland, New Zealand. Evolutionary geneticists have long claimed that altering a species’ appearance or metabolism requires many incremental changes in DNA. But the fossil record doesn’t contain all the intermediate forms that might be expected in such a scenario, suggesting instead that major new innovations may pop up suddenly.

The story of Ara-3 shows how both can be true. Many genetic changes — some still unknown — had to happen over several years to allow the bacteria to consume citrate, but the ability seemed to appear virtually overnight. With such laboratory evolution experiments, says Rainey, “One can really get at the inner workings of evolution.” 

Debunked science
Some exciting finds from years past fell by the wayside in 2012, as new studies disproved some ideas and poked major holes in others.

No arsenic life  When researchers claimed two years ago to have found a bacterium that could survive on arsenic in place of phosphorus, other scientists had trouble replicating the results. New work put nails in the coffin, showing the bug is great at tolerating arsenic but, like all other life, needs phosphorus (SN: 2/25/12, p. 10).

Not-so-speedy neutrinos  A loose cable served as an embarrassing end to an exciting tale: that neutrinos could travel faster than light, violating the theory of special relativity. Faulty wiring to a GPS unit made the trip appear shorter than it actually was (SN: 4/7/12, p. 9).

Chronic fatigue not from virus  The 2011 retraction of a key finding linking XMRV virus to chronic fatigue syndrome took most of the life out of this proposal. This year a large study funded by the National Institutes of Health concluded that the theory is truly dead.

Supersolidity not solid  Findings of “supersolidity,” in which atoms in solid helium slosh around without friction, appear to have been wrong. Instead, researchers may have just observed the normal stiffening of a material (SN Online: 10/12/12).

Death still hard to cheat  Go ahead, have a pat of butter. A study found that rhesus monkeys live no longer than normal on a very low-calorie diet. Evidence so far suggests that calorie restriction can benefit health, but effects on human longevity are still unknown (SN: 10/6/12, p. 8).

Return to 2012 Science News Top 25

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.