Evolving E. coli follow ‘tortoise-hare’ pattern

"This finding provides insights into fundamental evolutionary constraints; in fact, the presence of a Tortoise-Hare pattern confirms that the fitness landscape is hilly and rugged," says Joshua Nahum. "If it were a smooth landscape, the hare would win every time." (Credit: oldbilluk/Flickr)

To explain new findings about the pace of evolution, researchers turn to the classic story of the tortoise and the hare.

Their work finds that limiting migrations among populations of bacteria produced better adaptations.

The cost, however, was that the bacteria evolved slower. Taking your time, however, isn’t always a bad thing, says Joshua Nahum, a biocomputational research associate at Michigan State University.

“We name this the Tortoise-Hare pattern, as it is the slow-and-steady population with low migration that ultimately wins the fitness race,” says Nahum, who is part of the university’s BEACON Center for the Study of Evolution in Action.

“Understanding this effect is important, especially for understanding the evolution of disease, reducing the evolution of antibiotic resistance, and predicting how populations respond to climate change.”

E. coli populations

All living organisms rely on evolution by natural selection to better adapt to their environment. This adaptation requires mutations, or changes in DNA, that improve reproductive success, referred to as fitness.

Rather than a racetrack, though, these tortoises and hares are competing on a landscape riddled with hills—elevations that represent populations with the highest level of fitness.

For the study in the Proceedings of the National Academy of Sciences, the team manipulated migration rates of populations of gut bacteria, E. coli. They created a grid of 96 populations and had some amble into neighboring territory, which simulated slow migration. Then, to recreate speedier migration, they had others that raced all over the grid regardless of distance.

Why ‘hares’ don’t win

The team found that a population with rampant migration is likely to all get trapped on the same hill, which, more times than not, is not the tallest peak. Why? Because shortly after their summit, beneficial mutations sweep across other populations. This traps the sprinters at the peak, preventing them from climbing other hills. Meanwhile, populations with limited migration will likely take their time and reach a wider variety of peaks.

The tortoises evolve slower, but they can better adapt to their environment because some of the explored peaks may be higher in fitness, having higher reproductive rates, than the peak that was filled by a less-structured, albeit speedier, population.

Sprinting all together to a single peak does, however, provide brief glory for the shortsighted hares. These organisms will hold a fleeting advantage over the slower evolving tortoises, but the race isn’t over just yet. The slower organisms that didn’t place all of their evolutionary marbles into one basket opt instead to methodically climb many hills, amassing many beneficial adaptions.

“This finding provides insights into fundamental evolutionary constraints; in fact, the presence of a Tortoise-Hare pattern confirms that the fitness landscape is hilly and rugged,” Nahum says. “If it were a smooth landscape, the hare would win every time.”

Researchers from the University of Washington, City University of New York, University of Sydney (Australia), University of Chicago, and University of Wisconsin also contributed to the study.

The National Science Foundation funds Michigan State’s BEACON Center.

Source: Michigan State University