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American Association for the Advancement of Science

In evolution, the tortoise beats the hare

Petri dishes containing colonies of two strains of E. coli bacteria that can be tracked by their different colors. By watching the change over time in the relative abundance of the two types, Woods et al. were able to track new beneficial mutations as they arose.
[Jeffrey Barrick]

A new study of E. coli bacteria shows that, even during evolution, a slow and steady pace can win the race. Robert Woods and colleagues performed an experiment in which a sub-population of slowly mutating bacteria eventually took over an entire colony of more rapidly mutating bacteria -- the same way the tortoise beat the hare in Aesop's fable.

The researchers monitored a population of E. coli bacteria that was made up of four genetically distinct sub-populations, checking for specific mutations as the bacteria grew and divided. Sometimes, the researchers would start new colonies with clones of the bacteria and replay their evolution again and again.

Flasks containing E. coli bacteria that have been evolving as part of a long term experiment. In their Science paper, Woods et al. found that, in the population labelled Ara 1, a particular lineage prevailed because it was more flexible or evolvable over the long term than another lineage that was more fit over the short term.
[Brian Baer]

Each time they evolved, though, the bacterial clones that mutated slowly -- and at first appeared less "fit" than the others -- still out-competed the E. coli bacteria that mutated quickly. In the end, the bacteria that seemed to have the advantage at first were overtaken by the bacteria that collected mutations slowly.

Woods and his colleagues suggest that this is because the rapidly mutating bacteria collected many mutations that helped their fitness in the short-term -- but all of those early mutations prevented them from collecting more important mutations later, for the long-term. The early mutations hindered the bacteria's overall "evolvability," the researchers say.

Instead, the slowly mutating sub-population of bacteria missed out on some of the initial beneficial mutations, but they were able to collect very important mutations later on, allowing them to eventually take over the entire population.