In Evolution, Adaptability Beats Fitness

The longest continuous evolution experiment was started in 1988 and is still ongoing. The study, examining the “evolvability” of Escherichia coli (E. coli), has recently surpassed 52,000 generations and has had a sample of the population frozen and saved every 75 days (every 500 generations). The wealth of data obtained is fantastic and these frozen ancestors have been the focus of a recent study that set out to find whether the eventual “evolutionary winners” displayed signs of their genetic superiority hundreds of generations earlier.

To the researcher’s surprise, the bacterial winners in fact showed the absolute opposite: they were far inferior to the strains of bacteria that died out in later generations. To explain this they discovered that while these ancestors were conventionally less evolutionarily fit (they reproduced at a much slower rate), these “evolutionary winners” were much better at adapting to circumstances and at taking advantage of beneficial mutations. Adaptability trumped fitness.

“[The idea of] selection for evolvability has been in the air for a long time, but this is one of the first real systematic and explicit demonstrations of this actually happening,” said evolutionary biologist and population geneticist Michael Desai of Harvard University […]

The first surprise came when the team compared the fitness of four strains — two EWs [eventual winners] and two ELs [eventual losers] — and found that while all four strains had significantly higher fitness than the ancestral strain, the ELs appeared more fit than the EWs. Comparing the four strains directly confirmed the result: The two EW strains were at a significant disadvantage to the ELs. If these strains had not accumulated any more mutations, the researchers estimated the EWs would have gone extinct in just 350 additional generations. […]

The results suggested that the EWs, while initially at a disadvantage, prevailed in the long-term because they were more likely to acquire more beneficial mutations. In other words, the EWs had greater evolvability.

This seems like evolutionary evidence for the premise of Tim Harford’s latest book, Adapt.

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