Lenski says most populations followed the same pattern but around the 31,500th generation he noticed that one of the twelve populations suddenly acquired a new ability:
Mostly, the patterns Lenski saw were similar in each separate population. All 12 evolved larger cells, for example, as well as faster growth rates on the glucose they were fed, and lower peak population densities.Lenski and his colleagues had saved samples of each population every 500 generations, these allowed him to revive the bacteria and "replay" evolution:
But sometime around the 31,500th generation, something dramatic happened in just one of the populations – the bacteria suddenly acquired the ability to metabolise citrate, a second nutrient in their culture medium that E. coli normally cannot use.
Indeed, the inability to use citrate is one of the traits by which bacteriologists distinguish E. coli from other species. The citrate-using mutants increased in population size and diversity.
"It's the most profound change we have seen during the experiment. This was clearly something quite different for them, and it's outside what was normally considered the bounds of E. coli as a species, which makes it especially interesting," says Lenski.
The replays showed that even when he looked at trillions of cells, only the original population re-evolved Cit+ – and only when he started the replay from generation 20,000 or greater. Something, he concluded, must have happened around generation 20,000 that laid the groundwork for Cit+ to later evolve.More details about the experiment at NewScientist.
Lenski and his colleagues are now working to identify just what that earlier change was, and how it made the Cit+ mutation possible more than 10,000 generations later.