Bacteria bred for 2000 generations settle long-term evolution mystery

A 2000-generation experiment with bacteria suggests existing variation makes a smaller contribution to evolution than we had thought

By Michael Le Page

There is a long-standing debate in biology about the relative importance of existing variation versus new mutations in evolution. Now this has been put to the test in an experiment with bacteria that ran for 2000 generations – and new mutations quickly won out.

After 500 generations, the pre-existing variation no longer mattered and all further evolution was due to new mutations, says Minako Izutsu at Michigan State University.

“Five-hundred generations in our experiment is 75 days. It’s really short,” says Izutsu. “I thought the effects …