Evolution in Action

In order to study the evolution of single-celled organisms to multicellularity, researchers at the University of Minnesota in St. Paul set out to force an evolution of unicellular brewer's yeast into multicellularity in their lab, reports New Scientist's Bob Holmes. The team, led my William Ratcliff, had a simple approach — grow yeast in a liquid, centrifuge each culture once a day, and inoculate the next batch with the selected yeast that clumped together in each tube, Holmes says. And within 60 days, or about 350 generations, each of the yeast cultures had evolved a clumped "snowflake" form consisting of cells that remained connected to one another after division. "This relatedness provides the conditions necessary for individual cells to cooperate for the good of the whole snowflake," Holmes says. The key step in the evolution to multicellularity, Ratcliff says, is for the organism to select groups of cells over single cells. After several hundred generations of selection, the yeast snowflakes began to show "a rudimentary division of labor," Holmes says, with some cells undergoing cell death to provide weak points for other cells to break off, allowing the snowflake to create offspring while leaving the clump strong enough to survive. "Snowflake lineages exposed to different evolutionary pressures evolved different levels of cell death. Since it is rarely to the advantage of an individual cell to die, this is a clear case of cooperation for the good of the larger organism," Holmes says. The researchers believe this indicates that the snowflakes are evolving as a unit.