|(Image by AZAdam)
(From the Discover Magazine blog)
Natural selection is the process by which organisms well-suited to a particular environment reproduce at a faster rate than their competitors. In this experiment, the scientists engaged in artificial selection by creating an environment which made it very difficult for lone yeast cells to survive, thus encouraging the proliferation of cells with adaptations conducive to colonial life. They did this by suspending the cells in tubes of liquid and then centrifuging them, which forced clumped cells down to the bottom of the tube, while lighter single cells floated. They discarded the supernatant (liquid with single cells), and repeated the experiment with the clumped cells until they had high concentrations of cells that had an affinity for clumping.
By this process, they were able to isolate 10 different strains of yeast cells with adaptations conducive to colonial life. They grew these cells in Petri dishes where they grew as snowflake-shaped colonies that resembled and behaved like multicellular organisms. When a snowflake got too large, part of it broke off and formed new snowflakes. There were also signs of division of labor, with some cells committing suicide (apoptosis) to create weak points for the snowflakes to break when they were ready to reproduce.
While it may seem surprising that yeast could evolve multicellularity so rapidly, there is evidence that yeast evolved from multicellular fungi. Thus, they may have retained genes necessarily for multicellular life and reactivated them under the selective pressure of this experiment.