Organisms mutate all the time, and yeast are no different. Those mutations are random—some may be beneficial, others not. Those mutations that give the organism an advantage in its environment pass on to future generations, while disadvantageous ones do not.
Enter the method called adaptive lab evolution: Through this process, a laboratory controls the environment in which the organism lives. The mutations are still random, but since the environment is controlled the surviving mutations are those that thrive in that environment. The lab, meanwhile, can select the mutations it deems most useful.
“You can think of it as strength training for yeast,” says Richard Preiss, founder of Escarpment Laboratories in Guelph, Ontario. “You put the yeast through a series of stresses until it mutates. … Each time DNA replicates, you get a chance of mutation. If the mutation has a favorable advantage, it will grow and outcompete other mutations, and you end up with a different population.”