Evolutionary change presents a number of enigmas.
In particular, scientists have long puzzled over how traits that require several independent genetic changes can emerge so suddenly. Those individual changes are likely to be disadvantageous to the organism, so natural selection ought to weed them out. Researchers at the University of Chicago examined prions in yeast cells. These prions fold incorrectly in the same telltale way as do the mammalian variety, and the mutations are likewise passed down from generation to generation. But yeast prions do not harm their hosts.