|Hint: Altruistic, fermentative, willing to leaven|
In multicellular organisms, a cell might undergo programmed cell death, or apoptosis, if the cell's genetic information has substantial mutation. If the death of the single cell would seemingly benefit the entire organism, then it makes sense that the cell would commit suicide. As Abraham Lincoln said (in regard to committing small injustices in order to eliminate a larger injustice, slavery), it is sometimes necessary to sacrifice an arm to save the body.
But in unicellular yeast, there are still cases of apoptosis. If there is no whole organism comprised of these cells, for what reason would it be worth it to program cell death rather than try to survive, regardless of the severity of the mutation?
One theory suggests that programming cell death allows for more control over how the cell dies. If the cell were to try and survive despite severe mutations, it risks dying later as a result of the mutations. Programmed cell death gives the cell a bit more control, and it can break down any enzymes it contains before undergoing apoptosis. If it were to die as a result of mutations, the cell may not have the chance to break down the enzymes. In this case, it may burst and the enzymes could harm other yeast in the colony. Although the cells are not of one same organism, they are of the same population and do still have a vested interest in the survival of the population.
Another suggestion is that the cells will burst if they are not at optimal fitness level in order to provide nutrients for those with that are better adapted. If a cell has severe mutations, it may undergo apoptosis in order to burst, selectively releasing its stored nutrients to the population.
Either way, yeast are altruistic. A cell is willing to sacrifice itself to prevent harm or benefit its more well-adapted fellows in a population.