I almost wanted to ask this in Cooking SE, but thought it would be better fit here.
So I understand than when you mix yeast with sugar and warm water, you get CO2. When using baking powder, it's a standard redox reaction, but what is it with yeast? Is the yeast still present after whatever reaction happens there? I don't think so, because that would mean that the reaction would go on forever. But what evolutionary sense is there for yeast to evaporate into CO2 when confronted with sugar and warm water?