Well, for a starter, when a population has a lot of genetic variation, it has a better chance of surviving. Let's make an example; let's say a group doesn't have much genetic variation, and one conducts a disease, and it's unable to fight it off, meaning it dies. Then, the others can conduct this disease, and since they have the same genetic makeup as the others, they won't be able to fight it off either. I hope this helps! ~Mia