What causes genetic variation to be preserved or eliminated
how does that affect evolution?

genetic variation is a mutation or a change in the sequence of DNA. It can be preserved if that sequence is needed for a species to survive ( evolution). It can also be preserved if that sequence is strong enough to be passed down from mother to child. The mutation can be eliminated if it causes distress to the animal or if it makes the animals chances of survival smaller. If an extra fin on a dolphin helped it to better escape its predator and better obtain food then this trait may stick around because it is beneficial to the species. If the fin cause the dolphin to swim slower then the dolphin may not survive to reproduce or the trait will not pass on.

Explanation:

cancer   is   essentially   uncontrolled   mitosis   of specialized cells