Answer:
D. Small changes add up over many generations.
Explanation:
It's not A because not all animals in a population can always reproduce. Some may be infertile.
It's not B because genetic variety is something that occurs naturally. Because of genetic variety, random mutations can occur (these mutations later become adaptations).
It's not C because adaptations can be passed down to later generations. These adaptations can lead to natural selection which eventually leads to evolution.
