Fact 1: Mutations are essential for evolution to occur.

Fact 2: Viruses can cause a mutation in a gene.

Given the two facts above, answer the question "What can cause evolution to occur?" with the following options: 
- heat
 - viruses
 - alleles
 - pollen
 - Pesticides
 - extinction
 - A computer
 - animals


 

  viruses       