Answer:
Atheism and Evolution are two different things that are often miscombined. Atheism is a belief or philosophy that there are no Deities. Evolution is a biological process whereby the structure and genetic code of organisms change over many generations, as particularly unsuccessful organisms die without offspring and particularly successful organisms are likely to have more offspring. The characteristics of the successful organism thus become the norm.
There is no particular reason to link them, other than the fact that atheists are less likely to reject evolution on the basis of their philosophy.