Over a long period of time, helpful variations in a species replace the unfavorable ones. This change can lead to
fossilization.

evolution.

geographic isolation.

genetic mutations

This change is termed as genetic mutations

Explanation:

It is due to the genetic mutation a permanent change occurs in the DNA sequence of a gene,this helpful variation in a species replaces the unfavorable ones in many cases .

For example: Disease like sickle cell anemia,color-Blindness are few example of genetic Mutations

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.