Overtime organisms change sometimes they change so much of the sentence may be completely different for their ancestors what must happen for this process to occur

In order to survive in the changing environmental conditions.

Explanation:

Changes occur in the body of an organism which make them different from their ancestors because these organisms evolve with the passage of time due to difference in environmental conditions. If these organisms can't evolve, they can't tolerate that environment and climatic condition and also become extinct so this process occur in the organisms in order to survive that different environmental conditions so we can say that this change is necessary in the organisms to survive.

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.

idk lol use google m8