Answer:
<em>Evolution</em>
Explanation:
Evolution can be described as the changes which occur in a population over a period of time. Over a period of time, organisms tend to evolve those characteristics which make them better adapted to live in an environment. Such changes which occur over a long period of time are termed as evolution. Charles Darwin was the scientist to bring forward the theory of evolution.
The change in the tail length of a rat over a course of many years is an example of evolution.
