A population is defined as a group of the organism of the same species living in a specified area within a given period of time.
All human beings in one geographical area constitution a population of human beings. Also, all cowpea plant in one geographical area at a given time form the population of cowpea plants. The Population can only be formed by organisms of the same species.
Variation is defined as the differences which exist between individuals of the same species. All members of a population have some characteristic in common, by which they can be recognized as a member of the species.
At the same time, member of the same species have individuals different. For example, all the student in a school belonging to the same human population and therefore have the same human features, but each student differs from the others, hence each student can easily be recognized.
