Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?

Why are human beings who look so different from each other in terms in size, colour and looks said to belong to the same species?

Why are human beings which look so different from each other in terms of size. Colour and looks are said to be belonging to the same species?

Why do they differ from each other ?

Why are the different parts of birds different from each other ?

In a class individual learners differ from each other in terms of

All human races like Africans, Asians, Europeans, Americans and others look so different from each other still they belong to the same species. Give three points to justify the statement.