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?

Explanation

A species is a group of organisms that are capable of interbreeding to produce a fertile offspring. Skin colour, looks, and size are all variety of features present in human beings. These features are genetic but also environmentally controlled. Various human races are formed based on these features. All human races have more than enough similarities to be classified as same species.
Therefore, all human beings are a single species as human of different colour, size, and looks are capable of reproduction and can produce a fertile offspring.