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

Organisms of different species lose their ability to interbreed. Humans, though they look so different from each other in terms of looks, colour and size, they are able to interbreed and produce healthy offsprings. Hence all human beings belong to a single species. These differences that we see could be due to mutation, accumulated variation and other environmental factors.