must be able to mate with one another in nature and produce fertile offspring
Species is often defined as the group of individuals that can potentially interbred. Species is also considered as the biggest gene pool in natural conditions. The definition of species is still unclear. In bacteria they do not produce and mate they commonly just divide asexually. In some condition different species can also hybdridize and form hybdrids. Debate and researches is still on going in defining a species.