Appearance alone fails to define a species.
The definition of a species must consider variation among individuals and recognize that species can be distinct, even if they appear very similar to one another. The Biological Species Concept defines a species as groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups . So, a species is the largest group within a population in which gene flow is possible. And members of the same species must be capable of interbreeding with one another. For example, Paleontologists have no way of knowing if fossils organisms were capable of interbreeding.