Identification of groupings of different species
- A monophyletic grouping is one in which all species share a common ancestor and all species derived from that common ancestor are included. This is the only form of grouping accepted as valid by cladists. (For example, turtles, lizards, crocodilians and birds are all derived from a shared common ancestor. Thus a monophyletic grouping would place all of these together, rather than placing birds into a separate group.)
- A paraphyletic grouping is one in which all species share a common ancestor, but not all species derived from that common ancestor are included (for example, grouping turtles, lizards and crocodiles as "reptiles" and separating that grouping from the birds).
- A polyphyletic grouping is one in which species that do not share an immediate common ancestor are lumped together, while excluding other members that would link them (for example, a hypothetical group the "lizmams" made by grouping together the lizards and the mammals).