Monophyly

A group in a biological taxonomy is monophyletic if all organisms in that group are known to have developed from a common ancestral form.

For example, all organisms in the genus Homo are known to have come from the same ancestral form in the family Hominidae. Therefore, the genus Homo is monophyletic. Conversely, if it were discovered that Homo habilus developed from a different ancestor than Homo sapiens, the genus Homo would no longer be monophyletic. Since biologists prefer groupings to be monophyletic, this discovery would likely lead biologists to create a new grouping.

In fact, biologists are discussing this issue with regards to the genus Homo at this very moment, though for different reasons. For example, H. heidelbergensis and H. neanderthalensis are thought to be subspecies of H. sapiens. If this is true, Homo is not strictly monophyletic!