Monophyly

Monophyly means common descent from a single ancestor. Biologists have introduced a kind of taxonomy called cladistics. The aim of this is to make biological classification fit what we know about the way organisms evolved.

If there is a group made of a common ancestor, and all its descendants, that group is monophyletic.^[1][2] (Greek: "of one race").

A group which contains organisms, without their common ancestor, is called polyphyletic. Convergent evolution is a usual cause of such groups. If a group contains the common ancestor, but leaves out some of its descendants, it is called paraphyletic. All living things on this planet are thought, on the basis of DNA analysis, to be monophyletic, that is, to have one origin, not many.

Example: all species in the genus Homo have come from the same ancestral form in the family Hominidae, and no other descendants are known. Thus the genus Homo is monophyletic.

If, on the other hand, it were discovered that Homo habilis had developed from a different ancestor from Homo sapiens, and this ancestor were not included in the genus, then the genus would be polyphyletic. Since biologists by and large tend to prefer monophyletic groups, in this case, they would likely either split the genus or broaden it to include the additional forms.

Example of polyphyletic group: Sea slugs.

1. ↑ King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press, 285. ISBN 0-19-530761-5
2. ↑ Futuyma, Douglas J. 2005. Evolution. Sinauer Associates: Sunderland, Massachusetts, 45. ISBN 0-87893-187-2