Monophyly
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In phylogenetics, a group is monophyletic (Greek: "of one race") if it consists of a common ancestor and all its descendants. A taxonomic group that contains organisms but not their common ancestor is called polyphyletic, and a group that contains some but not all descendants of the most recent common ancestor is called paraphyletic.
Image:Monophyletic.png For example, all organisms in the genus Homo are believed to 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 than Homo sapiens, and this ancestor were not included in the genus, then the genus would be polyphyletic. Since biologists by and large prefer groups to be monophyletic, in this case, they would likely either split the genus or broaden it to include the additional forms. Taxonomists run into trouble with polyploidy in plants, as there is evidence that some polyploid species are of multiple orgins, i.e. the species has arisen more than once. For example Iragopogen miscellus has formed as many as 20 times in eastern Washington.
Some evolutionary taxonomists use the term holophyletic to describe this sort of group and instead use monophyletic in its older sense, where it refers to both holophyletic and paraphyletic groups.
[edit] See also
da:Fylogenetisk systematik et:Monofüleetiline rühm es:Monofilético fr:Monophylétique it:Monofiletico he:מונופילטיות hu:Monofiletikus nl:Monofyletisch pt:Monofilético ru:Монофилия sv:Monofyletisk wa:Monofiletike zh:單系群
