#paraphyly

"Paraphyletic" is a word of convenience. It helps evolutionary biologists describe a group of creatures that have something meaningful in common, even if it means excluding another group.

When you or I say the word "ape," we usually mean every ape except humans, even though we are apes, an uncomfortable truth those racist speciesist Planet of the Apes apes would prefer to ignore.

So "ape" may mean chimps (both kinds), gorillas, orangutans, and those grumpy-faced B-listers, the gibbons, pictured above. Used this way, ape is paraphyletic. It includes all the descendants of some long-ago ape grandparent, but not one group. People.

We often use the word "animal" in a paraphyletic way, too -- again, excluding ourselves, but meaning all the other things we call animals.

So, I thought in addition to blurbs about particular species, I'd also post now and then on some important topics related to paleozoology. This is the first one of those, about the distinction between monophyletic, paraphyletic, and polyphyletic groups.

Cladistics is the study of clades, and a clade is a group consisting of a species and all of its evolutionary descendants. A monophyletic group is one that includes a species and all of its descendants. All clades are therefore monophyletic. Since we have not been able to directly observe evolution throughout its three billion-year history however, many of the groups that have been established are not technically clades, leading to groups that are para- or polyphyletic. Systematists (scientists who study various species and their relationships to one another, essentially cladistics) strive to establish only monophyletic groups, but this is not always possible due to the extensive gaps in the fossil record.

Paraphyletic groups are formed when some, but not all, of a species' descendants are included. These groups are extremely common, and are often established when there is not enough information available to establish a proper clade. An example is reptiles, as they are traditionally defined and as most people think of them. That is, most people will say that mammals and birds are not reptiles. In reality, reptiles only constitute a monophyletic group if mammals and birds are included, as mammals descended from the mammal-like reptiles that flourished until the Cretaceous and birds are descended from dinosaurs.

Polyphyletic groups are those composed of a group of descendants, with the most recent common ancestor excluded. An example of this kind of group is warm-blooded animals. It was once thought that birds and mammals were very closely related because they shared this trait, which we now know to be the result of convergent evolution. However, trying to construct this as a legitimate phyletic group must exclude reptiles, making it polyphyletic.