>>I thought a paraphyletic group was a group of species that share a common ancestor, but with some of the descendents of that common ancestor excluded. Woudn't that be the case for a group consisting of birds and frogs?>>



Birds and frogs do not form a paraphyletic group because they do not share a single nearest common ancestor. All birds share a nearest common ancestor, which is the first archosaurian reptile that evolved feathers. All frogs also share a nearest common ancestor, which is a lissamphibian. The nearest common ancestor of all frogs and all birds are different. So these two groups cannot be lumped into a single monophyletic group. Lumping them would result in a polyphyletic group.



When you start with a group that consists of the descendants of a single nearest common ancestor, then systematists of all schools (Darwinian, cladistic, or phenetic) would agree that such a group is monophyletic. When you remove species from a monophyletic group, you end up with at least 2 different groups. The tetrapods form a monophyletic group because all members share a single common ancestor. If you remove the frogs from the tetrapods, you end up with 2 different groups: "frogs" and "all the other tetrapods but frogs". "All other tetrapods but frogs" (even though this group is now missing some of the descendants of their common ancestor) still share a nearest single common ancestor. Therefore "all other tetrapods but frogs" would be monophyletic according to the Darwinians and pheneticists. This same group is considered paraphyletic by the fastidious cladists.



If you then remove the birds from the group "all other tetrapods but frogs", then you end up with three different groups: birds, frogs, and "all other tetrapods but frogs and birds". The group "all other tetrapods but frogs and birds" would still share a nearest common ancestor (namely the first tetrapod that evolved). Therefore the group of "all other tetrapods but frogs and birds" would be monophyletic according to the Darwinians and pheneticists but it would be paraphyletic according to the fastidious cladists.



Since frogs share a nearest common ancestor, it is also a monophyletic group. The same is true of the birds. Birds also form a monophyletic group. However, if you then lump the birds and the frogs into a single group, you end up with a group with 2 different nearest common ancestors. Such a group would of course be polyphyletic.



Hence you are correct that a paraphyletic group is a group that is missing some of the descendants of a common ancestor. However, a paraphyletic group is also a monophyletic group according to the Darwinians and the pheneticists because this group shares a nearest common ancestor. A polyphyletic group, however, does not share a single nearest common ancestor. Since birds and frogs do not share a single nearest common ancestor, birds plus frogs would be considered polyphyletic, not paraphyletic or monophyletic.



As you can see, paraphyletic groups and polyphyletic groups are very different, and they should not be lumped into the umbrella term "not monophyletic," as some dishonest cladists have done. In fact, a paraphyletic group is merely one flavor of a monophyletic group, just as red giants, neutron stars and white dwarfs are different flavors of what we call stars. Hence Darwinians and pheneticists consider paraphyletic taxa valid. It is the cladists who have a weird dogma of intolerance of paraphyletic taxa. Such quasi religious dogma really has no place in science, and the cladists' intolerance has resulted in taxonomic chaos for decades to come.

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