426 CHAPTER 16birds. Lizards, formerly classified as order Lacertilia, are not a monophyletic group
because “Lacertilia” excludes one branch of the lizard tree, the snakes (see Figure
16.23). A polyphyletic taxon includes species from two or more different ances-
tors, but excludes other descendants that are placed in different taxa. The falcons,
hawks, and eagles have similar adaptations for predation, such as a short, hooked
bill and strong grasping feet with sharp, curved claws. Because of those shared
characters, they were long classified together as the order Falconiformes. However,
it has recently become clear from DNA evidence that these similarities result from
convergent evolution. Falcons are more closely related to parrots and songbirds,
and the hawks and eagles are a distantly related clade, which has been named the
order Accipitriformes. Falconiformes, as previously used to include both falcons
and eagles, was a polyphyletic order [19, 28]. It is now used to refer only to falcons,
a monophyletic group.
Although most evolutionary biologists prefer to give names to taxa that are
monophyletic, informal names are sometimes used to refer to paraphyletic assem-
blages of species. Eukaryotes are a monophyletic group, but “prokaryotes” are
paraphyletic because eukaryotes are nested among them. Despite this phylogenetic
situation, which was only recently discovered, many biologists continue to use the
familiar term “prokaryote” to contrast these organisms with eukaryotes.
No system of classification is without difficulties. The phylogenetic boundaries
of a taxonomic rank, such as family, are often arbitrary. For example, the great
apes include the Asian orangutans (Pongo) and the African gorillas, chimpanzee,
bonobo, and human. A “splitter” might recognize two families, Pongidae for the
orangutans and Hominidae for the others. A “lumper” might demote those to sub-
families, and combine them all into one family, Hominidae. (This scheme is widely
used; see http://tolweb.org/Catarrhini/16293) Another issue is that a taxon com-
posed of extinct species (e.g., Dinosauria) will be paraphyletic if it does not include
living descendants of that group (e.g., birds), so the classification may not admit a
formal name only for the extinct group. Often, the living members of a group (such
as birds), together with their last common ancestor, are called a crown group, and
the larger clade of related extinct lineages (e.g., the various theropod dinosaurs) is
called a stem group. Despite these various complications, a genealogical classifica-
tion is both meaningful and useful, for it usually conveys a lot of information. A
zoologist, reading that a species is in the Coleoptera (beetles), immediately knows
a great deal about the organism. A geneticist, learning that horseradish (Armora-
cia) is in the Brassicaceae along with Arabidopsis, will expect its genome to be rather
familiar.
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