Evolution__3rd_Edition

482 PART 4 / Evolution and Diversity
Figure 16.5
Phylogeny of the main
vertebrate groups. Reptiles are a
paraphyletic group made up of
turtles, lizards, snakes, and
crocodiles in this figure.
Reptiles are a paraphyletic group
Fish are another paraphyletic group
Fish Amphibia Turtles Mammals Lizards Snakes Crocodiles Birds
reptiles are given equal taxonomic rank as classes. What has happened is that two
groups, mammals and birds, have independently undergone relatively rapid phenetic
evolution and have come to look very different from reptiles. The different reptilian
lineages have changed more slowly and have been left looking more like each other
than like birds or mammals. Crocodiles and lizards, for instance have cold blood,
scales, four legs, and walk with a reptilian gait; birds are hot blooded, have feathers,
two legs and two wings, and they fly. Yet crocodiles share a more recent common
ancestor with birds than with lizards. The characters of crocodiles and lizards (scales,
and so on) are ancestral for the group as a whole; and the paraphyletic group was
formed because ancestral characters were used for definition.
Paraphyletic groups become a danger whenever one or more subgroups have evolved
relatively quickly and left their former relatives behind. If the cladistic philosophy is
accepted, paraphyletic groups have to be ruled out. They are defined phenetically (by
ancestral characters) and their recognition is inevitably subjective. The class Reptilia,
therefore, is disbanded in cladistic classification. One subgroup of the former Reptilia,
called Archosauria, includes the crocodiles and birds (and dinosaurs). Another subgroup,
called Lepidosauria, contains the lizards, snakes, and probably turtles.
The cladistic classification of the tetrapods can seem odd. The Reptilia were
recognized in almost every formal classification before cladism; but cladism rules them
out. There are many other examples of paraphyletic groups in non-cladistic classifications:
fish are one of them. The tetrapods evolved from one particular group of fish, the
lobe-finned fish (Section 18.6.1, p. 540). If we consider the relations of any tetrapod
(such as a cow), any lobe-finned fish (such as a lungfish), and any ray-finned fish (such
as a salmon), the cow and the lungfish share a more recent common ancestor than do
the lungfish and the salmon. The category “fish” (containing the lungfish and salmon,
but excluding the cow) does not exist in a cladistic classification.
Some cladists have been rather fanatical in their insistence on ruling out fish and
reptiles. In a way, what we do in practice in these cases does not matter much. Their
paraphyletic status is well known, and can do little damage; it is not worth getting
worked up about. In other, less well known, cases, it is more important to avoid paraphyletic
groups. If a classification contains an unspecified mixture of monophyletic
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