Vertebrata

bone skull

CHORDATA tail

Figure 5.8 Reconstructing the phylogeny of vertebrates by cladistic methods. (a) Are the defining features of vertebrates the possession of bone, a skull and a tail? (b) The tail is found in a wider group, termed the Chordata, but the skull and bone define the Vertebrata.

which the most closely related species are joined most closely. The branches in the clado-gram join at branching points, or nodes, each of which marks the base of a clade.

Hennig invented a rather complex terminology for cladistics, but some terms are commonly used, and should be mentioned. He called phylogenetically informative characters apomorphies, or derived characters (and distinguished them from plesiomorphies, the characters present in wider groups). Apomor-phies shared by two or more species are termed synapomorphies. Apomorphies are features that arose once only in evolution, and therefore diagnose all the descendants of the first organism to possess that new character. Synapomorphies of parrots, the bird Order Psittaciformes include the deep, hooked beak and the unusual foot in which two toes point forward and two back.

The concept of an apomorphy actually corresponds to an older distinction between homology and analogy in evolution. The fore-limb of vertebrates is a good example of a homology: even though the arm of a human is very different from the wing of a bird or the paddle of a whale, the detailed anatomy of each is the same, and they clearly arose from the same ancestral structure. On the other hand, the swimming limbs of vertebrates differ from group to group: in detail it can be shown that the paddles of ichthyosaurs, whales, plesiosaurs and seals (Fig. 5.9) are not homologs; they are merely analogs.

Cladistics might seem relatively straightforward, but it is not in practice (Box 5.5). There

Figure 5.9 Swimming forepaddles of a variety of reptiles (a-d) and mammals (e-g): (a) Archelon, a Cretaceous marine turtle; (b) Mixosaurus, a Triassic ichthyosaur; (c) Hydrothecrosaurus, a Cretaceous plesiosaur; (d) Plotosaurus, a Cretaceous mosasaur; (e) Dusisiren, a Miocene sea-cow; (f) Allodesmus, a Miocene seal; and (g) Globicephalus, a modern dolphin. The forelimbs are all homologous with each other, and with the wing of a bird and the arm of a human. However, as paddles, these are all analogs: each paddle shown here represents a separate evolution of the forelimb into a swimming structure.

Figure 5.9 Swimming forepaddles of a variety of reptiles (a-d) and mammals (e-g): (a) Archelon, a Cretaceous marine turtle; (b) Mixosaurus, a Triassic ichthyosaur; (c) Hydrothecrosaurus, a Cretaceous plesiosaur; (d) Plotosaurus, a Cretaceous mosasaur; (e) Dusisiren, a Miocene sea-cow; (f) Allodesmus, a Miocene seal; and (g) Globicephalus, a modern dolphin. The forelimbs are all homologous with each other, and with the wing of a bird and the arm of a human. However, as paddles, these are all analogs: each paddle shown here represents a separate evolution of the forelimb into a swimming structure.

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