Page 143 - Introduction to Paleobiology and The Fossil Record

130 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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 termi-
nology for cladistics, but some terms are com-
monly used, and should be mentioned. He
called phylogenetically informative characters
apomorphies, or derived characters (and dis- (a) (b) (d)
tinguished them from plesiomorphies, the
characters present in wider groups). Apomor- (c)
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 (f)
and the unusual foot in which two toes point
forward and two back. (e)
The concept of an apomorphy actually cor- (g)
responds to an older distinction between Figure 5.9 Swimming forepaddles of a variety of
homology and analogy in evolution. The fore- reptiles (a–d) and mammals (e–g): (a) Archelon,
limb of vertebrates is a good example of a a Cretaceous marine turtle; (b) Mixosaurus, a
homology: even though the arm of a human Triassic ichthyosaur; (c) Hydrothecrosaurus, a
is very different from the wing of a bird or Cretaceous plesiosaur; (d) Plotosaurus, a
the paddle of a whale, the detailed anatomy Cretaceous mosasaur; (e) Dusisiren, a Miocene
of each is the same, and they clearly arose sea-cow; (f) Allodesmus, a Miocene seal; and (g)
from the same ancestral structure. On the Globicephalus, a modern dolphin. The forelimbs
other hand, the swimming limbs of vertebrates are all homologous with each other, and with
differ from group to group: in detail it can be the wing of a bird and the arm of a human.
shown that the paddles of ichthyosaurs, However, as paddles, these are all analogs: each
whales, plesiosaurs and seals (Fig. 5.9) are not paddle shown here represents a separate
homologs; they are merely analogs. evolution of the forelimb into a swimming
Cladistics might seem relatively straightfor- structure.
ward, but it is not in practice (Box 5.5). There

Box 5.5 Cladistic analysis

There are three steps in drawing a cladogram: character analysis, outgroup comparison and tree
calculation. The character analysis is the process of listing characters that vary among the organisms
of interest, and identifying those that are possible apomorphies. Characters are often coded as pres-
ence/absence or 0/1. So, for example, in a character analysis of the apes, a possible apomorphy might
be “possession of an opposable thumb, used for gripping”. This would be coded as present (or 1)
in humans, and as absent (or 0) in chimpanzees and gorillas.
The outgroup comparison is the phase when characters and their codings are tested for validity.
If the group under study, the ingroup, consists of the apes, then the outgroup might be monkeys.
The outgroup could really be oak trees and worms, but they are so distantly related to the apes that
comparisons would be largely meaningless (oak trees and worms do not even have thumbs). Com-