Page 142 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 129

this is true of biologists as well. Nearly all this is not a helpful character in sorting out
studies of ecology, behavior and evolution are the phylogeny of parrots because all other
tied to a phylogenetic tree of the organisms birds have beaks too. True parrots have blue
involved. Since 1990 phylogenetic trees have and green feathers that have a special irides-
been springing up everywhere, both because cent quality not seen in the feathers of cocka-
of new techniques for discovering trees and a toos. But such special light-refl ecting feathers
realization that nothing in biology means any- are seen also in many other bird groups, and
thing without a tree. so are plesiomorphic for parrots.
Phylogenetically informative characters
identify clades, or monophyletic groups. These
Cladistics: reconstructing life’s hierarchy
are groups that had a single origin and include
For centuries, biologists have struggled with all the descendants of that common ancestor.
the search for the true pattern of relationships A good example of a clade is the Psittaci-
among organisms: how is the tree of life to be formes, the parrots, a group that has long

discovered? Debates about whether birds been identified as real and distinct from all
originated from dinosaurs, whether annelids others by naturalists. Clades are distinguished
and arthropods are close relatives or not, or from two kinds of non-natural groups: (i)
whether the gorilla or chimp is the closest paraphyletic groups, which had a single
relative of humans, all hinge on the need to common ancestor, but do not include all
identify patterns of relationships correctly. descendants, such as the Reptilia, which
If you had the task of sorting out the rela- excludes birds and mammals; and (ii) poly-
tionships among 100 species of parrots, where phyletic groups, which are random assem-
would you begin? You might note the color blages of organisms that arose from more
of their feathers, and classify them into a blue than one ancestor, and so have no place in the
group, a red group and a green group. But search for the tree of life.
then you might notice that body size or beak Willi Hennig (1913–1976), an eminent
shape gives a different classifi cation. If you German entomologist, realized the difference
then looked at the internal anatomy of the between phylogenetically informative and
100 parrots, you might fi nd an entirely differ- uninformative characters, and between mono-
ent classification based on the shape of the phyletic and paraphyletic/polyphyletic groups.

skull, the bones of the wing, or the arrange- He stressed the need to develop a new, more
ment of muscles or arteries. Up to 1960, sys- objective method in systematics, which has
tematists had a hard task in seeking to decide come to be called cladistics. The fundamental
which characters were “good” and which aim of cladistics is to identify clades, and
were “bad”. Good characters are phyloge- so to discover, or reconstruct, the tree of life.
netically informative, that is, indicative of the Patterns of relationships are shown as branch-
true phylogeny, but what about the bad, or ing diagrams, or cladograms (e.g. Fig. 5.8), in
uninformative, characters?
Phylogenetically uninformative characters
fall into two main categories: convergences sea squirt amphioxus
and plesiomorphies. Convergence in evolu- shark salmon frog shark salmon frog
tion is when features, or organisms, evolve to
look the same perhaps because they live the
same way. The marsupial mole of Australia VERTEBRATA VERTEBRATA
looks just like the northern hemisphere mole, ?bone bone
skull
with great paddle-like limbs, poor eyesight ?skull CHORDATA
?tail
and an excellent sense of smell, because they (a) (b) tail
both burrow and eat worms, and yet they are
not closely related. Two species of parrots Figure 5.8 Reconstructing the phylogeny of
might have convergently evolved a red patch vertebrates by cladistic methods. (a) Are the
of feathers on their wings as a signal. Plesio- defining features of vertebrates the possession of

morphies are characters that are shared by the bone, a skull and a tail? (b) The tail is found in
organisms of interest, say parrots, but also by a wider group, termed the Chordata, but the
other groups. So, all parrots have beaks, but skull and bone defi ne the Vertebrata.