Evolution__3rd_Edition

480 PART 4 / Evolution and Diversity
Figure 16.4
Different kinds of character and
taxonomic group. Homologies
are characters shared between
species that were present in the
common ancestor. They can be
derived or ancestral. (a) Shared
derived homologies are found
in all the descendants of the
common ancestor, and are
distributed in monophyletic
groups. (b) Shared ancestral
homologies are found in some
but not all the descendants of
the common ancestor, and are
distributed in paraphyletic
groups. (c) Homoplasies are
characters shared between
species that were not present
in the common ancestor, and
fall into polyphyletic groups.
See Table 16.1 for the way the
different characters are used
by the different schools of
classification. The crucial
difference between paraphyletic
and polyphyletic groups, in
terms of the shaded gray zone,
is at the bottom and not at the
top of the tree: the paraphyletic
group contains the ancestor
whereas the polyphyletic group
does not. The pattern at the top,
in which the polyphyletic group
seems to miss out a species
between the included species
whereas the paraphyletic group
seems to contain a set of
contigous species, is an
accident of the way the pictures
are drawn a by revolving
appropriate nodes it would be
possible to make the species
in the polyphyletic group
contiguous or to intrude a gap
in the paraphyletic group.
(a) Derived homology (b) Ancestral homology (c) Homoplasy
Species 1 2 3 4 1 2 3 4 1 2 3 4
Characters a' a' a a
a' a a a
a' a a' a
a'
a' a'
Kinds of
a'
a'
a
a
a
a
a
a a
characters
a
Kinds of
taxonomic
group
a
a' is a shared derived character a is a shared ancestral
in species 1 and 2
character in species 2–4
Monophyletic
Homology Homoplasy
Paraphyletic
Polyphyletic
Cladistic classification has the advantage of objectivity. The phylogenetic hierarchy
exists independently of the methods we use to discover it, and is unique and unambiguous
in form. When different techniques for inferring phylogenetic relations disagree,
there is always the external reference point to appeal to. When we cannot work out the
phylogeny of some group or other, we do at least know that a solution exists to aim at.
With the phenetic system there is no such external solution. There is no single natural
phenetic hierarchy analogous to the phylogenetic hierarchy.
When a pair of species, like 5 and 6 in Figure 16.3b, are classified together cladistically,
that means they share a more recent common ancestor than with any other
species. Cladistic relations are fundamentally ancestral relations. In practice, the inference
of ancestral relations (that is, the phylogeny in Figure 16.3a) can be difficult, and
cladistic classification can be uncertain.
In Chapter 15, we saw that the main evidence for phylogenetic relations comes
from a particular kind of character called shared derived homologies. We can use the
distinction between the different kinds of characters to clarify the different schools
of classification (see Table 16.1). Characters can be divided into homoplasies and
homologies, and homologies into derived homologies and ancestral homologies.
(The distinctions are reillustrated in Figure 16.4.) Only derived homologies indicate
phylogenetic relations, and a cladistic classification is based on derived homologous
characters, not on ancestral homologies or on homoplasies. Numerical phenetic classification
groups species using as many characters as possible, and averaging over them
regardless of their evolutionary meaning. Phenetic classification uses all three kinds of
a
a
a' is a homoplasy
in species 1 and 3
..