Evolution__3rd_Edition

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Cladism admits only monophyletic
groups
CHAPTER 16 / Classification and Evolution 481
characters. Later in the chapter we shall discuss a third school of classification, called
evolutionary classification, that uses homologies (both ancestral and derived) but
rejects homoplasies.
16.6.2 Cladists distinguish monophyletic, paraphyletic, and
polyphyletic groups
The groups of cladistic classifications are monophyletic in the sense that they contain all
the descendants of a common ancestor: the group has a common ancestor unique to
itself (Figure 16.4a). Cladism rejects paraphyletic and polyphyletic groups. A paraphyletic
group contains some, but not all, of the descendants from a common ancestor
(Figure 16.4b). The members that are included are the forms that have changed little
from the ancestral state and the excluded species are those that have changed more. A
paraphyletic group therefore contains the rump of conservative descendants from an
ancestral species. Polyphyletic groups are formed when two lineages convergently
evolve similar character states (Figure 16.4c). The key difference between paraphyletic
and polyphyletic groups is that paraphyletic groups contain their common ancestor,
whereas polyphyletic groups do not.
As Figure 16.4 shows, the different kinds of group are defined by the different kinds
of characters. Derived homologies fall into monophyletic groups; ancestral homologies
fall into both monophyletic and paraphyletic groups; and homoplasies fall into polyphyletic
groups. Paraphyletic groups are defined when some descendants of a common
ancestor retain their ancestral characters, while others evolve new derived character
states. The difference between paraphyletic and polyphyletic groups therefore arises
because ancestral characters would have been present in a group’s common ancestor,
but convergent characters would not.
Cladistic classifications only include monophyletic groups because only they have
the unambiguous hierarchical arrangement of the phylogenetic tree. Only monophyletic
groups are formed in the cladistic conversion of a phylogenetic tree into a
classification (Figure 16.3). Monophyletic groups are defined unambiguously by their
branching relations: they contain all the branches below a given ancestor, and nothing
has to be said (or indeed be known) about the phenetic evolution of species within each
branch. But in order to define paraphyletic and polyphyletic groups, we do need to
know about phenetic similarity. We have to decide which species to include and which
to exclude, and this is done by including in paraphyletic or polyphyletic groups only
phenetically similar species. Because of the subjectivity of measures of phenetic similarity,
the tree of life cannot be unambiguously divided into paraphyletic or polyphyletic
groups. (Figure 16.7 below illustrates this point.)
Taxonomists had known about evolutionary convergence and polyphyletic groups
for a long time before cladism, but paraphyletic groups proved a more insidious problem.
Hennig was the first to recognize them clearly, and his work was not widely known
until it was translated into English in 1966.
We can see how paraphyletic groups tend to crop up by means of an earlier example:
reptiles (see Figure 16.1c). The phylogenetic relations of the main tetrapod groups
are probably those of Figure 16.5. In traditional classifications, mammals, birds, and