Evolution__3rd_Edition

426 PART 4 / Evolution and Diversity
Different characters conflict with
each other
Cladism aims to distinguish reliable
from unreliable characters
shall take the characters and character states as the starting point. They are usually
represented by symbols, such as a and a′ (where a might stand for oviparity and a′ for
viviparity); a and a′ are two states of one character. The states of a second character
might be symbolized by b and b′.
Phylogenetic inference is not simple, mainly because not all the characters for which
we have evidence will point to the same phylogeny. In an easy case, all the characters
will agree. For example, suppose we want to know the phylogeny of three species a
humans, chimpanzees, and a species of worm. Some character states are shared
between humans and chimpanzees; many character states are shared between all three
species; practically no character states are shared between worms and either chimpanzees
(but not humans) on the one hand, or between worms and humans (but not
chimpanzees) on the other. Humans and chimpanzees, we conclude, share a more
recent common ancestor than either does with the worm. If all cases were this easy, we
could simply read phylogenetic relations from the character states. Cladistics would
hardly need to have been invented.
But suppose now that we are studying the phylogeny of humans, a bat, and a bird.
Some character states are similar in birds and bats: both have wings and other skeletal
adaptations for flight. Other character states are similar in humans and bats: both are
viviparous and lactate. Which evidence should we rely on? Figure 15.2 shows another
famously problematic example, from the relations of birds and reptiles. Suppose we are
studying the phylogeny of a crocodile, a bird, and a lizard. The crocodile and lizard
share many similarities: they have scales and walk on four legs, whereas birds have
feathers and walk with two of their appendages and fly with the other two. But a
detailed study of the skull shows that birds and crocociles have important similarities
there, whereas lizards have a different skull anatomy. Which evidence should we rely
on? These two examples illustrate a general problem. In most phylogenetic research,
different characters point to different phylogenies. (I should stress the word research in
the previous sentence. Easy cases a such as humans, chimpanzees, and worms, or
humans, gorillas, and oak trees a have all been solved. We know their phylogeny. The
cases left for research are the ones that are not easy. They are not easy either because we
have practically no knowledge of the character states in the species, and phylogenetic
research has yet to begin, or because of conflict among characters.)
When different characters point to conflicting phylogenies, we can be sure that at
least some of the characters are misleading. A set of species has only one phylogeny: the
phylogeny that represents the ancestral relations that those species possess. A set of
species can no more have multiple phylogenetic relations than a human family can
have more than one family tree. If a human family has two conflicting family trees in its
possession, at least one of them must be wrong. Likewise, if two characters suggest
incompatible phylogenies, something is wrong with at least one of them.
The techniques of cladistics work by distinguishing between reliable and unreliable
characters. The unreliable characters, once identified, can be discarded. The amount of
character conflict in the shortened list of reliable characters should be reduced a and in
a good case the conflict will be reduced to zero, and all the reliable characters will agree
on the same phylogeny. The analysis of characters, to distinguish reliable from unreliable
characters, proceeds in two stages: we first distinguish homologies from homplasies,
and then distinguish derived homologies from ancestral homologies.
..