Evolution__3rd_Edition

436 PART 4 / Evolution and Diversity
Figure 15.9
(a) The ancestral state of a
character (a) must have evolved
before its derived state (a′).
(b) If the fossil record is
relatively complete, the
ancestral state will be preserved
in earlier fossils; (c) but if it is
incomplete, the derived state
may (ii) or may not (i, iii) be
preserved earlier than the
ancestral state.
Cladistic analysis reduces character
conflict
Time
(a) Pattern of
evolution
a a'
imperfect record, the evidence may be practically worthless. Most real cases somewhere
between, and an intermediate level of confidence is appropriate.
15.6.3 Other methods
a
(b) Fossil evidence from
relatively complete record
a and a'
(c) Fossil evidence from
relatively incomplete record
Outgroup comparison and the fossil record are not the only ways to determine character
polarity. A third classic technique uses embryonic development, and we shall
encounter a fourth (and recently invented) technique in Section 15.13 when we look at
paralog rooting.
15.7 Some character conflict may remain after cladistic
character analysis is complete
a
a
The cladistic techniques are intended to infer the phylogenetic relations of a group of
species from conflicting evidence. The conflict in the raw evidence arises because some
of the characters are homoplasies, some ancestral homologies, and some derived
homologies. The cladistic analysis boils down the initial evidence to a list of derived
homologies and should reduce the conflict relative to an unanalyzed list of characters,
for the theoretical reasons given in Section 15.5. In an ideal case, the conflict should be
reduced to zero, because real derived homologies cannot conflict. However, the actual
level of conflict is likely to be reduced to something more than zero because the techniques
can all make mistakes. Convergence can be deceptively exact, and homoplasies
can be mistaken for homologies. The criteria for determining character polarities may
be inapplicable (if the character lacks a fossil record, or its phylogenetic relations with
nearby outgroups are obscure), and even when they can be used they are still fallible.
Moreover, the existence of more than one criterion may increase the uncertainty. If a
character can be studied by more than one of the criteria, they can be played against
each other: if the criteria agree, that increases the plausibility of the conclusion, but if
they disagree, we have another problem of deciding which evidence to trust.
Suppose, for instance, we began with a list of 100 characters, of which 30 pointed
to one phylogeny (which we can call a), 30 to a second phylogeny b, 20 to a third
a
(i)
a
(ii)
a'
a'
a
(iii)
..