Evolution__3rd_Edition

432 PART 4 / Evolution and Diversity
Time
Lizard
Ancestral homology can be
misleading
Phenetic appearance
Derived homologies are reliable
evidence
Crocodile Bird
Figure 15.6
The evolution of birds, crocodiles, and lizards illustrates how if
one lineage undergoes rapid evolution, members of the other two
lineages are left looking relatively similar even though they are
phylogenetically distant. A crocodile looks more like a lizard than
a bird; but a crocodile has a more recent common ancestor with a
bird than with a lizard.
distinction implies that we are comparing the two species with at least one other
species. Then whether the homology is ancestral or derived depends on what that third
species is.
Ancestral homologies are most dangerous for phylogenetic inferences in cases such
as a bird, a crocodile, and a lizard (Figure 15.6). Here one lineage within a group of
species has undergone rapid evolution. Birds have evolved wings and other skeletal and
physiological adaptations for flight. The lineages to the crocodiles and lizards have
evolved slowly in comparison. They have both retained ancestral reptilian characters
such as scales and walking on four legs. The crocodile and lizard have been left looking
relatively similar compared to birds because of the evolutionary spurt of the latter. But
the similarity between crocodiles and lizards is ancestral similarity. It is for characters
that were present in the common ancestor of all three groups. The similarity is not evidence
that crocociles and lizards share a more recent common ancestor with each other
than either does with birds.
The complete analysis of a character has two stages, first distinguishing homoplasies
from homologies and then distinguishing ancestral from derived homologies. In all, a
character can belong to any one of three types (Figure 15.7). The distinction matters
because, of the three kinds of shared character, only derived homologies are evidence
that the two species share a more recent common ancestor with each other than with
any other species under investigation.
Phylogenies, therefore, should not be inferred from simple phenetic similarity.
Phenetic similarity mixes reliable similarity (in derived homologies) with unreliable
similarity (in homoplasies and ancestral homologies). That phenetic similarity is misleading
in the case of convergence is widely appreciated; but ancestral homologies
cause the same problem, and more insidiously. We have seen it in the reptiles (though
they are not the only example): a crocodile looks more like a lizard than a bird, but is
phylogenetically closer to a bird than to a lizard. The point of these two examples is not
that phenetic similarity never indicates phylogenetic relationships, but that it is unreliable.
If we sift the evidence, and concentrate on derived homologous similarity, we
should make fewer mistakes in phylogenetic inference.
In Section 15.2 we saw that phylogenetic inference faces the problem of character
conflict, that different characters suggest different phylogenies. The conflict is caused
by homoplasies and ancestral homologies, both of which can fall into incompatible sets
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