Evolution__3rd_Edition

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Figure 15.7
Shared characters divide into
homoplasies, ancestral
homologies, and derived
homologies. (a) a′ is a
homoplasy: it is not in the
common ancestor of the species
that share it. (b) a is an ancestral
homology: it is in the common
ancestor of the species that
share it, but has been lost in
some descendants of that
common ancestor. (c) a′ is a
derived homology: it is in the
common ancestor of the species
that share it, and in all its
descendants. Notice that
only derived homologies
always indicate phylogenetic
relationships. Figure 16.4
(p. 480) and Table 16.1 (p. 475)
show the kinds of taxa that are
defined by these three kinds of
character.
Cladists infer character polarities
CHAPTER 15 / The Reconstruction of Phylogeny 433
All characters
that are similar
in different species
Homoplasies
Homologies
Ancestral
Derived
(a) a' is a homoplasy (b) a is an ancestral
(c) a' is a derived
in species 1 & 3
homology in species
homology in species
2, 3 & 4
1 & 2
Species 1 2 3 4 1 2 3 4 1 2 3 4
a' a a' a a' a a a a' a' a a
a a a a a' a
a a a
of species. If we successfully identify the homoplasies and ancestral homologies, and
discard them, the problem of character conflict should be removed. Correctly identifed
derived homologies must all agree on the same phylogeny. They all evolved in the same
phylogenetic tree, and should all fall into the same pattern of groups (“horizontal”
transfer of characters between lineages is an exception). If, in a number of characters,
the homologies and character polarities have been correctly identified, it is impossible
for different derived homologies to suggest incompatible phylogenies. The same is not
true for homoplasies and ancestral homologies. Ten different homoplasies, or 10 different
ancestral homologies, can fall into up to 10 different, and conflicting, groupings
of species.
In summary, we have divided characters into three kinds and considered theoretically
how each relates to groups of species in a phylogeny. Only shared derived homologies
consistently reveal phylogenetic groups. But how can we distinguish in practice
between ancestral and derived states of characters?
15.6 The polarity of character states can be inferred by
several techniques
The question of how to distinguish derived from ancestral homologies has the following
general form. A character has two states, which we can call a and a′: we need to
know whether a evolved from a′, or the other way round. In this section, we discuss
two of the methods. The distinction between ancestral and derived character states is
sometimes referred to as character polarity. Analyzing a character to work out which of
its states are ancestral and which derived is also working out the “polarities” of the
character states.