Evolution__3rd_Edition

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We distinguish ancestral from
derived homology
CHAPTER 15 / The Reconstruction of Phylogeny 431
15.5 Derived homologies are more reliable indicators of
phylogenetic relations than are ancestral homologies
Dog Horse Frog Fish
A' A" A' A
A'
A' → A"
A'
A → A'
A
The next stage is to divide the homologies into ancestral and derived homologies.
Consider the number of digits on the feet of a frog, a dog, and a horse. The frog and dog
have standard tetrapod feet, with five digits. This is the ancestral state for all tetrapods.
(Tetrapods are the group of amphibians, reptiles, birds, and mammals.) Horses have
reduced the number of their digits, and have only one of the five digits left. The similarity
of the dog and frog is not evidence that they share a more recent common ancestor
with each other than either does with a horse. Indeed, both the dog and the horse are
mammals, and share a more recent common ancestor than either does with a frog. In
the group of the frog, dog, and horse, the state of having five digits per foot is a homology
in the dog and frog, but not evidence of a phylogenetic relationship.
We therefore need to distinguish ancestral from derived homologies (Figure 15.5).
To see the distinction, first take the set of species under analysis. A homology that is
present in the common ancestor of that group is an ancestral homology and is useless
for determining phylogenetic relations within the group. The character state A′ in
Figure 15.5 is like the five-digit tetrapod foot in the group of the frog, dog, and horse.
However, if we are studying instead the relations of a frog, a dog, and a fish, the fivedigit
foot is no longer the ancestral state. It was not present in the common ancestor of
the three species. For these three species, the five-digit foot is a derived homology. It
evolved within the group of species that we are studying and tells us something about
the phylogeny. It tells us that the frog and dog share a more recent common ancestor
with each other than either does with the fish.
Ancestral homologies are characters that were present in the common ancestor of
the group of species under study. Derived homologies are homologies that evolved
after the common ancestor, within the group of species under study. The distinction
between ancestral and derived homologies is meaningless if we are only talking about
two species by themselves: any homology for the two species is simply a homology. The
Figure 15.5
Ancestral and derived homologies. A′ is an ancestral homology
if we are studying the phylogeny of the dog, horse, and frog. A′
is a derived homology if we are studying the phylogeny of the
dog, horse, and fish. An ancestral homology was present in the
common ancestor of the group of species under study; a derived
homology evolved more recently that the common ancestor of the
group of species being studied. The distinction between derived
and ancestral homologies is relative to the group of species.
Derived homologies reliably indicate phylogenetic relations;
ancestral homologies do not.