Evolution__3rd_Edition

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Parsimony underlies outgroup
comparison
Character polarities can be inferred
from the fossil record
character a evolved into a′, or a′ into a. We look at a closely related species and infer
that the state there is ancestral in the group of four. If the outgroup had a we should
infer that species 2 and 4 share a more recent common ancestor with each other than
with any of the other species; the relations of 1 and 3 remain uncertain (as is spelled out
further below).
The underlying assumption of outgroup comparison is that evolution proceeded via
the least possible steps. This is the assumption of “parsimony,” which we look at in
more detail later (Section 15.9.4). In Figure 15.8, if the character in the outgroup (a) is
ancestral in the group of species 1–4, there must have been at least one evolutionary
event in the phylogeny: a transition from a to a′ before the ancestor of species 2 and 4.
If, having observed a in the outgroup, we had reasoned that a′ was the ancestral state of
species 1–4, we should need at least two events: a change from a′ to a somewhere
between the outgroup and species 1–4, and then a change from a′ back to a in species 1
and 3. If the character state in the outgroup is ancestral, the fewest evolutionary events
are required.
Outgroup comparison, like all techniques of phylogenetic inference, is fallible.
Sometimes, one possible outgroup will suggest that one character state is ancestral, but
another outgroup will suggest that a different character state is ancestral. The result will
then depend on which outgroup we rely on. The method is most reliable when the
closely related species that could be used as outgroups all suggest the same inference,
but it is possible to be led astray by the method in particular cases. The inference should
be treated with caution, and if possible tested against other evidence.
Before we can use outgroup comparison, we need to know something about the phylogeny.
We needed to know that fish and amphibians were outside the Amniota in
order to use them as “outgroups.” In practice this is not a major problem. Outgroup
comparison cannot be used when we are absolutely ignorant, but if we know something
about the phylogeny of a group (for example that amphibians are not amniotes,
but are closely related to them) we can build on that knowledge to find out more (in
this case, more about the phylogeny within the amniotes).
15.6.2 The fossil record
CHAPTER 15 / The Reconstruction of Phylogeny 435
In the evolution of mammals from mammal-like reptiles, many characters changed
(Section 18.6.2, p. 542). Posture evolved from a “sprawling” to an “upright” gait, and
jaw articulation and circulatory physiology also changed. Some, though not all, of these
characters leave a fossil record, and we can infer which character states were ancestral
and which derived by seeing which is found in the earlier fossils.
The reasoning could hardly be easier. The ancestral state of a character must have
preceded the derived states in fact, and therefore the earlier state in the fossil record is
likely to be ancestral. In the case of the mammal-like reptiles, the criterion is reliable,
because the fossil record is relatively complete. If the record is less complete, a derived
character could be preserved earlier than its ancestral state (Figure 15.9), and the paleontological
inference will be the opposite of the truth.
For a whole fossil series like the mammal-like reptiles, we can be reasonably sure
which states are ancestral. At the other extreme, where there are few fossils and a highly