Evolution__3rd_Edition

434 PART 4 / Evolution and Diversity
Outgroup comparison infers
character polarity from the states
in related species
(a) Observations
Species
Character state
(b) Phylogenetic inference
15.6.1 Outgroup comparison
Amniotes are the group made up of reptiles, birds, and mammals; all these animals possess
an egg membrane, called the amnion, during their development. It is known that
amniotes are a monophyletic group, that is they all share a unique common ancestor.
Here, we will assume that the amniotes are indeed known to be a good phylogenetic
group but that we do not know the relations among the different amniotes. For
instance, in a set of six amniotic species (such as a mouse, a kangaroo, a bird of paradise,
a robin, a crocodile, and a tortoise), does the kangaroo share a more recent common
ancestor with a mouse, a bird of paradise, or what?
Suppose we have established homologies in various characters, including reproductive
physiology. The kangaroo and mouse are viviparous, and the other four species are
oviparous. Did the ancestor of the group of six species breed viviparously, in which case
viviparity was ancestral and oviparity derived, or did it breed oviparously, in which case
evolution went the other way round? By the method of outgroup comparison, the
answer is found by looking at a closely related species which is known to be phylogenetically
outside the group of species we are studying. The character state in that outgroup
is likely to have been ancestral in the group under consideration.
In this case, we might look at a salamander, a frog, or even a fish. They are all near relatives
of the amniotes, but are not amniotes themselves. These “outgroup” species
almost all breed oviparously. The inference by outgroup comparison, therefore, is that
oviparity is ancestral in the amniotes. Viviparity, in the kangaroo and mouse, would
then be a shared derived character and oviparity in the other four a shared ancestral
character.
In the abstract, there could be two species, species 1 and 3, sharing homology a and
two others, species 2 and 4, with homology a′ (Figure 15.8). We wish to know whether
1 2 3 4 Outgroup
a a' a a' a
2 4 1 3 Outgroup
a' a'
a
a
a
a'
a
a
Figure 15.8
(a) Species 1–4 have the character states as given. We wish to
know whether a or a′ was the state in their common ancestor.
(b) We look at a closely related species, the outgroup. It has state
a, and we infer that was the state in the ancestor of species 1–4.
The gray lines for species 1 and 3 indicate their branching
relations remain uncertain.
..