Evolution__3rd_Edition

440 PART 4 / Evolution and Diversity
A
B
A B C D B A C D A B C D D C B A C D B A
Figure 15.10
Unrooted and rooted trees. One unrooted tree for four species
is compatible with five rooted trees. An unrooted tree is a
timeless picture of branching relations and does not specify
where the ancestor (or root) of the tree is. The root could be
Unrooted trees can be rooted
cladistically
rooted tree is correct. In general, the root could be in any one of the internal branches.
A four-species unrooted tree has five internal branches. A five-species unrooted tree
has seven internal branches and is compatible with seven rooted trees.
Unrooted trees can be thought of as part of the internal workings of molecular phylogenetic
techniques. The unrooted tree links the species according to the evidence that
is used to infer the phylogeny, but it does not show ancestral relations. Once a technique
has found the unrooted tree for a set of species, some further evidence is used to
find the root, and thus the ancestral relationships between the species. This further evidence
often consists of one of the cladistic techniques for determining character polarity
(Section 15.6 above). For instance, in Figure 15.10 we could look at the molecular
sequence in some closely related species (or “outgroup”). If it was most similar to
species A, that would suggest the root of the tree lies in the branch leading to A. The
rooted tree would be the one at the left in Figure 15.10. In some cases however, the location
of the root cannot be found, or an analysis can proceed with an unrooted tree
alone. Then the unrooted tree is the final product of the molecular phylogenetic study.
15.9.2 One class of molecular phylogenetic techniques uses
molecular distances
C
D
Unrooted tree
Rooted trees
anywhere in it, and there are five topological possibilities, as
drawn below. In general, any one unrooted tree of s species has
2s − 3 internal branches and therefore 2s − 3 possible rooted
trees. (Here, as elsewhere in the chapter, we confine ourselves
to strictly bifurcating trees.)
Imagine that we know the sequences of a particular 100-nucleotide stretch of DNA in
four species, A, B, C, and D. For any pair of species, such as A and B, the nucleotides will
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