Evolution__3rd_Edition

446 PART 4 / Evolution and Diversity
Figure 15.14
Phylogenetic inference by
parsimony. (a) The inference
uses observations such as DNA
sequence data, shown here for
five sites. (b) We then count
the minimum number of
evolutionary changes implied
by the sequence data for all
possible phylogenies (or
unrooted trees, to be exact).
The three possible unrooted
trees for four species are shown.
The marks within each branch
indicate the location of an
evolutionary change. For
instance, the top row shows
where the changes must be for
the first two sites (AA in species
1 and 2, TT in species 3 and 4).
In the left-hand tree, two
changes is the minimum that
can produce this pattern, and
the two changes must be in the
internal branch. We finally
sum the number of changes
for all the trees, and the tree
requiring the fewest changes
(seven, in this case) is inferred
to be correct. The fifth site is
ignored in the counting in
(b) because it is the same in
all species and does not help
us to infer the phylogeny. Sites
like this, which are equally
compatible with all possible
trees, are called uninformative.
Sites (such as 1–4) that require
different numbers of events in
different trees are called
informative.
(a) Sequence data
(b) Counting evolutionary changes
1 3
2 4
1
2
1
2
3
4
3
4
Species DNA sequence
1 A A AA A
2 A A T T A
3 T T T C A
4 T T AGA
1 2
3 4
1 2
3 4
1 2
3 4
1 1
For the characters shared between humans and chimps, the argument is particularly
powerful. Chimps and humans share whole complex organ systems like hearts and
lungs, eyes, brains, and spinal cords. The initial evolution of each of these characters
required improbable mutations, and natural selection operating over millions of generations.
It is evolutionarily improbable to the point of near impossibility that the same
changes would have evolved independently in the two lineages after their common
ancestor. By contrast, there is nothing improbable about postulating that the characters
could have been passed on in passive inheritance from the common ancestor of
chimps and humans to the modern descendants.
For some characters other than the complex morphological characters shared
between humans and chimps, the argument is less powerful. At the other extreme, if we
find one nucleotide, at a particular site in the DNA, shared between two species, there is
a 25% probability that it could be shared by chance and the principle of parsimony does
not strongly suggest that the nucleotide has not changed through all the evolutionary
intermediates between the two.
The argument is more powerful in some cases than others. But evolutionary change
in all characters is improbable to some extent, as compared with simple inheritance,
and the principle of parsimony therefore has a sound evolutionary justification. In
4
3
1 2
4
3
1 2
4 3
7 9 8
Sites 1 and 2
Site 3
Site 4
Total number
of evolutionary
changes
..