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Evolution__3rd_Edition

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446 PART 4 / <strong>Evolution</strong> and Diversity<br />

Figure 15.14<br />

Phylogenetic inference by<br />

parsimony. (a) The inference<br />

uses observations such as DNA<br />

sequence data, shown here for<br />

five sites. (b) We then count<br />

the minimum number of<br />

evolutionary changes implied<br />

by the sequence data for all<br />

possible phylogenies (or<br />

unrooted trees, to be exact).<br />

The three possible unrooted<br />

trees for four species are shown.<br />

The marks within each branch<br />

indicate the location of an<br />

evolutionary change. For<br />

instance, the top row shows<br />

where the changes must be for<br />

the first two sites (AA in species<br />

1 and 2, TT in species 3 and 4).<br />

In the left-hand tree, two<br />

changes is the minimum that<br />

can produce this pattern, and<br />

the two changes must be in the<br />

internal branch. We finally<br />

sum the number of changes<br />

for all the trees, and the tree<br />

requiring the fewest changes<br />

(seven, in this case) is inferred<br />

to be correct. The fifth site is<br />

ignored in the counting in<br />

(b) because it is the same in<br />

all species and does not help<br />

us to infer the phylogeny. Sites<br />

like this, which are equally<br />

compatible with all possible<br />

trees, are called uninformative.<br />

Sites (such as 1–4) that require<br />

different numbers of events in<br />

different trees are called<br />

informative.<br />

(a) Sequence data<br />

(b) Counting evolutionary changes<br />

1 3<br />

2 4<br />

1<br />

2<br />

1<br />

2<br />

3<br />

4<br />

3<br />

4<br />

Species DNA sequence<br />

1 A A AA A<br />

2 A A T T A<br />

3 T T T C A<br />

4 T T AGA<br />

1 2<br />

3 4<br />

1 2<br />

3 4<br />

1 2<br />

3 4<br />

1 1<br />

For the characters shared between humans and chimps, the argument is particularly<br />

powerful. Chimps and humans share whole complex organ systems like hearts and<br />

lungs, eyes, brains, and spinal cords. The initial evolution of each of these characters<br />

required improbable mutations, and natural selection operating over millions of generations.<br />

It is evolutionarily improbable to the point of near impossibility that the same<br />

changes would have evolved independently in the two lineages after their common<br />

ancestor. By contrast, there is nothing improbable about postulating that the characters<br />

could have been passed on in passive inheritance from the common ancestor of<br />

chimps and humans to the modern descendants.<br />

For some characters other than the complex morphological characters shared<br />

between humans and chimps, the argument is less powerful. At the other extreme, if we<br />

find one nucleotide, at a particular site in the DNA, shared between two species, there is<br />

a 25% probability that it could be shared by chance and the principle of parsimony does<br />

not strongly suggest that the nucleotide has not changed through all the evolutionary<br />

intermediates between the two.<br />

The argument is more powerful in some cases than others. But evolutionary change<br />

in all characters is improbable to some extent, as compared with simple inheritance,<br />

and the principle of parsimony therefore has a sound evolutionary justification. In<br />

4<br />

3<br />

1 2<br />

4<br />

3<br />

1 2<br />

4 3<br />

7 9 8<br />

Sites 1 and 2<br />

Site 3<br />

Site 4<br />

Total number<br />

of evolutionary<br />

changes<br />

..

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