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Evolution__3rd_Edition

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

Percentage difference in DNA<br />

0<br />

1<br />

2<br />

3<br />

4<br />

5<br />

6<br />

7<br />

8<br />

Common chimp<br />

Pygmy chimp<br />

Human<br />

Gorilla<br />

Distance methods revised the great<br />

ape phylogeny<br />

Orang-utan<br />

Common gibbon<br />

Siamang gibbon<br />

Old World monkeys<br />

0<br />

5<br />

10<br />

15<br />

20<br />

25<br />

30<br />

35<br />

BP (Myr)<br />

Figure 15.12<br />

Phylogenetic relations of hominoids, as revealed by DNA<br />

hybridization. This result contains the evidence that the DNA of<br />

humans is 98.5% similar to that of chimpanzees. We meet another<br />

example of a classic distance method in Section 15.13 below.<br />

Redrawn, by permission of the publisher, from Sibley & Ahlquist<br />

(1987).<br />

Figure 15.12 mainly illustrates a distance method in action, but it has three particular<br />

features that are worth noting. One is that the DNA of humans and chimpanzees is<br />

98.5% identical: DNA hybridization is the main evidence for this frequently encountered<br />

observation. 4 Secondly, humans and chimpanzees seem to have a more recent<br />

common ancestor than either has with gorillas. Thirdly, the human lineage banches<br />

from our nearest ape relatives a little over 5 million years ago. The second and third features<br />

matter for a controversy that we shall look at later in Section 15.13.<br />

In practice, distance methods rarely use the simple fraction of sites that are identical<br />

between the DNA of two species, as the raw measurements of distance have first to be<br />

corrected for a problem known as “multiple hits.” This problem comes up in some<br />

form or other in all molecular phylogenetic methods and we look at it next.<br />

15.9.3 Molecular evidence may need to be adjusted for the problem of<br />

multiple hits<br />

Multiple hits refers to the following problem. Imagine two species just after they have<br />

split from a common ancestor. Our 100-nucleotide stretch of DNA will probably be<br />

identical to them so the molecular distance between them is zero (Figure 15.13, at time<br />

zero). After a while, the nucleotide may change at one site in one of the species. Maybe<br />

it was initially T, and changed to C in one of the species. The molecular distance is now<br />

1%. A while later a second change occurs, and then a third, and so on. The molecular<br />

4 Britton (2002) have recently revised the figure down to about 95%, taking account of insertions and deletions.<br />

However, the lower figure does not alter the inferred time of human origin, because the distances<br />

between all species pairs are likely to be subject to similar adjustments.<br />

..

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