Evolution__3rd_Edition

442 PART 4 / Evolution and Diversity
Percentage difference in DNA
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Common chimp
Pygmy chimp
Human
Gorilla
Distance methods revised the great
ape phylogeny
Orang-utan
Common gibbon
Siamang gibbon
Old World monkeys
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BP (Myr)
Figure 15.12
Phylogenetic relations of hominoids, as revealed by DNA
hybridization. This result contains the evidence that the DNA of
humans is 98.5% similar to that of chimpanzees. We meet another
example of a classic distance method in Section 15.13 below.
Redrawn, by permission of the publisher, from Sibley & Ahlquist
(1987).
Figure 15.12 mainly illustrates a distance method in action, but it has three particular
features that are worth noting. One is that the DNA of humans and chimpanzees is
98.5% identical: DNA hybridization is the main evidence for this frequently encountered
observation. 4 Secondly, humans and chimpanzees seem to have a more recent
common ancestor than either has with gorillas. Thirdly, the human lineage banches
from our nearest ape relatives a little over 5 million years ago. The second and third features
matter for a controversy that we shall look at later in Section 15.13.
In practice, distance methods rarely use the simple fraction of sites that are identical
between the DNA of two species, as the raw measurements of distance have first to be
corrected for a problem known as “multiple hits.” This problem comes up in some
form or other in all molecular phylogenetic methods and we look at it next.
15.9.3 Molecular evidence may need to be adjusted for the problem of
multiple hits
Multiple hits refers to the following problem. Imagine two species just after they have
split from a common ancestor. Our 100-nucleotide stretch of DNA will probably be
identical to them so the molecular distance between them is zero (Figure 15.13, at time
zero). After a while, the nucleotide may change at one site in one of the species. Maybe
it was initially T, and changed to C in one of the species. The molecular distance is now
1%. A while later a second change occurs, and then a third, and so on. The molecular
4 Britton (2002) have recently revised the figure down to about 95%, taking account of insertions and deletions.
However, the lower figure does not alter the inferred time of human origin, because the distances
between all species pairs are likely to be subject to similar adjustments.
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