Evolution__3rd_Edition

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CHAPTER 7 / Natural Selection and Random Drift 177
Table 7.6
Rates of evolution for synonymous and non-synonymous (that is, amino acid-changing)
substitutions in various genes. Rates are expressed as inferred number of base changes per
10 9 years. These data were used to calculate the introductory figures in Table 7.1. Modified
from Li (1997).
Gene Non-synonymous rate Synonymous rate
Albumin 0.92 5.16
a-globin 0.56 4.38
b-globin 0.78 2.58
Immunoglobin V H 1.1 4.76
Parathyroid hormone 1.0 3.57
Relaxin 2.59 6.39
Ribosomal S14 protein 0.02 2.16
Average (45 genes) 0.74 3.51
Box 7.3
Using Pseudogenes to Infer the Total Mutation Rate
Nachman & Crowell (2000) estimated the rate of evolution in
18 pseudogenes that are present in both humans and chimpanzees.
The average rate of evolution was about 2.5 × 10 −8 per nucleotide
site per generation. This can be multiplied by the diploid size of the
human genome, about 6.6 × 10 9 nucleotides, to give the total
number of mutations per human reproductive event. Nachman
and Crosswell gave a range of estimates, and 175 mutations per
generation is a representative figure.
The exact estimate for the total human mutation rate depends
on what figure is used for the human generation length, the time
since the common ancestor of humans and chimpanzees, and
the ancestral population size. However, the results are usually
somewhere in the 150–300 range for the human mutation rate.
Numbers in this range were first deduced from pseudogene
sequence data in the late 1980s (pseudogenes had not even
been discovered before 1981), and the early estimates usually
gave the figure as 200. When the number first appeared,
it was much higher than had previously been supposed,
but most human geneticists now accept that something like
200 mutations occur every time a new human being is reproduced.
There may be some extra mutations, not estimated from the
pseudogene sequence data a such as mutations in chromosome
numbers or chromosome structure. But the 175–200 mutations
estimated from pseudogenes probably includes most human
mutations.
The inference assumes: (i) that the mutation rate in pseudogenes
is representative of the genome as a whole; and (ii) all mutations
in pseudogenes are neutral (that is, no selective constraints exist
on them at all). The second assumption may not be valid (see
Section 7.8.5 on codon bias).
7.6.2 Both natural selection and neutral drift can explain the trend
for proteins, but only drift is plausible for DNA
The neutral explanation for the relation between evolutionary rate and functional
constraint is as follows. In the active site of an enzyme, an amino acid change will