Evolution__3rd_Edition

..
Long-generation
species,
e.g., whale
Short-generation
species,
e.g., mouse Outgroup
a b c
A generation time effect is seen in
synonymous evolution ...
CHAPTER 7 / Natural Selection and Random Drift 169
Figure 7.4
Wilson et al.’s (1977) method to test for a generation time effect
on the rate of protein evolution. a, b, and c are the numbers of
evolutionary changes in the three segments of the tree; they are
estimated from the pairwise molecular differences between the
species using the method of Box 7.2. The “outgroup” can be any
species known to have a more distant common ancestor with the
pair of species being compared. The evidence suggests that a ≈ b
for many molecules and species pairs, whereas a would be less
than b if generation time influenced evolutionary rate.
generation time and the other had a long generation time. Wilson et al. used a relative
rate test (Box 7.2), and found that the amount of change was similar in the two lineages.
The result now looked awkward for the neutral theory. At the time, a neutralist could
easily argue that mutations occur at a probabilistically constant rate in absolute time,
and the result was as expected.
When DNA evidence became available, it showed a different picture, at least for synonymous
changes. (Synonymous changes are nucleotide changes that do not alter the
amino acid. Nucleotide changes that do alter the amino acid are called non-synonymous.
Synonymous changes are possible because of the redundancy in the genetic code a
Section 2.5, p. 28.) Rodents, such as mice and rats, have shorter generation times than
primates and artiodactyls (such as cows). For synonymous substitutions, evolution is
faster in rodents than in artiodactyls, and faster in artiodactyls than in primates (Table 7.4).
Synonymous substitutions occur faster in species with shorter generation times.
Table 7.4
Rates of evolution in silent base sites are faster in groups with shorter generation times.
There are estimates for various pairs of species, and each estimate is an average for a number of
proteins; the number of sites is the total number of base sites (for all proteins) that have been
used to estimate the rate. The divergence times, which are in millions of years, are uncertain;
a range of estimates (in parentheses) have been made. Modified from Li et al. (1987).
Number of Number Rate Generation
Species pairs proteins of sites Divergence (× 10 −9 years) time
Primates
Man vs chimp 7 921 7 (5–10) 1.3 (0.9–1.9) 5
Man vs orang-utan 4 616 12 (10–16) 2 (1.5–2.4) 6 Long
Man vs OW monkey 8 998 25 (20–30) 2.2 (1.8–2.8) 7
Artiodactyls
Cow vs goat
Cow/sheep vs goat
3
3
297
1,027
17 (12–25)
55 (45–65)
4.2 (2.9–6)
3.5 (3.0–4.3)
5
6 Medium
7
Rodents
Mouse vs rat 24 3,886 15 (10–30) 7.9 (3.9–11.8) } Short