Evolution__3rd_Edition

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Insulin illustrates the effect of
functional importance
Figure 7.6
The insulin molecule is made
by snipping the center out of
a larger proinsulin molecule.
The rate of evolution in the
central part, which is discarded,
is higher than that of the
functional extremities. From
Kimura (1983). Redrawn with
permission of Cambridge
University Press, © 1983.
CHAPTER 7 / Natural Selection and Random Drift 175
However, a fundamental distinction remains. For any evolutionary change, in which
one version of a gene is substituted for another, we can ask whether the force driving
that change was natural selection or random drift. In the nearly neutral theory, just
as in the original neutral theory, the force driving molecular evolution is neutral
drift. Natural selection against disadvantageous mutations has a subtler, more flexible
form in the nearly neutral theory than in the purely neutral theory. Drift and selection
combine in different ways in the two theories to explain the observed facts of molecular
evolution. But a crucial similarity remains: both theories explain evolution by drift.
Natural selection has only a negative role, acting against disadvantageous mutations.
This contrasts with all “selectionist” theories of molecular evolution, in which
molecular evolutionary change occurs because natural selection favors advantageous
mutations.
7.6 Evolutionary rate and functional constraint
7.6.1 More functionally constrained parts of proteins evolve
at slower rates
A protein contains functionally more important regions (such as the active site of an
enzyme) and less important regions. The rate of evolution in the functionally more
important parts of proteins is usually slower. For example, insulin is formed from a
proinsulin molecule by excising a central region (Figure 7.6). The central region is discarded,
and its sequence is probably less crucial than that of the outlying parts which
form the final insulin protein. The central part evolves six times more rapidly than the
outlying parts. The same result has been found by comparing evolutionary rates in the
active sites and in other regions of enzymes; the surface of a hemoglobin, for example,
may be functionally less important than the heme pocket, which contains the active
site. The evolutionary rate is about 10 times faster in the surface region (Table 7.5).
A similar tendency may underlie differences in the rates of evolution of whole genes,
or proteins. In Table 7.1 we saw that some proteins evolve faster than others. One
Insulin
A
S S
S
B
Proinsulin (pig)
(30 aa) (33 aa) (21 aa)
S
B
Evolutionary rate
0.4 × 10 –9 /aa/yr
A
C peptide
Evolutionary rate
2.4 × 10 –9 /aa/yr