Evolution__3rd_Edition

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Amino acid differences
1.0
0.9
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0.1
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0 100 200 300
Time (Myr)
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Kimura argued that drift explains
the molecular clock, whereas
selection does not
Morphological evolution is not
clocklike
CHAPTER 7 / Natural Selection and Random Drift 165
Figure 7.3
The rate of evolution of hemoglobin. Each point on the graph is
for a pair of species, or groups of species, with the value for that
pair being obtained by the method of Figure 7.2. Some of the
points are for α-hemoglobin, others for β-hemoglobin. From
Kimura (1983). Redrawn with permission of Cambridge
University Press, © 1983.
What does a constant rate imply about whether molecular evolution is mainly driven
by natural selection or neutral drift? Kimura reasoned that constant rates are more
easily explained by neutral drift than selection. Neutral drift has the property of a
random process and its rate will show the variability characteristic of a random process.
Neutral mutations crop up at random intervals, but if they are observed over a
sufficiently long time period the rate of change will appear to be approximately constant.
Neutral drift will drive evolution at a fairly constant rate. Natural selection,
Kimura argued, does not produce such constant change. Under selection, the rate of
evolution is influenced by environmental change as well as the mutation rate; and it
would require a surprisingly steady rate of environmental change, over hundreds of
millions of years, in organisms as different as snails and mice and sharks and trees to
produce the constant rate of change seen in Figure 7.3.
Moreover, if we look at characters, such as any adaptive morphological characters,
that have undoubtedly evolved by natural selection, they do not seem to evolve at
constant rates. Kimura (1983) discussed the evolution of the bird wing as an example.
Before the wing evolved, there was a long period during which the vertebrate limb
remained relatively constant (in the form of the tetrapod limb of amphibians and
reptiles). Then came a shorter period when the wing originated and evolved. Finally,
there was a long period of fine tuning a more or less finished wing form.
The wings of birds undoubtedly evolved by natural selection. The rate of change during
wing evolution fluctuated between fast and slow. The rate of molecular evolution
appears to be relatively constant, compared with morphological evolution. This observation
is also Kimura’s reason for confining the neutral theory to molecules, and not
applying it to the gross phenotypes of organisms. Molecular evolution does appear to
have a fairly constant rate, as would be expected for a random process. Morphological
evolution has a different pattern, and is probably driven by the non-random process of
selection.
Molecular evolution in “living fossils” provides a striking example both of the
constant rate of molecular evolution and of the independence between molecular and