Evolution__3rd_Edition

164 PART 2 / Evolutionary Genetics
Molecular evolution seems to show
a molecular clock
Table 7.2
Amounts of variation in natural populations. Variation can be measured as percentages of
polymorphic loci (P) and average percent heterozygosity per individual (H). Also given is the
number of loci used to estimate P and H. For meaning of H, see Box 6.3 (p. 149). Modified
from Nevo (1988).
Species Number of loci P (%) H (%)
Phlox cuspidata 16 11 1.2
Liatris cylindracea 27 56 5.7
Limulus polyphemus 25 25 5.7
Balanus eburneus 14 67 6.7
Homarus americanus 28–42 18 3.8
Gryllus bimaculatus 25 58 6.3
Drosophila robusta 40 39 11
Bombus americanorum 12 0 0
Salmo gairdneri 23 15 3.7
Bufo americanus 14 26 11.6
Passer domesticus 15 33 9.8
Homo sapiens 71 28 6.7
7.3 Rates of molecular evolution are arguably too constant
for a process controlled by natural selection
The rate of molecular evolution can be measured for any pair of species by the method
shown in Figure 7.2. Each pair of species needs a figure for the number of molecular differences
and the time to their common ancestor. We can plot the point defined by these
two numbers for many pairs of species; Figure 7.3 is an example for α-hemoglobin. The
striking property of the graph is that the points for the different species pairs fall on a
straight line. Molecular evolution appears to have an approximately constant rate per
unit time; it is therefore said to show a molecular clock. Evolutionary change at the
molecular level ticks over at a roughly constant rate, and the amount of molecular
change between two species measures how long ago they shared a common ancestor.
(Molecular differences between species can be used to infer the time of events in the
evolutionary past, as we shall see in Parts 4 and 5 of this text.)
A graph such as Figure 7.3 requires a knowledge of the time to the common ancestor
for each species pair. These times are estimated from the fossil record and are uncertain
(Chapter 18); the results are therefore not universally trusted. However, we can also test
the constancy of molecular evolution by another method, which does not require absolute
dates, and this other test also suggests that molecular evolution is fairly clock-like
(Box 7.2). There is empirical controversy as to how constant the molecular clock is, but
the statistical details are involved and we shall not enter into them here. We can reasonably
conclude at present that the rate of molecular evolution is constant enough to
require explanation.
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