Evolution__3rd_Edition

168 PART 2 / Evolutionary Genetics
The molecular clock may or may
not be predicted to depend on
generational time, depending on
the mutational process
The real mutational process does
depend on generation length
clear-cut. In particular, we should look at whether the clock runs relative to absolute
time (in years) or generational time. Mice have shorter generations than elephants: but
do molecules in mice show the same amount of evolutionary change per million years
as equivalent molecules in elephants?
The prediction of the neutral theory depends on the mutational process. The rate of
neutral evolution equals the neutral mutation rate (Section 6.3, p. 144). If species with
short generation times have more mutations per year than species with long generation
times, we expect species with short generations to evolve faster. We can distinguish
three possibilities. One is that most mutations have external, environmental causes,
such as UV-rays or chemical mutagens. Environmental mutagens probably hit organisms
at an approximately constant rate through time. An organism that breeds after
1 year will have been hit by about 12 times as many mutagens as an organism that
breeds after 1 month. The neutral theory then predicts the molecular clock will tick
according to absolute time.
Secondly, at the opposite extreme, most mutations might occur during the disruptive
events of meiosis. Meiosis happens only once per generation in all species, whether
their generation times are long or short. The number of mutations per generation
would then be similar in elephants and in shrews. The neutral theory predicts that the
molecular clock should tick according to generational time.
Thirdly, mutations might mainly happen when DNA is replicated. The mutation
rate would depend on the number of times DNA is replicated per generation, which
equals the number of mitotic cell divisions in the cell lines that produce gametes. (The
cell lines that produce the gametes are called the “germ line.”) Species with long generation
times do have more germ line cell divisions than species with short generation
times, but the number is not proportional to generation time. For instance, a 30-yearold
human female has 33 cell divisions behind each of her eggs, since the time when she
was herself a zygote. A 30-year-old man has about 430 cell divisions behind each of his
sperm. The average of the man and woman is about 230 cell divisions. A mature female
rat has 29 cell divisions behind each egg, and a male rat about 58 cell divisions behind
each sperm, giving an average of 43 cell divisions. The ratio of germ line cell divisions
in a human to a rat is 230 : 43 or about five. The human generation length is about
30 years, the rat’s about 1 year. The ratio of generation lengths in years is about 30,
but humans have only about five times as many cell divisions in the germ line.
If mutations mainly happen at mitosis, the neutral theory predicts that the rate
of evolution will be slower per year in species with longer generations than in species
with shorter generation times, but not as slow as the ratio of their generation times
(expressed in years) would predict.
For much of the twentieth century, mutations were thought mainly to have environmental
causes. This belief followed from the discovery in the 1920s that X-rays and
certain chemicals could cause mutations. But by the late twentieth century it had been
established that most mutations are internal copying errors during DNA replication
rather than externally caused. Thus, the third possibility is the most realistic. The
neutral theory predicts there should be a generation time effect in the molecular clock.
Now let us turn to the evidence. What kind of time do real molecular clocks keep?
For proteins, an important early paper by Wilson et al. (1977) strongly suggested that
the clock runs relative to absolute time for protein evolution. Figure 7.4 shows their
method. They picked a number of pairs of species. In each pair, one species had a short
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