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Evolution__3rd_Edition

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168 PART 2 / <strong>Evolution</strong>ary Genetics<br />

The molecular clock may or may<br />

not be predicted to depend on<br />

generational time, depending on<br />

the mutational process<br />

The real mutational process does<br />

depend on generation length<br />

clear-cut. In particular, we should look at whether the clock runs relative to absolute<br />

time (in years) or generational time. Mice have shorter generations than elephants: but<br />

do molecules in mice show the same amount of evolutionary change per million years<br />

as equivalent molecules in elephants?<br />

The prediction of the neutral theory depends on the mutational process. The rate of<br />

neutral evolution equals the neutral mutation rate (Section 6.3, p. 144). If species with<br />

short generation times have more mutations per year than species with long generation<br />

times, we expect species with short generations to evolve faster. We can distinguish<br />

three possibilities. One is that most mutations have external, environmental causes,<br />

such as UV-rays or chemical mutagens. Environmental mutagens probably hit organisms<br />

at an approximately constant rate through time. An organism that breeds after<br />

1 year will have been hit by about 12 times as many mutagens as an organism that<br />

breeds after 1 month. The neutral theory then predicts the molecular clock will tick<br />

according to absolute time.<br />

Secondly, at the opposite extreme, most mutations might occur during the disruptive<br />

events of meiosis. Meiosis happens only once per generation in all species, whether<br />

their generation times are long or short. The number of mutations per generation<br />

would then be similar in elephants and in shrews. The neutral theory predicts that the<br />

molecular clock should tick according to generational time.<br />

Thirdly, mutations might mainly happen when DNA is replicated. The mutation<br />

rate would depend on the number of times DNA is replicated per generation, which<br />

equals the number of mitotic cell divisions in the cell lines that produce gametes. (The<br />

cell lines that produce the gametes are called the “germ line.”) Species with long generation<br />

times do have more germ line cell divisions than species with short generation<br />

times, but the number is not proportional to generation time. For instance, a 30-yearold<br />

human female has 33 cell divisions behind each of her eggs, since the time when she<br />

was herself a zygote. A 30-year-old man has about 430 cell divisions behind each of his<br />

sperm. The average of the man and woman is about 230 cell divisions. A mature female<br />

rat has 29 cell divisions behind each egg, and a male rat about 58 cell divisions behind<br />

each sperm, giving an average of 43 cell divisions. The ratio of germ line cell divisions<br />

in a human to a rat is 230 : 43 or about five. The human generation length is about<br />

30 years, the rat’s about 1 year. The ratio of generation lengths in years is about 30,<br />

but humans have only about five times as many cell divisions in the germ line.<br />

If mutations mainly happen at mitosis, the neutral theory predicts that the rate<br />

of evolution will be slower per year in species with longer generations than in species<br />

with shorter generation times, but not as slow as the ratio of their generation times<br />

(expressed in years) would predict.<br />

For much of the twentieth century, mutations were thought mainly to have environmental<br />

causes. This belief followed from the discovery in the 1920s that X-rays and<br />

certain chemicals could cause mutations. But by the late twentieth century it had been<br />

established that most mutations are internal copying errors during DNA replication<br />

rather than externally caused. Thus, the third possibility is the most realistic. The<br />

neutral theory predicts there should be a generation time effect in the molecular clock.<br />

Now let us turn to the evidence. What kind of time do real molecular clocks keep?<br />

For proteins, an important early paper by Wilson et al. (1977) strongly suggested that<br />

the clock runs relative to absolute time for protein evolution. Figure 7.4 shows their<br />

method. They picked a number of pairs of species. In each pair, one species had a short<br />

..

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