Evolution__3rd_Edition

..
Log fitness of organisms
Number of mutations in organism
. . . that may or may not confirm the
mutational theory of sex
Work on a second prediction is also
inconclusive
3
2
1
CHAPTER 12 / Adaptations in Sexual Reproduction 323
Figure 12.6
Three different relations between the fitness of an organism
and the number of deleterious mutations it carries. The y-axis is
logarithmic. 1, Synergistic epistasis: multiple mutations have an
increasingly damaging effect on the organism. 2, Independent,
or multiplicative, fitness effects. 3, Multiple mutations have a
decreasingly damaging effect. Deleterious mutations are purged
more (slope 1), equally (slope 2), and less (slope 3), efficiently
with sexual, than asexual, reproduction. The relative positions of
lines 1, 2, and 3 up the y-axis is irrelevant; only the slope matters.
If slope 1 was drawn below 2 it would still be synergistic epistasis.
deleterious mutation rates, U is less than one in bacteria and more than one in great
apes, including humans. Kondrashov’s theory correctly predicts the absence of sex in
bacteria. The problematic area is around flies and worms. They reproduce sexually
and Kondrashov’s theory predicts U > 1. On the high estimate, the prediction is
upheld; on the low estimate, it is falsified. Thus, further research is needed on the fraction
of mutations that are deleterious. However, it is worth noting that U is > 1 in
humans whether the correct figure is 2 or 30 deleterious mutations per generation. If
Kondrashov’s theory turns out to be wrong, and sex does not help the selective purge of
deleterious mutations, we shall be left with a paradox a how can humans exist, given
their high deleterious mutation rate?
The second prediction of Kondrashov’s theory concerns the relation between the
fitness of an organism and the number of deleterious mutations it contains. Three sorts
of relation are theoretically possible (Figure 12.6). Kondrashov’s theory only works if
the graph slopes down a a condition called synergistic epistasis. Experimenters are also
trying to test this prediction, but no conclusive results are yet available.
In conclusion, the mutational theory suggests that sex exists to help life cope with
its load of deleterious mutations. The theory has been worked out, and is internally
consistent. It makes two predictions about real sexual creatures: they should have
deleterious mutation rates of one or more, and their fitness relations should show
synergistic epitasis. These predictions have inspired a major research programme a
one of the most active and important in modern evolutionary biology a but it is
currently inconclusive. The next few years of work should tell us whether U is > 1 in
fruitflies, but we do not yet know.
12.2.3 Coevolution of parasites and hosts may produce rapid
environmental change
The second theory we shall look at ignores the effect of deleterious mutations and concentrates
on external environmental change. Sex is more likely to be advantageous if
environments change rapidly: the problem is to work out how environments could