Evolution__3rd_Edition

..
. . . as illustrated by a motor car
analogy
A controversial prediction about the
genomic deleterious mutation rate
Mutation–accumulation
experiments are so far ambiguous
between the cars, creating one car with two good components, at the expense of the
second car with two (rather than one) bad components. This is an improvement. You
have created a car that goes out of two that did not. If a car is a wreck, it does not much
matter whether it contains one, two, or 20 defects. So you can load a second bad component
into an already broken-down car without making things worse.
In genetic terms, imagine a simple haploid model with two loci and two alleles. At
each locus, there is a good version of the gene, symbolized by 1, and a bad (deleterious)
version, symbolized by 0. Four haplotypes are possible: 11, 01, 10, 00. (Remember these
are combinations of alleles at two loci, not the more familiar diploid genotypes at one
locus. See Section 8.4, p. 199, on haplotypes.) Sex, as in the car analogy, helps when two
individuals with single complementary defects interbreed: that is, an 01 × 10 mating.
That will produce some 11 offspring (or grandchildren) at the expense of some 00 offspring.
The advantage of sex is that it increases the number of deleterious mutations
removed in one death. If an 01 individual clones itself, one death among its offspring
removes one bad gene. If it reproduces sexually, one death of an 00 offspring removes
two bad genes. The average quality of the surviving offspring can be increased.
Kondrashov’s theory requires two conditions in order for natural selection to favor
sex despite the 50% cost. We can look at them in turn.
12.2.2 The mutational theory predicts U >1
CHAPTER 12 / Adaptations in Sexual Reproduction 321
The first condition is that the deleterious mutation rate is high enough. If deleterious
mutations are rare, any advantage of sex will be minor. If they are common, sex may be
more advantageous. The deleterious mutation rate is expressed as a genomic figure,
that is the average number of new deleterious mutations that occur in each offspring. It
is the sum of the deleterious mutations carried into that offspring by the sperm plus the
number carried by the egg. The genomic deleterious mutation rate is symbolized by U.
Sex becomes advantageous relative to cloning if U is more than about one. This is the
most controversial prediction of Kondrashov’s theory, because deleterious mutation
rates have historically been thought to be much lower. Measurements of deleterious
mutation rates are being attempted by two methods at present, though neither has yet
yielded a conclusive result.
One method is the mutation–accumulation experiment, pioneered by the Japanese
geneticist Terumi Mukai. The experimenter attempts to create conditions in which
selection does not act against mutation. Mutations will then accumulate over time at
the same rate as they occur. From time to time, the fitness of individuals in the experimental
population are measured relative to control individuals. Any decline in fitness
of the experimental line can be used to estimate the deleterious mutation rate. Mukai’s
original experiment produced a dramatic decline (Figure 12.5), suggesting that the
deleterious mutation rate (U ) could be one, or even more than one, in fruitflies.
Since then, these experiments have been created several times in several species. Some
experiments produce high U, like Mukai’s; others produce negligibly low U. Mutation–
accumulation experiments are an active area of research, but currently ambiguous.
The second method uses rates of DNA sequence evolution. We begin with a region of
DNA, such as a pseudogene, that evolves in a completely neutral manner. This DNA