Evolution__3rd_Edition

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But selection may have removed
heritable variation
Other processes may recreate
variation
CHAPTER 12 / Adaptations in Sexual Reproduction 335
than average) produce offspring who also have higher than average fitness. If high
quality males do not produce high quality offspring, there is no point in picking them
as mates.
The conditions in the two theories face a common difficulty. While selection operates
on any character, it reduces its heritability (see, for example, the Illinois maize
experiment, Table 9.2, p. 239, and Figure 9.7, p. 238). In a population in which some
individuals possess good genes and others bad genes, selection acts to fix the good genes
a and once it has done so there will be no variation in genetic quality left. Zahavi’s
theory then would not work.
In fact, the amount of variation in genetic quality for species in nature is unknown,
and no firm conclusion can be drawn. However, there are three arguments to be aware
of. One is the possibility just noted, that Zahavi’s theory may not work in species
in which males transfer only sperm because there is not enough variation in genetic
quality.
Alternatively, enough genetic variation may exist for the handicap process to
operate. The variation in genetic quality is likely to exist because of one or other of the
two factors we looked at earlier in the chapter: deleterious mutation and host–parasite
coevolution. At any one locus, mutation contributes little genetic variation. But if the
highest estimates of the genomic deleterious mutation rate (see Table 12.1) turn out to
be correct, then a substantial amount of variation in genetic quality will exist. Females,
by picking males with long tails, may be picking mates with relatively few bad genes.
Likewise, if the cyclic gene association proposed in the parasitic theory are correct (see
Figure 12.7), a population will contain variation in genetic quality for this reason.
Females, by picking males with long tails, may be picking mates with good parasiteresistance
genes; may be only healthy males are able to grow long tails.
Some evidence exists that female choice is influenced by genetic quality. For
instance, Welch et al. (1998) did a particularly clear experiment with gray tree frogs
(Hyla versicolor) in Missouri. Welch et al. fertilized half a female’s eggs with sperm of
preferred males, and half with sperm of unpreferred males. The offspring of the preferred
males had higher fitness. Research at present aims to test how widespread these
female preferences for good genes are, and what the explanation is for the differences in
genetic quality between males.
12.4.7 Natural selection may work in conflicting ways on males
and females
The evolution of sex opened up the possibility of future evolutionary conflicts between
males and females. Consider an animal such as a fruitfly, in which females usually mate
with a number of different males over a period of time. The female stores sperm after
each mating. She produces eggs steadily, at a certain rate, over time, and draws on her
store of sperm to fertilize the eggs as she lays them. One male, therefore, fertilizes most
of the eggs that a female lays between when he mates with her and when the next male
mates with her.
Several selective forces will be at work in male and female fruitflies. For instance,
selection favors a male who can accelerate a female’s egg production immediately after