Evolution__3rd_Edition

336 PART 3 / Adaptation and Natural Selection
A hypothesis about conflicting
selection pressures on males and
females ...
. . . is supported by ingenious
experimental results
mating with him, because that male will then fertilize more eggs. Male fruitflies seem to
transfer chemicals with their sperm that act as hormones in the female and accelerate
egg production. The accelerated egg production may not be in the interest of the
female. Her optimal rate of egg production will be some trade-off between her survival
and reproduction. If she produces extra eggs now, it will be by allocating less energy to
maintaining her body. Survival will decrease, and her total lifetime output of eggs will
also decrease. The male gains extra eggs in the short term, at the cost to the female of
reduced lifetime fitness. The cost is not paid by the male, because the later eggs that a
female loses because she dies younger would have been fertilized by another male.
Natural selection on females favor resistance to the male techniques of accelerating egg
production. Females may evolve counterhormones or other methods of restoring the
optimal egg production rate.
The selective forces differ if there is lifetime monogamy. Now the “interests” of the
male and female are identical. If a male causes a female to accelerate her short-term egg
production, but reduces her lifetime fitness, his own fitness will go down by the same
amount.
Holland & Rice (1999) tested this reasoning experimentally with fruitflies. Fruitflies
usually mate with several members of the opposite gender. Holland and Rice allowed
some fruitflies to mate normally, as controls. In their experimental lines, they imposed
monogamy by selecting at random one individual to be the only mate of another individual
(of the opposite gender). They bred lines of these experimentally monogamous
fruitflies for 47 generations.
As predicted, the male fruitflies in the monogamous lines evolved to be less harmful
to their mates and females evolved to be less resistant to males. Figure 12.11 shows some
of Holland and Rice’s results. After 47 generations of monogamy, males had evolved
lower rates of courting (Figure 12.11a). Also, the total reproduction output per female
increased under monogamy (Figure 12.11b). This result suggests that the conflict
between males and females in normal fruitflies is reducing the fitness of the average
individual by 20% or so. The main interest of the results is to illustrate the evolutioanry
theory of intersexual conflict, but the results have other interests too. Intersexual
conflict is a further factor that can be added to the list of causes of adaptive imperfection
in Chapter 10. Sexual selection, including intersexual conflict, may also underlie
the relatively rapid evolution of genes that are expressed in the reproductive system a a
phenomenon noted in Sections 7.8.2 (p. 182) and 14.12 (p. 417).
12.4.8 Conclusion: the theory of sex differences is well worked out but
incompletely tested
The theory of sexual selection is at a more advanced stage than the theory of why sex
exists. The models, such as those of Fisher and Zahavi, may be correct, and some work
has been done to test them. The tests, however, are at an early stage. There are several
pieces of evidence for open-ended female choice in species with extravagant, costly
male characters. This suggests Darwin was right to explain those characters by female
choice. But less work has been done on the other crucial theoretical variable: the inheritance
of genetic quality.
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