Evolution__3rd_Edition

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Courtship bouts
Net reproductive rate
(a)
100
75
50
25
0
(b)
20
15
10
5
0
A B
Replicate
Group selection favors a femalebiased
sex ratio
Monogamy
Control
Monogamy
Control
CHAPTER 12 / Adaptations in Sexual Reproduction 337
Figure 12.11
Experimentally imposed monogamy causes the evolution of
reduced reproductive conflict in fruitflies. (a) Individual males
were put with individual females and the amount of courtship
behavior was measured. Courtship rates were reduced in the
monogamous fruitflies. Results are shown for two replicate lines
(A and B) of males sampled from the experimental line after
45 generations of monogamy and from a control line. (b) The total
reproductive output per female increased (this was measured as
the number of mature progeny per female). It is shown here for
the experimental and control lines for the final three generations
of the 47-generation experiment. Redrawn, by permission of the
publisher, from Holland & Rice (1999).
Many other consequences of sexual selection are also being investigated. One hot
topic is the experimental study of intersexual conflict. The evolutionary forces of intersexual
conflict depend on the mating system. By experimentally altering the mating
system from polygamy to monogamy, for example, it is possible to produce a predictable
reduction in male–female conflict over evolutionary time.
12.5 The sex ratio is a well understood adaptation
12.5.1 Natural selection usually favors a 50 : 50 sex ratio
The sex ratio is one of the most successfully understood adaptations. The main idea is
again due to Fisher. In most species, the sex ratio at the zygote stage is about 50 : 50.
Fisher explained the 50 : 50 sex ratio as an equilibrium point: if a population ever comes
to deviate from it, natural selection will drive it back.
At first glance, the 50 : 50 sex ratio might seem inefficient. Most species do not have
parental care and are not monogamous, meaning one male can fertilize several females.
It would be more efficient for the species to produce more females than males. The extra
males are not needed to fertilize the females of the species and do not increase its reproductive
rate. (This is another “group selection” argument, see Section 11.2.5, p. 301.)
However, imagine what would happen to a population with a persistently female-