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Evolution__3rd_Edition

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336 PART 3 / Adaptation and Natural Selection<br />

A hypothesis about conflicting<br />

selection pressures on males and<br />

females ...<br />

. . . is supported by ingenious<br />

experimental results<br />

mating with him, because that male will then fertilize more eggs. Male fruitflies seem to<br />

transfer chemicals with their sperm that act as hormones in the female and accelerate<br />

egg production. The accelerated egg production may not be in the interest of the<br />

female. Her optimal rate of egg production will be some trade-off between her survival<br />

and reproduction. If she produces extra eggs now, it will be by allocating less energy to<br />

maintaining her body. Survival will decrease, and her total lifetime output of eggs will<br />

also decrease. The male gains extra eggs in the short term, at the cost to the female of<br />

reduced lifetime fitness. The cost is not paid by the male, because the later eggs that a<br />

female loses because she dies younger would have been fertilized by another male.<br />

Natural selection on females favor resistance to the male techniques of accelerating egg<br />

production. Females may evolve counterhormones or other methods of restoring the<br />

optimal egg production rate.<br />

The selective forces differ if there is lifetime monogamy. Now the “interests” of the<br />

male and female are identical. If a male causes a female to accelerate her short-term egg<br />

production, but reduces her lifetime fitness, his own fitness will go down by the same<br />

amount.<br />

Holland & Rice (1999) tested this reasoning experimentally with fruitflies. Fruitflies<br />

usually mate with several members of the opposite gender. Holland and Rice allowed<br />

some fruitflies to mate normally, as controls. In their experimental lines, they imposed<br />

monogamy by selecting at random one individual to be the only mate of another individual<br />

(of the opposite gender). They bred lines of these experimentally monogamous<br />

fruitflies for 47 generations.<br />

As predicted, the male fruitflies in the monogamous lines evolved to be less harmful<br />

to their mates and females evolved to be less resistant to males. Figure 12.11 shows some<br />

of Holland and Rice’s results. After 47 generations of monogamy, males had evolved<br />

lower rates of courting (Figure 12.11a). Also, the total reproduction output per female<br />

increased under monogamy (Figure 12.11b). This result suggests that the conflict<br />

between males and females in normal fruitflies is reducing the fitness of the average<br />

individual by 20% or so. The main interest of the results is to illustrate the evolutioanry<br />

theory of intersexual conflict, but the results have other interests too. Intersexual<br />

conflict is a further factor that can be added to the list of causes of adaptive imperfection<br />

in Chapter 10. Sexual selection, including intersexual conflict, may also underlie<br />

the relatively rapid evolution of genes that are expressed in the reproductive system a a<br />

phenomenon noted in Sections 7.8.2 (p. 182) and 14.12 (p. 417).<br />

12.4.8 Conclusion: the theory of sex differences is well worked out but<br />

incompletely tested<br />

The theory of sexual selection is at a more advanced stage than the theory of why sex<br />

exists. The models, such as those of Fisher and Zahavi, may be correct, and some work<br />

has been done to test them. The tests, however, are at an early stage. There are several<br />

pieces of evidence for open-ended female choice in species with extravagant, costly<br />

male characters. This suggests Darwin was right to explain those characters by female<br />

choice. But less work has been done on the other crucial theoretical variable: the inheritance<br />

of genetic quality.<br />

..

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