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Evolution__3rd_Edition

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..<br />

in Sexual<br />

Reproduction<br />

12Adaptations<br />

T his chapter mainly concentrates on three related research<br />

questions: the questions of how sex, sex differences, and<br />

sex ratios are adaptive. Biologists do not understand why<br />

sexual, as opposed to asexual, reproduction exists and we<br />

look at four hypotheses a genetic constraint, group selection,<br />

deleterious mutation, and parasite–host coevolution. Once<br />

sex had evolved, natural selection favored different sets of<br />

adaptations in males and females. The theory of sexual<br />

selection aims to explain male–female differences. We look at<br />

female choice for bizarre male traits, such as the peacock’s<br />

tail, and at evolutionary conflicts between the sexes. Thirdly,<br />

we turn to the sex ratio (the ratio of males to females). The<br />

sex ratio is one of the best understood adaptations, and we<br />

look at a recent study of a successfully predicted deviation<br />

from a 50 : 50 sex ratio in a bird species with helpers at the<br />

nest.

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