Evolution__3rd_Edition

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Full Contents xiii
11.4 The two senses of “unit of selection” are compatible: one
specifies the entity that generally shows phenotypic adaptations,
the other the entity whose frequency is generally adjusted by
natural selection 310
Summary Further reading Study and review questions
12. Adaptations in Sexual Reproduction 313
12.1 The existence of sex is an outstanding, unsolved problem in
evolutionary biology 314
12.1.1 Sex has a 50% cost 314
12.1.2 Sex is unlikely to be explained by genetic constraint 315
12.1.3 Sex can accelerate the rate of evolution 316
12.1.4 Is sex maintained by group selection? 318
12.2 There are two main theories in which sex may have a short-term
advantage 320
12.2.1 Sexual reproduction can enable females to reduce the
number of deleterious mutations in their offspring 320
12.2.2 The mutational theory predicts U > 1 321
12.2.3 Coevolution of parasites and hosts may produce rapid
environmental change 323
12.3 Conclusion: it is uncertain how sex is adaptive 327
12.4 The theory of sexual selection explains many differences between
males and females 327
12.4.1 Sexual characters are often apparently deleterious 327
12.4.2 Sexual selection acts by male competition and
female choice 328
12.4.3 Females may choose to pair with particular males 329
12.4.4 Females may prefer to pair with handicapped males,
because the male’s survival indicates his high quality 331
12.4.5 Female choice in most models of Fisher’s and Zahavi’s
theories is open ended, and this condition can be tested 332
12.4.6 Fisher’s theory requires heritable variation in the male
character, and Zahavi’s theory requires heritable variation
in fitness 333
12.4.7 Natural selection may work in conflicting ways on males
and females 335
12.4.8 Conclusion: the theory of sex differences is well worked out
but incompletely tested 336
12.5 The sex ratio is a well understood adaptation 337
12.5.1 Natural selection usually favors a 50 : 50 sex ratio 337
12.5.2 Sex ratios may be biased when either sons or daughters
disproportionately act as “helpers at the nest” 339
12.6 Different adaptations are understood in different levels of detail 341
Summary Further reading Study and review questions