Evolution__3rd_Edition

340 PART 3 / Adaptation and Natural Selection
The sex ratio is a successfully
understood adaptation
Table 12.4
The remarkable adjustable sex ratio of the Seychelles warbler. (a) The sex ratio depends on
territory quality (based on 118 nests in 3 years). (b) Parents adjust their offspring sex ratio after
experimental translocation from low to high quality territories. Control pairs did not adjust
their sex ratio after translocation between similar quality territories. The sex ratios in (a) and
(b) were measured both molecularly, in eggs, and in nestlings. Territory quality was measured
by insect sampling. Simplified from data in Komdeur (1996).
(a) The sex ratio and territory quality.
Territory quality Number of sons Number of daughters Percent sons
Low 44 13 77
Medium 14 13 55
High 4 32 12.5
(b) Translocation experiment.
Before translocation After translocation
Sex ratio Sex ratio
Territory Territory Number
quality Female Male quality Female Male of pairs
Low 2 18 High 29 5 4
High 15 4 High 16 4 3
which parents adjust the sex ratio of their offspring? Molecular evidence suggests that
the sex ratio biases are already present when the eggs are laid. What is going on at earlier
stages, when the sex ratio bias is estabished, is unknown.
In summary, when one gender of offspring enhance parental reproduction, natural
selection favors parents who produce more offspring of that gender. When one offspring
gender reduces parental reproduction, natural selection favors parents who produce
less of that gender. Both these predictions have been successfully tested in the
Seychelles warbler.
Local resource competition, and local resource enhancement, are two examples in
which deviations from a 50 : 50 sex ratio have been successfully predicted. Some other
examples are even more detailed. For instance, quantitative differences in the sex ratio
produced by different ants nests can be predicted from the genetic relatedness within
each nest. However, the example of the Seychelles warbler is enough to illustrate how
the theory of adaptive sex ratios can be remarkably successful in explaining both the
normal 50 : 50 sex ratio and deviations from it. The basic theory has inspired various
different kinds of test. The theory is quantitative in its predictions; and the key variable
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