Evolution__3rd_Edition

342 PART 3 / Adaptation and Natural Selection
Summary
1 For many characters, it is not obvious how (or
whether) they are adaptive.
2 Adaptation can be studied by comparing the
observed form of an organ with a theoretical prediction,
by experimentally altering the organ, and by
comparing the form of the organ in many species.
3 Sex has a 50% fitness disadvantage relative to
asexual reproduction.
4 Sexually reproducing populations will evolve faster
than a set of asexual clones, provided that the rate of
favorable mutation is high enough.
5 The taxonomic distribution of asexual reproduction
suggests that asexual forms have a higher extinction
rate than sexual forms. However, it is generally
doubted that sex is maintained by group selection.
6 Two modern theories of why sex exists propose that
it is favored by: (i) the large numbers of deleterious
mutations, which are more efficiently removed by sexual
than asexual reproduction; and (ii) the coevolutionary
arms race of parasites and hosts. The problem
of why sex exists has not been finally solved.
7 Males in many species have bizarre and deleterious
secondary sexual characters; the peacock’s train is an
example.
8 Darwin explained the evolution of strange secondary
sex characters by sexual selection: the characters
reduce their bearers’ survival, but increase their success
in reproduction; sexual selection in most species
works by male competition and by female choice.
Further reading
9 The greater sexual dimorphism of polygynous species
than monogamous species suggests the importance of
sexual selection.
10 The preference of females for males with deleterious
characters is theoretically puzzling. It may
be explained by Fisher’s theory, in which deleterious
characters were formerly advantageous and are maintained
by majority preference, or by a Zahavi’s handicap
theory, in which the costly character indicates
superior genetic quality.
11 There can be conflicting forces of selection on
males and on females. The conflict depends on the
mating system, and can be studied by experimentally
altering the mating system and allowing the population
to evolve to a new adaptive state.
12 The sex ratio is usually 50 : 50 because the reproductive
success of all the males in a population must
equal the reproductive success of all the females. If the
population sex ratio deviates from 50 : 50, natural
selection favors individuals that produce more offspring
of the rarer sex.
13 The theory of sex ratio has correctly predicted
when the ratio should differ from 50 : 50. It has been
tested experimentally in the case of “helpers at the
nest” in the Seychelles warbler.
14 The functions of sex, sexual selection, and sex ratio
are three of the most important areas of research on
adaptation. They have reached different stages of
theoretical advance.
On sex, my popular book (Ridley 2001) explains the basic problem and Kondrashov’s
theory. The Fisher–Muller theory can be traced through Barton & Charlesworth (1998),
Burt (2000), and Otto & Lenormand (2002). Butlin (2002) reviews ancient asexuals,
and the Meselsohn test. Rice (2002) reviews experimental work on the evolution of sex.
For the mutational theory, two thorough reviews of the research on U are by
Keightley & Eyre-Walker (1999) and Lynch et al. (1999). (Note that Lynch uses U for
the genomic rate, Keightley for the gametic rate. The latter is half the former.) These
can now be updated by the exchange between Kondrashov and Eyre-Walker &
Keightley in Trends in Genetics (2001), vol. 17, pp. 75–8, and by Shabalina et al. (2001).
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