Evolution__3rd_Edition

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Genetic cycles in snails are
consistent with the theory
CHAPTER 12 / Adaptations in Sexual Reproduction 327
environmental change is more erratic, or open ended, such that once a genotype has
been eliminated it is unlikely to be useful again, sex is not advantageous.
The parasitic theory has not inspired such a large research program as the mutational
theory. The parasitic theory is plauible, not least because parasite–host coevolution is
widespread in life. But we do not know whether the specific prediction of the theory a
cycles in the associations between resistance genes in hosts a is correct. The most direct
test so far is by Dybdahl & Lively (1998), which is part of a long-term study of sex in
New Zealand snails by Curt Lively, where several genetic clones of the aquatic snail
Potamopyrgus antipodarum have been distinguished. The snail exists in both sexual and
asexual forms. The main parasite of the snails in a trematode (Microphallus) which, as
its name suggests, is a parasitic castrator.
The clones underwent frequency cycles in the 1990s (Figure 12.8). Moreover,
Dybdahl and Lively showed experimentally that the parasites were best able to infect
the snail clone that had the highest frequency in the previous year. This suggests that
the parasites are adapting to penetrate the commonest host genotypes. (In Section 5.13,
p. 127, we saw evidence for frequency-dependent selection in this system.) The results
are all consistent with the parasite theory of sex. However, further work is needed to
show that the genetic cycles are of the right type to explain the existence of sex.
12.3 Conclusion: it is uncertain how sex is adaptive
Both deleterious mutation and parasite–host coevolution are reasonable theories of
sex, but it has not been conclusively shown for either of them that they really maintain
sex in nature. They are afloat in evolutionary biology today as stimulating hypotheses
that are inspiring much research. They are not mutually exclusive ideas, and both factors
could turn out to be contributing to the selective advantage of sex. Other hypotheses
exist too, and some of them are highly ingenious.
Today, the question of why sex exists remains an “outstanding puzzle.” Evolutionary
biologists are not confident the question has been satisfactorily answered. Maybe we
need some radically new idea that has not yet been put forward or lies unappreciated.
Alternatively, the gist of the answer may lie in the theories we have discussed and the
problem is more one of showing how they apply in nature. Whatever the answer turns
out to be, it is likely to tell us something about the safety, or otherwise, of cloning technology
(Box 12.1).
12.4 The theory of sexual selection explains many
differences between males and females
12.4.1 Sexual characters are often apparently deleterious
For the most part, the characters of organisms are adaptive: they increase the organisms’
chances of surviving to reproduce. However, there are some characters that do
the opposite, and (as Darwin was well aware) natural selection does not explain why