Evolution__3rd_Edition

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In host–parasite relations, the
fitness of a genotype may depend
on frequency
Snails and their parasite provide an
example
Figure 5.10
Frequency-dependent
selection. (a) Negative
frequency-dependent fitness
means that the fitness of a
genotype decreases as the
frequency of the genotype
increases. (b) Positive
frequency-dependent fitness
means that the fitness of a
genotype goes up as its
frequency increases. In general,
frequency dependence refers to
any case in which the graph is
anything other than flat. A flat
line, with fitness constant for all
genotype frequencies, means
that selection is not frequency
dependent.
CHAPTER 5 / The Theory of Natural Selection 127
random, the near lethality of SS means that disassortative mating will be unimportant;
however, the assumption that the genotypes have equal fertility may well be false.
5.13 The fitness of a genotype may depend on its frequency
The next interesting complication is to consider selection when the fitness of a genotype
depends on its frequency. In the models we have considered so far, the fitness of
a genotype (1, 1 − s, or whatever) was constant, regardless of whether the genotype
was rare or common. Now we consider the possibility that the fitness of a genotype
goes up or down as the genotype frequency increases in the population (Figure 5.10).
Frequency-dependent selection means that natural selection is acting and the fitnesses
of the genotypes vary with the frequency of the genotypes. The two main kinds are
negative frequency dependence, in which the fitness of a genotype goes down as its
frequency goes up, and positive frequency dependence, in which the fitness of a genotype
goes up as its frequency goes up.
Negative frequency dependence can arise in host–parasite interactions. For instance,
two genotypes of a host may differ in their ability to keep out two genotypes of a parasite.
This kind of set-up is like a lock and key. It is as if the two host genotypes are like
two different locks, and the two parasite genotypes are like two different keys. One of
the parasite keys fits one of the host locks and the other parasite key fits the other host
lock. Then, if one of the host genotypes is in high frequency, natural selection will favor
the parasite genotype that can penetrate that common kind of host. The result is that
a high frequency automatically brings a disadvantage to a host genotype, because it
creates an advantage for the kind of parasite than can exploit it. As the frequency of
a host genotypes increases, its fitness soon decreases.
Lively & Dybdahl (2000) recently described an example where the host is a snail,
Potamopyrgus antipodarum, which (as its name hints at) lives in New Zealand, in
freshwater habitats. The snail suffers from various parasites, of which a trematode
called Microphallus is the most important (it is a parasitic castrator). The authors distinguished
several strains (or clones) of the snail host and measured the frequency of
each clone. They then measured, in an experiment, the ability of Microphallus to infect
each clone. Figure 5.11 shows the infection rates achieved by parasites collected from
two lakes, when experimentally exposed to snails taken from one of the two lakes. The
local parasites infected the common clones better than the rare clones. It was the high
Fitness of AA
(a)
0 1
Frequency of genotype AA
Fitness of AA
(b)
0 1
Frequency of genotype AA