Evolution__3rd_Edition

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Figure 12.7
Frequency changes of host and
parasite genotypes. (a) As H 2
becomes commoner, there
is selection to increase the
frequency of P 2 , which in turn
selects against H 2 , and
H 1 increases in frequency.
(b) Plotted against time, the
frequency of each genotype
oscillates cyclically.
The theory predicts cyclical changes
in gene associations
Frequency of P 2
P2
P 1
H 1
CHAPTER 12 / Adaptations in Sexual Reproduction 325
(a) (b)
Frequency of H 2
H 2
Frequency of H 1
Selection of this sort generates cyclic changes in gene frequency (Figure 12.7). As a
genotype increases in frequency, its fitness (after a time lag) decreases. If parasite genotype
P 1 is commoner, host genotype H 1 will be favored and will increase in frequency;
the fitness of P 1 then goes down as more hosts are resistant to it. Then, as P 2 becomes
commoner, the fitness of H 1 decreases. When H 1 becomes rarer, the frequency of P 1
will in turn increase again. Cycles of gene frequency are driven by corresponding cycles
of gene fitness.
We need a more complex, and probably more realistic, model to produce an advantage
for sex. Imagine now that resistance and counterresistance are controlled by two
loci. Again, a haploid model is simplest. With two loci and two alleles at each, there are
four haplotypes, AB, Ab, aB, and ab. There will be complimentary sets in the host and
parasite; if A H B H , A H b H , a H B H , and a H b H are the host genotypes, then we could write the
parasite genotypes as A P B P , A P b P , a P B P , and a P b P . A H B H and A P B P are analogous to H 1
and P 1 in the previous model. As a concrete example, A H and B H might control two cell
surface molecules used by the parasite to penetrate the host. Hosts with allele A H are
efficiently penetrated by parasites with a P , but not A P ; B H hosts are penetrated by b P
parasites but not B P . b P parasites are therefore favored if the hosts are mainly B H ; likewise,
b H is a gene for resistance to b P parasites. Haplotype frequencies at both loci will
oscillate for the same reasons as did H 1 and P 1 in the simpler model. In the sexual parasites
and the sexual hosts, alleles at the two loci can recombine, whereas in asexual parasites
and hosts, they cannot. A third locus determines whether reproduction is sexual or
asexual.
How can sex be advantageous? As the frequencies of the four haplotypes oscillate
through time, there will be some chance that any one of them will be lost at the trough
of its frequency cycle. Suppose, for example, that the frequency of A H b H is driven so low
in one cycle that it is lost from both the asexual and sexual populations. In the asexual
population it has been lost forever, whereas in the sexual population it will be recreated
by recombination between the other three genotypes. As the frequency of the parasites
that specialize in attacking A H b H hosts increase again, sexual reproduction will be an
advantage as it is more often associated with the resistant genotype. Thus sex has an
advantage because it maintains in reserve an ability to recreate multilocus genotypes
that have been disadvantageous, but may be needed again. The cycles of host–parasite
coevolution are exactly the kind of circumstances in which this ability is favored. If the
Time