Evolution__3rd_Edition

324 PART 3 / Adaptation and Natural Selection
Sex may be advantageous in
changeable environments ...
. . . such as in parasite–host
coevolution
possibly be changing rapidly enough. It is not difficult to believe that environments
might change fast enough to make sex advantageous every few hundred years, but
how could they be changing fast enough to make it advantageous every generation?
Remember, the environment would have to be changing so rapidly that an average sexual
female’s daughters must be twice as fit as those of an average asexual female. We
cannot take it for granted that ordinary environmental change will be enough. If we are
to explain the existence of sex by environmental change, we have some work to do.
One promising suggestion is that the coevolution between parasites and hosts may
generate fast enough environmental change to make sex advantageous in the short
term. The “environment” here, for the parasite, is the host’s resistance mechanism and,
for the host, the parasite’s method of penetrating its defenses. Several authors have suggested
that parasite–host coevolution may be important in the maintenance of sex, and
Hamilton is the best known of them.
The theory can be made more exact by a simple model. Some parasite–host relationships
have gene-for-gene matching systems such that one host genotype is adapted for
resisting one parasite genotype, another host genotype for another parasite genotype,
and so on. The best understood example is from wheat and parasitic rusts, and similar
selection may operate in the human HLA system (Section 8.6, p. 203).
The simplest genetic model for host–parasite coevolution is haploid, with two alleles
in each of the host and parasite species. One parasite allele is adapted to penetrate hosts
with one of the host alleles, the other parasite allele penetrates the other (Table 12.2).
Table 12.2
A simple model of gene-for-gene matching in a pair of host and parasite
species. The numbers in the table are the fitnesses of the genotypes.
(a) Fitness of parasite genotype in two types of host.
H 1
Host genotype
Parasite genotype
P1 0.9 1
P2 1 0.9
(b) Fitness of host genotype against two types of parasite.
P 1
Parasite genotype
Host genotype
H1 1 0.9
H2 0.9 1
H 2
P 2
..