fixed for a given habitat or there may be facultative adjustment by the
foundress according to the precise conditions encountered. In all cases,
natural selection is expected to operate on the sex-allocation strategies as
long as there is heritable variation in sex allocation; chromosomal sex
determination, for example, constrains the evolution of unequal sex
allocation. An appreciation of Charnov’s (1982) sex-allocation theory is
best achieved with reference to the theories it unites and develops upon.
The following sections therefore largely discuss parasite sex determina-
tion in terms of classical sex-allocation theory. As will become clear,
although it is one of the best verified evolutionary theories and many of
the model systems used to test and further its development involve
parasites and parasitoids, the absence of a host–parasite perspective,
whether epidemiological (population level) or immunological (individual
level), is striking. Although this certainly reflects the bias of data collected
from parasite–invertebrate host systems, parasite–vertebrate host systems
do bring different parameters into play. Whilst these additions can be
considered within the sex-allocation framework, most notably that of
host quality, the conclusions reached can modify those based solely on
resource-allocation theory as it stands to date.Fisher’s Principle
Ronald Fisher (1930) developed the first major insight in the development
of the sex-ratio theory, presenting an explanation as to why equal invest-
ment in male and female progeny, hence an equal sex ratio, is commonly
observed in nature. His argument essentially shows that, if the cost of
producing females and males were equal, a sex ratio of 1 : 1 would
maximize the number of grand-offspring (F2 descendants) attributable
to an individual. The fitness of a sex-ratio genotype depends on its
frequency in a population, and any deviation from an equal investment in
the sexes (to either a male- or female-biased sex investment) will provide
a selective advantage for a mutant genotype that invests conversely in the
sexes, i.e. if a population consists of individuals that produce a male-
biased offspring sex ratio, females are the more valuable sex because they
will all mate, whereas the males will not. Therefore any individual that
produces more females will have a selective advantage and therefore the
population sex ratio will tend to move towards equal numbers of male
and females. Likewise, when the sex ratio is female-biased, production of
males is advantageous and once again the population will tend towards
an equal sex ratio. Since then, both game theory and population-genetic
models have extended Fisher’s (1930) insight. An equal sex ratio is the
only one that cannot be invaded by mutants and has thus been termed an
unbeatable strategy (Hamilton, 1967) or an evolutionarily stable strategy
(Maynard-Smith and Price, 1973). Furthermore, although an equal
population sex ratio could consist of a mixture of sex-ratio phenotypes
(i.e. at the extreme, some individuals producing just females and someParasite Sex Determination 203