just males), there is weak selection for all individuals to invest equally in
males and females and population-genetic models have shown that, no
matter how many alleles are present at a sex-determining locus, an even
sex ratio is the only one that is stable to novel mutations that affect sex
determination and that, if the equilibrium sex ratio is not 1 : 1, only
mutations that render the equilibrium closer to 1 : 1 will spread – the
evolutionary genetic stability of the even sex ratio (Eshel and Feldman,
1982).
For Fisher’s (1930) principle to apply, several assumptions must be
met: (i) panmixia; (ii) fitness returns from increased investment in either
sex must be linear (both sexes benefit to the same degree from increased
investment); (iii) the mother or foundress must be equally related to both
sex offspring; and (iv) there must be heritable variation in the sex ratio,
allowing evolution to an equilibrium or locally optimal strategy. The
majority of work on sex ratio has considered what happens when these
assumptions are not met.Habitat Population Substructure – Local Mate Competition
Violation of the assumption of panmixia is common in organisms whose
population is highly substructured, such as that resulting from low
dispersal or habitat isolation. This is particularly relevant to parasites,
whose populations are necessarily structured according to their hosts.
When mating is not random and there is a certain degree of inbreeding, an
equal sex ratio is not predicted to be the optimal investment and a
female-biased sex ratio is predicted. At the extreme, when an individual
finds itself alone in an isolated habitat, investing equally in male and
female function is a suboptimal use of resources. This is because males
can fertilize several females and, as all males from a single mother will be
brothers, by producing just enough males to fertilize the females, the
foundress decreases competition among brothers for mates and increases
the number of daughters to be fertilized and produce the next generation
and resources are not wasted on unnecessary males. This principle is
known as local mate competition (LMC) and was originally developed to
explain the female-biased sex ratios observed in many insect species,
notably arrhenotokous parasitoids (Hamilton, 1967): when a small
number of foundresses produce offspring that are to mate with each other,
the optimum proportion of sons (sex ratio),r, is:
r= (n−1)/2n
wherenis the average number of foundresses contributing to the mating
pool (Fig. 10.1)
Many parasitoids are haplodiploid (where the males are haploid and
females diploid) and the sex of offspring is determined at conception.
This enables flexibility in the offspring or brood sex ratio, and the ecology
of parasitoids frequently results in competition among brothers for mates,204 R.E.L. Paul