Biology Reference
In-Depth Information
behaviour. To understand interesting patterns of sex allocation, it is first necessary to
explain why equal investment in males and females is the null model.
Fisher's theory of equal investment
If one male can fertilize the eggs of dozens of females why not produce a sex ratio of, say,
one male for every 20 females? With this ratio the reproductive success of the population
would be higher than with a 1:1 ratio since there would be more eggs around to fertilize.
Yet in nature the ratio is often very close to 1:1, even when males do nothing but fertilize
the female. As we saw in Chapter 1, the adaptive value of traits should not be viewed as
being 'for the good of the population', but 'for the good of the individual' or, more
precisely, 'for the good of the gene' that controls that trait. Darwin struggled with why a
1:1 sex ratio should be favoured, but a clear answer was provided by R.A. Fisher (1930).
Suppose a population contained 20 females for every male. Every male has 20 times the
expected reproductive success of a female (because there are on average 20 mates per
male) and, therefore, a parent whose children are exclusively sons can expect to have
almost 20 times the number of grandchildren produced by a parent with mainly female
offspring. A female-biased sex ratio is, therefore, not evolutionarily stable because a gene
which causes parents to bias the sex ratio of their offspring towards males would rapidly
spread, and the sex ratio will gradually shift towards a greater proportion of males than
the initial 1 in 20. But now imagine the converse. If males are 20 times as common as
females a parent producing only daughters will be at an advantage. Since one sperm
fertilizes each egg, only one in every 20 males can contribute genes to any individual
offspring; therefore, females have 20 times the average reproductive success of a male. So
a male-biased sex ratio is not stable either. The conclusion is that the rarer sex always has
an advantage, and parents which concentrate on producing offspring of the rare sex will,
therefore, be favoured by selection. Only when the sex ratio is exactly 1:1 will the expected
success of a male and a female be equal and the population sex ratio stable. Even a tiny bias
favours the rarer sex: in a population of 51 females and 49 males where each female has
one child, an average male has 51/49 children. This
average
value is the same whether one
male does most of the fathering or whether fatherhood is spread equally among the males.
One way to test Fisher's theory is by perturbing the sex ratio away from 1:1 and then
examining whether it evolves back towards this point. Alexandra Basolo (1994) did
this, taking advantage of the unusual sex determination in the southern platyfish
Xiphorus maculatus
. In this species, sex is determined by a single locus with three sex
alleles, with three female (WX, WY, XX) and two male (YY, XY) genotypes. She showed
that if the relative frequency of the different alleles is varied, to set up populations with
biased sex ratios, then selection favours the rarer sex, as predicted, and quickly moves
the sex ratio back to a 1:1 ratio (Fig. 10.1).
The above argument, that the sex ratio should be 1:1, can be refined by re-phrasing it
in terms of resources invested. In the above discussion, we have implicitly assumed that
sons and daughters are equally costly to produce. However, suppose sons are twice as
costly as daughters to produce because, for example, they are twice as big and need twice
as much food during development. When the sex ratio is 1:1 a son has the same average
number of children as a daughter. But since sons are twice as costly to make they are a
bad investment for a parent: each of its grandchildren produced by a son is twice as
All else being
equal, a 1:1 sex
ratio will be
favoured by
natural selection
If the sex ratio is
perturbed from
1:1, it will evolve
back to this point
