Biology Reference
In-Depth Information
(a)
(b)
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
0
1
2
3
0
1
2
3
Generation
Generation
Fig. 10.1 Sex ratio evolution in the southern platyfish. When the sex ratio is perturbed away from equal numbers of
males and females, it quickly evolves back to this point. The perturbation is towards a female (a) or a male (b) biased
sex ratio. Different lines are different replicates. From Basalo (1994). Reprinted with permission from the University of
Chicago Press.
costly as one produced by a daughter. It would, therefore, pay the parents to concentrate
on making daughters. As the sex ratio swings towards a female bias, the expected
reproductive success of a son goes up until at a ratio of two females to every male an
average son produces twice the number of children produced by an average daughter. At
this point sons and daughters give exactly the same return per unit investment; a son
costs twice as much to make but yields twice the return. This means that when sons and
daughters cost different amounts to make, the stable strategy in evolution is for the
parent to invest equally in the two sexes and not to produce equal numbers. An example
to illustrate this point is Bob Metcalf 's (1980) study of the sex ratio of two species of
wasp: Polistes metricus and P. variatus . In the former females are smaller than males,
while in the latter they are similar in size. As predicted, the population sex ratio is biased
in P. metricus and not in P. variatus . In both species the investment ratio is 1:1.
More precisely,
equal investment
is favoured in the
two sexes
Sex allocation when relatives interact
Fisher's theory assumes that related individuals do not interact, either cooperatively or
competitively. If such interactions occur, then individuals can be favoured to bias their
offspring sex ratios, to reduce competition between relatives, or increase cooperation
(Box 10.2).
Local resource competition
Anne Clark (1978) found that the African bushbaby, Galago crassicaudatus , has a male-
biased investment ratio among its offspring. She pointed out that this could be explained
by the species' life history. As with most mammals, female Galago do not disperse as far
as males, and often end up competing both with their mother and with each other for
rich sources of food, such as gum and fruit trees in the mother's home range. This local
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