Biology Reference
In-Depth Information
BOX 10.2 SEX RATIOS WHEN RELATIVES INTERACT
The fitness of a female will be equal to her fitness through daughters plus her
fitness through sons, which will be given by the equation:
fitness = (number of daughters produced multiplied by the fitness of each
daughter) + (number of sons produced multiplied by the fitness of each son)
Consider the case of where daughters compete for resources, and so the mean
fitness of each daughter decreases with the number of daughters produced.
Such local resource competition would favour a male-biased sex ratio, to lessen
competition between daughters and, hence, increase the fitness of each
daughter produced (Fig. B10.2.1a). As the sex ratio becomes more male biased,
it will decrease the average number of mates obtained by each male, and hence
decrease the fitness return of producing sons. The evolutionarily stable (ES) sex
ratio would be that at which these two forces cancels out.
In contrast, if daughters cooperate, then the average fitness of daughters
increases with the number of daughters produced. Such local resource
enhancement would favour a female-biased sex ratio, to increase cooperation
between daughters and, hence, increase the fitness of each daughter produced (Fig.
B10.2.1b). As the sex ratio becomes more female biased, it will increase the average
number of mates obtained by each male, and hence increase the fitness return of
producing sons. The ES sex ratio would be that at which these two forces cancels out.
(a)
Local resource competition
Moving from an unbiased to a male-biased sex ratio
reduces LRC between females
(b) Local resource enhancement
Moving from an unbiased to a female-biased sex ratio
increases the amount of cooperation between females
Fig. B10.2.1
Selection for biased sex ratios when relatives interact. (a) If sisters
compete for resources, then a male-biased sex ratio is favoured to reduce competition
between sisters. (b) If sisters cooperate with each other, then a female-biased sex ratio is
favoured to facilitate cooperation. Males are blue, females are pink. From West (2009).
