Evolution, 4th Edition

All AbouT SEx 261

If the sex ratio can evolve, why does natural selection seem to favor equal num-

bers of males and females? At first glance, that situation seems odd: a population

can grow faster if there are more females than males, since only females produce

offspring. The answer results from the simple fact that every individual has one

father and one mother. As a result, whenever there are fewer males than females, a

male will on average leave more offspring to the next generation than will a female

(FIGURE 10.17). Any mutation that causes females to produce more sons there-

fore has an evolutionary advantage and will spread. The advantage disappears

when the sex ratio reaches an equal number of males and females. If a population

has an excess of males rather than females, a mutation causing mothers to make

more daughters is favored. In short, natural selection pushes the sex ratio to evolve

toward equal numbers of males and females.

The equal sex ratio predicted by this theory is seen in many animals and plants,

but there are spectacular exceptions. These actually give us a deeper understand-

ing of sex ratios and, more generally, of how natural selection works. Much of our

understanding of this topic was developed by William D. Hamilton, who drew

attention to the remarkable biology of fig wasps [20]. In these minute insects, the life

cycle is tightly bound to the fig trees they pollinate (FIGURE 10.18). A fig is a strange

inflorescence that resembles a sunflower that has closed in on itself. The interior

is hollow, and lined with flowers. When the flowers are mature, a female fig wasp

enters the fig through a small opening. There she does two things: she lays her eggs

in the flowers, and she pollinates the flowers with pollen that she carried into the fig

with her. With those missions accomplished, the female dies.

Some weeks later, a first wave of offspring hatch, and they are all sons. Male fig

wasps are the stuff of science fiction. They have no wings or digestive system. Two

organs, however, are greatly enlarged: their jaws and their testes. The males literally

fight to the death. The next wave of eggs to hatch are all female, and the lone surviv-

ing male mates with all of them. The females then exit from the fig, picking up pol-

len on their way out. They begin another life cycle, leaving the male behind to die.

In these fig wasps, many more females than males are born. What could explain

that bias? The answer comes from considering the relationship among the indi-

viduals inside a fig: they are typically a single family of brothers and sisters. A

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.17.ai Date 12-15-2016

Generation 1 Generation 2

Later generations Generation 3

FIGURE 10.17 Selection usually favors a sex ratio with nearly

equal numbers of males and females. In this example, females

initially produce more daughters than sons, so the sex ratio is

biased toward an excess of females. Generation 1: a mutation

occurs in one female (red) that causes her to produce only

sons. Generation 2: the mutation is now carried by the son of

the female that carried the mutation in generation 1. There

are only three males, and each of them will be the father to

one-third of the individuals in the next generation. Genera-

tion 3: the mutation has increased in frequency because each

male in the previous generation is father to one-third of the

population, and each of his offspring has a probability of 1/2

of inheriting the mutation. Later generations: the mutation will

continue to spread until there are equal numbers of males

and females. At that equilibrium, the mutation will be more

common in males because females with the mutation pro-

duce only sons, while males with the mutation produce both

sons and daughters that carry the mutation.

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