Evolution, 4th Edition

262 CHAPTER 10

mutation that causes females to make more sons will be selected against because

only females leave the fig to start the next generation. Here selection favors the

families that produce the largest numbers of females, and those are families that

have strongly female-biased sex ratios. This prediction differs from the theory of

equal sex ratios described earlier because selection is now acting on the family

rather than on the individual (see Chapter 12).

The prediction changes, however, if several females lay their eggs in the same

fig. As the number of females increases, natural selection favors them to produce

sex ratios that are closer and closer to equal numbers of sons and daughters. That

is because competition among individuals, rather than families, now occurs within

each fig. The theory of equal sex ratios described above then comes into play. A

mathematical model can be used to predict how the sex ratio favored by natural

selection changes depending on the number of females that lay their eggs in a fig.

This model’s prediction has been tested with data from natural populations of

fig wasps. The sex ratios produced by females largely agree with the theoretical

prediction (FIGURE 10.19). Recall that wasps have haplodiploid sex determination,

so females can change the sex ratio of their offspring behaviorally. By unknown

means, female wasps are somehow able to sense how many other females are in

the fig and produce an appropriate number of sons and daughters. The agreement

between the theory and experiment is a wonderful example of the predictive power

of evolutionary theory.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.18.ai Date 12-20-2016

Female

wasp

Male

wasp

Fig

inorescence

Female laying

eggs

Females emerge,

mate with the

surviving male,

collect pollen, and

y to other gs to

repeat the cycle.

A female wasp enters a g,

pollinates the owers, and

lays eggs in some of them.

Male wasps emerge rst,

and ght each other to

the death.

The young wasps

develop inside the

owers.

FIGURE 10.18 The fig wasp Tetrapus

costaricensis (inset) is highly sexually di-

morphic. Females lay their eggs inside

a fig. When the offspring hatch, the

males fight each other, and the winner

then mates with the females inside the

fig—many or all of whom are his sisters.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.19.ai Date 12-20 -2016 01-24-17

Sons (%)

1 2 3

Number of mothers

4 5 6

20

10

30

0

50

40

FIGURE 10.19 A mathematical model predicts that natural selection will favor female

fig wasps to produce few sons if only one female lays her eggs in a fig, but to pro-

duce increasingly equal numbers of sons and daughters as the number of females

that lay eggs in the fig increases (shown by the blue curve). In nature, the number of

females that enter a fig to lay eggs varies. When only a single female lays her eggs

in a fig, fewer than 25 percent of her offspring are male. With increasing numbers of

females, the sex ratio tends toward equal numbers of sons and daughters (red dots),

in good agreement with the theoretical prediction. (After [22].)

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