Evolution, 4th Edition

All AbouT SEx 253

total lifetime fitness. That optimal value of the trait shifts if the strength of natu-

ral or sexual selection changes. Increased predation, for example, can select for

decreased male traits. Male crickets (Teleogryllus oceanicus) scrape together their

wings to make a song that attracts females. The song also attracts a fly (Ormia

ochracea) that parasitizes the crickets. After the arrival of the fly on the island of

Kauai in the Hawaiian Islands, the crickets evolved modified wings that do not

produce song after only 20 generations [51].

Attracting females is one way that males increase their mating success. Darwin

pointed out there is a second way: males can directly interfere with other males’

access to females. We will explore these two modes of sexual selection shortly. But

before doing that, we will first consider the basic question of why sexual selection

acts more frequently on males than females.

Why are males sexually selected?

Widowbirds and túngara frogs illustrate a very general pattern. In most cases, it

is the male’s secondary sexual traits that are exaggerated. This shows that sexual

selection is more common and intense on males than females. Why should that be?

The solution to this puzzle comes from considering the fundamental differences

in the reproductive biology of males and females [3]. Because a male makes a large

number of sperm, he is often capable of fertilizing a large number of females. A

trait that increases the number of mates that he can acquire is favored by selection

and so will spread. This creates the opportunity for selection on traits that increase

male mating success. In contrast, a female can often fertilize all her eggs with a

single mating. A trait that increases the number of mates she acquires therefore

has no fitness advantage.

This logic is called Bateman’s principle in honor of the geneticist who dem-

onstrated it experimentally with Drosophila melanogaster [7]. Bateman put several

males and females together in bottles for several days. These individuals carried

genetic markers that allowed Bateman to determine how many of the offspring

that later hatched were produced by each parent. He concluded that the num-

ber of offspring sired by a male increased in proportion to the number of females

he mated. In contrast, the number of offspring produced by a female did not

increase with the number of males she mated. Bateman also observed that there

was greater variance in reproductive success among males than among females.

That suggests there is greater opportunity for sexual selection on males than on

females.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.07.ai Date 12-20-2016 01-30-17

Mean female mating preference

Mean male trait

Survival

Mating

success

Lifetime

tness

FIGURE 10.7 Selection favors mating displays that maximize

a male’s lifetime fitness, which is a compromise between what

maximizes survival and what maximizes mating success. In this

schematic, male survival is maximized by a tail length that optimizes

aerodynamics, resulting in stabilizing selection for tails of interme-

diate length. Male mating success depends on the mean female

mating preference (on the x-axis). If most females prefer long tails,

then males with long tails will have the greatest mating success (at

right). Tails will evolve to an equilibrium that is longer than what

maximizes survival. If females prefer very short tails (at left), tails will

evolve to a length shorter than what maximizes survival. (After [27].)

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