Evolution, 4th Edition

All AbouT SEx 251

Sexual Selection

It is easy to understand how some traits have evolved to become sexually dimorphic.

An individual without gonads will leave no genes to the next generation. Gonads

and genitalia are called primary sexual traits. But Darwin pointed out that other

kinds of traits, such as the male peacock’s spectacular train of feathers, are more dif-

ficult to explain. These are secondary sexual traits, which differ between the sexes

but do not play a direct role in reproduction. A male peacock’s spectacular feathers

make flight more difficult and attract predators, so we might expect them never to

have evolved by natural selection. Interestingly, secondary sexual traits are often

among the most rapidly evolving phenotypic characters. In many groups of animals

and plants, they are the only traits that can reliably distinguish species.

Darwin reasoned that secondary sexual traits must evolve by something other

than selection for survival and the production of gametes. He made several key

observations. Traits with extreme sexual dimorphism tend to be exaggerated much

more often in males than in females. Often these traits are not expressed in imma-

ture males. Most important, these traits are used by males in the mating season

when they interact aggressively with other males and court females.

These observations lead Darwin to propose that these traits have evolved by

sexual selection, which is selection caused by competition for mates among indi-

viduals of the same sex. A male that prevents other males from mating will leave

more copies of his genes to the next generation than they will. Likewise, a male

that attracts many females to mate with him will be more genetically successful

than a male that attracts few or none. Sexual selection can cause the evolution of

traits that decrease survival if the reproductive advantage they produce compen-

sates for that cost. In short, a trait can evolve by sexual selection if it increases a

male’s overall fitness, even if it decreases survival.

Sexual selection is one of Darwin’s most ingenious ideas. He developed it

without the benefit of any data showing whether these traits in fact do decrease

survival but increase male reproductive success. Was Darwin right? Many studies

since his time have verified that secondary sexual traits have exactly these effects.

A simple and elegant experiment with the long-tailed widowbird (Euplectes

progne) shows that the male’s extremely long tail attracts females [1]. Males estab-

lish territories on the African savannah, and females nest on the territory of the

male that attracts them. Males were captured on their territories and divided into

four groups. The tail feathers of the first group were cut to half of their original

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.04.ai Date 11-02-2016

FIGURE 10.4 Anglerfishes (suborder Ceratioidei) have

extreme sexual dimorphism. When a male finds a female,

he bites and attaches himself permanently to her. His

body fuses with hers and his organs largely degenerate

except for his testes, which grow to fill most of his body.

This female has two males attached to her (indicated by the

arrows).

10_EVOL4E_CH10.indd 251 3/22/17 2:25 PM