1.4 Sexual selection

Darwin noticed a puzzle: some traits seem to hurt survival. The peacock's tail is heavy, conspicuous, and metabolically expensive. Why would selection build it? His answer was a second engine of evolution.

Two kinds of competition

Sexual selection is selection for traits that improve mating success rather than survival. It operates through two mechanisms:

• Intrasexual selection — members of one sex compete directly with each other for access to mates (think of antlers and male-male combat).
• Intersexual selection — members of one sex choose mates based on certain traits (think of peahens choosing the showiest tail).

A trait can spread even if it shortens life, as long as it raises reproduction enough to compensate.

The handicap principle

Why should a costly ornament signal quality? Amotz Zahavi's handicap principle answers: a signal is reliable precisely because it is costly. Only a genuinely healthy, well-fed peacock can afford to grow and haul around an enormous tail. The cost guarantees the signal cannot be faked. Honest signals must be expensive.

Why this matters for the human mind

Many human traits — humor, music, generosity, creativity, and aspects of language — may be partly products of sexual selection, advertising the quality of the underlying brain. This reframes courtship displays not as cultural frills but as fitness indicators.

A grounding example

The long tail of the male widowbird was tested directly by Malte Andersson. He cut tails short on some males and glued the cut pieces onto others, making them artificially long. Males with experimentally lengthened tails attracted far more mates than shortened ones, even though the long tail offered no survival benefit. Female choice alone was driving the trait — sexual selection caught in the act.