What mechanism did Darwin propose in 'The Descent of Man' as key to human evolution?

In The Descent of Man, Charles Darwin proposed sexual selection as a key mechanism in human evolution. He described it as a form of selection connected to reproduction: traits could spread because they helped individuals attract mates or compete with rivals, even when those traits did not improve survival.

Darwin used examples such as elaborate plumage, courtship displays, weapons, and differences between males and females. He argued that mate choice and competition could shape bodies, behavior, aesthetic preferences, and some human differences. The mechanism complemented natural selection rather than replacing it.

The Descent of Man was first published in 1871. Darwin had developed his ideas about sexual selection since the 1850s, but he gave them much fuller treatment in this book than in On the Origin of Species. A common mix-up is to treat sexual selection as unrelated to evolution by natural selection; Darwin presented it as a specialized evolutionary process driven by reproductive success.