This is a very useful introduction to Charles Darwin’s ‘other’ great idea: sexual selection.
Nowadays, we tend to think of sexual selection as a special sub-category of natural selection. But Darwin was careful to keep the two separate: while natural selection concerned the struggle for survival, sexual selection concerned the struggle for mates. It’s an important distinction. To leave offspring, individuals who reproduce sexually need to survive long enough to mate, and they actually need to mate with one or more other individuals. Darwin’s theory of sexual selection explains the myriad physical and behavioural adaptations species have evolved to assist individuals in their struggles to mate.
It’s long been my personal hunch that sexual selection is far more important in the evolution of new species than it’s generally given credit for. Be that is it may (or may not), this short book is a very nice introduction to the subject.