The Red Queen effect: in evolution as a species evolves the species it
is in competition with also evolves to keep up with each improvement
it makes. Also called evolutionary arms races.
The name comes from the animated Red Queen chess piece in Lewis Carroll's "Alice Through the Looking Glass". The Queen is forced to run continually but never moves because everything else in the landscape is also running and so keeps pace with her.
Ridley argues the main race is not between animals and there predators (eg rabbits and foxes) but between species and their parasites particularly bacteria and virus diseases. The other great race is between individuals within the same species.
The first part of the book deals with why sex exists, why it evolved (or failed to evolve out of existence), why there are two sexes and why these give two genders. (This is interesting but I'm not sure I was convinced on a first reading).
The second deals with the evolution of human intelligence over the last million years. If I have understood correctly, the book argues that intelligence evolved in humans primarily due to mate selection, rather than the need to outwit animals. Ie competition for mates between men and between women. In many animals mate selection strongly drives the species to evolve in a particular direction. Ridley deals with the classic example, the Peacock's tail and many other examples.
When asked what people look for in ideal mates the common answers are women look for witty men and men look for beautiful (and young) women. Ridley argues that female homid apes with large heads and other features resemble young female apes and so (other things being equal) would be more likely to attract good mates. Thus there is an evolutionary force based on male mate selection which drove early homid apes to become more human (bigger heads etc). Also there is was a complementary mate selection from females who preferred clever men (ie who used the bigger brains inherited from their mothers).
1993, Penguin Books ISBN 0140167722
W.B.Langdon 23 March 2001 (last update 25 Apr 2009)