170 years ago, almost to the day, Charles Darwin opened a book that was to change the world.

Thomas Robert Malthus' Essay on the Principle of Population was already well-known, indeed highly controversial, in 1838. But when Darwin read it in October that year, he saw something that no-one else had.

Malthus' argument was deceptively simple. Populations increase geometrically (1, 2, 4, 8, 16, etc) whereas the resources on which they depend increase arithmetically (1, 2, 3, 4, 5, etc.). In a moment of inspiration, Darwin recognised that the resulting "resource scarcity" selected some organisms (those better adapted to survival and reproduction) over others. Evolution by natural selection was born.

170 years later it is apparently over - at least for humans - or so reasoned Steve Jones in this newspaper last week. Fewer older fathers, more interbreeding between previously separated communities and greater resistance to nature's processes of selection means that, for humans, natural selection's engine has been switched off.

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