Next year marks the 150th anniversary of the publication of Charles Darwin's "On the Origin of Species". Darwin's insight was to recognize the workings of the process of natural selection, the cornerstone of the theory of evolution. During natural selection, an organism's favourable characteristics become more common in successive generations. This is because a successful organism is much more likely to breed and multiply than a less successful one.

This is the origin of the term 'survival of the fittest', which was coined in the 1860s by the British sociological theorist Herbert Spencer after he read Darwin's book. Now, though, a new study on wild Scottish deer carried out by Edinburgh University's Loeske Kruuk and her colleagues suggests the concept of 'survival of the fittest' is something of a myth. Why? Because scientists had previously overlooked the fact that among many animals there are two sexes, and what works well genetically for one sex won't necessarily wash with the other.

The researchers made the discovery after several years studying red deer on the Isle of Rum. During their analysis, the team paternity-tested the deer to work out who was related to whom, and then logged how many offspring each animal produced and how many young were born to those offspring. The results showed that the daughters of stags who had fathered large numbers of young (of both sexes) tended to be relatively unsuccessful in the deer dating stakes and produced fewer calves. Sons of successful stags, on the other hoof, were unaffected.

But 'survival of the fittest' suggests the successful males should have successful daughters as well as successful sons. Why didn't they? It's because the genes that single out a successful stag include those for large stature, impressive antlers and the ability to roar the loudest: not much use to a female. "So the idea that some genes are better than others is too simplistic," says Kruuk. "Instead, it depends on the sex of the individual animal carrying the genes."

Is the same true for successful females? Do they have wet blankets for sons? "It's slightly more complicated," says Kruuk, "because females not only contribute their genes to their offspring's later performance, they also provide maternal care. So a good-quality female, although she may be passing on genes that are detrimental in a son, will also provide very high-quality maternal care. These two effects seem to balance one another out."

Does this leave 'survival of the fittest' for dead? perhaps not, because the effect seen in the deer has an unexpected spin-off: it contributes to genetic variation, which makes the population fitter as a whole since a larger gene pool means a better ability to cope with life's challenges down the track.