Sex in a changing environment

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Some evolutionary biologists have a problem with sex, but mathematical models may help. The problem with sexual reproduction is its high 'cost' compared with other methods; so why is the practice so widespread? Writing in the current issue of Genetics, David Waxman and Joel Peck of the Centre for the Study of Evolution in BIOLS present the fruits of a mathematical investigation which show that sex can justify its costs when the environment is changing.

At first sight the most straightforward and economical way of passing on genes to future generations (and therefore achieving fitness in the evolutionary sense) is asexual cloning. Sexual processes and behaviour involve extra effort and decrease the potential number of copies of genes that can be passed on. If the organism has a good set of genes and the environment is constant, there seems to be little to be gained by shuffling the pack; Joel and David have already pointed out (in Nature in 1998) that this situation applies in hostile environments where there are genetic dangers in mating with incoming migrants from more sultry climes. In the harsh north, they say, it pays to be asexual and to keep yourself to yourself.

So, what are the advantages of sex that outweigh the extra cost? Using the best genetic information available, David and Joel modelled environments populated either by sexual hermaphrodites or by asexuals. The sexual hermaphrodites had to divide their resources between 'male effort' and 'female effort' while the asexuals could put all their eggs into one basket. They chose to use hermaphrodites because they most closely resemble asexuals and this type of comparison is particularly appropriate for many plant species. But, they say, their calculations also apply (more or less) to populations with separate sexes.

A most dramatic result emerging from their calculation is that changing the environment brings about an enormous increase in genetic variability. This happens in all populations, but the effect is significantly greater in those which reproduce sexually. Selection favours advantageous new mutations, but sexual reshuffling gives a better chance that these will spread through the population and come together in a variety of new combinations. Their model, unlike previous models, shows that even a low rate of environmental change can produce a large increase in genetic variability in the population. While a modest rate of change can be very damaging to an asexual population, it leaves a sexual population virtually unscathed.

So in a changing world, like the Red Queen we have to keep running to stay in the same place. Global warming promises 'interesting' changes and it looks as if sex is here to stay.

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Friday 19th November 1999

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