From the Times Online:
The fight against malaria could eventually be transformed by releasing into disease-ridden areas genetically modified mosquitoes that cannot transmit the infection.
Scientists in America have engineered a species of mosquito which is resistant to the malaria infection. Its ability to block the infection suggests that it could come to dominate mosquito populations if released into the wild.
Environmentalists are objecting, because it involves replacing an existing species with a genetically modified one, and no one knows the effect of introducing a new gene into the wild.
The scientists found that while these modified mosquitoes have no advantage when feeding on uninfected blood, they are much better adapted when blood carries the malaria parasite. Infection with the Plasmodium organism does not kill normal mosquitoes, but it does reduce breeding efficiency.
The GM mosquitoes did not suffer from this and over nine generations (several months) they grew in number to make up 70 per cent of a laboratory population, compared with 50 per cent at the outset.
Now, without working the math, it seems to me the population of GM mosquitos will increase up to a point. Above that point, their numbers will have a significant impact on the spread of malaria. This will mean there's less malaria-infected blood to feed on, and the fitness advantage begins to fade.
I can see four possible outcomes:
• Malaria goes extinct, and so do the modified mosquitos, having lost their fitness advantage. The mosquito population returns to normal.
• Malaria goes extinct, but so do the wild-type mosquitos. The modified population is less fit, but there are no wild-type mosquitos to compete with.
• Malaria becomes relatively scarce. The modified mosquitos die out as their fitness advantage becomes more trivial. Malaria rebounds.
• The mosquito population and the population of malaria-infected animals reach an equilibrium. Malaria stabilizes at a lower frequency, and the frequency of the modified gene stabilizes at a frequency between 0% and 100% in the population.
But without knowing the shapes of the fitness curves, I can't say much more about it.