The clever technique of decreasing a mosquito population by releasing large numbers of male mosquitos with a fatal gene has flaws in practice.

One flaw is that some viable offspring do develop, containing just part of the added gene. This has the effect of transferring other genetic differences coming from the strain from which the fatal males were made.

Another is that eventually the population evolves resistance, in the form of genes that make females mate preferentially with the nonmodified males, even though they are fewer.