9.3 Predicting how the gene drive alters evolution

If the drive has no effect on the fitness of its carriers, how might we expect its frequency to change across generations in a population?

How will a gene drive with no effect on fitness change in frequency across generations?

As we did with our imaginary songbirds, we will follow a mosquito population for a full turn around its life cycle—starting and ending with the gene pool. Imagine that the frequency of the gene drive, allele D, is 0.5, as is frequency of the + allele.

When we draw gametes at random from the gene pool to make zygotes, a quarter are DD, half are D+, and a quarter are ++.

Because it’s easier (for the author!) to think in numbers than in frequencies, we imagine that our zygotes develop into 100 larvae: 25 homozygotes for the gene drive, 50 heterozygotes, and 25 homozogytes for the non-drive allele. And we let all these larvae grow up to be adults.

All that remains is to let the adults make, let’s say, 10 gametes each. This is straightforward for the homozogotes:

Gamete production in homozygotes is just like for an ordinary gene.

But what about in the heterozygotes? Remember, all the gametes they produce carry the gene drive:

Gene drive heterozygotes produce all gene-drive-carrying gametes.

So what are the allele frequencies in the new gene pool?

Calculate the new frequencies of the two alleles.

They are 0.75 for D and 0.25 for +:

The frequency of the gene drive has risen dramatically.

The population has evolved, rather substantially, even though the gene drive has no effect on the fitness of the individuals who carry it.