Alternatively, we might choose animals' mates for them, but if they come from different species they are unlikely to produce fertile progeny. This was a difficult question. In these examples, the selected trait continued to respond to selection for many generations, even after its value was very much higher than that measured in the original stock. Selection over time increases the frequency of alleles conferring high bristle number, and, with their increased frequency, the chance of getting combinations of such alleles for high bristle number gradually increases. First, the chance that all 1000 flies in the original sample were heterozygotes is very close to zero.
H is the average heterozygosity in the population.
P is the proportion of polymorphic loci. We know that at least enzymes B, D and E were polymorphic. This is 3 loci out of 5, or 0.6. Of course, if one homozygote is less fit than the other, selection might eventually purge that gene from the population, thus reducing population variability. However, this is not a direct consequence of selfing.