These models will be described in chapter 22. The fundamental basis for adaptive evolution. A disadvantageous mutation will disappear due to purifying selection. A character with even a minuscule selective advantage will be fixed by natural selection. The mean fitness increases as evolution proceeds.
Resistant rats are less fit if there is no pesticide present, so the allele rapidly disappears.
Persistence of deleterious alleles in natural populations Deleterious alleles are often repeatedly reintroduced by recurrent mutation or by gene flow from environments where they are favored. These processes can result in a stable equilibrium between elimination by natural selection and reintroduction by mutation or gene flow. That allele frequency is the square root of the mutation rate divided by the selection coefficient. This is if the allele is recessive. Huntington's persists because the Hh heterozygous condition is not expressed until about age 35.