Work Step by Step
Since farmers and breeders are focused on production (number of eggs, most milk, etc.) and gaining the most monetary value from the domestic species, they select the most productive animals to breed, increasing their earnings. By doing so, they remove the genes found in less productive individuals, narrowing the genetic diversity. While this might be the most monetarily effective choice for breeders, it doesn't help domestic species overall, as traits such as virus resistance are passed over in favor of increased productivity.