It is often asked why natural selection doesn't eliminate genetic variation for disease resistance. Complex host parasite interactions guide the evolution of both hosts and parasites, and are one of the major reasons for the maintenance of genetic variation in natural populations.1-7-1 Combining genetic theory and epidemiology gives insight into why genetic variation in resistance persists. First, selection pressures, especially those for disease resistance, will differ across time and environments. Second, natural selection will not make populations completely resistant to infection; as natural selection moves a host population toward resistance, the selection pressure for resistance decreases. A certain proportion of susceptible animals can be carried without exposing the population as a whole to risks of epidemics. Once the number of genetically susceptible animals falls below this level, selection pressure for resistance ceases. Thirdly, modern domestic livestock populations have been selected for other characteristics, with disease impacts masked by non-genetic control measures.
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