Selection pressures

In the absence of the parasite, the selection pressures for horizontal resistance were negative, and the level of resistance declined to the minimum. Once the parasite appeared, the selection pressures for horizontal resistance were positive, and the level of horizontal resistance increased to the maximum, in areas where the parasite had the maximum epidemiological competence.

The mechanism of these selection pressures is one of reproductive advantage. The horizontal resistance has a genetic cost. In the absence of the parasite, individuals with a reduced genetic cost (i.e., a low resistance) have a reproductive advantage, and the level of the resistance declines. Once the parasite appears, the resistant individuals have a reproductive advantage. In the following generation, there is a higher proportion of resistant individuals, and the levels of resistance have increased. This process can continue until the maximum levels of horizontal resistance are reached for the entire population.

However, the law of diminishing returns operates. When the resistance is low, the selection pressures are great, and the rate of accumulation of resistance is high. As resistance accumulates, the selection pressure decreases, and the rate of accumulation of resistance also decreases. Eventually, a balance is reached, and it is maintained at a level of resistance at which the positive and negative selection pressures cancel each other.

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