Predicting the Response to Selection

When a quantitative characteristic is subjected to natural or artificial selection, it will increase with the passage of time, provided there is genetic variation for that characteristic in the population. Suppose a dairy farmer breeds only those cows in his herd that have the highest milk production. If there is genetic variation in milk production, the mean milk production in the offspring of the selected cows should be higher than the mean milk production of the original herd. This increased production is due to the fact that the selected cows possess more genes for high milk production than does the average cow, and these genes are passed on to the offspring. The offspring of the selected cows possess a higher proportion of genes for greater milk yield and therefore produce more milk than the average cow in the initial herd.

The extent to which a characteristic subjected to selection changes in one generation is termed the response to selection. Suppose that the average cow in a dairy herd produces 80 liters of milk per week. A farmer selects for increased milk production by breeding the highest milk producers, and the progeny of these selected cows produce 100 liters of milk per week on average. The response to selection is calculated by subtracting the mean phenotype of the original population (80 liters) from the mean pheno-type of the offspring (100 liters), obtaining a response to selection of 100 — 80 = 20 liters per week.

The response to selection is determined primarily by two factors. First, it is affected by the narrow-sense heri-tability, which largely determines the degree of resemblance between parents and offspring. When the narrow-sense her-itability is high, offspring will tend to resemble their parents; conversely, when the narrow-sense heritability is low, there will be little resemblance between parents and offspring.

The second factor that determines the response to selection is how much selection there is. If the farmer is very stringent in the choice of parents and breeds only the highest milk producers in the herd (say, the top 2 cows), then all the offspring will receive genes for high-quality milk production. If the farmer is less selective and breeds the top 20 milk producers in the herd, then the offspring will not carry as many superior genes for high milk production, and they will not, on average, produce as much milk as the offspring of the top 2 producers. The response to selection depends on the phenotypic difference of the individuals that are selected as parents; this phenotypic difference is measured by the selection differential, defined as the difference between the mean phenotype of the selected parents and the mean phenotype of the original population. If the aver-

Canis ^ familiaris metris-optimae

Canis ^ familiaris metris-optimae

Persian sheepdog

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