A reason for not meeting the expected response to selection may be that the population is reaching a selection limit. A selection limit indicates that the population has reached the point where further change is no longer feasible. This can be due to the fact that there is no more genetic variation, but there are other reasons.
Limit due to opposing natural selection
In Figure 2 you see an example of a population that apparently has reached a selection limit. The high line is still responding to selection and is growing bigger and bigger. The low line, on the other hand, has linearly decreased in size for approximately 25 generations, but then further decrease was no longer feasible. Even though selection was on the lightest chickens every generation, the next generation was not getting any lighter anymore. It is not clear why this is the case. Selection results are always expressed at phenotypic level. It could be that the genetically smallest birds showed the same phenotype as the genetically larger birds, so directional selection was no longer possible. In that case this selection limit represents a physiological limit, rather than limited genetic variation. Which of the two was the case could be tested by selecting the light birds upwards again. If that is still possible then the genetic variation is still present. Another reason for the reached selection limit could be that the smallest birds were no longer capable of reproduction. That would be a typical example of natural selection working in the opposite direction of artificial selection. Selection limits due to opposing natural selection usually are hard to undo. In some cases an improvement of the environment may take away the natural selection limit.
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Limiting environment to express genetic potential
For example, in order to show growth potential, animals require sufficient nutrient intake. If those nutrients are unavailable, they can’t show their genetic potential and only grow as the feed intake allows them. This is shown in a long term selection experiment in quail, where at some point a selection plateau occurred (see figure 3, the solid line). A selection plateau seems to be the same as a selection limit, only it can be elevated by changing the environment. This is not possible with a real selection limit. The selection plateau in the quail was elevated by improving the feed quality. With this experiment it was clearly shown that limits to selection in some cases actually may be plateaus because they are caused by limits in the environment, rather than by genetics.
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