Perhaps more important than the theory are the results of quantitative genetic experiments, that is, artificial selection experiments. Every generation you select the more extreme parents, and they transmit part of that extreme values to their offspring. But there are often limits. But often you get to a plateau. 15 One possibility is that there's no more genetic variation.
Selection tends to consume genetic variability.
Another possibility is that, although there is still some genetic variability, it can't be expressed until there is a recombination event. At this stage there can be no selection. However, eventually there will be a chance recombination event on the chromosome. Correlated responses to selection. Another barrier to indefinite improvement of a trait.