Polygenetic selection describes the changes in a gene pool in response to adaptation of a polygenetic trait.
Prerequisite
Let us assume we consider a polygenetic trait that is determined by several independent loci.
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The alleles that favor the trait are marked red and numbered CVL1-8.
Let us further assume that there is a linear relationship between the number of CLV allele per individual and the probability to phenotypically expose the trait.
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The green line in this graph depicts such a linear relationship. I'm fully aware that this is an oversimplification. The relationship might be non-linear convex as the yellow line or concave as the red one. Next, the relationship always is a statistical one. That is our line depicting relationship is only the correlation function of the cloud of single observations (black dots). Finally, I'm fully aware that this models neglects probability functions that exist for all alleles, and have to be described by a set of conditional probabilities.
All this can be neglected for the sake of comprehension only. The principle of polgenetic selection would hold even if we consider a quite more complex model. (The pioneers of polygenetic inheritance, including Waddington, considered a model with even a simple threshold function. (Unfortunately, many people still believe that all genetic inheritance works like Mendelian traits, which hinders them to understand evolutionary principles.)
Selection
The selection of polygenetic traits leads to an accumulation of alleles that favor the trait that is positively selected.
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In this picture two populations are shown. In the population on the left the the alleles that code the trait are scattered scarcely while after the selection process, the population on the right, the alleles are much more common. Each individual is shown as a single chromosome. It really doesn't matter if these alleles summon on a single chromosome by crossing over or on different chromosomes of the same individual.
Experimental Example
The Waddinton experiment is an example of slecetion of a polygenetic trait, the crossveinlessness. Waddington enhanced the process of selection by arteficially increasing the expression of the phenotype even in individuals with a lower number of specific trait alleles.
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Graphical representation of his radiation experiment would be an upward shift of the curve. Again, it doesn't matter whether this shift in probability of expression is caused by a linear shift (blue line) or by the curve becoming more convex (cyan line).
