FIGURE 3.

Polygenic adaptation model. Multiple loci (4 in this case) are responsible for a single trait. Most loci (loci 1, 2, 3) harbor mutations, later becoming beneficial, while there are also loci (locus 4) gaining new advantageous mutations after the onset of selection. The selected mutations are present at different frequencies in the population before selection as a result of random drift. Upon environmental change, selection acts on all these beneficial mutations and elevates their frequency. However, each mutation only needs to have a slight increase in frequency.