An experiment showing the process of natural selection has shown that certain traits will diminish through predation. Broad-horned flour beetles are eaten by Assassin bugs, which target the larger beetles for maximum nutrition. Because of this, scientists found that predation of the beetles resulted in lower fitness for males and much greater fitness and reproductive success for females. Additionally, males that survived would have smaller bodies and mouthparts, resulting in more generations of smaller males. In this way, scientists found that sexual dimorphism diminished in more severe environments, supporting that natural selection directly resulted in the dimorphisms seen in the wild types.
This is less directly genetic, but it is interesting to note how the fitness of the species under certain conditions can influence the genetic variations and phenotypes expressed. This is a study that propels the understanding of natural selection forward (despite this actually being an artificial selection event).