How do researchers think that natural selection has acted on this gene during recent human evolution (in the last ~ 85,000 years)?
Humans originated in Africa, where a lot of melanin (G allele) would be beneficial because it would let some UV light to penetrate allowing them to synthesize vitamin D, without giving them too much UV radiation. Those with a mutation causing them to have lighter skin would no do well in this environment. However, when humans began to migrate to colder climates, those with the mutation would have an advantage. Individuals carrying the 'a' allele had less melanin, which allowed for more UV light to penetrate their skin, and would have been able to synthesize vitamin D better.