Genetic drift is essentially random. Alleles at loci more than a few hundred thousand base pairs away are unaffected.

In contrast, an allele under selection pressure drags along large chunks of the chromosome on which it occurs. Statistically, the effect is different. You start to see long-range correlations across the chromosome.

The MHC region, which contains genes that govern many functions of the immune system, has been under continuous selection pressure resulting in extraordinary variability, but also very strong long range correlations spanning the region. It's unlike anywhere else in the human genome. This is a clear signature of evolution.