There should also be noted a second level of evolution is at play here. Instead of genes fighting for survival it is a survival of memes or ideas instead. Call it "progress" instead of "evolution". An environment of cooperation is highly benefical to the propagation of ideas so any idea which helps ideas themselves propagate , such as "lets cooperate on this" naturally propagates itself better than a "let's not trade anything." kind of idea.
Genes or Memes, either way its self replicating self adjusting information and the outcome,evolution or progress, is quite similar.
Evolutionists often cite this as supporting their theory but there are a couple of problems with it:
1. If the whole poulation had the trait then it would reproduce 1/4 less than other populations of the same species. (check out genetics 101).
2. Even bigger problem is that we have never found a population with as much as 50% of it carrying this trait. ( I think 25% or so is the highest). Now the question is, how does the rest of the population manage to survive malaria? If this mutation was the only way to survive it then it would exist in 100% of the population in malarial areas. Clearly there are other reasons why people survive malaria so this is not the reason for it. Let's note that the black death, one of the worst illnesses known, killed between 1/4 and 1/3 of Europe's population by all accounts, yet there was no 'immunity' to it in this fashion. Clearly, human beings have the ability to fight many diseases and survive, this is part of the inherent strenght of the genome pool of a species.