I saw a documentary on the telly the other day, about this village in Brittain that had a very high percentage of survivors during the plague years that swept through a large part of Europe (in ye olde medieval days). A scientist looked a bit closer at the case, and noticed that a lot of the people living there today were descendents of the original plague-survivors. It also turned out that they had a genetic "defect", which is linked to the immune system. Apparently, this gene makes it impossible for the plague virus to enter white blood cells, stopping it in it's tracks.
Interestingly, the relatives of those that did not get the plague at all have two copies of said gene, while the relatives of those who got the plague but recovered only have one.
Even more interestingly, it turns out that accross Europe, the population in areas visited by the plague has a much higher occurance of this specific gene, when compared to the people in the area *not* exposed to the plague.
Now, if you're still with me... Does this not make it very likely that Darwin was right? After all, the "unfit" died during the plague, while those who were "fittest" survived. And all of that because of a slight difference in their genes.
(another thing: the AIDS virus attacks the body in pretty much the same way as the Plague does. This means that people who are better at fighting the plague are also better at fighting AIDS... There are people in the US who have no AIDS, even though they "should" have had it, considering their situation - partner of AIDS-victim, etc. Again, the same gene was found... It turns out that some 40% of the US population has a higher resistance to AIDS, because they have one copy of the gene; and some appear to be immune because they have two copies.)