Here's a simplified illustration of genetic drift: A group of people leave their home and sail to another land. They meet with disaster at sea and only half of the people make it to the new land. By coincidence, all the survivors happen to have blue eyes. Therefore, while the proportion of blue eyes relative to other colors was only 50% of the original group, it is now 100% of the surviving group and therefore 100% of the genetic stock that will reproduce at the new land.
Ahh, ok.
So in this case, if you were to imagine a evolutionary biologst
in the future studying these people, if he had to explain everything
via natural selection, he would assume that blue eyes imparted
some survivability advantage in the past, and he would be wrong.
Sometimes shit just happens, and a gene ("good" or "bad") gets
popular by basically winning a lotto rather than "working" their
way up.
Also, if you took the same situation, except all the brown-eyed
people were specifically killed by, say, a racist, then in that case
it WOULDN'T be drift, right? Because even though it only happened
once, blue eyes did in fact increase their survivability.
[inkling]