One of the Hardy Weinberg Assumptions is: "No Genetic Drift"; what does that mean?
No Genetic Drift- The key to this is SMALL POPULATION. There are two types:
Founder effect: A tiny portion of a population leave and found their own population (for the old population, this is gene flow. We are now focusing on the tiny new population, which has very different genetics than the old population. (Example: Lehi leaving Jerusalem. Whoever said that, you are brilliant.)
Bottle-neck: Taking a population and wiping out most of them so only a few are remaining.