What are the two extreme causes of genetic drift?

The Hardy Weinberg Genetic Equilibrium requires a number of factors to insulate a population from genetic drift - naemly that if a population is sufficiently large, it's gene pool will be largely unaffected by random events. When a small population is subject to a random event (a natural disaster or a few number of individuals leaving a disproportionately large or small number of offspring), the gene pool is impacted to a much greater degree. think of peeing in a pool versus an ocean - the pool is much more affected due to its small size

When a small number of individuals colonize a new habitat, their allele frequencies determine the composition of the new gene pool - it is probable that their frequencies would not match the gene pool of the parent population. think of Mars being populated only by astronauts under the height 5'6" (due to transport constraints) - would a human population on mars have the same average height as earth?

The two extreme causes of genetic drift are the founder effect and population bottleneck.