How does genetic drift work?

Answer 1

Genetic drift is the term used for random evolutionary developments that have equal "survival-value" with respect to natural selection. Natural selection defines broad parameters for what kind of traits organisms existing in a certain environment should ideally have, but within these parameters there is a lot of room for random drift.

Answer 2

Genetic drift is the spread of specific random variations throughout the gene pool in the absence of specific selection pressures. There's always random variation in the population, but there aren't always changes in the environment for the population to adapt to. So natural selection, in stead of moving the population towards adaptation, might select from that random variation to move 'sideways', as it were, to a state that's equally well-adapted to the environment as what came before, but different. As random variation may produce many variants that are, more or less, equally well-adapted to their environment, the direction of evolution that results is more or less random. Because variations may spread throughout small populations faster than throughout large populations, and because a large gene pool has a stabilizing effect on the spread of variations, small populations drift faster than large populations.