Genetic drift — one of the basic mechanisms of evolution — is simply the evolutionary equivalent of a sampling error.

Imagine a game in which you have a bag holding 100 marbles, 50 of which are brown and 50 green. You are allowed to draw 10 marbles out of the bag. Now imagine that the bag is restocked with 100 marbles, with the same proportion of brown and green marbles as you have just drawn out. The game might play out like this:

It’s clear that the ratio of brown to green marbles “drifts” around (5:5, 6:4, 7:3, 4:6 . . .)

This drifting happens in populations of organisms. Due to many random factors, the genes in one generation do not wind up in identical ratios in the next generation, and this is evolution. It is possible for the frequency of genes for brown coloration to increase in a population of beetles without the help of natural selection. While this is evolution, it is evolution due to chance, not selection.

Genetic drift has several important effects on evolution:

1. Drift reduces genetic variation in populations, potentially reducing a population’s ability to evolve in response to new selective pressures.
2. Genetic drift acts faster and has more drastic results in smaller populations. This effect is particularly important in rare and endangered species.
3. Genetic drift can contribute to speciation. For example, a small isolated population may diverge from the larger population through genetic drift.