Genetic Drift.

 Genetic drift is something that happens randomly. For example, if I were driving my car down the street and happened to hit some butterflies it would be pure chance what color of butterflies I hit, and the ones I didn't would be lucky.

On the other hand, if I were to go out with a butterfly net and purposely catch purple butterflies because they stood out more in nature, that would be natural selection and NOT genetic drift because it did not happen randomly.