An evolutionary force. It is a stochastic process best understood by an example of error in sampling. Suppose there is a population of tree frogs with two types of genetically distinct individuals: red and green. Now suppose that these tree frogs all live in a big forest and that every month all the trees are hit by lightning except one. Beginning with a half red, half green forest, after several months the proportions will be quite different -- by chance, the surviving tree may have more red than green frogs, or vice versa. This is how a population evolves by drift. NB: Here we assume that colour is heritable -- that it has a genetic basis -- otherwise, there is no genetic change and evolution does not occur.