Amechanism of evolution that suggests changes in the gene pool of a population are due to chance
genetic drift
natural selection
gene flow
evolution

genetic drift

A mechanism of evolution that suggests changes in the gene pool of a population are due to chance is genetic drift. A genetic drift is a natural  phenomena  in which changes occur in allele frequencies in a population from one generation to the next generation and subsequent generations. The main reason for genetic drift is the selection of allele for the next generation a kind of sampling error. Although genetic drift can occur in populations of all sizes, it's effects are stronger in small populations. In a genetic drift a beneficial allele may be lost or a harmful allele can become fixed in a gene pool of the population purely by chance.

and after water is usually warmed by the sun the water temperature raises and then evaporates .

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms