Living things reproduce with variation and are subject to natural selection. Reproductive variation results from random genetic mutations. Natural selection is the non-random screening (via death or survival of individuals) of those mutations which aid the life form in its struggle to survive long enough to reproduce. For example, even a slight improvement in eyesight will tend to aid in survival and thus help to propagate the genes that carry that trait within the gene pool of a species. Eventually, a single species will diverge into new ones (speciation), each better adapted to its particular environment, which includes geography, competition, predation, etc. Over geological time, this process has produced all the species that have ever existed (until humans began artificial selection and even direct synthesis of new species).
Evolution is perhaps the simplest, most profound, and most verified scientific theory ever conceived.