What accounts for the diversity of millions of species on earth?

The answer is genetic variations. In addition, Genetic variation means that biological systems in which individuals and populations are different over space. In a genetic variation, the genes of organisms in inhabitants change. The Gene alleles control separate traits that can be handed on from parents to offspring. The gene variation is significant to the procedure of natural selection in which the genetic variations that stand up in a population occur by chance but the process of natural selection does not.