If isolated populations evolve (change) enough to where they are no longer able to reproduce, they are considered two separate species
Over billions of years, organisms have changed so much from ancestral forms to warrant classification in higher taxa (i.e. Domain, Kingdom, Phylum, etc.)
Some unstable isotopes are deemed "radioactive" when the nucleus spontaneously decays into more stable daughter isotopes and ejects high-energy radiation
Cytochrome c amino acid sequence differences between human and different eukaryotes reveal less differences between more closely related species (less time to change independently since species diverged)