NASA: The term "unnatural selection" will be used loosely to describe evolution of nucleic acids or other replicatable polymers in vitro. The term "Natural Selection" will be reserved for the evolution of living organisms and their viruses. Natural Selection usually involves the coevolution of nucleic acids and proteins, while "unnatural selection", as practiced so far, allows replicating nucleic acids to evolve but holds constant the enzymes that catalyze replication. It is widely believed that biology based on DNA, RNA, and proteins was preceded by the biology of an "RNA world" in which enzymes were composed of RNA alone. The origin of RNA replication is thus the central puzzle of the origins of life. Consequently, RNA-catalyzed RNA replication is presently one of the main goals of experimental work on unnatural selection. However, there is also a more distant goal, namely, to achieve replication and selection in systems unrelated to RNA.