Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others.
The half-life of a radioisotope is the amon the symbol 42He, the subscript 2 is the mass number for helium and the superscript 4 is the atomic number for helium.
