Once the offspring have been produced by selection, recombination and mutation of individuals from the old population, the fitness of the offspring may be determined. If less offspring are produced than the size of the original population then to maintain the size of the original population, the offspring have to be reinserted into the old population. Similarly, if not all offspring are to be used at each generation or if more offspring are generated than the size of the old population then a reinsertion scheme must be used to determine which individuals are to exist in the new population. Pure Reinsertion is the simplest reinsertion scheme.
Every individual lives one generation only.
This scheme is used in the simple genetic algorithm. However, it is very likely, that very good individuals are replaced without producing better offspring and thus, good information is lost. The elitist combined with fitness-based reinsertion prevents this losing of information and is the recommended method. On the other hand the best individuals can live many generations. However, every generation some new individuals are inserted.