Replica plating and indirect selection of bacterial mutants

Elective enrichment is an indispensable technique in bacterial physiology and genetics (van Niel, 1949). Specific biotypes are most readily isolated by the es-tablishment of cultural conditions that favor their growth or survival. It has been repeatedly questioned, however, whether a selective environment may not only select but also direct adaptive heritable changes. In accord with similar discussions in evolutionary biology (Huxley, 1942), we may denote the concepts of spontaneous mutation and natural selection in contrast to specific induction as "preadaptation " and "directed mutation", respectively. Many lines of evi-dence have been adduced in support of preadaptation in a variety of system