Although changes due to single gene mutations may involve sudden and large changes (like that of Ancon
breed of sheep), they can also be so small that one may not be able to notice them. Therefore, earlier concept that, mutations could involve sudden and big changes only, was not correct. Mutations could also be observed which do not involve any change in morphology but only modify nutritional requirements as in case of Neurospora
(Mutations : 2. Biochemical Level (Biochemical and Microbial Genetics)
). similarly, mutations could be observed in pathogens, as a result of which a pathogen could lose or acquire virulence against a particular host.