Specimens taken from a patient sample usually contain a mixture of different bacteria. But sample with only one type of bacteria, are necessary to study a bacterial species accurately. Therefore, it is necessary to isolate all of the possible disease causing organisms in order to study the sample by itself. Once isolation of individual colonies is done, then these colonies can be used for identification tests. A sample of the specimen is move very fast onto an agar plate in order to physically separate the bacterial colonies.
The key distinction is that in genetic drift allele frequencies change by chance, whereas in natural selection allele frequencies change by differential reproductive success
