## Scientific method in biology

As an example of the same scientific procedure in a completely different discipline – biology, we can investigate the experiment known as the "Fluctuation test". This experiment was carried out in 1943 by Max Delbrück and Salvador Luria, who were interested in the mechanism by which mutations occur (you can read more about mutations in chapter 5). The basic goal of the experiment was to find out whether mutations arise randomly or are part of a regulated response of the organism to changing environmental conditions. Delbrück and Luria cultured bacteria sensitive to infection with a virus (bacteriophage) and then plated equal numbers of bacteria on Petri dishes (culture vessels) containing the growth medium as well as the virus (Figure 2.3). After a few days, they observed how many bacterial colonies grew on the dishes (cells that grow on the dishes must have undergone a mutation that makes these cells resistant to virus).

If mutations arise as an organism's response to changes in the environment (the presence of a virus), then approximately the same number of colonies should grow on each dish (all dishes are identical, as well as the type and number of cells). However, if the mutations were to occur randomly, it would be possible to observe different numbers of colonies on different plates. Since the results of the experiment showed that different numbers of colonies grew on different dishes, it was possible to conclude that mutations arise randomly. Dishes without virus, on which colonies grew uniformly, served as a control. The results of this simple experiment were crucial not only for geneticists and molecular biologists, but also helped to explain some questions of evolutionary biology. You can read more about the role that the fluctuation test played in the study of evolution in Chapter 14.