This paper describes a simple, rapid and low-cost technique for growing bacteria (or other microbes) in an environmental gradient, in order to determine the tolerance of the microbial population to varying concentrations of sodium chloride ions, and suggests how the evolutionary response of a microbial population to the selection pressure of the saline can be measured. This system can be used to quantify and follow the consequences of natural selection on successive generations of bacteria. A list of suggested investigations appropriate for post-16 school pupils is provided. (Contains 9 figures.)