In which scenario could natural selection most obviously occur?

Natural selection is likely when there are stronger selection pressures. For example, a perpetual selection pressure is the fact that organisms have to compete for food and resources, meaning the best adapted ones survive. However, a stronger selection pressure may cause natural selection to occur more obviously. An example of this is when the number of white peppered moths decreased during industrialization in England due to the soot collecting on the trees. The change in population was very fast and obvious.