Do populations evolve or do individual organisms evolve

Individual organisms don't evolve. Populations evolve. Because individuals in a population vary, some in the population are better able to survive and reproduce given a particular set of environmental conditions.

https://www.pbs.org/wgbh/evolution/library/faq/cat01.html

Due to the amount of pressure

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.