A study was carried out to investigate natural selection in the plant evening primrose, Oenothera biennis. Physiological changes associated with resistance to grazing by herbivorous insects were measured over 5 years in 16 experimental populations of evening primrose. For 8 of the experimental populations the plants were regularly sprayed with chemical insecticide, and the other 8 experimental populations were not sprayed. In each of the 5 years some older plants died after producing seeds and some new plants grew from seed. Observations after the study were: • sprayed populations: o The length of time that the plants produced flowers (flowering period) became longer. o Flowering started earlier in each generation. o The mean concentration of natural insect-deterring chemicals in the plants was relatively low. • non-sprayed populations: o The flowering period remained the same over the 5 years. o The mean concentration of natural insect-deterring chemicals in the plants increased. Analysis showed that genetic differences were responsible for the differences in flowering time and concentration of natural insect-deterring chemicals in the plants. The researchers concluded that natural selection was acting on both groups of plants.