Natural and artificial selection — practice question

The monkey flower, Mimulus guttatus, is pollinated by bumblebees through cross-pollination, and it does not self-pollinate. Because bumblebee numbers are declining in many parts of the world, an experiment was done in Kansas to find out what effect the loss of pollinators would have on these plants. 1600 Mimulus plants were grown in a field. Another 1600 Mimulus plants were grown in a glasshouse that bumblebees could not enter. Seeds were collected and sown repeatedly for several generations at each site. Initially, the glasshouse plants produced few seeds, but after five generations they were able to self-pollinate and the number of seeds produced was almost the same as that of the field plants. After five generations, the flowers of the glasshouse plants were significantly smaller than those of the field plants.