Scientists believe that a shift from pollination by insects to pollination by birds occurred several times over the course of angiosperm evolution. Two researchers designed an experiment to investigate how these shifts might evolve using two species of monkey flower (Mimulus spp.). M. lewisii has violet-pink flowers and is pollinated by bumblebees. M. cardinalis has orange-red flowers and is pollinated by hummingbirds. The researchers switched flower-color genes between the two species. As a result of the gene transfer, they produced a variation of M. cardinalis with dark pink flowers (instead of the original orange-red) and a variation of M. lewisiis with orange flowers (instead of the original violet-pink). Plants of both genetically altered varieties were placed in their original habitats and observed. The genetically altered variety of M. cardinalis was visited by bumblebees 74 times more often than plants with the original color flowers. The genetically altered variety of M. lewisii was visited by hummingbirds 68 times more often than plants with the original color flowers. The evolution of easily modified flower colors that can make plants attractive to animals was an important factor in angiosperm evolution because: