Where Phlox drummondii lives by itself, it has a periwinkle blue blossom. But where its range overlaps with Phlox cuspidata, which is also light blue, drummondii flowers appear darker and more red. Some individual butterflies prefer light blue blossoms and will go from blue to blue, avoiding the dark reds. Other individual butterflies prefer the reds and will stick with those. This “constancy” prevents hybrid crosses.
Hybrid offspring between drummondii and cuspidata turn out to be nearly sterile, making the next generation a genetic dead-end. The persistent force of natural selection tends to push the plants toward avoiding those less fruitful crosses, and encourages breeding true to type. In this case, selection apparently worked upon floral color. source