Rocky Mountain National Park visitors often wonder why there are so many different types of flowers in the park, especially when they learn we are home to about 1000 different species of vascular (higher) plants. There are many reasons for this diversity. The most obvious are that the park spans almost 7,000 ft. in altitude from 7,624 ft. to 14,259 ft., is one third tundra and two-thirds forested, and is located on both sides of the Continental Divide. This means that plants can grow in a very large number of different habitats as defined by temperatures, moisture, sun exposure, length of growing season, and other site factors.
Recent research conducted outside the park* suggests there may be at least one other reason why some animal-pollinated plant groups may be very diverse - the shape of the flowers. Most flowers can be classified as bilaterally or radially symmetrical. Bilaterally symmetrical flowers can be divided into two equal halves by only one line through the middle. Orchids are an excellent example. Only one line through the flower will divide the flower in such a way that each half is a mirror image. Radially symmetrical flowers can be divided into similar parts by any number of lines drawn across the center of the flower. They are arranged like a wheel with spokes (petals and other flower parts) arranged around a center point.
It takes much greater precision on the part of the animal pollinator to gather nectar or pollen (and also to deposit pollen in the correct spot to fertilize the plant's seeds) in a bilaterally symmetrical flower than in a radially symmetrical flower, thus reducing the number of pollinators capable of visiting the flower. Fewer, more specialized pollinators mean the plant is subjected to less pollen from other types of plants and more likely to receive pollen capable of fertilizing its seeds - thus fewer pollinators take nectar and/or pollen and return nothing of use to the plant. This can provide a real benefit to the plant. Also, with fewer species of pollinators, small mutations in bilaterally symmetrical plants can further restrict the species of pollinators capable of visiting the flower and fertilizing seeds, leading to less gene flow between the normal and the slightly different plants, and ultimately to the development of separate species. A botanist has found evidence* that this actually occurs in nature by comparing several closely related groups of plants with bilaterally and radially symmetrical flowers. The botanist discovered that in 15 out of 19 comparisons, groups that produced bilaterally symmetrical flowers also contained more species than the closely related groups with radially symmetrical flowers.
So the next time you visit Rocky Mountain National Park in the summer and wonder why there are so many different types of flowers, one reason may well be because of the shape of the flowers!
* Proc. R. Soc. Lond. B (2004) 271, 603-608