In 1793 the German botanist christian Konrad Sprengel first suggested that the colors in flowers existed not to please our senses but to please those of bees and other insects. He observed that as insects fed on nectar in different flowers, they helped plants reproduce by conveying pollen from the stamen, or male part, of one plant to the pistil, or female part, of another. To lure pollinating insects, he reasoned, flowers must use color signals to advertise their nectar. When later researchers found that the sensitivity of bee color vision precisely matched the hues in floral color displays, another inference seemed obvious: the debut of flowering plants, or angiosperms, some 100 million years ago probably shaped the evolution of color vision in bees, a well-established group that adapted to exploit this new food source.
Now biologist Lars Chittka of the State University of New York at Stony Brook has used a novel research approach to reverse this long-held notion. Flowers, he says, adapted their coloration to the bee visual spectrum, not the other way around.
The idea was simple, Chittka explains. To see whether flower color influenced the tuning of bee photoreceptors [color-sensitive cells in the eye], we had to look at whether ancestors of bees had a different set of photoreceptors before there were angiosperms. Of course, there is no direct way to prove what receptors they had back then, Chittka admits--the fossil record just doesn’t contain that kind of information. But there is, he says, an indirect way. By studying traits--such as types of photoreceptors--that bees share with existing, related species, one can infer information about the traits held by a common ancestor.
Chittka looked at color vision in living arthropods, a group that includes insects, spiders, and crustaceans, all of which had evolved before flowers existed. In particular, he collected available data on the light- sensitive pigments found in the photoreceptors of these different creatures. Such pigments absorb light best in a particular narrow band of wavelengths, with declining sensitivity to other wavelengths. Thus, a green receptor picks up hues other than green but is especially sensitive to green shades in its peak zone. The brain integrates the ranges of the different receptors to recognize hundreds of colors.
Chittka then plotted the photoreceptor pigment ranges of living arthropods onto the group’s evolutionary tree, something no one had ever done before. The results were astonishing: despite diverse diets and habitats, essentially all arthropods possess photoreceptor pigments sensitive to the same range of light as bees--ultraviolet, blue, and green. In fact, Chittka traced this universality to the very base of the tree, where the ancient chelicerates--ancestors of spiders, scorpions, and horseshoe crabs, which had just green and ultraviolet photoreceptors-- parted evolutionary company with the crustaceans and insects, which possessed all three photoreceptor pigments, more than 500 million years ago.
The implication, Chittka maintains, couldn’t be more clear: the ancestors of bees--and apparently many other ancient arthropods--had the equipment for perceiving flower color 400 million years before the first flower bloomed. The precise matching of bee visual acuity to blossom colors, he says, is probably due to flowers having adapted to pollinator vision, and not to the spectral tuning of bees to flowers, as had been supposed.
Left unanswered is the question of what did trigger the evolution of color vision in bees, if flowers did not. Specialized feeding habits, Chittka suggests, may have had nothing to do with the development, given that they were probably shared by so many arthropods occupying different ecological niches. Instead, Chittka speculates, color vision may have been a general response to very basic environmental conditions. One idea, he says, is that the green receptor is matched to the most common background in natural habitats: green foliage. Similarly, the ability to see blue and ultraviolet might have helped early arthropods pick out predators--or prey- -against the sky’s blue backdrop as they flitted through a landscape without flowers.