Anyone who has looked on the undersides of leaves of plants at the edge of a meadow or in the garden is familiar with aphids, tiny grain-sized insects that feed on sap. Here is a photograph of pea aphids, from the April 30 issue of Science. Pea aphids come in two colors, red and green. The colors are caused by molecules known as carotenoids, which serve other necessary functions in animals, including immune system regulation and vision (Eat your carrots!). Unlike most (all?) other animals, aphids have the genes to make their own carotenoids, which they apparently got by a lateral gene transfer from a fungal pathogen 30 to 80 million years ago. A single point mutation in the genes accounts for the difference between the red and green coloration.
Well, now, that's a bit of interesting genetics, the sort of science that has become possible with the widespread -- and increasing fast and cheap -- sequencing of genomes. But here's the thing I think is neat. Among the aphids' natural enemies, ladybugs prefer to gobble red aphids, and parasitic wasps prefer to deposit their eggs in green aphids. Which may have something to do with maintaining the different color populations.
It is, when you think of it, a Dr. Seussian sort of world. Red aphids, green aphids. Ladybugs, wasps. Sap-mopping, stem-hopping, gene-swapping pests. Add the ants that farm aphids, protecting and tending them, milking them of their honeydew by stroking them with antennas. If all of this is the product of an Intelligent Designer, then he has a wonderful sense of whimsy. One fish, two fish. Red fish, blue fish. And to think that I saw it on Mulberry Street.
In his autobiography, the brilliant physicist John Archibald Wheeler makes this confession of faith: "Whatever can be, is." He goes further: "Whatever can be, must be." Anything not prohibited by the laws of nature, exists, he says. That may not be exactly true in the biological world; one has to work with the genes that one's got. But there can be a bit of cutting and pasting, and pretty soon you have red aphids running from ladybugs and green aphids running from wasps.