How to spread diversity

Natural History, June, 2004 by Stephan Reebs

Why do some plant families blossom forth with a wealth of species, whereas others have so few? What contributes to the emergence of new plant species? One factor, according to Risa D. Sargent, a zoologist at the University of British Columbia in Vancouver, is symmetry: it makes a difference whether the flowers are bilaterally symmetrical (the Left half mirroring the right half, as in orchids) or radially symmetrical (the same pattern all around, as in lilies).

Sargent, who studies the interactions between pollinators and flowers, compared nineteen pairs of closely related plant families. One member of each pair had bilaterally symmetrical flowers, the other, radially symmetrical. In fifteen such pairs, the bilaterally symmetrical families were the more species-rich.

New species usually arise from mutations, but at first the mutated genes may have trouble getting a foothold in the genetic pool of the population. Evidently, though, if the pollen (the male agent) of a mutated plant reaches a plant belonging to the same species--instead of being wasted on another, incompatible species--those mutations will spread more easily. What makes that scenario more likely? Well, one good way for a plant to spread mutations is to rely exclusively on a specialized pollinator, usually an insect. Bilaterally symmetrical flowers are good at inducing such specialization, because they constrain pollinators to take exactly the right position on the flower to bring the pollen into contact with the stigma, gateway to the next generation. ("Floral symmetry affects speciation rates in angiosperms," Proceedings of the Royal Society of London B 271:603-608, March 22, 2004)