Search Theory, Random Motion, and the Convergent Evolution of Pollen and Spore Morphology in Aquatic Plants

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Abstract
A consideration of the nature of random motions and the theory of random search suggests that search efficiencies would be much greater for aquatic spores with large long axes and which move in planes, such as at the water surface, rather than in 3 dimensions. Since such adaptations also increase the risk of inbreeding a greater incidence of dioecism is predicted in aquatic environments. These predictions are supported by data compiled on the aquatic spores of a variety of unrelated angiosperms, pteridophytes, bryophytes and fungi. Nearly all aquatic spores had either extremely large long axes relative to nonaquatic spores, or were transported in search vehicles with large long axes. Many spores were equipped with flotation devices, and many of the taxa were dioecious. These trends in aquatic spore morphologies can best be explained as convergent evolution toward an efficient search vehicle. Several alternative, but not mutually exclusive, hypotheses are also examined as potential explanations for this apparent convergence.