Selection on pollen morphology: a game theory model

Variation in pollen aperture number has been widely described among and within angiosperm species, and fossil data indicate that aperture number has increased over geological time. Within species, variation in aperture number is very often found within individual plants (heteromorphism), with the genetic variation being expressed in morph proportions produced by individuals. We present models of the evolution of pollen aperture number motivated by previous results on Viola diversifolia in which higher aperture number results in more rapid germination and lower longevity. The models demonstrate that under certain conditions selection results in a mixed evolutionarily stable strategy (ESS), in which all plants produce all types of pollen. In contrast, selection cannot easily maintain a standard genetic polymorphism with each pollen morph produced on a different plant. These predictions match the available evidence on patterns of natural variation in pollen aperture number. The models and data lead to an unusual evolutionary prediction: a population-level increase in the mean number of pollen apertures should occur by a progressive increase in the proportion of higher aperture-number morphs produced on individual plants, rather than by an increase in the proportion of individuals that consistently produce pollen with different aperture numbers.