EVOLUTION OF INFLORESCENCE DESIGN - THEORY AND DATA

Very low fruit set in milkweeds and other flowering plants often has b een attributed to greater sexual selection on inflorescence size via m ale, rather than female, reproductive success. Although this explanati on has been generally accepted, alternate explanations have been prese nted, and recently the ''male function'' or ''pollen donation'' hypoth esis has been sharply criticized. In this paper, we make the distincti on between selection on total flower number and on the size of inflore scence units, both of which have been termed ''inflorescence size.'' W e present an ESS model for the evolution of inflorescence design that considers reproductive success through male and female function. The m odel predicts that selection will balance the proportional changes in female and male reproductive success resulting from changes in inflore scence-unit size. We conducted a field study of selection on the size of inflorescence units (umbers) by manipulating umbel size and number in a natural population of Asclepias tuberosa, in southeastern Arizona , during two reproductive seasons. We found that the male fitness func tion reached a maximum at an intermediate umbel size in both years (al though not significantly different from the smallest umbel size in eit her year), whereas the female fitness function was highest for the sma llest umbel size in one year, but was constant across umbel sizes in t he other year. We also found that pollinator visitation rate correspon ded well with male, but not female, function, and that between-year Va riation in the male reproductive success of different umbel sizes corr esponded with variation in the composition of the pollinator pool. Our empirical results, when inserted in the model, predict ESS umbel size s similar to those observed in the study population and the species th roughout its range.