Evolution of inflorescence design: Theory and data

Mark Fishbein, David L Venable

Research output: Contribution to journalArticle

54 Scopus citations


Very low fruit set ill milkweeds and other flowering plants often has been attributed to greater sexual selection on inflorescence size via male, rather than female, reproductive success. Although this explanation has been generally accepted, alternate explanations have been presented, and recently the 'male function' or 'pollen donation' hypothesis has been sharply criticized. In this paper, we make the distinction between selection on total flower number and on the size of inflorescence units, both of which have been termed 'inflorescence size.' We present an ESS model for the evolution of inflorescence design that considers reproductive success through male and female function. The model predicts that selection will balance the proportional changes in female and male reproductive success resulting from changes in inflorescence-unit size. We conducted a field study of selection on the size of inflorescence units (umbels) 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 function reached a maximum at an intermediate umbel size in both years (although not significantly different from the smallest umbel size in either year), whereas the female illness function was highest for the smallest umbel size in one year, but was constant across umbel sizes in the other year. We also found that pollinator visitation rate corresponded well with male, but not female, function, and that between year variation in the male reproductive success of different umbel sizes corresponded with variation in the composition of the pollinator pool. Our empirical results, when inserted in the model, predict ESS umbel sizes similar to those observed in the study population and the species throughout its range.

Original languageEnglish (US)
Pages (from-to)2165-2177
Number of pages13
Issue number6
Publication statusPublished - Dec 1996



  • Asclepias tuberosa
  • ESS model
  • inflorescence design
  • inflorescence size
  • milkweed
  • pollen transfer
  • sexual selection
  • size-number trade-offs

ASJC Scopus subject areas

  • Agricultural and Biological Sciences(all)
  • Agricultural and Biological Sciences (miscellaneous)
  • Ecology, Evolution, Behavior and Systematics
  • Genetics
  • Ecology
  • Genetics(clinical)

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