Stabilizing selection and the structural variability of flowers within species

Zoophilous flowers often appear to be precisely formed for pollen transfer and exhibit relatively little Variability in structure within species. Functional optimization by the seemingly exacting requirements of pollen transfer may account for these observations. I used the results of a literature survey to examine the levels of intraspecific Variation in flowers across a wide range of taxa. The least variable attributes were those potentially affecting the mechanical fit between flower and pollinator, which are potentially constrained by selection for pollination performance. I discuss six mechanisms by which plant-pollinator interactions could generate stabilizing selection on flowers. In addition, I consider the stabilizing roles of limiting resources and also two functionally-neutral mechanisms. Further work is required to identify the actual mechanisms by which selection stabilizes the evolution of flowers. (C) 1998 Annals of Botany Company.