Pollinatormediated selection on flowertube length in a hawkmothpollinated Gladiolus (Iridaceae)

Darwin's mechanistic model whereby selection favours plants with flower tubes that exceed the tongue length of the primary pollinator, was tested using unmanipulated plants of the hawkmothpollinated South African iris, Gladiolus longicollis. The study population was characterized by exceptionally large phenotypic variation in flowertube length (range 56129mm). Directional selection on tube length was revealed by a significant positive relationship between this trait and both fruit and seed set. Selection was attributed to the effect of tube length on pollen receipt, as supplemental hand pollinations showed fruit and seed set in the population to be pollen limited. Indirect selection on tube length may also occur through the correlation of this trait with inflorescence height, although direct selection on the latter trait was significant only for seed set. The main pollinators at the study site were individuals of the large hawkmoth Agrius convolvuli that had tongue lengths of 85135mm. Other hawkmoths had ...