Abstract
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 hawkmoth-pollinated South African iris, Gladiolus longicollis. The study population was characterized by exceptionally large phenotypic variation in flower-tube length (range 56-129 mm). 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 85-135 mm. Other hawkmoths had tongues that were much too short to reach the nectar in G. longicollis flowers and seldom carried pollen of G. longicollis. Flowers with tubes shorter than the tongues of A. convolvuli are apparently not effectively pollinated because stigmas do not contact the moth's head effectively. This study demonstrates that selection may occur among plants with natural phenotypic variation in flower-tube length, and supports Darwin's model of pollinator-mediated selection.
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- Efron B., Tibshirani R. Statistical data analysis in the computer age. Science. 1991 Jul 26;253(5018):390–395. doi: 10.1126/science.253.5018.390. [DOI] [PubMed] [Google Scholar]
- Hoekstra H. E., Hoekstra J. M., Berrigan D., Vignieri S. N., Hoang A., Hill C. E., Beerli P., Kingsolver J. G. Strength and tempo of directional selection in the wild. Proc Natl Acad Sci U S A. 2001 Jul 24;98(16):9157–9160. doi: 10.1073/pnas.161281098. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Johnson SD, Steiner KE. Generalization versus specialization in plant pollination systems. Trends Ecol Evol. 2000 Apr;15(4):140–143. doi: 10.1016/s0169-5347(99)01811-x. [DOI] [PubMed] [Google Scholar]
- Maad J. Phenotypic selection in hawkmoth-pollinated Platanthera bifolia: targets and fitness surfaces. Evolution. 2000 Feb;54(1):112–123. doi: 10.1111/j.0014-3820.2000.tb00012.x. [DOI] [PubMed] [Google Scholar]