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Pollination by Euglossine Bees

Robert L. Dressler
Evolution
Vol. 22, No. 1 (Mar., 1968), pp. 202-210
DOI: 10.2307/2406664
Stable URL: http://www.jstor.org/stable/2406664
Page Count: 9
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Pollination by Euglossine Bees
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Abstract

The term "euglossine pollination" is used for the relationships in which flowers are visited only by male euglossine bees, which, attracted by odor, "brush" on the surface of the flower. These relationships are shown by at least four genera of euglossine bees, a number of orchid genera and a few genera of Araceae and Gesneriaceae. Among the orchids, these pollination systems have permitted the evolution of unusual physical mechanisms of pollination, in which the pollinaria are expelled with great force, or the bees are so placed that they fall in a predetermined path and receive the pollinaria. The placement of the bee within the flower and the placement of the pollinarium on the bee are often very precise. The attraction of the bees by flower odor is markedly specific, in many cases attracting only a single bee species. Pollination relationships are found to be, in general, more specific among species which have close sympatric allies than among geographically isolated species. The specificity of the odors has provided effective isolating mechanisms for many sympatric species pairs. As a rule, distinct, closely related species occur sympatrically only when they are pollinated by different bee species or when they are pollinated by different physical mechanisms. When different physical mechanisms are involved, a given bee species may pollinate several sympatric orchid species (usually not closely related). Euglossine pollination appears to be efficient in terms of fruit-set, but more quantitative data are needed. This system also appears to impose a high degree of "fidelity" on the bees. Euglossine pollinated orchids have various devices which promote cross-pollination, and the strong-flying bees are evidently capable of cross-pollination over considerable distances. Some plant species are evidently limited geographically by the occurrence of their pollinators, but the bee distributions are not well known. Euglossine pollination is a system favorable for speciation. Some plant species may find pre-adapted pollinators in new areas after long-distance dispersal. This pollination system offers an unusually good model for sympatric speciation, based on odor-modifying mutation. Shifting to markedly larger or smaller pollinators may cause severe selection pressure and rapid morphological change.

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