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Journal Article

Pollination Systems as Isolating Mechanisms in Angiosperms

Verne Grant
Evolution
Vol. 3, No. 1 (Mar., 1949), pp. 82-97
DOI: 10.2307/2405454
Stable URL: http://www.jstor.org/stable/2405454
Page Count: 16
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Pollination Systems as Isolating Mechanisms in Angiosperms
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Abstract

Floral isolating mechanisms consist of barriers to interspecific pollination in angiosperms imposed by structural contrivances of the flowers which prevent the pollen of one species from being conveyed to the stigmas of the other (mechanical isolation), or imposed by the constancy of the pollinators to one kind of flower (ethological isolation). A necessary precondition for the rise of floral isolating mechanisms in any group of angiosperms is that the plants shall be non-promiscuous, that is, pollination shall be effected exclusively by certain types of animals. Such non-promiscuous plants are shown to be differentiated into species on floral characters to a greater extent than angiosperms pollinated promiscuously by miscellaneous insects. 40 per cent of the taxonomic characters in bee and longtongued fly plants pertain to the floral parts exclusive of calyx, and the corresponding figure in bird plants is 37 per cent, as compared with only 15 per cent of floral characters in promiscuous, entomophilous angiosperms, and 4 per cent in wind-pollinated angiosperms (cf. table 1). The great importance of floral characters for the classification of non-promiscuous angiosperms may perhaps be a consequence of the operation of floral isolating mechanisms. Floral isolation of the mechanical type may exist in plants pollinated by birds, butterflies, moths, bees, or long-tongued flies. Ethological isolation must be confined to plants pollinated by those insects which exhibit habits of flower constancy, chiefly bees, but also some long-tongued flies and hawkmoths. As an example of ethological isolation under artificial conditions some observations and experiments with Gilia capitata (Polemoniaceae) are reported. Three subspecies of Gilia capitata, two with linear corolla lobes and sweet-scented nectar, and one with oval corolla lobes and creosote-scented nectar, were grown intermixed in a garden where they were pollinated by honeybees. The bees were observed in general to remain either with the linear-lobed or the oval-lobed subspecies, and not to cross-pollinate them; in the progeny of the garden plants a low percentage (0 to 21.7 per cent) of F1 hybrids between the two groups of gilias was encountered. The most probable conclusion that can be drawn from existing knowledge of bee psychology, from the known characters of the plants, and from the observed behavior of the bees on the flowers is that the bees were distinguishing the two groups of gilias by the form and venation of their corolla lobes and/or by the odor of their nectar. Ethological isolation may have two roles in the evolution of bee plants: the initiation of a primary evolutionary divergence as a result of the selective pollination of mutant floral types arising within a population; and the prevention or reduction of interbreeding between sympatric populations which have acquired different floral characters during a previous period of geographical separation. In the light of present knowledge, the latter role, or the augmentation of the efficiency of allopatric speciation, is probably of considerable evolutionary importance; whereas the former role, the initiation of sympatric speciation, is largely of academic interest. In any case, bee plants, in so far as they possess a method of isolation not available to promiscuous plants, are in a position to undergo more rapid evolution than the less specialized groups of angiosperms. There are, in the flora of Southern California, an average number of 5.94 species per genus in bee plants, as opposed to 3.38 species per genus in promiscuous, entomophilous angiosperms.

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