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Natural Selection for Reproductive Isolation in Phlox
Donald A. Levin and Harold W. Kerster
Vol. 21, No. 4 (Dec., 1967), pp. 679-687
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2406765
Page Count: 9
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Populations of the eastern species of Phlox typically are composed of organisms with pigmented corollas. A small percentage of these populations show color dimorphism, i.e., they contain pigmented and non-pigmented or white forms. The white phase often comprises less than 1% of such populations. Exceptions to the normal color structure occur in a number of species, especially P. pilosa. Discrete populations or population systems are known to occur in which the white phase is the predominate or sole form. Although such populations are infrequent, they often grow in habitats supporting a congener with similar ecological requirements and flowering period. This congener often is P. glaberrima. The relationship of color displacement and congener presence was studied in an assemblage comprised of P. glaberrima and the two phases of P. pilosa. The pollen of both species is well-marked so that an objective, quantitative appraisal of interspecific pollen flow can be made with facility. An examination of the stigma P. glaberrima and P. pilosa disclosed that there was considerable interspecific pollen flow. It was most enlightening to find that while 30% of the pigmented phase of P. pilosa bore alien pollen, only 12% of the white phase bore such pollen. Moreover, 4.8 times as many grains were transported from P. glaberrima to flowers of the pigmented phase as to equal numbers of flowers of the white phase. Thus it appears that color divergence aids pollinator discrimination and conserves reproductive potential. Character displacement in the eastern Phlox alliance occurs irrespective of the species' abilities to exchange genes. Displacement occurs between species which are incapable of hybridization, but are capable of exchanging pollen. We conclude that Phlox can ill afford the loss of pollen typically incurred in bispecific populations, and that color displacement is the product of selection for enhanced ethological isolation. The presence of a congener which competes for domestic pollen ostensibly has served as the stimulus for color divergence.
Evolution © 1967 Society for the Study of Evolution