Reproductive Character Displacement and Floral Diversity in Solanum Section Androceras

Abstract

Flowers of several Solaurum species are conspicuously divergent in regions of distributional overlap in Mexico. S. grlyi and S. lumtiholtziatnumi are similar in floral size throughout most of their respective r-anges, but in an extensive zonle of sympatr-y flowers of S. gr ayi ai-e much smaller thani elsews here. Similarpatter-nis of displacemiient are observed in sympatric local populations of other species. The unusual heteranidrous morphology of section Androceras flox ers appear-s to restrict the size i-anige of bee visitors that can effectively pollinate them, and co-existing species with flow\ers of disparate size may exploit quite different segments of the available pollinator fauna. Species of section Andro-eras display an extr-aordinary diversity of flor-al sizes in comparison with other Solanumn alliances. Floral diver-sity in the section may have evolved via historical episodes of char-acterdisplacemenit, driven by a potenitial formechanical isolation. Floral differences between related plant species often appear to be adaptive, sometimes reinforcng reproductive isolation by imposing mechanical, ethological, or phenological barriers (V. Grant, 1949; Levin, 1971, in press), and sometimes relaxing competitive pressures by permitting efficient subdivision of limited pollinator pools (Levin and Anderson, 1970; Mosquin, 1971; Heinrich, 1975; Levin, in press). If differences among co-existing species in floral traits are really advantageous, then floral character displacement ought to be commnonly encountered in regions of species sympatry. Brown and Wilson (1956) formualized the concept of character displacement (which dates back under the name 'character divergence to Darwin, 1859), and a plethora of examples have since been described and disputed in the zoological literature (P. Grant, 1972). It is surprising that there have been relatively few r-eports of character displacement in plants, and taxonomists observing variation patterns possibly attributable to character displacement would do a service by calling attention to them. Currently known cases of floral character displacement can be briefly summarized. The ranges of two montane species of Fu(hsia overlap in central Mexico (Breedlove, 1969). Where allopatric, the species have similar, generalized morphologies and are visited by both bees and birds. Where sympatric, their flowers are sharply divergent, and one is a bee specialist, the other a bird specialist. Two subspecies of Polanisia dode(andra appear to have undergone character displacement in floral size at their zone of contact near the U.S.-Mexican border (Iltis, 1958). The author suggests no explanation. Corolla color displacement occurs in Illinois populations of Phlox pilosa where that species co-exists with P. L. H. Bailey Hortorium and Section of Ecology and Systemnatics, C(ornell University, Ithaca, New York 14853.