POLLINATOR CHOICE AND STABILIZING SELECTION FOR FLOWER COLOR INDELPHINIUM NELSONII

Abstract

Bright colors are characteristic of the flowers of many higher plants. For the last century these colors have been interpreted as adaptations by which flowers signal their presence to animal pollinators (Darwin, 1876; Muller, 1883; Plateau, 1899; Clements and Long, 1923; Kugler, 1943; Daumer, 1958; Heinrich, 1975; Jones, 1978; Kevan, 1978). If this is correct, the exact color(s) found within populations of single species, and the array of colors represented in an assemblage of sympatric species, may be explicable largely as evolutionary consequences of pollinator foraging behavior. Several plant species in the Rocky Mountains of Colorado exhibit polymorphisms well suited for studies of relationships between flower color, pollinator behavior, and plant fitness. Although flowers of these species are normally brightly colored, we have noticed rare individuals whose flowers are white or pale. Such color changes are likely to have a genetic basis, as we shall discuss later. Species with such flower color polymorphisms include Delphinium nelsonii Greene (Ranunculaceae), Lupinus argenteus Pursh (Leguminosae), Ipomopsis aggregata (Pursh) V. Grant (Polemoniaceae), Iris missouriensis Nutt. (Iridaceae), and Linum lewisii Pursh (Linaceae). The low frequencies of individuals in populations of these species suggest an equilibrium between their production by spontaneous mutation and some form of selection against them (cf. Crow and Kimura, 1970; Harding, 1970). Selection against albinos could take at least two forms. Mutations causing reduced flower pigmentation might be associated with physiological or developmental changes (e. g., reduced ovule number, stigma size, or receptivity) which lower intrinsic fecundity. Alternatively, pollinators might discriminate against albinos and thus exert stabilizing selection for flower color. In this paper we describe experiments and observations with one of the species listed above, Delphinium nelsonii, a small perennial herb whose flowers are normally deep blue. Near the Rocky Mountain Biological Laboratory (RMBL) in west-central Colorado, D. nelsonii is pollinated primarily by broad-tailed hummingbirds (Selasphorus platycercus Swainson) and bumblebee queens (B ombus appositus Cresson, B. flavifrons Cresson, B. californicus F. Smith, and B. nevadensis Cresson) (Waser, 1978; Price and Waser, 1979). We present evidence that paleor white-flowered albinos are rare in populations near the RMBL; that selection against albinos occurs in the form of reduced seed sets relative to normal blueflowered individuals; that intrinsic fecundity of albinos is not lower than that of normal individuals; that both hummingbirds and bumblebees discriminate against albinos in a way sufficient to explain their low seed sets and consequent rarity; and that discrimination may be due to difficulty pollinators experience in extracting nectar from albino flowers.