An Early Pleistocene Eagle from Nebraska

Abstract

Grant and Grant (1968) have proposed an explanation for the reciprocal evolution of hummingbirds and the plants upon which they feed. According to this interpretation most hummingbird-pollinated flowers, especially temperate species, have evolved from bee flowers (Grant 1961; Grant and Grant 1965). The process involves an incipient stage during which a primitive hummingbird or progenitor already “preadapted” to feed on a particular bee flower (in the sense of securing insects within the corolla, or nectar, or both), causes at first occasional but then increasingly frequent pollinations. This stage is followed by a resultant eventual elongation (and presumably a decrease in diameter) of the floral tube and thus an increasing exclusiveness for bird pollination. The following discussion assumes that the evolutionary history of P. kunthii follows the model, i.e., that this Mexican highland species has evolved from a relatively unspecialized bee-pollinated form to its present condition as a hummingbird-pollinated species with a high degree of exclusiveness. More specifically, the Diglossa-Bombus exploitation could aid in the selection of P. kunthii as a bird flower, as indicated in the following chronological schema.