Abstract
The interaction between plants and extremely diverse habitats and environmental conditions of California has resulted in rapid evolution in many plant groups. A small number of studies suggest that evolution in many of these groups has been accompanied and perhaps paralleled by evolution in particular groups of bees (Linsley and MacSwain, 1957, 1959; Linsley et al., 1963a, 1963b; 1964; Thorp, 1969). The interaction between these bees and their pollen plants is oligolectic. An oligolectic relationship is defined by Linsley and MacSwain (1957, 1958) as one . . in which the individual members of the population, throughout its range and in the presence of other pollen sources, consistently and regularly collect pollen from but a single plant species or from a group of similar or related plant species, turning to other sources, if at all, only when there is a local shortage or absence of that pollen. Oligolectic relationships may be nonselective or co-evolved. Included in the first class are Andrena linsleyi Timberlake, which collects pollen of Oenothera deltoides Torr. and Frem. in the morning after sphinx-moths have pollinated the flowers (Linsley et al., 1963; Gregory, 1964), and Andrena nemophilae Ribble which is discussed below. Such a bee must adapt to the evolutionary vicissitudes of the flower and its pollinator(s). The second class includes those bees which are pollinators of the host plant. Thus they may influence the evolution of the plant as well as being influenced by the plant. This group of bees and their pollen plants are of particular interest to the student of pollination biology and evolution. Although a number of studies have dealt with oligolectic bees, the basis of the relationship remains an enigma. The flight period of most oligolectic bees is short and the adult bee's only previous contact with a particular pollen source is the pollen and nectar it consumed as a larva. Three explanations have been suggested to explain oligolecty. First, it is genetically controlled (Thorp, 1969). Second, a preimaginal conditioning from the pollen and nectar that the larva consumes (Linsley, 1958). Third, some combination of the above, i.e., genetical control modified by conditioning (Thorp, 1969). In this paper my objectives are (1) to present a broad picture of the plant Nemophila menziesii H. and A. s.l. and its pollinators and (2) to re-examine particular questions concerning evolution within oligolectic bees. Specifically, I discuss the distribution, floral behavior and breeding system of the plant, Nemophila as a bee plant, the oligolectic Nemophila bees, the role of polylectic bees in pollination, and fly pollination. In the final section I discuss host plant recognition and evolution in oligolectic bees
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