Robertson (1914) cited by Antonini (1995) states that, in addition to the synchrony between the plant's flowering period and the bee's reproductive activity, the effort to avoid competition, short flights and nesting in areas close to the plants used in nest provisioning are factors that lead to the evolution of oligolecty.
First, oligolecty is defined as specialization at the level of plant genus or family, not species.
Interestingly, a recent phylogenetic mapping of pollen use within the tribe Anthidiini of the Megachilidae suggests that polylecty (generalization in plants visited for pollen) has evolved repeatedly from oligolecty, but not vice versa (Muller 1996).
Oligolecty appears to be less prevalent in more temperate regions (28% in southern Illinois, Robertson as cited in Michener 1979; 23% in Germany, Westrich 1989) and lowest in the tropics (17% in dry forest of Costa Rica, Heithaus 1979; 1-2% in moist tropical French Guiana, Michener 1979).
A cladistic analysis of the bee species involved allowed me to trace the host-plant specializations in an evolutionary context (see Armbruster 1992) and to address the question of whether oligolecty or polylecty represents the derived foraging strategy in this bee group.
A change from oligolecty to polylecty is thus equally parsimonious as a transition from polylecty to oligolecty.
caturigense as oligolectic due to its highly developed predilection for Leguminosae leads to a clear support for an evolutionary transition from oligolecty to polylecty.
Nevertheless, the ancestral state is assumed to be oligolectic inferring a transition from oligolecty to polylecty within this group for the following reason.