Specific pollen placement by zygomorphic flowers on pollinators is one of the key innovations of angiosperm evolution . In most phylogenetic lineages that have evolved zygomorphic flowers, reproductive organs are positioned either in the lower or upper part of the flower. Although these specific positions largely enhance pollen economy, they also represent architectural constraints such that flowers are able to place pollen only on the dorsal or ventral part of pollinators' bodies . Such constraints can lead to interspecific pollen placement in situations where phylogenetically related species with the same floral architecture share pollinators .
Bartoš, M., & Janeček, Š. (2014, September 8). Pollinator-induced twisting of flowers sidesteps floral architecture constraints. Current Biology. Cell Press. https://doi.org/10.1016/j.cub.2014.07.056