Hasta fechas muy recientes, la mayor??a de los estudios te??ricos y emp??ricos sobre la evoluci??n de la relaci??n entre las plantas y sus polinizadores han asumido que estos sistemas est??n altamente especializados, es decir, cada especie vegetal es visitada por s??lo unos cuantos polinizadores relacionados filogen??ticamente como fruto de la selecci??n impuesta por ellos. La omnipresencia de especializaci??n se ha derivado de aceptar el principio del polinizador m??s eficiente, seg??n el cual una planta debe especializarse en el polinizador m??s eficaz y/o m??s abundante cuando su disponibilidad es predecible. Muchos estudios han encontrado realmente que los polinizadores ejercen selecci??n fenot??pica sobre rasgos reproductivos de las plantas. Sin embargo, igual de evidente es el hecho de que muchas especies vegetales son polinizadas por conjuntos numerosos y taxon??micamente diversos de polinizadores. Estas observaciones sugieren que en realidad los sistemas de polinizaci??n son generalistas. La generalizaci??n es producida y mantenida por varios factores ecol??gicos entre los que destacan la variaci??n espacial y la fluctuaci??n temporal de la identidad de los polinizadores m??s importantes, la similitud en la eficiencia y preferencia por los rasgos florales por parte de polinizadores diferentes, la existencia de factores externos que reducen la capacidad de los polinizadores de afectar al fitness de la planta, y la existencia de selecci??n indirecta.A controversy exists about the importance of generalization versus specialization in pollination systems. Whereas the evolutionary and ecological theory speculates that the plant-pollinator systems should be highly specialized, many evolutionary ecologists now acknowledge that generalization is frequent in natural systems. There is still no consensus about which ecological factors can promote this counter-intuitive and paradoxical result. I propose that generalization in plant-pollination systems can arise by several non-exclusive factors. One kind of factors are related to the ability of pollinators of acting as true selective agents. Despite that many studies have shown that pollinators can provoke phenotypic selection on floral traits, only occasionally it has been shown that this selection actually produces any response to selection. Two main reasons can explain why pollinators cannot induce adaptations in many plants: (1a) a significant spatial and temporal variability in pollinator abundance and identity highly reduces the possibilities of congruent selection on floral traits; (1b) the occurrence of extrinsic factors acting during another stages of the plant reproductive and recruitment processes can also decrease the actual effect that pollinators has on fitness. A second group of factors are those related with the fact that, to specialization occurs, it is necessary that two different pollinators can constitute as completely separated selective agents, by differing in their per-visit efficiencies and/or floral trait preferences. By contrast, similarity in efficiency and preference will constrain specialization even although pollinators act as selective pressures, since plant will have not any criteria to benefit a given pollinator species with respect to the other