Criticisms of the syndromes

Whilst it is clear that pollination syndromes can be observed in nature, there has been much debate amongst scientists as to how frequent they are and to what extent we can use the classical syndromes to classify plant-pollinator interactions.[26] Although some species of plants are visited only by one type of animal (i.e. they are functionally specialized), many plant species are visited by very different pollinators.[25][27] For example, a flower may be pollinated by bees, butterflies, and birds. Strict specialization of plants relying on one species of pollinator is relatively rare, probably because it can result in variable reproductive success across years as pollinator populations vary significantly.[25] In such cases, plants should generalize on a wide range of pollinators, and such ecological generalization is frequently found in nature. A study in Tasmania found the syndromes did not usefully predict the pollinators.[28] This debate has led to a critical re-evaluation of the syndromes, which suggests that on average about one third of the flowering plants can be classified into the classical syndromes.[3] This reflects the fact that nature is much less predictable and straightforward than 19th Century biologists originally thought. Pollination syndromes can be thought of as extremes of a continuum of greater or lesser specialization or generalization onto particular functional groups of pollinators that exert similar selective pressures"[4] and the frequency with which flowers conform to the expectations of the pollination syndromes is relatively rare. In addition, new types of plant-pollinator interaction, involving "unusual" pollinating animals are regularly being discovered, such as specialized pollination by spider hunting wasps (Pompilidae) and fruit chafers (Cetoniidae) in the eastern grasslands of South Africa.[18] These plants do not fit into the classical syndromes, though they may show evidence of convergent evolution in their own right. An analysis of flower traits and visitation in 49 species in the plant genus Penstemon found that it was possible to separate bird- and bee- pollinated species quite well, but only by using floral traits which were not considered in the classical accounts of syndromes, such as the details of anther opening.[29] Although a recent review concluded that there is "overwhelming evidence that functional groups exert different selection pressures on floral traits",[4] the sheer complexity and subtlety of plant-pollinator interactions (and the growing recognition that non-pollinating organisms such as seed predators can affect the evolution of flower traits) means that this debate is likely to continue for some time. Penstemon ( /?p?nst?m?n/),[1] Beard-tongue, is a large genus of North American and East Asian flowering plants formerly placed in the Scrophulariaceae family (Cronquist system). Due to new genetic research, it has now been placed in the vastly expanded family Plantaginaceae.