Floral morphology can vary considerably from one individual to the next within a species. Two of the factors shown to drive this variation are pollinators and climate. Pollinators exert a direct selection pressure on flowers through their aesthetic preferences and ability to physically interact with the flower, while climate can affect morphology either directly, via temperature, radiation, moisture, and other abiotic factors; or indirectly, by affecting what pollinators are available in the plant’s range. The relative contributions of these factors isn’t well understood.
In a recent study published in Annals of Botany, Urs K. Weber and colleagues sought to assess floral variation as it relates to climate and pollinators for four species of the genus Calceolaria, which displays specialized pollination. The genus occurs mostly in the Andes and is pollinated by oil-bees that collect floral oils as a reward. The authors undertook a large-scale sampling of both the plants and their pollinators, took morphological measurements, and studied the corresponding climatic data. The morphological measurements consisted of 25 or more characters including several dimensions of floral lobe size, stigma length, and colour. The combined data were analyzed using spatial approaches, as well as multivariate and structural equation modelling approaches.
The researchers found that the pollinators belonged to three different groups of insect, each with different morphological preferences: two genera of oil-bees and a type of non-oil-collecting fly. Proportions of the three groups of pollinators were quite different from one species to the next, and each group was seen most often on flowers with particular characteristics corresponding to their known preferences. In at least one species, however, analyses suggested that climate does have an indirect effect on morphological variation. As the authors write, the results “show that climate can indirectly explain the variation in floral morphology, probably contributing to floral evolution even in plants highly specialized on a small number of pollinators.”
The four species appear to have different levels of pollinator specialization, a fact the authors attribute to different climatic niche sizes. The climate was also found to significantly explain the differences in floral visitors across a range, supporting “the idea that different climates expose plants to different faunas, either by attracting different floral visitors or by modulating their survival or behaviour at a given locality,” the authors write. “In the future, long-term floral evolution studies should be done in the system, explicitly measuring selection at localities displaying different abiotic and biotic conditions. Such a study will allow a direct quantification of the effect of the variables considered here on plant fitness, as well as an evaluation of whether the variation observed here is heritable.”