Selection of trait combinations through bee and fly visitation to flowers of Polemonium foliosissimum

Abstract

Pollinators are known to exert natural selection on floral traits, but the extent to which combinations of floral traits are subject to correlational selection (nonadditive effects of two traits on fitness) is not well understood. Over two years, we used phenotypic manipulations of plant traits to test for effects of flower colour, flower shape and their interaction on rates of pollinator visitation to Polemonium foliosissimum. We also tested for correlational selection based on weighting visitation by the amount of conspecific pollen delivered per visit by each category of insect visitor. Although bumblebees were the presumed pollinators, solitary bees and flies contributed substantially (42%) to pollination. In manipulations of one trait at a time, insects visited flowers presenting the natural colour and shape over flowers manipulated to present artificial mutants with either paler colour or a more open or more tubular flower. When both colour and shape were manipulated in combination, selection on both traits arose, with bumblebees responding mainly to colour and flies responding mainly to shape. Despite selection on both floral traits, in a year with many bumblebees, we saw no evidence for correlational selection of these traits. In a year when flies predominated, fly visitation showed a pattern of correlational selection, but not favouring the natural phenotype, and correlational selection was still not detected for expected pollen receipt. These results show that flower colour and shape are subject to pollinator-mediated selection and that correlational selection can be generated based on pollinator visitation alone, but provide no evidence for correlational selection specifically for the current phenotype.

Quantifying hummingbird preference for floral trait combinations: The role of selection on trait interactions in the evolution of pollination syndromes

“…Hummingbirds favored wide corolla tube width and vertical presentation of the flowers, neither of which is found in S. virginica, but this preference was trait context dependent. Association of contrary characters states with greater pollinator visitation has also been documented in at least one other study utilizing two trait manipulations (Campbell et al 2014). The question is: why would present-day S. virginica exhibit traits that do not consistently improve attraction?…”

“…The microevolutionary equivalent of our experiment (here we manipulated traits across macroevolutionary scales) is quantifying the role of correlational selection to phenotypic selection, that is, the detection of selection to increase trait covariance. In the just cited references and other phenotypic selection studies, there is little evidence of correlational selection acting on the flower (Sletvold et al 2012;Kulbaba and Worley 2012), and when correlational selection is detected, it is often limited to a subset of two-trait interactions (Conner et al 1996a,b;Valdivia and Niemeyer 2006;reviewed in Campbell 2009;Nattero et al 2010;Vanhoenacker et al 2010;Agren 2010, 2010;Bartkowski and Johnston 2012;Campbell et al 2014). However, when dimension reduction techniques are used, for example canonical analysis (Reynolds et al 2010a, b), selection acting on trait interactions is often observed.…”

“…There is a long history of testing the adaptive hypotheses of traits in plant evolutionary studies through experimental manipulation of the flower (Clements and Long 1923) and these approaches are still in use (Cresswell and Galen 1991;Campbell et al 1996;Fulton and Hodges 1999;Schemske and Bradshaw 1999;Temeles and Rankin 2000;Gomez et al 2006;Sletvold and Agren 2011;Campbell et al 2014). Simple trait manipulations have also been used successfully in animal studies (e.g., Ketterson et al 1996;Sinervo and Svensson 2002;McGlothlin et al 2005;de Heij et al 2011;Balmford et al 2013) to test adaptive hypotheses of traits.…”

Spatial variation in pollinator‐mediated selection on phenology, floral display and spur length in the orchidGymnadenia conopsea

“…Pollinators may exert selection not only on single floral traits but also on combinations of traits, that is, correlational selection. In particular, correlational selection is expected when one trait influences the rate of pollinator visitation and a second trait influences pollinator effectiveness per visit, because pollination success will depend on the product of these two components (Sletvold & Agren, 2011;Campbell et al, 2014). In the orchid Dactylorhiza lapponica, phenotypic manipulations documented nonadditive effects on reproductive success of floral display and spur length, a trait affecting pollination efficiency (Sletvold & Agren, 2011).…”

Wise flies: a pre‐dispersal seed predator prefers hermaphrodites over females in the gynodioecious Polemonium foliosissimum

“…Finally, because Hylemya does not pollinate the flowers on which it oviposits, larvae will perish if their host flower is not pollinated. Thus the most important cues for Hylemya may be those used by pollinators such as corolla size (Brody, ) or a combination of floral traits (Campbell et al , ). Interestingly, although petal size and sepal size were not correlated here on P. foliosissimum , they are correlated on I. aggregata , the alternative host for Hylemya (Brody, ; Campbell et al , ).…”