SELECTION ONPOLEMONIUM BRANDEGEEI(POLEMONIACEAE) FLOWERS UNDER HUMMINGBIRD POLLINATION: IN OPPOSITION, PARALLEL, OR INDEPENDENT OF SELECTION BY HAWKMOTHS?

Abstract

We then compared the patterns of selection by hummingbirds with our previous study examining selection by hawkmoths. We documented contrasting selection on sex organ positioning through female function, with hummingbirds selecting for stigmas exserted beyond the anthers and hawkmoths selecting for stigmas recessed below the anthers. Furthermore, hummingbirds selected for longer and wider corolla tubes, and hawkmoths selected for narrower corolla tubes. Therefore, contrasting selection by hawkmoths and hummingbirds may account for variation in sex organ arrangements and corolla dimensions in P. brandegeei.We documented how floral traits under selection by multiple pollinators can result in either an intermediate "compromise" between selective pressures (sex organs) or apparent specialization (corolla tube length) to one pollinator.

Convergent evolution of floral shape tied to pollinator shifts in Iochrominae (Solanaceae)*

“…The insect pollinators are principally bees except in A. arborescens and I. ellipticum, which have generalized systems with contributions from multiple hymenopterans, dipterans, and lepidopterans (Smith et al 2008b). A wide variety of pollinators has been shown to exert selection on corolla shape in microevolutionary studies (e.g., Galen 1996;Gomez et al 2006;Kulbaba and Worley 2013). Nonetheless, our study appears to be the first to examine this relationship on a macroevolutionary scale, treating both pollination system and floral shape as quantitative traits (see Gomez et al (2015) for a discrete example).…”

“…Shape variation may also influence the interaction between the pollinator and the floral reproductive organs, leading to associated variation in pollen deposition or removal. This link between flower shape and pollinator efficiency has well-documented consequences for both male and female components of fitness (e.g., Campbell et al 1996;Kulbaba and Worley 2013;La Rosa and Conner 2017). Importantly in the context of the pollinator-shift model, different pollinators often exert different selective pressures on flower shape (Campbell 2004;Muchhala 2007;Gomez et al 2008), suggesting that shifts among pollinators, whether within a single species or across a clade, will lead to corresponding shifts in multiple aspects of shape.…”

The evolvability of herkogamy: Quantifying the evolutionary potential of a composite trait

“…Indeed, evolutionary theory predicts reduced additive genetic variance and hence reduced evolvability in highly selfing populations (Charlesworth and Charlesworth 1995;Lande and Porcher 2015). In such species, we expect stabilizing selection on flower architecture if the pollinator community is constant across years (Cresswell 2000;Armbruster et al 2009a), while among-year variation in the pollinator community might lead to fluctuating selection (Sahli and Conner 2011;Kulbaba and Worley 2013;Campbell and Powers 2015). For mixed-mating species, temporal variation in pollinator communities may generate variation in outcrossing rates (Eckert et al 2009) and fluctuating selection on herkogamy.…”

A meta‐analysis of the agents of selection on floral traits

“…In contrast to trait type, less is known about whether the strength of selection on floral traits differs predictably between populations pollinated by different taxa. Few studies have estimated selection by different pollinator taxa on floral traits (Sahli and Conner ; Kulbaba and Worley ), and these studies have focused on differences in the direction of selection, rather than the strength of selection. But all things being equal, floral traits should evolve more rapidly in response to pollinator taxa that exert strong selection than to taxa that exert weak selection (Sahli and Conner ).…”