Trade-off mitigation: a key concept to understand the evolution and function of convergent floral phenotypes
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Proposed Symposium Title: Trade-off mitigation: a key concept to understand the evolution and function of convergent floral phenotypes
Kazuharu Ohashi1, Ayumu Tanaka1, Andreas Jürgens2, James D. Thomson3
Affiliations: 1 University of Tsukuba, Tsukuba, Japan 2 Technische Universität Darmstadt, Darmstadt, Germany 3 University of Toronto, Toronto, Canada
Flowers are often thought to have adapted to pollination by specific animals due to the trade-off that one flower cannot adapt to diverse pollinators. If this is true, why do most flowers harbor various visitors, while maintaining distinct phenotypes among ecotypes, subspecies, and congeners? Because strong phenotypic trade-offs leading to disruptive selection have only been observed in highly specialized systems, we proposed a hypothesis that flowers can adapt to diverse pollinators simultaneously by evolutionarily mitigating trade-offs (Ohashi et al. 2021). In this talk, we explore three ideas about how floral phenotypic convergence evolves and functions from the perspective of trade-off mitigation. First, we demonstrate that certain phenotypic convergence in flowers—floral color change, compact inflorescences, and nocturnal anthesis—could have resulted from adaptive generalization for particular pollinator communities rather than adaptive specialization for specific pollinators. Next, based on the fact that even specialist flowers are often visited by minor pollinators, we discuss that any pollination system can be positioned along a continuum of "generalized systems for specific sets of pollinator groups." With this in mind, we propose three types of functional traits that constitute a floral syndrome: those that improve pollination by primary visitors, those that exclude flower visitors that act as conditional parasites, and those that modify trait-fitness relationships to mitigate trade-offs among pollinators. Finally, we suggest that the concept of trade-off mitigation may provide an explanation for the uniformity of floral phenotypes in some clades. Flowers attracting a wide range of pollinators, such as compact inflorescences in Apiaceae, may have achieved "super-mitigating" phenotypes that could eliminate fitness valleys among all types of pollinators. In such flowers, any local shift in pollinator composition will not exert disruptive selection pressure but rather result in a slight change of the adaptive peak.