Floral specialization and angiosperm diversity: phenotypic divergence, fitness trade-offs and realized pollination accuracy.

W Scott Armbruster

doi:10.1093/aobpla/plu003

W Scott Armbruster

Open Access

https://doi.org/10.1093/aobpla/plu003

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Abstract

Plant reproduction by means of flowers has long been thought to promote the success and diversification of angiosperms. It remains unclear, however, how this success has come about. Do flowers, and their capacity to have specialized functions, increase speciation rates or decrease extinction rates? Is floral specialization fundamental or incidental to the diversification? Some studies suggest that the conclusions we draw about the role of flowers in the diversification and increased phenotypic disparity (phenotypic diversity) of angiosperms depends on the system. For orchids, for example, specialized pollination may have increased speciation rates, in part because in most orchids pollen is packed in discrete units so that pollination is precise enough to contribute to reproductive isolation. In most plants, however, granular pollen results in low realized pollination precision, and thus key innovations involving flowers more likely reflect reduced extinction rates combined with opportunities for evolution of greater phenotypic disparity (phenotypic diversity) and occupation of new niches. Understanding the causes and consequences of the evolution of specialized flowers requires knowledge of both the selective regimes and the potential fitness trade-offs in using more than onepollinator functional group. The study of floral function and flowering-plant diversification remains a vibrant evolutionary field. Application of new methods, from measuring natural selection to estimating speciation rates, holds much promise for improving our understanding of the relationship between floral specialization and evolutionary success.

Highlights

Flowering plants are the most abundant and diverse autotrophic organisms on land
Given that only a tiny amount of inter-morph pollination will swamp any divergence except that driven by very strong selection (Wright 1951; Roughgarden 1979; Waser 2001; Armbruster and Muchhala 2009), it is important to assess the degree of reproductive isolation that can result from different kinds of floral specialization
Any feature that increases the likelihood of reproductive isolation may have major effects on both diversification and phenotypic disparity

Summary

Introduction

Flowering plants are the most abundant and diverse autotrophic organisms on land. Reproduction by means of flowers and fruits is often invoked as one of the main causes of this evolutionary success (Stebbins 1974; Regal 1977), but the mechanisms of the putative causal link between various angiosperm innovations and the group’s evolutionary success remain elusive (Crepet and Niklas 2009). Selection to speed up the life cycle in highly seasonal environments may result Another approach to assessing fitness trade-offs and possible connections between floral specialization and flowering-plant diversification involves adaptive accuracy theory. This approach, which can be used in this context to quantify pollination accuracy, is useful for assessing the ability of the flowers of sympatric species to generate segregated pollen flow and maintain reproductive isolation (or not). That the potential role of natural selection in gradual shifts between pollinators does not, by itself, demonstrate its importance

Discussion

Literature Cited