Floral specialization and angiosperm diversity: phenotypic divergence, fitness trade-offs and realized pollination accuracy - PubMed (original) (raw)

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

W Scott Armbruster. AoB Plants. 2014.

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 one pollinator 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.

Keywords: Adaptive accuracy; Collinsia; Dalechampia; Pedicularis; Stylidium.; fitness trade-offs; pollination; realized precision.

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Figures

Figure 1.

Figure 1.

Causes of differences in species diversity of lineages with different character states, here, for example, bearing flowers with (petaliferous) and without (apetalous) petals.

Figure 2.

Figure 2.

Complex links between floral specialization, floral ‘isolation’ and clade species diversity/richness. Specialized flowers may increase the likelihood of floral isolation in the strict sense and/or in the broad sense. Increased occurrence of floral isolation, in the strict sense, may increase the speciation rate and hence clade species richness and diversity. Alternatively, floral ‘isolation’, in the broad sense, may decrease the rate of extinction and lead to increases in clade species richness and thereby diversity. In turn, increased species diversity of clades may increase the number of clade members occurring in sympatry, thereby selecting for improvements in floral isolation in the broad sense, which may be manifested by increased floral specialization.

Figure 3.

Figure 3.

Adaptive ‘ridge’ (a series of concentric cigar-shaped volumes) in three-trait space, capturing fitness trade-offs in Dalechampia spp. with resin rewards. Adaptation to small-bee pollinators precludes visitation by large bees, while adaptation to large-bee pollinators precludes small-bee pollinators. The darker the shade of blue, the higher the fitness in that volume. Off-diagonal volumes (clear) experience lowest fitness. Region 1 is a volume of low fitness because only small bees are attracted, but only large bees contact stigmas and anthers. Region 2 is a volume of low fitness because resin costs exceed pollination benefits, and/or greater interspecific pollination occurs. Region 3 is a volume of low fitness because, although the small bees attracted touch the stigmas, they do not carry pollen because they do not contact the anthers.

Figure 4.

Figure 4.

Flower of Collinsia sparsiflora with pollinating male Eucera (Tetralonia) bee (Apidae: Anthophorinae). The keel of the lower lip is partially depressed, with the fertile part contacting the underside of the thorax.

Figure 5.

Figure 5.

Stylidium dichotomum flower in female phase retrieving pollen from Leioproctus sp. which has sprung the column while obtaining nectar.

Figure 6.

Figure 6.

Number of papers on evolutionary aspects of pollination published in 5-year periods from 1989 to 2013. Data from Thomson ISI Web of Science, searching on “evolution AND pollination” on 30 Oct. 2013. Note that papers published prior to 1988 were excluded from the analysis because they did not include searchable abstracts.

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