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[交流] 一篇关于花的演化过程中的一些超越传统理论！

Evolution of floral form: electrostatic forces, pollination, and adaptive compromise
W. Scott Armbruster

Do floral size and shape affect the electrostatic properties of flowers? Do electrostatic properties affect the delivery of pollen to, and capture from, pollinators? The first detailed analysis of how variation in floral form may affect electrostatically 'assisted' pollination is provided by Vaknin et al. (2001) in this issue (see pp. 301–306). They used an experimental approach to study this question by examining the electrostatic pollination of model flowers of different sizes and shapes. This study not only illustrates a new way to analyse floral form and function experimentally, but also demonstrates the potential importance of a largely unrecognized selective force that may significantly influence the evolution of floral form.
A cornerstone of the evidence that Darwin mustered to support his new hypothesis of natural selection in 1859 was the adaptive interpretation of variation in form of the flowers of orchids (Darwin, 1859, 1862) and other angiosperm species (Darwin, 1877). Since Darwin's time the adaptive analysis of floral form has been an active area of evolutionary research (Waser, 1983; Wilson & Thomson, 1996). Indeed, floral study systems are arguably more amenable to adaptive analysis than most other comparative systems because many features of floral form are directly and obviously related to floral function, and because floral function is closely connected to reproductive fitness. For example, flower colour interacts directly with animal colour perception, and floral fragrance with animal olfaction, to attract pollinators, while flower size and shape interact with animal size and behaviour to achieve pollen deposition and pickup (Wilson & Thomson, 1996; Chittka et al., 2001; Cresswell, 2001).
'The process of adaptive compromise in flowers may be more complex than we previously realised'
Electrostatic pollination
The first suggestions that electrostatic forces may be involved in the pickup and deposition of pollen by pollinators were theoretical discussions by Hardin (1976), Corbet et al. (1982), Erikson & Buchmann (1983), and by Buchmann & Hurley (1978) in connection with flowers that release pollen to buzzing bees ('buzz pollination'). Inductive charging causes movement of charge through plant tissues when a body of opposite charge approaches a structure, such as the flower. Thus it can be expected that as a pollinator approaches a flower, the flower (and its pollen) develop an opposite (generally negative) charge to that of the animal (and its pollen), potentially causing animal-borne pollen to be attracted to floral structures and vice versa. This process may significantly enhance deposition of pollen on pollinators and on stigmas.
Empirical evidence for the effects of electrostatic forces on pollination has been obtained only relatively recently (Gan-Mor et al., 1995). Furthermore, up until now we have had no empirical evidence that such processes influence the evolution of floral form. The experiments by Vaknin et al. (2001), using apple pollen and metal models of apple flowers of varying size and shape, provide the first empirical evidence supporting the idea that the size and shape of flowers influence their electrostatic properties and, thereby, potentially the rate of arrival of pollinator-borne pollen on their stigmas. Thus, these new data show that floral electrostatic properties might sometimes influence the evolution of floral form.
Adaptive compromise in floral evolution
One of the first to point out that the parts of a flower may play roles in multiple functions was Sprengel (1793). He noted that structures, such as petals and petal appendages, may be involved in protecting floral nectar from dilution by rain water, in addition to attracting pollinators. Darwin (1859, 1862) drew on Sprengel's observations, not only in his adaptive interpretation of floral design, but as an example of how organs with multiple functions must reflect the net effect of multiple, often conflicting, selective pressures. Organisms respond to the sum of combined selective pressure generated by the different effects of variation in floral form on seed production. The net effect of opposing selective forces is usually declining fitness at both extremes, with highest fitness at some intermediate value (e.g. fitness trade-offs leading to stabilizing selection; Fig. 1). A phenotype produced by such opposing selective forces can be said to reflect evolutionary or adaptive 'compromise' (Armbruster, 1996; Brody, 1997).
Sprengel's (1793) observations suggest to an evolutionist (he himself was not, of course, since his work preceded that of both Lamarck and Darwin) that selection for attraction of pollinators and protection of nectar may be in conflict with one another. Similarly, selection generated by the need to attract pollinators and by the need to hide or protect flowers from flower-eating or seed-eating herbivores may often be in conflict (Armbruster, 1997; Brody, 1997). More subtle adaptive compromise may result from selection for longer styles that promote pollen-tube competition, thereby allowing female mate choice (Lankinen & Skogsmyr, 2001), vs selection for short or intermediate style length generated by the need to fit the size and shape of the principal pollinators (Armbruster, 1996, Fig. 2). To this list we can now add the observation by Vaknin et al. (2001), that selection for longer, more exerted styles is potentially generated by the electrostatic enhancement of pollen arrival on the stigma, while the fit of flowers with pollinators may generally select for short or intermediate style lengths (Cresswell, 2001; Figs. 1, 2). Vaknin et al. (2001) also show that selection generated by electrostatic enhancement of pollination may favour wider, more-open corollas, while selection generated by pollinators and nonpollinating floral visitors (e.g. nectar thieves) may favour other corolla characteristics. Thus the process of adaptive compromise in flowers may be more complex than we previously realized.

Analysis of complex evolution through study of floral form
The relatively direct relationship between floral form and function has allowed the discipline of floral biology to attempt analyses of more complex and subtle functional and evolutionary processes than can usually be addressed with empirical systems. These include studies of pollen arrival and departure dynamics involving pollinators (Harder & Thomson, 1989) or electrostatic properties (Vaknin et al., 2001). They also include challenging issues in evolution, such as the roles of drift, constraint, and adaptive compromise in floral evolution (Armbruster, 1996; Wilson & Thomson, 1996; Chittka et al., 2001). Recent studies of floral form and function are showing that the evolutionary process is more complicated and idiosyncratic than simple adaptive models suggest (Gould & Lewontin, 1979). For example, integrated comparative studies of flower colour in relation to sensory abilities of pollinating insects suggest that diversification in flower colour may have been influenced by genetic constraints, chance events, exaptation (preadaptation), and indirect selection manifested through floral-vegetative pleiotropy (Chitka et al., 2001). And, as alluded to already, we can now suspect that the length and shape of angiosperms styles may sometimes reflect adaptive compromise to selection generated by pollinator morphology and behaviour, mate choice in plants, and the electrostatic forces influencing pollination (Fig. 2). These new insights demonstrate that experimental study of the relationship between floral form and function remains a fruitful avenue for comparative and functional analyses of the evolutionary process.

Summary
Darwin utilized the form and function of flowers as a prime example of the workings of natural selection. Today, the evolution of floral form remains an exciting area of research. New investigations into the effects of floral shape and size on the electrostatic properties of flowers and electrostatically 'assisted' pollination complement previous and ongoing studies of the effect of floral form on plant mate choice, attraction of animals, and the placement of pollen on, and capture of pollen from, pollinators.
References
1. Armbruster WS. 1996. Evolution of floral morphology and function: an integrated approach to adaptation, constraint, and compromise in Dalechampia (Euphorbiaceae). In: Lloyd DG, Barrett SCH, eds. Floral biology. New York, USA: Chapman & Hall, 241–272.
2. Armbruster WS. 1997. Exaptations link the evolution of plant-herbivore and plant–pollinator interactions: a phylogenetic inquiry. Ecology 78: 1661–1674.
3. Brody AK. 1997. Effects of pollinators, herbivores, and seed predators on flowering phenology. Ecology 78: 1624–1631.
4. Buchmann SL, Hurley JP. 1978. A biophysical model for buzz pollination in angiosperms. Journal of Theoretical Biology 72: 639–657.
5. Chittka L, Spaethe J, Schmidt A, Hickelsberger A. 2001. Adaptation, constraint, and chance in the evolution of flower color and pollinator color vision. In: Chittka L, Thomson JD, eds. Cognitive ecology of pollination. Cambridge, UK: Cambridge University Press, 106–126.
6. Corbet SA, Beament L, Eisikowitch D. 1982. Are electrostatic forces involved in pollen transfer? Plant, Cell & Environment 5: 125–129.
7. Cresswell JE. 2001. Manipulation of female architecture in flowers reveals a narrow optimum for pollen deposition. Ecology 81: 3244–3249.
8. Darwin C. 1859. On the origin of species by means of natural selection. London, UK: Murray.
9. Darwin C. 1862. On the various contrivances by which British and foreign orchids are fertilized by insects, and on the good effects of intercrossing. London, UK: Murray.
10. Darwin C. 1877. The different forms of flowers on plants of the same species. London, UK: Murray.
11. Erikson EH, Buchmann SL. 1983. Electrostatics and pollination. In: Jones CE, Little RJ, eds. Handbook of experimental pollination biology. New York, UK: Van Nostrand Reinhold, 173–184.
12. Gan-Mor S, Schwartz Y, Bechar A, Eisikowitch D, Manor G. 1995. Relevance of electrostatic forces in natural and artificial pollination. Canadian Journal of Agricultural Engineering 37: 189–194.
13. Gould SJ, Lewontin R. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society, Series B 205: 581–598.
14. Harder LD, Thomson JD. 1989. Evolutionary options for maximizing pollen dispersal of animal-pollinated plants. American Naturalist 133: 325–334.
15. Hardin GB. 1976. Better charge, better pollination. Agricultural Research 25: 15.
16. Lankinen A, Skogsmyr I. 2001. Evolution of pistil length as a choice mechanism for pollen quality. Oikos 92: 81–90.
17. Sprengel CK. 1793. Das Entdeckte Geheimniss der Natur Im Bau und in der Befruchtung der Blumen (Reprinted 1972). New York, USA: Weldon & Wesley.
18. Vaknin Y, Gan-Mor S, Bechar A, Ronen B, Eisikowitch D. 2001. Are flowers morphologically adapted to take advantage of electrostatic forces in pollination? New Phytologist 152: 301–306.
19. Waser NM. 1983. The adaptive nature of floral traits: ideas and evidence. In: Real LA, ed. Pollination biology. New York, USA: Academic Press, 241–285.
20. Wilson P, Thomson JD. 1996. How do flowers diverge? In: Lloyd DG, Barrett SCH, eds. Floral biology. New York, USA: Chapman & Hall, 88–111.

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