Understanding how reproductive traits evolved in the past leads to important insights into how organisms adapted. In sex- ually reproducing organisms, traits associated with outcrossing

782 American Journal of Botany 97(5): 782–796, 2010; http://www.amjbot.org/ © 2010 Botanical Society of America Understanding how reproductive traits evolved in the past leads to important insights into how organisms adapted. In sexually reproducing organisms, traits associated with outcrossing are thought to be under strong selection because of their direct effect on reproductive success; the radiation of fl oral morphologies in angiosperms represents a great example of this (e.g., Darwin, 1871 ; Lloyd and Webb, 1992 ). Specifi cally in the case of animal-pollinated plant species, pollinator preference represents a strong selective pressure to fl ower traits ( Darwin, 1862 ; F æ gri and van der Pijl, 1966 ; Stebbins, 1970 ; Schemske and Bradshaw, 1999 ). Attraction of pollinators and successful pollen transfer represent the primary targets of selection during fl ower evolution, leading to repeated evolutionary shifts between pollinators and, consequently, to the diversifi cation of fl oral forms ( Darwin, 1862 ; F æ gri and van der Pijl, 1966 ; Stebbins, 1970 , 1974 ; Harder and Barrett, 2006 ). The association between particular pollinators and specifi c fl oral traits is thought to have led to the evolution of pollination syndromes, which correspond to suites of fl oral traits that are associated with the attraction of specifi c pollinators ( Vogel, 1954 ; F æ gri and van der Pijl, 1966 ; Stebbins, 1970 ; Fenster et al., 2004 ). In this context, homoplastic evolution of similar fl ower morphologies are thought to have resulted from strong selection pressures by similar pollinators ( F æ gri and van der Pijl, 1966 ). Despite this, differing opinions exist regarding the applicability of the syndrome concept (see Herrera, 1996 ; Strauss and Whittall, 2006 ; Waser, 2006 ). Nonetheless, the recognition of pollinators as functional groups defi ned by their ecological similarities is generally considered a valuable concept for the study of fl ower specialization to pollinators ( Fenster et al., 2004 ). Evolutionary specialization leads to shifts in fl oral morphologies that are associated with the use of a subset of pollinators compared to those visiting the ancestral morphology ( Armbruster et al., 2000 ; Fenster et al., 2004 ). The identifi cation of specifi c shifts in fl oral morphologies allows us to establish when signifi cant evolutionary changes took place, as well as to test specifi c hypotheses associated with the processes that may have led to such morphological shifts ( Grant and Grant, 1968 ; Armbruster and Webster, 1982 ; Armbruster et al., 1994 ; Johnson and Steiner, 1997 ; Hansen et al., 2000 ; Fenster et al., 2004 and references therein). Shifts among fl oral morphologies resulting from the selection exerted by specifi c pollinator groups can occur in three different ways ( Armbruster, 1993 ): (1) gradual quantitative shifts that correspond to those shifts proposed by Darwin ’ s coevolutionary race model (1862); (2) gradual qualitative shifts with intermediate stages among fl owers pollinated by different pollinators; and (3) qualitative shifts without intermediate morphologies ( Stebbins, 1970 , 1974 ; Armbruster, 1993 ). Despite the transient nature of the intermediate phases among fl oral morphologies, these morphologies may persist in plant populations when the frequencies of the effective pollinators fl uctuate ( Stebbins, 1970 , 1974 ). This condition is thought to represent the rule, rather than the exception, for shifts between pollination modes ( Stebbins, 1970 ). On the other hand, drastic variation in the frequencies of pollinators might lead to rapid changes in traits that determine pollinator specifi city, leading to shifts without intermediate morphologies. Little evidence is available as to how fl oral morphologies have shifted over time, making it diffi cult to evaluate the direction 1 Manuscript received 25 June 2009; revision accepted 25 February 2010. The authors thank B. Loeuille, M. Kaehler, R. Ree, S. Branco, R. Olmstead, S. Graham, and an anonymous reviewer for comments that greatly improved this manuscript. This paper is part of the thesis of S.A., which was supported by FAPESP (Grant 06/59916-0) and MBG (Elizabeth E. Bascom Fellowship). 2 Author for correspondence (e-mail: [email protected])