Integrative and Comparative Biology 2006 Delph

Integrative and Comparative Biology 2006 Delph - 465 Trait...

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Trait selection in flowering plants: how does sexual selection contribute? Lynda F. Delph 1,* and Tia-Lynn Ashman y * Department of Biology, 1001 East Third Street, Indiana University, Bloomington, IN 47405 USA; y Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260 USA Synopsis By highlighting and merging the frameworks of sexual selection envisioned by Arnold (1994) and Murphy (1998), we discuss how sexual selection can occur in plants even though individuals do not directly interact. We review studies on traits that influence pollen export and receipt in a variety of hermaphroditic and gynodioecious plants with the underlying premise that pollination dynamics influences mate acquisition. Most of the studies reviewed found that phenotypes that enhance pollen export are in harmony with those that enhance pollen receipt suggesting that in many cases pollinator visitation rates limit both male and female function. In contrast, fewer traits were under opposing selection; but when they were, the traits most often were associated with enhancing the specific aspects of a given sex function. Our review helps clarify and illustrate why sexual selection can be a component of trait evolution in hermaphrodite plants. Introduction In his influential book The Descent of Man and Selection in Relation to Sex (Darwin 1871), Darwin spends a chap- ter discussing secondary sexual characters in “lower classes of animals,” particularly hermaphroditic ones. However, he is fairly dismissive of applying sexual selec- tion to these hermaphrodites because they “have too imperfect senses and too low mental powers” to appre- ciate each other’s beauty and general characteristics. It is therefore not surprising that he did not include a chapter on plants. We have progressed considerably in our think- ing on sexual selection, and we now readily include aspects of sexual selection in studies of trait evolution of hermaphroditic animals. However, applying and embracing such thinking withplants,especiallyhermaph- roditic ones, has taken somewhat more time to achieve. Forexample,justoveradecadeago,Stanton(1994),ina symposium issue focused specifically on comparisons of sexual selection in plants and animals, stated that “some colleagues and reviewers continue to view the application of sexual selection thinking to plant reproduction with deep suspicion.” And following that symposium, Grant (1995) stated that because in his view (1) secondary sexual characters“arethedefiningfeatureofsexualselection”and (2) plants lack these characters, he called into question the notion of sexual selection operating in plants. Here, we discuss a framework of trait selection championed by Arnold (1994) that differs very little fundamentally from how Darwin viewed it, but that, nevertheless, is easily applicable to hermaphroditic flowering plants. It involves thinking about the various pathways to fitness and takes a selection-gradient approach. We combine this with a novel dichotomy
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Integrative and Comparative Biology 2006 Delph - 465 Trait...

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