Summary• How species evolve depends on the communities in which they are embedded. Here, we briefly review the ideas underlying concepts of diffuse coevolution, evolution, and selection.• We discuss criteria to identify when evolution will be diffuse. We advocate a more explicitly trait-oriented approach to diffuse (co)evolution, and discuss how considering effects of interacting species on fitness alone tells us little about evolution. We endorse the view that diffuse evolution occurs whenever the response to selection by one interacting species on a given trait is altered by the presence of a second interacting species.• Building on the work of others, we clarify and expand the criteria for diffuse evolution and present a simple experimental design that will allow the detection of diffuse selection.• We argue that a greater focus on selection on specific traits and the evolutionary response to that selection will improve our conceptual understanding of how communities affect the evolution of species embedded within them.New Phytologist (2005) 165 : 81-90 © New Phytologist (2004

Plant-pollinator interactions are one of the most important and variable mutualisms in nature. Multiple pollinators often visit plants and can vary in visitation rates, pollen removal and deposition, and spatial and temporal distribution, altering plant reproduction and patterns of pollinator-mediated selection. Although some visitors may not be effective pollinators, pollinator effectiveness is rarely estimated directly as seed set resulting from a single visit by each taxon visiting generalist plants. For two years, effectiveness of visitors to wild radish, Raphanus raphanistrum, was quantified by counting seeds set and pollen grains removed as a result of a single visit. We calculated a pollinator's importance to plant reproduction as the product of visitation rate and single-visit seed set, and regressed pollinator body size on pollen-removal and on seed set effectiveness. Although pollinators differed in effectiveness and visitation rates, pollinator importance was primarily determined by visitation rates. In contrast to similar 2-yr studies, pollinator assemblage composition varied little, suggesting pollinator-mediated selection can be consistent across years for this generalist. Larger pollinators were more effective than smaller at effecting seed set, but body size was a poor predictor of pollen removal ability. Instead, pollen-removal effectiveness may be more influenced by foraging behavior than size.

Although conflicting selection from different resources is thought to play a critical role in the evolution of specialized species, the prevalence of conflicting selection in generalists is poorly understood. Plants may experience conflicting selection on floral traits by different pollinators and between genders. Using artificial selection to increase phenotypic variation, we tested for conflicting and nonadditive selection on wild radish (Raphanus raphanistrum) flowers. To do this, we measured selection by each of the major pollinator taxa through both male and female fitness, and tested for a single-generation response to selection by a subset of these pollinators. We found some evidence for conflicting selection on anther exsertion--sweat bees exerted stabilizing selection and larger bees selected for increased exsertion. Stamen dimorphism was only under selection by honey bees, causing a response to selection in the next generation, and flower size was under similar selection by multiple pollinators. Selection differed significantly between genders for two traits, but there was no evidence for stronger selection through male fitness or for conflicting selection between genders. Our results suggest wild radish flowers can adapt to multiple pollinators, as we found little evidence for conflicting selection and no evidence for nonadditive selection among pollinators. Janzen's seminal paper, "When is it coevolution" (Janzen 1980), emphasized that selection on traits is likely more complex than simple pairwise interactions. Instead, evolution is diffuse when the selection on a given trait is dependent upon the broader community context in which a species is found (Hougen-Eitzman and Rausher 1994;Iwao and Rausher 1997;Inouye and Stinchcombe 2001). The majority of studies of diffuse evolution have measured mean fitness of the interacting species (Strauss and Irwin 2004); this does not provide any information about natural selection, defined as the slope of the relationship between fitness and a particular phenotypic trait (Strauss et al. 2005). Diffuse selection, where this slope changes depending on which species of selective agent are present, has been documented mainly in plant-herbivore systems (e.g., Pilson 1996;Juenger and Bergelson 1998;Stinchcombe and Rausher 2001) and in systems involving plant mutualists and antagonists (e.g., Gomez 2003;Cariveau et al. 2004;Irwin et al. 2004).There are at least four nonmutually exclusive ways in which multiple selective agents may affect trait evolution. First, adaptation in a heterogeneous environment may be constrained if conflicting selection by different selective agents causes a tradeoff, whereby an adaptation that is beneficial in the presence of one selective agent is deleterious in the presence of another (e.g., Irwin 2006;Lankau 2007;Gomez 2008). In an extreme example of conflicting selection, exactly opposing selection gradients could lead to no net selection on a trait (Fig. 1A). Second, multiple selective agents may select on the same trait in the same manne...

Despite the development of diversity indices in community ecology that incorporate both richness and evenness, pollination biologists commonly use only pollinator richness to estimate generalization. Similarly, while pollination biologists have stressed the utility of pollinator importance, incorporating both pollinator abundance and effectiveness, importance values have not been included in estimates of generalization in pollination systems. In this study, we estimated pollinator generalization for 17 plant species using Simpson's diversity index, which includes richness and evenness. We compared these estimates with estimates based on only pollinator richness, and compared diversity estimates calculated using importance data with those using only visitation data. We found that pollinator richness explains only 57-65% of the variation in diversity, and that, for most plant species, pollinator importance was determined primarily by differences in visitation rather than by differences in effectiveness. While simple richness may suffice for broad comparisons of pollinator generalization, measures that incorporate evenness will provide a much more accurate understanding of generalization. Although incorporating labor-intensive measurements of pollinator effectiveness are less necessary for broad surveys, effectiveness estimates will be important for detailed studies of some plant species. Unfortunately, at this point it is impossible to predict a priori which species these are.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. abstract: Genetic correlations are the most commonly studied of all potential constraints on adaptive evolution. We present a comprehensive test of constraints caused by genetic correlation, comparing empirical results to predictions from theory. The additive genetic correlation between the filament and the corolla tube in wild radish flowers is very high in magnitude, is estimated with good precision ( ), and is caused by pleiotropy. Thus, evolu-0.85 ‫ע‬ 0.06 tionary changes in the relative lengths of these two traits should be constrained. Still, artificial selection produced rapid evolution of these traits in opposite directions, so that in one replicate relative to controls, the difference between them increased by six standard deviations in only nine generations. This would result in a 54% increase in relative fitness on the basis of a previous estimate of natural selection in this population, and it would produce the phenotypes found in the most extreme species in the family Brassicaceae in less than 100 generations. These responses were within theoretical expectations and were much slower than if the genetic correlation was zero; thus, there was evidence for constraint. These results, coupled with comparable results from other species, show that evolution can be rapid despite the constraints caused by genetic correlations.