Insect herbivores are hypothesized to be major factors affecting the ecology and evolution of plants. We tested this prediction by suppressing insects in replicated field populations of a native plant, Oenothera biennis, which reduced seed predation, altered interspecific competitive dynamics, and resulted in rapid evolutionary divergence. Comparative genotyping and phenotyping of nearly 12,000 O. biennis individuals revealed that in plots protected from insects, resistance to herbivores declined through time owing to changes in flowering time and lower defensive ellagitannins in fruits, whereas plant competitive ability increased. This independent real-time evolution of plant resistance and competitive ability in the field resulted from the relaxation of direct selective effects of insects on plant defense and through indirect effects due to reduced herbivory on plant competitors.

Broad‐scale geographical gradients in the abiotic environment and interspecific interactions should select for clinal adaptation. How trait clines evolve has recently received increased attention because of anticipated climate change and the importance of rapid evolution in invasive species. This issue is particularly relevant for clines in growth and defense of plants, because both sets of traits are closely tied to fitness and because such sessile organisms experience strong local selection. Yet despite widespread recognition that growth and defense traits are intertwined, the general issue of their joint clinal evolution is not well resolved. To address heritable clinal variation and adaptation of growth and defense traits of common milkweed (Asclepias syriaca), we planted seed from 22 populations encompassing the species' latitudinal range in common gardens near the range center (New York) and toward the range edges (New Brunswick and North Carolina). Populations were differentiated in 13 traits, and six traits showed genetically based latitudinal clines. Higher‐latitude populations had earlier phenology, lower shoot biomass, more root buds and clonal growth, higher root‐to‐shoot ratio, and greater latex production. The cline in shoot biomass was consistent in all three locations. Selection on phenology was reversed in New Brunswick and North Carolina, with early genotypes favored in the north but not the south. We found no clines in foliar trichomes or toxic cardenolides. Annual precipitation of source populations explained variation in phenology, clonal growth, root‐to‐shoot ratio, and latex. Across four traits measured in New Brunswick and North Carolina, we found garden‐by‐latitude (and garden‐by‐precipitation) interactions, indicating plasticity in genetically based trait clines. In the two gardens with substantial herbivory (New York and North Carolina), northern populations showed higher resistance to insects. Resistance to aphids was driven by trichomes and water content, while resistance to monarch caterpillars was driven by latex. However, surveys of natural populations indicated that leaf damage and insect diversity on milkweed are low at the geographical extremes (New Brunswick and North Carolina) and higher toward the range center. We speculate that milkweed plants evolved clines in growth traits in response to climate, and that this set the template for tolerance to herbivory, which subsequently shaped the evolution of defensive traits.

Author contributions M.K. co-designed and implemented the overall strategy for the creation of the knock-in fly lines, designed and implemented the bioassays, the RT-qPCR experiments and the RMO analysis, performed statistical analyses and co-wrote the manuscript. S.C.G. designed and implemented the overall strategy for the creation of the knock-in fly lines, prepared the sequence data and metadata for the phylogenetic analyses, co-designed all other experiments, and co-wrote the manuscript. F.S. performed the structural modelling and docking site analyses. J.N.P. performed the phylogenetic, ancestral state and co-evolutionary analyses. K.I.V. conducted crosses, genotyping, and feeding experiments, and co-designed the qPCR experiments. J.M.A. and S.L.B. conducted crosses and genotyping, and feeding and sequestration experiments. A.P.H. performed the in vitro physiological experiments and sequestration analyses. T.M. conducted feeding experiments M.A. performed the RMO analysis with M.K., and conducted genotyping and feeding experiments. G.G. completed the RMO and ouabain dietary survival analyses. F.R. supervised the structural modelling and docking site analyses. S.D. oversaw and interpreted in vitro cell line analyses, helped to design the overall project and co-wrote the manuscript. A.A.A. helped to design the overall project, oversaw the in vitro physiological and sequestration experiments, and co-wrote the manuscript. N.K.W. led the overall collaboration, the project design and its integration, creation of fly lines and statistical analyses, and co-wrote the manuscript. Peer review information Nature thanks Joseph W. Thornton and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.Online content Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at

One signature of adaptive radiation is a high level of trait change early during the diversification process and a plateau toward the end of the radiation. Although the study of the tempo of evolution has historically been the domain of paleontologists, recently developed phylogenetic tools allow for the rigorous examination of trait evolution in a tremendous diversity of organisms. Enemy-driven adaptive radiation was a key prediction of Ehrlich and Raven's coevolutionary hypothesis [Ehrlich PR, Raven PH (1964) Evolution 18:586 -608], yet has remained largely untested. Here we examine patterns of trait evolution in 51 North American milkweed species (Asclepias), using maximum likelihood methods. We study 7 traits of the milkweeds, ranging from seed size and foliar physiological traits to defense traits (cardenolides, latex, and trichomes) previously shown to impact herbivores, including the monarch butterfly. We compare the fit of simple random-walk models of trait evolution to models that incorporate stabilizing selection (Ornstein-Ulenbeck process), as well as time-varying rates of trait evolution. Early bursts of trait evolution were implicated for 2 traits, while stabilizing selection was implicated for several others. We further modeled the relationship between trait change and species diversification while allowing rates of trait evolution to vary during the radiation. Species-rich lineages underwent a proportionately greater decline in latex and cardenolides relative to species-poor lineages, and the rate of trait change was most rapid early in the radiation. An interpretation of this result is that reduced investment in defensive traits accelerated diversification, and disproportionately so, early in the adaptive radiation of milkweeds.Asclepias ͉ cardenolides ͉ coevolution ͉ macroevolutionary trends ͉ latex T he tempo of evolution is a key parameter in describing the diversification of life and is fundamental to the concept of adaptive radiation (1). It has been argued that an initially high rate of trait evolution followed by the slowing of trait change is a signature of adaptive radiation (1-3), because this pattern is concordant with niche-filling models where high rates of phenotypic change occur during ecological diversification. However, comparatively few studies have investigated the rate of trait evolution through time (4), with the notable exception of paleontological approaches (5). Now, the rapidly expanding palette of phylogenetic tools supports the rigorous test of historical hypotheses, such as rate change and directionality in character evolution, even for those taxonomic groups completely lacking a fossil record (4,(6)(7)(8). Recently developed models of adaptive radiation predict that trait change should be concentrated at early speciation events during clade diversification, followed by a decline in rate as the number of species increases (9, 10).In this study, we assess the tempo of trait evolution in American milkweeds (Apocynaceae, Asclepias), a speciose clade (Ϸ130 species) of toxi...

The extent to which evolutionary change occurs in a predictable manner under field conditions and how evolutionary changes feed back to influence ecological dynamics are fundamental, yet unresolved, questions. To address these issues, we established eight replicate populations of native common evening primrose (Oenothera biennis). Each population was planted with 18 genotypes in identical frequency. By tracking genotype frequencies with microsatellite DNA markers over the subsequent three years (up to three generations, ≈5,000 genotyped plants), we show rapid and consistent evolution of two heritable plant life-history traits (shorter life span and later flowering time). This rapid evolution was only partially the result of differential seed production; genotypic variation in seed germination also contributed to the observed evolutionary response. Since evening primrose genotypes exhibited heritable variation for resistance to insect herbivores, which was related to flowering time, we predicted that evolutionary changes in genotype frequencies would feed back to influence populations of a seed predator moth that specializes on O. biennis. By the conclusion of the experiment, variation in the genotypic composition among our eight replicate field populations was highly predictive of moth abundance. These results demonstrate how rapid evolution in field populations of a native plant can influence ecological interactions.