Forum EditorialBlackwellOxford,NPHNew0028-646X1469-8137©297410.1111/j.1469-8137.2009.02974.xJune0497???501???Commentary The 2009Phytologist Authors UK Publishing (2009). Ltd Journal compilation © New Phytologist (2009) Darwinism renewed: Co-evolutionary interactions contemporary studies of The interactions of plants with their pathogens, parasites and pollinators can generate reciprocal selection pressures and plant adaptation consequently a distinctive co-evolutionary process. L. G. Barrett et al. (this issue, pp. 513–529) analyze this process for the complex spectrum of associations between plants and their One hundred fifty years after On the origin of species was microbial pathogens. In contrast to the specific, highly virulent first published (Darwin, 1859), biologists continue to share pairwise interactions that have been emphasized to date in co- Darwin’s fascination with the adaptive traits of organisms, evolutionary models, they argue that host-pathogen associations and to theoretically and empirically expand his initial form a continuum from specialists to generalists, with widely formulation of natural selection as the process that shapes varying fitness effects (and therefore selective outcomes) those traits. In this special issue of the journal, New Phytologist depending on environmental and genetic factors that jointly recognizes this continuing legacy by bringing together current influence the expression of both microbial virulence and host ideas and findings about plant adaptation from an accom- resistance genes. These factors can interact at several levels to plished, multi-national group of researchers studying a wide affect selection pressures, for instance in the case of co-infection array of plant systems. Many of these studies make use of the by multiple pathogens that interact within the host to modify formidable set of research tools now available to examine the its defence gene expression. Godsoe et al. (pp. 589–599) focus molecular and genomic bases of adaptive traits and their on a very different aspect of plant co-evolutionary dynamics, selective dynamics, and these tools have certainly provided a well-studied pollination mutualism. Using GIS-based niche critical insights. But the most exciting aspect of contemporary modeling and well-resolved phylogenetic data, they determine work on adaptation is not these new techniques per se, but that the specialized biotic interaction between Yucca brevifolia rather the way researchers are combining diverse tools in and its moth pollinators, rather than climatic variables, have robust, richly informative interdisciplinary approaches. led to evolutionary divergence in this system. Genomic data from model organisms are being used in new ways to inform studies of naturally evolved systems, and a Floral and mating system evolution rigorous phylogenetic context has become standard across sub-disciplines to refine population, species, and higher-level Floral features such as pollination syndromes and organ place- comparisons. ment provide clear examples of adaptation. Harder & Johnson The studies presented here address questions at all levels (pp. 530–545) comprehensively review evidence as to whether from the molecular to the macroevolutionary, drawing on floral and inflorescence traits fit a Darwinian model of gradual, information ranging from transgenic functional assays, tran- consistent selective change. Although data from manipulation scriptomics, and quantitative genetics trait matrices to GIS- experiments generally confirm that these ‘beautiful contrivances’ based niche modeling, field explant studies of recombinant are indeed adaptations that evolved for pollen transfer, the inbred lines, and distribution data from herbarium sheets. results of phenotypic selection analyses indicate that directional This new work makes clear how these sophisticated interdis- selection on these traits may be relatively weak and inconsistent ciplinary approaches are illuminating some of the most fund- in natural populations. Instead, adaptive floral features may amental and long-standing questions about plant adaptation be shaped largely during episodes of strong selection rather and the process of selective change. It also reflects an increas- than by continual gradual selection as Darwin envisioned. This ing awareness that the study of adaptation has important meta-analysis reveals limits to phenotypic selection analysis bearing on biodiversity conservation, invasion biology, and as a definitive test for adaptive function. Armbruster et al. potential constraints on adaptive evolution in the face of (pp. 600–617) pose a complementary question about the rapidly changing environments. Here, a brief overview of the evolution of floral adaptations: does floral diversification at feature points to some of the key findings with respect to the macroevolutionary level reflect adaptation to pollinators? several areas of shared focus: co-evolutionary interactions; They determine an optimal-fitness ‘adaptive surface’ relating floral and mating system evolution; geographic patterns of pollination performance to anther and stigma position, and adaptive evolution; genetic architecture of adaptation; and test against this prediction the realized distribution of floral evolution of functional traits. traits in three phylogenetically diverse genera. Their analysis © The Author (2009) New Phytologist (2009) 183: 497–501 497 Journal compilation © New Phytologist (2009) www.newphytologist.org 497 498 Forum Editorial reveals clade-specific departures from the optimum reflecting plasticity and adaptive divergence of populations. (The paper both lack of floral integration and conflicting selection pressures is graciously dedicated to the late Fakhri A. Bazzaz, whose for other outcrossing features. Fenster et al. (pp. 502–506) draw work deeply enhanced our understanding of plasticity and its our attention to vertical versus horizontal floral orientation as ecological implications.) Levin reviews the evolutionary ecology a significant influence on pollinator effectiveness that has been literature to establish that individuals encountering novel largely ignored for the past century. Their initial dataset shows conditions often displace the timing of flowering. He argues that hummingbird approach behavior and floral contact vary that, when a population colonizes a new habitat, such an in response to differently oriented artificial flowers, a result environmentally-induced phenological shift will lead to that argues for future pollination studies to include this aspect temporally assortative mating that effectively isolates the of floral adaptation. colonizing population from its source. As a result, phenological Gender expression and the evolution of plant mating systems plasticity will facilitate local adaptation to the new habitat that have long been a focus of theoretical and empirical interest. would otherwise be impeded by gene flow from the source S. C. H. Barrett et al. (pp. 546–556) present a well-developed population. Along with intriguing implications for ecological case study on the evolution of selfing based on phylogeo- range expansion, this paper adds a new dimension to contem- graphic analysis of multilocus nuclear DNA sequences (SNPs) porary ideas about the possible role of individual plasticity in from populations sampled across the Neotropical range of the evolutionary diversification. sexually polymorphic species Eichornia paniculata. Their data Questions of dispersal and range also hold immediate reveal multiple independent transitions from outcrossing to implications for extinction risks as natural habitats are increas- selfing in populations of this colonizing species. Preliminary ingly disrupted. For instance, effective dispersal can contribute results also suggest that both genetic factors and environmental to species persistence in fragmented landscapes by allowing stress contribute to developmental instability in the early stages re-colonization of habitat patches and outcrossing. However, if of selfing, which may facilitate this evolutionary transition. habitat fragmentation in itself imposes selection for reduced Randle et al. (pp. 618–629) investigate the ecological impli- dispersal ability (as occurs in island populations), this evolu- cations of self-fertilization: does selfing promote colonization tionary feedback will worsen its negative impact on species’ and hence range expansion, or does the lower genetic diversity distributions and persistence. Riba et al. (pp. 667–677) find a of selfing species instead restrict range size? They test the rela- negative correlation between the degree of landscape fragmen- tion of selfing ability to size of realized geographic range in tation and achene dispersal ability at both local and regional Collinsia, combining a phylogenetically controlled species-pair spatial scales in the wind-dispersed European herb Mycelis comparison with precise measures of floral form and self- muralis. Together with a common garden experiment con- fertilization activity. This elegant study establishes empirically firming a partial genetic basis to this trait, these results indicate that species with the highest proficiency for autonomous selfing that fragmentation may have negative evolutionary as well as also have significantly larger ranges, linking an individual ecological consequences. reproductive trait with ecological distribution including, Geographic patterns of variation can also provide insights as predicted by Herbert Baker, the spread