Annual Review of Ecology, Evolution, and Systematics - Volume 39, 2008

Over the past decade a proliferation of research has enriched and dramatically altered our understanding of the biology of figs, their pollinator wasps, and the myriad of other organisms that depend on them. Ecologically, this work underscores the crucial role that fig fruits play in sustaining and shaping tropical frugivore communities. More generally, this work addresses several key issues in evolutionary ecology, including evolution of breeding systems (shifts between monoecy and dioecy), factors that promote the stability of mutualisms, precision of adaptation, and trajectories of community assembly and coevolution in systems with multiple interacting partners. Moreover, both the pollinating and nonpollinating wasps associated with figs provide unparalleled opportunities for examining how different population structures can differentially affect sex allocation, kin selection, the evolution of parasite virulence, and many fundamental parameters of population genetics (e.g., levels of genetic variation and rates of silent and nonsilent base substitutions).

Considerable progress has been made in documenting evidence of very early plants starting in the basal Ordovician employing dispersed spore, phytodebris, and mesofossil data. Macrofossil evidence is sparse until Late Silurian, but recent new data are improving our understanding of aspects of earliest plants. The considerable information about the possible source of cryptospores and trilete spores especially from the well-preserved mesofossils of the Late Silurian and Early Devonian is summarized. Promising avenues of research are the study of spore ultrastructure, and neo-paleo comparisons between newly discovered resistant components of extant bryophytes and fragmentary fossil remains. Recent macrofossil discoveries in the Late Silurian advance our understanding of early events in plant evolution and raise new questions about the timing of evolution or relationships among earliest (mostly vascular) plants.

Foodwebs are important units of biodiversity, and yet, our knowledge of their spatial dynamics is sketchy at best. Here I attempt to synthesize existing knowledge into a framework that can both identify crucial gaps in the theory as well as facilitate empirical investigations. The synthesis is based on two major axes, foodweb complexity and type of movement, and considers two types of spatial effects, foodweb persistence via a reduction in local extinction and foodweb diversity via an increase in species coexistence. It highlights both invariant properties that are robust to increasing foodweb complexity and emergent properties that result from the interplay between foodweb dynamics and type of movement. It underscores the need for a comparative theoretical framework that can yield testable predictions.

Species selection in the broad sense—also termed species sorting—shapes evolutionary patterns through differences in speciation and extinction rates (and their net outcome, often termed the emergent fitness of clades) that arise by interaction of intrinsic biological traits with the environment. Effect-macroevolution occurs when those biotic traits, such as body size or fecundity, reside at the organismic level. Strict-sense species selection occurs when those traits are emergent at the species level, such as geographic range or population size. The fields of paleontology, comparative phylogenetic analysis, macroecology, and conservation biology are rich in examples of species sorting, but relatively few instances have been well documented, so the extent and efficacy of the specific processes remain poorly known. A general formalization of these processes remains challenging, but approaches drawing on hierarchical covariance models appear promising. Analyses integrating paleontological and neontological data for a single set of clades would be especially powerful.

Recent years have seen a rapid expansion in the scope of quantitative genetic analyses undertaken in wild populations. We illustrate here the potential for such studies to address fundamental evolutionary questions about the maintenance of genetic diversity and to reveal hidden genetic conflicts or constraints not apparent at the phenotypic level. Trade-offs between different components of fitness, sexually-antagonistic genetic effects, maternal effects, genotype-by-environment interactions, genotype-by-age interactions, and variation between different regions of the genome in localized genetic correlations may all prevent the erosion of genetic variance. We consider ways in which complex interactions between ecological conditions and the expression of genetic variation can be elucidated, and emphasize the benefits of conducting selection analyses within a quantitative genetic framework. We also review potential developments associated with rapid advances in genomic technology, in particular the increased availability of extensive marker information. Our conclusions highlight the complexity of processes contributing to the maintenance of genetic diversity in wild populations, and underline the value of a quantitative genetic approach in parameterizing models of life-history evolution.

Floral scent constitutes an ancient and important channel of communication between flowering plants, their pollinators, and enemies. Fragrance is a highly complex component of floral phenotype, with dynamic patterns of emission and chemical composition. The information content of specific volatile compounds is highly context dependent, and scent can function in direct and indirect ways from landscape to intrafloral scales. Floral scent promotes specialization in plant–pollinator relationships through private channels of unusual compounds, unique ratios of more widespread compounds, or through multicomponent floral filters. Floral scent also promotes outcrossing and reproductive isolation through floral constancy, via appetitive conditioning and discrimination on the basis of diverse mechanisms, including pheromone mimicry, odor intensity, complexity, composition, and synergy with visual stimuli. Finally, floral scent is a sexual signal and should be subject to the same selective pressures and modes of signal evolution as animal display, including signal honesty, sensory drive, and sensory exploitation.

The traditional notion of a gap between fishes and amphibians has been closed by a wealth of fish-like fossil tetrapods, many discovered since the mid 1980s. This review summarizes these discoveries and explores their significance relative to changing ideas about early tetrapod phylogeny, biogeography, and ecology. Research emphasis can now shift to broader-based questions, including the whole of the early tetrapod radiation, from the divergence from other lobed-finned fishes to the origins of modern amphibians and amniotes. The fish-to-tetrapod morphological transition occurred within the Upper Devonian; the divergence of modern tetrapod groups is an Early Carboniferous event. Modern tetrapods emerged in the aftermath of one of the five major extinction episodes in the fossil record, but the earlier Devonian tetrapod radiation is not well understood. Tetrapod limbs, paired fins, and comparative developmental data are reviewed; again, research emphasis needs to change to explore the origins of tetrapod diversity.

Introduced exotic earthworms now occur in every biogeographic region in all but the driest or coldest habitat types on Earth. The global distribution of a few species (e.g., Pontoscolex corethrurus) was noted by early naturalists, but now approximately 120 such peregrine species are recognized to be widespread from regional to global scales, mainly via human activities. Species adapted to human transport and to colonization of disturbed habitats are most widespread and are the principal invasive species. We identify a number of endogenous and exogenous factors that may contribute to the successful establishment and spread of peregrine species. Quantification of these factors may help to determine why certain species become invasive while others do not. Recent advances in theory and modeling of biological invasions and in molecular techniques should prove fruitful in improving our understanding of invasive earthworms, as well as in predicting their impacts on ecosystems.

Trait-based approaches are increasingly used in ecology. Phytoplankton communities, with a rich history as model systems in community ecology, are ideally suited for applying and further developing these concepts. Here we summarize the essential components of trait-based approaches and review their historical and potential application to illuminating phytoplankton community ecology. Major ecological axes relevant to phytoplankton include light and nutrient acquisition and use, natural enemy interactions, morphological variation, temperature sensitivity, and modes of reproduction. Trade-offs between these traits play key roles in determining community structure. Freshwater and marine environments may select for a different suite of traits owing to their different physical and chemical properties. We describe mathematical techniques for integrating traits into measures of growth and fitness and predicting how community structure varies along environmental gradients. Finally, we outline challenges and future directions for the application of trait-based approaches to phytoplankton ecology.

Though the distribution of global vegetation can generally be predicted from climate, grasslands are an exception. C4 grassy biomes cover vast areas that are warm enough and wet enough to support closed forests. The extent of this climate mismatch has been revealed by physiologically based global vegetation simulations and by large empirical data sets. Reasons for the existence of grassy biomes have long been debated, polarized into bottom-up (resources) or top-down (fire, herbivory) arguments. Recent studies indicate that both are important, especially in suppressing woody recruits. Grasses are formidable competitors belowground, create highly flammable fuels, and can support large herbivore densities. The net effect on trees is rare and episodic recruitment of adults in tree-fall gaps. The implication is that ecosystem structure and function depend on demographic transitions. Tree cover is increasing and grass/forest boundaries are changing. These changes can have large feedbacks to the earth-atmosphere system. Though progress has been made, there is still great uncertainty in predicting the future of C4 grassy biomes.