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- Volume 28, 1997
Annual Review of Ecology, Evolution, and Systematics - Volume 28, 1997
Volume 28, 1997
- Review Articles
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Molecular Population Genetics of Social Insects
Vol. 28 (1997), pp. 1–25More LessThe life of social insects centers around sedentary colonies that can include individuals belonging to different patrilines or matrilines, with a turnover of reproductives. The colony is a scene for both cooperation and conflicts, and the conceptual framework for the evolution of social life and colony organization is provided by the kin selection theory. Variable molecular markers make it possible to dissect kinship within colonies, identifying patrilines and matrilines and estimating genetic relatednesses. Such markers have been used to test hypotheses on social conflicts between queens and workers (split sex ratio hypothesis), among workers (worker policing hypothesis), and among reproductive females (skew hypothesis). The data from several species of ants, bees, and wasps indicate that workers can obtain information on the genetic heterogeneity of their colonies and use that information to manipulate reproductive decisions. The social structure of colonies and the mode of colony founding affect the population-wide dispersal of sexuals. Populations with multi-queen colonial networks have limited dispersal; females stay in their natal colonies, and mating flights can be restricted. As a result, coexisting queens tend to be related to each other, maintaining the genetic integrity of colonies, and populations become spatially differentiated to an extent that can lead even to socially driven speciation.
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Evolution of Eusociality in Termites
Vol. 28 (1997), pp. 27–54More LessEusociality in Isoptera (termites) converges along many lines with colony organization and highly social behavior in the phylogenetically distinct insect order Hymenoptera (ants, bees, wasps). Unlike the haplodiploid Hymenoptera, however, both sexes of Isoptera are diploid. Termite families thus lack asymmetric degrees of genetic relatedness generated by meiosis and fertilization, so explanations for eusocial evolution based on such asymmetries are not applicable to Isoptera. The evolution of eusociality in termites likely occurred in small families in which most helpers retained developmental flexibility and reproductive options. A suite of ecological and life-history traits of termites and their ancestors may have predisposed them toward eusocial evolution. These characteristics include familial associations in cloistered, food-rich habitats; slow development; overlap of generations; monogamy; iteroparity; high-risk dispersal for individuals; opportunities for nest inheritance by offspring remaining in their natal nest; and advantages of group defense. Such life-history components are particularly persuasive as fostering social evolution because many are present in a broad group of eusocial taxa, including Hymenoptera, beetles, aphids, thrips, naked mole rats, and shrimp. The evolution of eusociality in Isoptera likely evolved in response to a variety of contributing elements and the selective pressures that they generated.
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Evolutionary Genetics and Genetic Variation of Haplodiploids and X-Linked Genes
Vol. 28 (1997), pp. 55–83More LessThe evolutionary genetics of haplodiploids and X-linked genes share many features and are different from diploid (autosomal) genes in many respects. For example, the conditions for a stable polymorphism, the amount of genetic load, and the effective population size are all expected to be quite different between haplodiploids or X-linked genes and diploids. From experimental data, the genetic load for X-linked genes is much less than autosomal genes and appears less for haplodiploids than for diploids. The observed amount of molecular variation for haplodiploids is much less than that for diploids, even more so than predicted from the differences in effective population size. Extensive recently published data suggest that the differences in variation for X-linked and autosomal genes for Drosophila, mice, and humans are consistent with the differences predicted theoretically based on the relative effective population sizes.
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Dissecting Global Diversity Patterns: Examples from the Ordovician Radiation
Vol. 28 (1997), pp. 85–104More LessAlthough the history of life has been characterized by intermittent episodes of radiation that can be recognized in global compilations of biodiversity, it does not necessarily follow that these episodes are caused by processes that occurred uniformly around the world. Major diversity increases could be generated by the cumulative effects of different mechanisms operating simultaneously at several geographic or environmental scales. The purpose of this review is to describe ongoing research on the manifestations, at several scales, of the Ordovician Radiation, which was among the most extensive intervals of diversification in the history of life. Through much of the period, diversity was concentrated most heavily near regions of active mountain building and volcanism; differences in diversity patterns from continent to continent, and among regions within continents, reflect this overprint. While this suggests a linkage of the Radiation and tectonic activity, this is by no means the only mediating agent. Outcrop-based research in North America has demonstrated that tectonic activity was detrimental to some biotic elements, in contrast to its effects on other organisms. Moreover, in the Great Basin of North America where the local stratigraphic record is of particularly high quality, biotic transitions characteristic of the period occurred far more rapidly than observed in global compilations of diversity, suggesting that the global rate of transition may represent the aggregate sum of transitions that occurred abruptly, but at different times, around the world. Finally, it has been demonstrated that, in concert with an increase in average age, the environmental and geographic ranges of Ordovician genera both increased significantly through the period, indicating a role for intrinsic factors in producing Ordovician biotic patterns.
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A Comparison of Alternative Strategies for Estimating Gene Flow from Genetic Markers1
Vol. 28 (1997), pp. 105–128More LessThe estimation of gene flow from the distribution of genetic markers in populations requires an indirect approach. Gene flow parameters are defined by demographic models, and population genetic models provide the link between these parameters and the distributions of genetic markers. Following the introduction of allozyme methods in the 1960s, a standard approach to the estimation of gene flow was developed. Wright's island model of population structure was used to relate the distribution of allozyme alleles in populations to Nem, the product of the effective population size and the rate of migration.
Alternative strategies for the estimation of gene flow have been developed using different genetic markers, different models of demography and population genetics, and different methods of parameter estimation. No alternative strategy now available is clearly superior to the standard approach based on Wright's model and allozyme markers. However, this may soon change as methods are developed that fully utilize the genealogical relationships of DNA sequences. At present, alternative strategies do fill important needs. They can provide independent estimates of gene flow, measure different components of gene flow, and detect historical changes in population structure.
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The Evolution of Morphological Diversity
Vol. 28 (1997), pp. 129–152More LessThe diversity of organismic form has evolved nonuniformly during the history of life. Quantitative morphological studies reveal profound changes in evolutionary rates corresponding with the generation of morphological disparity at low taxonomic diversity during the early radiation of many clades. These studies have also given insight into the relative importance of genomic and ecological factors in macroevolution, the selectivity of extinction, and other issues. Important progress has been made in the development of morphological spaces that can accommodate highly disparate forms, although this area still needs more attention. Other future directions include the relationship between morphological and ecological diversification, geographic patterns in morphological diversity, and the role of morphological disparity as a causal factor in macroevolution.
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Insect Mouthparts: Ascertaining the Paleobiology of Insect Feeding Strategies
Vol. 28 (1997), pp. 153–193More LessOne of the most intensively examined and abundantly documented structures in the animal world is insect mouthparts. Major structural types of extant insect mouthparts are extensive, consisting of diverse variations in element structure within each of the five mouthpart regions—labrum, hypopharynx, mandibles, maxillae, and labium. Numerous instances of multielement fusion both within and among mouthpart regions result in feeding organs capable of ingesting in diverse ways foods that are solid, particulate, and liquid in form. Mouthpart types have a retrievable and interpretable fossil history in well-preserved insect deposits. In addition, the trace-fossil record of insect-mediated plant damage, gut contents, coprolites, and insect-relevant floral features provides complementary data documenting the evolution of feeding strategies during the past 400 million years.
From a cluster analysis of insect mouthparts, I recognize 34 fundamental mouthpart classes among extant insects and their geochronological evolution by a five-phase pattern. This pattern is characterized, early in the Devonian, by coarse partitioning of food by mandibulate and piercing-and-sucking mouthpart classes, followed by a rapid rise in herbivore mouthpart types for fluid- and solid-feeding during the Late Carboniferous and Early Permian. Mouthpart innovation during the Late Triassic to Early Jurassic added mouthpart classes for fluid and aquatic particle-feeding. This ecomorphological expansion of mouthpart design was associated with the radiation of holometabolous insects, especially Diptera. The final phase of mouthpart class expansion occurred during the Late Jurassic and Early Cretaceous, with addition of surface-fluid-feeding mouthpart classes that subsequently became important during the ecological expansion of angiosperms. Conclusions about the evolution of mouthpart design are based on the mapping of phenetic mouthpart classes onto (ideally) cladistic phylogenies of lineages bearing those same mouthpart classes. The plotting of phenetic and associated ecological attributes onto baseline phylogenies is one of the most important uses of cladistic data.
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Haldane's Rule
Vol. 28 (1997), pp. 195–218More LessHaldane's rule—the preferential sterility or inviability of hybrids of the heterogametic (XY) sex—characterizes speciation in all known animals. Over the past decade, an enormous amount of experimental and theoretical work has been devoted to explaining this pattern. This work has falsified several once-popular theories and, more important, has produced a strong consensus on the likely causes of Haldane's rule. Experiments show that the dominance theory, which posits that “speciation genes” act as partial recessives in hybrids, can explain Haldane's rule for hybrid inviability. Dominance likely also contributes to Haldane's rule for sterility. Recent experiments further show that faster evolution of hybrid male steriles plays an important role. Faster evolution of X-linked loci may also contribute, though the evidence here is weaker. Evolutionary geneticists now largely agree that the simultaneous action of these forces explains Haldane's rule.
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Echinoderm Larvae and Phylogeny
Vol. 28 (1997), pp. 219–241More LessNew robust phylogenies for echinoderms, based on congruent patterns derived from multiple data sets, provide a sound foundation for plotting the evolution of life-history strategies and comparing rates and patterns of larval and adult morphological change. This approach demonstrates that larval morphology has been evolving independently of adult morphology, that larval morphology displays more homoplasy than adult morphology, and that early developmental patterns are remarkably flexible. Larval morphology on its own can mislead phylogenetic analysis, not because of lateral gene transfer among distantly related taxa, but because of massive convergence in the form of nonfeeding larvae brought about by the loss of complex structures and the strong functional constraints on feeding larvae. The degree to which larval tissue is resorbed at metamorphosis is believed to be important in determining adult body plan. Although the correspondence is not precise, it does provide a model for understanding skeletal homologies among the classes.
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Preserving the Information Content of Species: Genetic Diversity, Phylogeny, and Conservation Worth
Vol. 28 (1997), pp. 243–268More LessA variety of phylogenetic measures have been proposed to quantify distinctiveness, often held to mark species of high conservation worth. However, distinctiveness of species and their numbers have different implications for conservation policy, depending on whether moral, esthetic, or utilitarian reasons are accepted as justifying conservation. The utilitarian position values species according to increasing numbers, and as they are more, as opposed to less, distinctive. The view is taken that conservation should seek to maximize the preserved information of the planet's biota, best expressed in terms of genetic information held in genes and not in portions of the genome of uncertain or no function. Gene number is thus an important component of assessing conservation value. Phylogenetic measures are better indicators of conservation worth than species richness, and measures using branch-lengths are better than procedures relying solely on topology. Distance measures estimating the differences between genomes are preferable to substitution distances. Higher-taxon richness is a promising surrogate for branch-length measures. Complete enumeration of biotas in terms of phylogeny is desirable to avoid uncertainties in the use of indicator groups, and this is achievable now for bacteria. Phylogenetic measures are already important for management of sets of populations within species and are applicable for sets of species. Measures incorporating extinction probabilities and decision costs are being developed, and these, in conjunction with the use of confidence limits on the conservation worth of alternative reserves, are vital for conservation decision-making.
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Theoretical and Empirical Examination of Density-Dependent Selection
Vol. 28 (1997), pp. 269–288More LessThe development of theory on density-dependent natural selection has seen a transition from very general, logistic growth-based models to theories that incorporate details of specific life histories. This transition has been justified by the need to make predictions that can then be tested experimentally with specific model systems like bacteria or Drosophila. The most general models predict that natural selection should increase density-dependent rates of population growth. When trade-offs exist, those genotypes favored in low-density environments will show reduced per capita growth rates under crowded conditions and vice versa for evolution in crowded environments. This central prediction has been verified twice in carefully controlled experiments with Drosophila. Empirical research in this field has also witnessed a major transition from field-based observations and conjecture to carefully controlled laboratory selection experiments. This change in approach has permitted crucial tests of theories of density-dependent natural selection and a deeper understanding of the mechanisms of adaptation to different levels of population crowding. Experimental research with Drosophila has identified several phenotypes important to adaptation, especially at high larval densities. This same research revealed that an important trade-off occurs between competitive ability and energetic efficiency.
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Toward an Integration of Landscape and Food Web Ecology: The Dynamics of Spatially Subsidized Food Webs
Vol. 28 (1997), pp. 289–316More LessWe focus on the implications of movement, landscape variables, and spatial heterogeneity for food web dynamics. Movements of nutrients, detritus, prey, and consumers among habitats are ubiquitous in diverse biomes and can strongly influence population, consumer-resource, food web, and community dynamics. Nutrient and detrital subsidies usually increase primary and secondary productivity, both directly and indirectly. Prey subsidies, by movement of either prey or predators, usually enhance predator abundance beyond what local resources can support. Top-down effects occur when spatially subsidized consumers affect local resources by suppressing key resources and occasionally by initiating trophic cascades. Effects on community dynamics vary with the relative amount of input, the trophic roles of the mobile and recipient entities, and the local food web structure. Landscape variables such as the perimeter/area ratio of the focal habitat, permeability of habitat boundaries, and relative productivity of trophically connected habitats affect the degree and importance of spatial subsidization.
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Settlement of Marine Organisms in Flow
Vol. 28 (1997), pp. 317–339More LessA feature common to many benthic marine plants and animals is the release of propagules that serve as the organism's only mechanism of dispersal. Successful dispersal depends to a large extent on the process of settlement—the transient phase between the pelagic life of the propagule and the benthic existence of the adult. The flow of water may affect settlement on three levels: 1. Flow can act by exerting hydrodynamic forces on settling propagules. These forces may affect the propagule's encounter with the substratum, its behavior following encounter, or both. 2. Flow may provide a settlement cue that induces active behavior of motile propagules. 3. Flow may act to mediate various settlement cues (e.g. sediment load and the concentration of attractants). We discuss these three levels of flow effects as a means of examining the potential importance of flow in the settlement process, and then we explore the ecological consequences of settlement in different flow-regimes in light of the direct effects of flow and flow-derived factors.
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Species Richness of Parasite Assemblages: Evolution and Patterns
Vol. 28 (1997), pp. 341–358More LessParasite communities are arranged into hierarchical levels of organization, covering various spatial and temporal scales. These range from all parasites within an individual host to all parasites exploiting a host species across its geographic range. This arrangement provides an opportunity for the study of patterns and structuring processes operating at different scales. Across the parasite faunas of various host species, several species-area relationships have been published, emphasizing the key role of factors such as host size or host geographical range in determining parasite species richness. When corrections are made for unequal sampling effort or phylogenetic influences, however, the strength of these relationships is greatly reduced, casting a doubt over their validity. Component parasite communities, or the parasites found in a host population, are subsets of the parasite fauna of the host species. They often form saturated communities, such that their richness is not always a reflection of that of the entire parasite fauna. The species richness of component communities is instead influenced by the local availability of parasite species and their probability of colonization. At the lowest level, infracommunities in individual hosts are subsets of the species occurring in the component community. Generally, their structure does not differ from that expected from a random assembly of available species, although comparisons with precise null models are still few. Overall studies of parasite communities suggest that the action of processes determining species richness of parasite assemblages becomes less detectable as focus shifts from parasite faunas to infracommunities.
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Hybrid Origins of Plant Species
Vol. 28 (1997), pp. 359–389More LessThe origin of new homoploid species via hybridization is theoretically difficult because it requires the development of reproductive isolation in sympatry. Nonetheless, this mode is often and carelessly used by botanists to account for the formation of species that are morphologically intermediate with respect to related congeners. Here, I review experimental, theoretical, and empirical studies of homoploid hybrid speciation to evaluate the feasibility, tempo, and frequency of this mode. Theoretical models, simulation studies, and experimental syntheses of stabilized hybrid neospecies indicate that it is feasible, although evolutionary conditions are stringent. Hybrid speciation appears to be promoted by rapid chromosomal evolution and the availability of a suitable hybrid habitat. A selfing breeding system may enhance establishment of hybrid species, but this advantage appears to be counterbalanced by lower rates of natural hybridization among selfing taxa. Simulation studies and crossing experiments also suggest that hybrid speciation can be rapid—a prediction confirmed by the congruence observed between the genomes of early generation hybrids and ancient hybrid species. The frequency of this mode is less clear. Only eight natural examples in plants have been rigorously documented, suggesting that it may be rare. However, hybridization rates are highest in small or peripheral populations, and hybridization may be important as a stimulus for the genetic or chromosomal reorganization envisioned in founder effect and saltational models of speciation.
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Evolutionary Genetics of Life Cycles
Vol. 28 (1997), pp. 391–435More LessThe life cycles of cellular species are reviewed from the genetic perspective. Almost all life cycles include stages during which only one genome is transmitted from a parent to its offspring. This, together with interorganismal gene exchange, which occurs regularly in at least some prokaryotes and in the majority of eukaryotes, allows selection to evaluate different alleles more or less independently. Regular genetic changes due to intraorganismal ploidy cycles or recombination may also be important in life cycles of many unicellular forms. Eukaryotic amphimixis is generally similar in all taxa, but the current data on the phylogeny and reproduction of unicellular eukaryotes are insufficient to determine whether it evolved several times or just once. Theoretically, gradual origin of amphimixis from apomixis, with each step favored by natural selection, is feasible. However, we still do not know how this process occurred nor what selection caused it. For reasons not entirely clear, some properties of amphimictic life cycles are much less variable and more conservative than the others. Evolution of many aspects of reproduction requires more theoretical studies, while the existing data are insufficient to choose among the currently competing hypotheses.
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Phylogeny Estimation and Hypothesis Testing Using Maximum Likelihood
Vol. 28 (1997), pp. 437–466More LessOne of the strengths of the maximum likelihood method of phylogenetic estimation is the ease with which hypotheses can be formulated and tested. Maximum likelihood analysis of DNA and amino acid sequence data has been made practical with recent advances in models of DNA substitution, computer programs, and computational speed. Here, we describe the maximum likelihood method and the recent improvements in models of substitution. We also describe how likelihood ratio tests of a variety of biological hypotheses can be formulated and tested using computer simulation to generate the null distribution of the likelihood ratio test statistic.
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Species Turnover and the Regulation of Trophic Structure
Vol. 28 (1997), pp. 467–494More LessTrophic structure, the partitioning of biomass among trophic levels, is a major characteristic of ecosystems. Most studies of the forces that shape trophic structure emphasize either “bottom-up” or “top-down” regulation of populations and communities. Recent work has shown that these two forces are not mutually exclusive alternatives, but efforts to model their interaction still often yield unrealistic predictions. We focus on the problems involved with modeling situations in which community composition, including both the number of trophic levels and the species composition within a trophic level, can change. We review the development of these ideas, emphasizing in particular how compositional change can alter theoretical expectations about the regulation of trophic structure. A comparison of studies on the effects of predators and resource productivity in limnetic ecosystems reveals an intriguing disparity between the results of manipulative experiments and those of correlational studies. We suggest that this contrast is a result of the difference in the temporal scales operating in the two types of studies. Ecosystem-level variables may appear to approach an equilibrium in short-term press experiments; however, processes such as invasion and extinction of species will not have time to play out in most such experiments. We found that the responses of ecosystems to short-term experimental treatments involve less change in species composition than is found in natural communities that have diverged in response to local conditions over longer periods. We argue that the results of short-term experiments support the predictions of models in which the species pool does not change, whereas correlational studies among systems support theories that incorporate compositional change.
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Extinction Vulnerability and Selectivity: Combining Ecological and Paleontological Views
Vol. 28 (1997), pp. 495–516More LessExtinction is rarely random across ecological and geological time scales. Traits that make some species more extinction-prone include individual traits, such as body size, and abundance. Substantial consistency appears across ecological and geological time scales in such traits. Evolutionary branching produces phylogenetic (as often measured by taxonomic) nesting of extinction-biasing traits at many scales. An example is the tendency, seen in both fossil and modern data, for higher taxa living in marine habitats to have generally lower species extinction rates. At lower taxononomic levels, recent bird and mammal extinctions are concentrated in certain genera and families. A fundamental result of such selectivity is that it can accelerate net loss of biodiversity compared to random loss of species among taxa. Replacement of vulnerable taxa by rapidly spreading taxa that thrive in human-altered environments will ultimately produce a spatially more homogenized biosphere with much lower net diversity.
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Tree-Grass Interactions in Savannas1
Vol. 28 (1997), pp. 517–544More LessSavannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other circumstances are mutually exclusive or unequal partners, yields theoretical insights and has practical implications.
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Previous Volumes
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Volume 54 (2023)
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Volume 53 (2022)
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Volume 52 (2021)
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Volume 51 (2020)
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Volume 50 (2019)
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Volume 49 (2018)
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Volume 48 (2017)
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Volume 47 (2016)
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Volume 46 (2015)
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Volume 45 (2014)
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Volume 44 (2013)
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Volume 43 (2012)
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Volume 42 (2011)
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Volume 41 (2010)
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Volume 40 (2009)
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Volume 39 (2008)
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Volume 38 (2007)
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Volume 37 (2006)
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Volume 36 (2005)
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Volume 35 (2004)
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Volume 34 (2003)
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Volume 33 (2002)
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Volume 32 (2001)
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Volume 31 (2000)
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Volume 30 (1999)
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Volume 29 (1998)
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Volume 28 (1997)
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Volume 27 (1996)
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Volume 26 (1995)
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Volume 25 (1994)
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Volume 24 (1993)
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Volume 23 (1992)
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Volume 22 (1991)
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Volume 21 (1990)
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Volume 20 (1989)
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Volume 19 (1988)
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Volume 18 (1987)
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Volume 17 (1986)
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Volume 16 (1985)
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Volume 15 (1984)
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Volume 14 (1983)
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Volume 13 (1982)
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Volume 12 (1981)
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Volume 11 (1980)
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Volume 10 (1979)
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Volume 9 (1978)
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Volume 8 (1977)
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Volume 7 (1976)
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Volume 6 (1975)
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Volume 5 (1974)
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Volume 4 (1973)
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Volume 3 (1972)
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Volume 2 (1971)
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Volume 1 (1970)
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Volume 0 (1932)