I Show (i + show)

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Distribution within Humanities and Social Sciences

Kinds of I Show

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  • Selected Abstracts

    Contesting the Curriculum: An Examination of Professionalism as Defined and Enacted by Australian History Teachers

    CURRICULUM INQUIRY, Issue 3 2007
    ABSTRACT In this article, I present an analysis of professionalism as defined and enacted by the History Teachers' Association of New South Wales (HTANSW). This analysis was part of a larger doctoral project (2000,2005) in which I employed critical qualitative inquiry to compare and contrast the contribution that two subject teaching associations (science and history) make to the project of teacher professionalism in Australia. My aim for this project was to explore what professionalism means in practice for a unique group of teachers: those who have made an active and fundamental commitment to their subject community by voluntarily serving on the executive committee of their subject-based professional association. In this article, I present findings from the case account of the HTANSW,an organization that operates locally as a professional teacher community and a representative organization for school-based history teachers. This case account details the manoeuvrings of an association that powerfully asserts an expansive role for history teachers as both contributors to, and critical commentators on, curriculum policy. In this article, I conceptualise the actions of this association as an enacted form of teacher professionalism. Drawing on study findings, I explicate my conception of professionalism as an enacted discourse of power and I show how this discourse is enacted in subject-specific ways. [source]

    Dewey's Conception of an Environment for Teaching and Learning

    CURRICULUM INQUIRY, Issue 3 2002
    David T. Hansen
    In this article, I examine the main contours of John Dewey's conception of an environment for teaching and learning. I show how his conception derives from two components of his philosophical anthropology: (1) his understanding of the nature of a growing self, and (2) his view of how human beings influence one another. With this background in place, I examine why Dewey argues that an environment for teaching and learning should be what he calls "simplified, purified, balanced, and steadying." I discuss how Dewey distinguishes an educative environment from what he calls "surroundings." Finally, I address why he argues that teachers should not focus directly on learning, but rather on the environment that obtains in the classroom. Throughout the article, I try to show how timely and powerful Dewey's conception of an environment remains,for teachers, teacher educators, and all who care about meaningful teaching and learning. [source]

    Thesis as Narrative or ,What Is the Inquiry in Narrative Inquiry?'

    CURRICULUM INQUIRY, Issue 2 2000
    Carola Conle
    I present elements of inquiry in a dissertation composed through experiential narrative. My account of the thesis process is interwoven with references to John Dewey's demonstrations of implicit inquiry in the creation and experience of art. Motivation, methodology, outcomes and literature review take on a narrative character and I show how aesthetic and reflective activities contributed to the inquiry. Conceptually, a ,tension-telos dynamic' characterizes the impetus for the work; ,resonance' is portrayed as the connecting principle among various narrative components of the thesis, and the function of a ,third term' in metaphorical relationships is presented as a structuring principle for these connections. Although my inquiry came about through personal stories, my narratives reached out to social, historical and philosophical contexts to gain a wider significance, academically and personally. [source]

    The evolutionary psychology of left and right: Costs and benefits of lateralization

    Giorgio VallortigaraArticle first published online: 2 AUG 200
    Abstract Why do the left and right sides of the vertebrate brain play different functions? Having a lateralized brain, in which each hemisphere carries out different functions, is ubiquitous among vertebrates. The different specialization of the left and right side of the brain may increase brain efficiency,and some evidence for that is reported here. However, lateral biases due to brain lateralization (such as preferences in the use of a limb or, in animals with laterally placed eyes, of a visual hemifield) usually occur at the population level, with most individuals showing similar direction of bias. Individual brain efficiency does not require the alignment of lateralization in the population. Why then are not left- and right-type individuals equally common? Not only humans, but most vertebrates show a similar pattern. For instance, in the paper I report evidence that most toads, chickens, and fish react faster when a predator approaches from the left. I argue that invoking individual brain efficiency (lateralization may increase fitness), evolutionary chance or direct genetic mechanisms cannot explain this widespread pattern. Instead, using concepts from mathematical theory of games, I show that alignment of lateralization at the population level may arise as an "evolutionarily stable strategy" when individually asymmetrical organisms must coordinate their behavior with that of other asymmetrical organisms. Thus, the population structure of lateralization may result from genes specifying the direction of asymmetries which have been selected under "social" pressures. © 2006 Wiley Periodicals, Inc. Dev Psychobiol 48: 418,427, 2006. [source]

    Avoidance by grazers facilitates spread of an invasive hybrid plant

    ECOLOGY LETTERS, Issue 2 2010
    E. Grosholz
    Ecology Letters (2010) 13: 145,153 Abstract Biological invasions greatly increase the potential for hybridization among native and non-native species. Hybridization may influence the palatability of novel hybrids to consumers potentially influencing invasion success; however, the palatability of non-native hybrids relative to the parent species is poorly known. In contrast, studies of native-only hybrids find they are nearly always more palatable to consumers than the parent species. Here, I experimentally demonstrate that an invasive hybrid cordgrass (Spartina) is dramatically less palatable to grazing geese than the native parent species. Using field and aviary experiments, I show that grazing geese ignore the hybrid cordgrass and preferentially consume native Spartina. I also experimentally demonstrate that reduced herbivory of the invasive hybrid may contribute to faster spread in a California estuary. These results suggest that biological invasions may increase future opportunities for creating novel hybrids that may pose a greater risk to natural systems than the parent species. [source]

    Ecological limits and diversification rate: alternative paradigms to explain the variation in species richness among clades and regions

    ECOLOGY LETTERS, Issue 8 2009
    Daniel L. Rabosky
    Abstract Diversification rate is one of the most important metrics in macroecological and macroevolutionary studies. Here I demonstrate that diversification analyses can be misleading when researchers assume that diversity increases unbounded through time, as is typical in molecular phylogenetic studies. If clade diversity is regulated by ecological factors, then species richness may be independent of clade age and it may not be possible to infer the rate at which diversity arose. This has substantial consequences for the interpretation of many studies that have contrasted rates of diversification among clades and regions. Often, it is possible to estimate the total diversification experienced by a clade but not diversification rate itself. I show that the evidence for ecological limits on diversity in higher taxa is widespread. Finally, I explore the implications of ecological limits for a variety of ecological and evolutionary questions that involve inferences about speciation and extinction rates from phylogenetic data. [source]

    Managing ecosystem services: what do we need to know about their ecology?

    ECOLOGY LETTERS, Issue 5 2005
    Claire Kremen
    Abstract Human domination of the biosphere has greatly altered ecosystems, often overwhelming their capacity to provide ecosystem services critical to our survival. Yet ecological understanding of ecosystem services is quite limited. Previous work maps the supply and demand for services, assesses threats to them, and estimates economic values, but does not measure the underlying role of biodiversity in providing services. In contrast, experimental studies of biodiversity,function examine communities whose structures often differ markedly from those providing services in real landscapes. A bridge is needed between these two approaches. To develop this research agenda, I discuss critical questions and key approaches in four areas: (1) identifying the important ,ecosystem service providers'; (2) determining the various aspects of community structure that influence function in real landscapes, especially compensatory community responses that stabilize function, or non-random extinction sequences that rapidly erode it; (3) assessing key environmental factors influencing provision of services, and (4) measuring the spatio-temporal scale over which providers and services operate. I show how this research agenda can assist in developing environmental policy and natural resource management plans. [source]

    On the relationship between niche and distribution

    ECOLOGY LETTERS, Issue 4 2000
    H.R. Pulliam
    Applications of Hutchinson's n -dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson's niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se. I introduce a simulation model called NICHE that embodies many of Hutchinson's original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions. [source]

    A Parsimonious Macroeconomic Model for Asset Pricing

    ECONOMETRICA, Issue 6 2009
    Fatih Guvenen
    I study asset prices in a two-agent macroeconomic model with two key features: limited stock market participation and heterogeneity in the elasticity of intertemporal substitution in consumption (EIS). The model is consistent with some prominent features of asset prices, such as a high equity premium, relatively smooth interest rates, procyclical stock prices, and countercyclical variation in the equity premium, its volatility, and in the Sharpe ratio. In this model, the risk-free asset market plays a central role by allowing non-stockholders (with low EIS) to smooth the fluctuations in their labor income. This process concentrates non-stockholders' labor income risk among a small group of stockholders, who then demand a high premium for bearing the aggregate equity risk. Furthermore, this mechanism is consistent with the very small share of aggregate wealth held by non-stockholders in the U.S. data, which has proved problematic for previous models with limited participation. I show that this large wealth inequality is also important for the model's ability to generate a countercyclical equity premium. When it comes to business cycle performance, the model's progress has been more limited: consumption is still too volatile compared to the data, whereas investment is still too smooth. These are important areas for potential improvement in this framework. [source]

    The Unemployment Volatility Puzzle: Is Wage Stickiness the Answer?

    ECONOMETRICA, Issue 5 2009
    Christopher A. Pissarides
    I discuss the failure of the canonical search and matching model to match the cyclical volatility in the job finding rate. I show that job creation in the model is influenced by wages in new matches. I summarize microeconometric evidence and find that wages in new matches are volatile and consistent with the model's key predictions. Therefore, explanations of the unemployment volatility puzzle have to preserve the cyclical volatility of wages. I discuss a modification of the model, based on fixed matching costs, that can increase cyclical unemployment volatility and is consistent with wage flexibility in new matches. [source]

    The Interaction between the Central Bank and a Single Monopoly Union Revisited: Does Greater Monetary Policy Uncertainty Reduce Nominal Wages?

    ECONOMIC NOTES, Issue 3 2007
    Luigi Bonatti
    Previous papers modelling the interaction between the central bank and a single monopoly union demonstrated that greater monetary policy uncertainty reduces the union's nominal wage. This paper shows that this result does not hold in general, since it depends on peculiar specifications of the union's objective function. In particular, I show that greater monetary policy uncertainty raises the nominal wage whenever union members tend to be more sensitive to the risk of getting low real wages than to the risk of remaining unemployed. This conclusion appears consistent with the evidence showing that greater monetary authority's transparency reduces average inflation. [source]

    Securitization of taxes implicit in PAYG pensions

    ECONOMIC POLICY, Issue 42 2005
    Salvador Valdés-Prieto
    SUMMARY Pay-as-you-go securities To preserve solvency, a pay-as-you-go (PAYG) pension system needs to adjust contribution rates and pension promises over time. Currently, it is not possible to hedge in the financial market against politically determined uncertainty as regards these parameters. I consider a policy reform whereby property rights are established on the implicit lifetime tax levied by PAYG finance, and are assigned to the pension institution. These property rights are well defined if the reform also features rule-based allocation of aggregate risk, in the form of defined-contribution or defined-benefit schemes. I show that a PAYG pension system may indeed be instantly restructured so as to minimize political risk and allow financial-market diversification of risk. A side benefit is securitization of human-capital flows, which are not traded in existing financial markets. The new securities, if traded in appropriately competitive financial markets, are complementary to the Notional Account reforms of the 1990s. However, fiscal instability can increase if securitization is implemented in the absence of initial solvency and credible adoption of rule-based methods to allocate aggregate risk. , Salvador Valdés-Prieto [source]

    Modelling Probabilities of Devaluations

    ECONOMICA, Issue 281 2004
    Gabriela Mundaca
    I show why, when the realized rates of depreciation within the exchange rate band are regressed on a given information set and conditioned on (ex post) actual no realignment (à la drift adjustment), a ,peso problem' is still encountered. The reason is that the frequency of realignments in the data need not be the same as the frequency of the (even small) subjective probabilities that a realignment may take place. I suggest an alternative approach to solve the peso problem and provide consistent estimates. My estimates of the expected realignment rates are greater than the ones obtained using the drift adjustment method. [source]

    Unemployment and Search Externalities in a Model with Heterogeneous Jobs and Workers

    ECONOMICA, Issue 273 2002
    Pieter A. Gautier
    This paper presents a matching model with low, and high,skilled workers and simple and complex jobs. I show that the degree to which low,skilled workers are harmed by high,skilled workers who are willing to temporarily accept simple jobs depends on the relative productivity of high, and low,skilled workers on simple jobs and on the quit rate of high,skilled workers. Under certain conditions, low,skilled workers can benefit from job competition with high,skilled workers. Within this framework, some explanations for the high and persistent unemployment rates of lower educated workers in the 1990s are evaluated. [source]

    Optimal At-will Labour Contracts

    ECONOMICA, Issue 270 2001
    Ed Nosal
    An at-will employment rule allows parties to sever their employment relationship for ,a good reason, a bad reason or no reason at all'[Schawb, S. (1993) Life-cycle justice: accommodating just cause and employment at will. Michigan Law Review, 92, 8--62]. A specific performance employment rule allows any party to force the other party to perform as specified in the contract. Although the theory of labour contracting generally assumes enforcement by specific performance, in practice, the vast majority of non-union employment relationships are mediated by an at-will rule. When employment contracts are enforced by an at-will rule, I show that the ,standard' counter-intuitive predictions generated by standard labour contracting models disappear. [source]

    Poverty Traps and Human Capital Accumulation

    ECONOMICA, Issue 270 2001
    Carlotta Berti Ceroni
    In this paper I show that persistent inequality in the distribution of human capital and a negative relation between initial inequality and steady-state aggregate output may follow from the fact that the poor require relatively higher returns to increase expenditure on education. Moreover, I show that poverty traps emerging in models where individual transitions do not depend on aggregate dynamics, though not robust to the introduction of idiosyncratic uncertainty, may still be relevant observationally, if idiosyncratic shocks occur with low probability. In this context, I also analyse the implications of introducing a public education system. [source]

    On the Endogenous Choice between Protection and Promotion

    ECONOMICS & POLITICS, Issue 1 2000
    D. Mitra
    In a model of strategic interaction between firms in lobbying activity, I show that capitalists might prefer tariffs (protection) to production subsidies (promotion). This is due to the congestion problem arising from the government's convex welfare costs of providing subsidies as opposed to both the free-rider problem and the congestion problem acting in opposite directions in the case of tariffs. If an industry association exists, coordination can be achieved when lobbying for tariffs, but not in the case of production subsidies. [source]

    Matricidal Madness in Foucault's Anthropology: The Pierre Rivière Seminar

    ETHOS, Issue 2 2007
    John M. Ingham
    I consider how Michel Foucault avoids trauma and tragedy while emphasizing power and discourse in his study of a 19th-century matricide in the Normandy countryside. Drawing on Jonathan Lear's discussion of knowingness, I show how Foucault misreads tragic drama as well as psychoanalysis to emphasize power, pleasure, and discourse. While seeming to acknowledge tragedy, his emphasis on will to power, violent madness, and male agonism more closely resembles Homeric epic. The attempts to displace psychoanalysis and refigure tragedy, however, are unconvincing, even self-defeating. Ironically, Foucault makes an inadvertent argument for psychoanalysis and tragedy and, thus, psychoanalytic anthropology. [source]

    From Hostage to Host: Confessions of a Spirit Medium in Niger

    ETHOS, Issue 1-2 2002
    Associate Professor Adeline Masquelier
    Spirit possession ostensibly solves problems by freeing the object of possession from certain responsibilities, yet it also creates a whole nexus of unavoidable obligations as the human host learns to cope with the social, financial, and moral demands of her powerful alter ego. Rather than simplifying situations, possession complicates them by introducing new relations and enabling new forms of communication. In this article, I explore what bori possession as communication entailed for a young Mawri woman from Dogondoutchi (Niger) when her possessing spirit made dramatic revelations that forced her to make changes in her life. I show that possession opens up a space of self-awareness for mediums as they struggle to gain progressive control over the terms of their relationships with spirits. In this space of reflexivity they help create and in their role as interlocutors, accusers, or diviners, spirits play a crucial role in the refashioning of human histories and identities. [source]


    EVOLUTION, Issue 5 2009
    Jason B. Wolf
    Interactions between cytoplasmic (generally organelle) and nuclear genomes may be relatively common and could potentially have major fitness consequences. As in the case of within-genome epistasis, this cytonuclear epistasis can favor the evolutionary coadaptation of high-fitness combinations of nuclear and cytoplasmic alleles. Because cytoplasmic factors are generally uniparentally inherited, the cytoplasmic genome is inherited along with only one of the nuclear haplotypes, and therefore, coadaptation is expected to evolve through the interaction of these coinherited (usually maternally inherited) genomes. Here I show that, as a result of this coinheritance of the two genomes, cytonuclear epistasis can favor the evolution of genomic imprinting such that, when the cytoplasmic factor is maternally inherited, selection favors maternal expression of the nuclear locus and when the factor is paternally inherited selection favors paternal expression. Genomic imprinting evolves in this model because it leads to a pattern of gene expression in the nuclear haplotype that is coadapted with (i.e., adaptively coordinated with) gene expression in the coinherited cytoplasmic genome. [source]


    EVOLUTION, Issue 2 2009
    Martin Willensdorfer
    Most conspicuous organisms are multicellular and most multicellular organisms develop somatic cells to perform specific, nonreproductive tasks. The ubiquity of this division of labor suggests that it is highly advantageous. In this article I present a model to study the evolution of specialized cells. The model allows for unicellular and multicellular organisms that may contain somatic (terminally differentiated) cells. Cells contribute additively to a quantitative trait. The fitness of the organism depends on this quantitative trait (via a benefit function), the size of the organism, and the number of somatic cells. The model allows one to determine when somatic cells are advantageous and to calculate the optimum number (or fraction) of reproductive cells. I show that the fraction of reproductive cells is always surprisingly high. If somatic cells are very small, they can outnumber reproductive cells but their biomass is still less than the biomass of reproductive cells. I discuss the biology of primitive multicellular organisms with respect to the model predictions. I find a good agreement and outline how this work can be used to guide further quantitative studies of multicellularity. [source]


    EVOLUTION, Issue 6 2006
    H. Allen Orr
    Abstract Several recent theoretical studies of the genetics of adaptation have focused on the mutational landscape model, which considers evolution on rugged fitness landscapes (i.e., ones having many local optima). Adaptation in this model is characterized by several simple results. Here I ask whether these results also hold on correlated fitness landscapes, which are smoother than those considered in the mutational landscape model. In particular, I study the genetics of adaptation in the block model, a tunably rugged model of fitness landscapes. Considering the scenario in which adaptation begins from a high fitness wild-type DNA sequence, I use extreme value theory and computer simulations to study both single adaptive steps and entire adaptive walks. I show that all previous results characterizing single steps in adaptation in the mutational landscape model hold at least approximately on correlated landscapes in the block model; many entire-walk results, however, do not. [source]


    EVOLUTION, Issue 11 2005
    John S. Heywood
    Abstract Starting with the Price equation, I show that the total evolutionary change in mean phenotype that occurs in the presence of fitness variation can be partitioned exactly into five components representing logically distinct processes. One component is the linear response to selection, as represented by the breeder's equation of quantitative genetics, but with heritability defined as the linear regression coefficient of mean offspring phenotype on parent phenotype. The other components are identified as constitutive transmission bias, two types of induced transmission bias, and a spurious response to selection caused by a covariance between parental fitness and offspring phenotype that cannot be predicted from parental phenotypes. The partitioning can be accomplished in two ways, one with heritability measured before (in the absence of) selection, and the other with heritability measured after (in the presence of) selection. Measuring heritability after selection, though unconventional, yields a representation for the linear response to selection that is most consistent with Darwinian evolution by natural selection because the response to selection is determined by the reproductive features of the selected group, not of the parent population as a whole. The analysis of an explicitly Mendelian model shows that the relative contributions of the five terms to the total evolutionary change depends on the level of organization (gene, individual, or mated pair) at which the parent population is divided into phenotypes, with each frame of reference providing unique insight. It is shown that all five components of phenotypic evolution will generally have nonzero values as a result of various combinations of the normal features of Mendelian populations, including biparental sex, allelic dominance, inbreeding, epistasis, linkage disequilibrium, and environmental covariances between traits. Additive genetic variance can be a poor predictor of the adaptive response to selection in these models. The narrow-sense heritability s,2A/s,2P should be viewed as an approximation to the offspring-parent linear regression rather than the other way around. [source]


    EVOLUTION, Issue 3 2004
    Abstract Theory suggests that frequency-dependent resource competition will disproportionately impact the most common phenotypes in a population. The resulting disruptive selection forms the driving force behind evolutionary models of niche diversification, character release, ecological sexual dimorphism, resource polymorphism, and sympatric speciation. However, there is little empirical support for the idea that intraspecific competition generates disruptive selection. This paper presents a test of this theory, using natural populations of the three-spine stickleback, Gasterosteus aculeatus. Sticklebacks exhibit substantial individual specialization associated with phenotypic variation and so are likely to experience frequency-dependent competition and hence disruptive selection. Using body size and relative gonad mass as indirect measures of potential fecundity and hence fitness, I show that an important aspect of trophic morphology, gill raker length, is subject to disruptive selection in one of two natural lake populations. To test whether this apparent disruptive selection could have been caused by competition, I manipulated population densities in pairs of large enclosures in each of five lakes. In each lake I removed fish from one enclosure and added them to the other to create paired low- and high-population-density treatments with natural phenotype distributions. Again using indirect measures of fitness, disruptive selection was consistently stronger in high-density than low-density enclosures. These results support long-standing theoretical arguments that intraspecific competition drives disruptive selection and thus may be an important causal agent in the evolution of ecological variation. [source]


    EVOLUTION, Issue 12 2003
    Abstract Accumulation of deleterious mutations has important consequences for the evolution of mating systems and the persistence of small populations. It is well established that consanguineous mating can purge a part of the mutation load and that lethal mutations can also be purged in small populations. However, the efficiency of purging in natural populations, due to either consanguineous mating or to reduced population size, has been questioned. Consequences of consanguineous mating systems and small population size are often equated under "inbreeding" because both increase homozygosity, and selection is though to be more efficient against homozygous deleterious alleles. I show that two processes of purging that I call "purging by drift" and "purging by nonrandom mating" have to be distinguished. Conditions under which the two ways of purging are effective are derived. Nonrandom mating can purge deleterious mutations regardless of their dominance level, whereas only highly recessive mutations can be purged by drift. Both types of purging are limited by population size, and sharp thresholds separate domains where purging is either effective or not. The limitations derived here on the efficiency of purging are compatible with some experimental studies. Implications of these results for conservation and evolution of mating systems are discussed. [source]


    EVOLUTION, Issue 3 2003
    Kari Lehtilä
    Abstract Tiffin and Inouye (2000) discussed the use of natural and imposed (controlled) damage in experiments of herbivore tolerance. They constructed a statistical model of the effect of herbivory on plant fitness, including damage level and an environmental factor as the independent factors, in which tolerance is defined as a slope of the regression line when damage level is regressed with plant fitness. They claim that while experiments with imposed damage are more accurate (i.e., they give a more correct estimate of tolerance), experiments with natural damage are more precise under a wide range of parameter values (i.e., tolerance estimates explain a larger part of variation in fitness). I show, however, that experiments with imposed damage are less precise only when an experimenter uses an experimental design that has weaker statistical power than in experiments with natural herbivory. The experimenter can nevertheless control the damage levels to optimize the experimental designs. For instance, when half of the experimental plants are left undamaged and the other half treated with maximal relevant damage level, experiments with imposed damage are almost always much more precise than experiments with natural damage. [source]


    EVOLUTION, Issue 12 2002
    Pärk. Ingvarsson
    Abstract., Partial self-fertilization is common in higher plants. Mating system variation is known to have important consequences for how genetic variation is distributed within and among populations. Selfing is known to reduce effective population size, and inbreeding species are therefore expected to have lower levels of genetic variation than comparable out crossing taxa. However, several recent empirical studies have shown that reductions in genetic diversity within populations of inbreeding species are far greater than the expected reductions based on the reduced effective population size. Two different processes have been argued to cause these patterns, selective sweeps (or hitchhiking) and background selection. Both are expected to be most effective in reducing genetic variation in regions of low recombination rates. Selfing is known to reduce the effective recombination rate, and inbreeding taxa are thus thought to be particularly vulnerable to the effects of hitchhiking or background selection. Here I propose a third explanation for the lower-than-expected levels of genetic diversity within populations of selfing species; recurrent extinctions and recolonizations of local populations, also known as metapopulation dynamics. I show that selfing in a metapopulation setting can result in large reductions in genetic diversity within populations, far greater than expected based the lower effective population size inbreeding species is expected to have. The reason for this depends on an interaction between selfing and pollen migration. [source]


    EVOLUTION, Issue 7 2002
    H. Allen Orr
    Abstract I describe several patterns characterizing the genetics of adaptation at the DNA level. Following Gillespie (1983, 1984, 1991), I consider a population presently fixed for the ith best allele at a locus and study the sequential substitution of favorable mutations that results in fixation of the fittest DNA sequence locally available. Given a wild type sequence that is less than optimal, I derive the fitness rank of the next allele typically fixed by natural selection as well as the mean and variance of the jump in fitness that results when natural selection drives a substitution. Looking over the whole series of substitutions required to reach the best allele, I show that the mean fitness jumps occurring throughout an adaptive walk are constrained to a twofold window of values, assuming only that adaptation begins from a reasonably fit allele. I also show that the first substitution and the substitution of largest effect account for a large share of the total fitness increase during adaptation. I further show that the distribution of selection coefficients fixed throughout such an adaptive walk is exponential (ignoring mutations of small effect), a finding reminiscent of that seen in Fisher's geometric model of adaptation. Last, I show that adaptation by natural selection behaves in several respects as the average of two idealized forms of adaptation, perfect and random. [source]


    EVOLUTION, Issue 2 2002
    Wade N. Hazel
    Abstract Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth-instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full-sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn. [source]


    EVOLUTION, Issue 3 2000
    Darren E. Irwin
    Abstract., Divergence of mating signals can occur rapidly and be of prime importance in causing reproductive isolation and speciation. A ring species, in which two reproductively isolated taxa are connected by a chain of intergrading populations, provides a rare opportunity to use spatial variation to reconstruct the history of divergence. I use geographic variation in the song of a likely ring species, the greenish warbler (Phylloscopus trochiloides) to reconstruct the microevolutionary steps that occurred during divergence of a trait that is often important in speciation in birds. Populations of a western Siberian (P. t. viridanus) and an eastern Siberian (P. t. plumbeitarsus) form of the greenish warbler meet, but do not interbreed in central Siberia; these forms are connected by a chain of interbreeding populations extending in a ring to the south around the treeless Tibetan Plateau. I show that: (1) song structure differs greatly between the two Siberian forms, which share the same habitat; (2) song structure changes gradually around the ring; (3) singing behavior is relatively simple in the Himalayas, but becomes increasingly complex to the north, both to the west and east of the Tibetan Plateau; and (4) song varies along independent axes of complexity in the western and eastern south-north clines. By comparing geographic variation in singing behavior and ecological variables, I distinguish among possible causes of song divergence, including selection based on the acoustic environment, stochastic effects of sexual selection, and selection for species recognition. I suggest that parallel south-to-north ecological gradients have caused a greater intensity of sexual selection on song in northern populations and that the stochastic effects of sexual selection have led to divergence in song structure. [source]