Home About us Contact
|
Home About us Contact | ||
|
|
||
Perspective
Kinds of Perspective Terms modified by Perspective Selected AbstractsTHE MUSEUM IN TRANSITION: A PHILOSOPHICAL PERSPECTIVE: Hilde S. HeinCURATOR THE MUSEUM JOURNAL, Issue 2 2003Peter Linett First page of article [source] CURRENT STATUS AND FUTURE PERSPECTIVE OF INTERVENTIONAL ENDOSCOPIC ULTRASOUND IN JAPANDIGESTIVE ENDOSCOPY, Issue 2009Mitsuhiro Kida Vilmann and Grimm made the first reports of endoscopic ultrasound (EUS)-fine needle aspiration (FNA), and since then EUS-FNA has become popular in the clinical fields, especially in Western countries. Furthermore interventional EUS such as pseudocyst drainage, EUS-guided biliary drainage, celiac plexus neurolysis, and dendritic cell injection, etc. have been introduced. We have investigated the current status and future perspectives of interventional EUS in Japan. Standardization of EUS-guided pseudocyst drainage has been achieved, but EUS-guided biliary dranage is still controversial, and EUS-fine needle injection (FNI) including EUS-celiac plexus neurolysis (CPN) and dendritic cell injection have been under investigation. In any case, EUS-FNA seems to be a promising future technique and new applications have to be invented. [source] DRUG SCHEDULING,SCIENCE AND CULTURAL PERSPECTIVEADDICTION, Issue 7 2010RAJAT RAY No abstract is available for this article. [source] BRIEF ALCOHOL INTERVENTION RESEARCH AND PRACTICE,TOWARDS A BROADER PERSPECTIVEADDICTION, Issue 6 2010PER NILSEN No abstract is available for this article. [source] FIGHTING FINANCIAL CRIME: A UK PERSPECTIVEECONOMIC AFFAIRS, Issue 1 2007Mike Bowron Financial crime has a devastating impact on individuals, companies and governments. Traditional methods of control, predominantly investigation and prosecution, have failed to abate the rise of both fraud and money laundering offences. Tackling financial crime is best approached from the perspective of prevention, an activity that requires co-operation between all those affected by this widespread and corrosive social problem. [source] LONG-TERM EFFECTS OF ALCOHOL POLICIES: AN ECONOMIC PERSPECTIVEADDICTION, Issue 3 2010PIETER VAN BAAL No abstract is available for this article. [source] A CHINESE PERSPECTIVE ON THE CHINA-AUSTRALIA FREE TRADE AGREEMENT AND POLICY SUGGESTIONSECONOMIC PAPERS: A JOURNAL OF APPLIED ECONOMICS AND POLICY, Issue 1 2008DAWEI CHENG This article provides an analysis of the potential economic effects of the China,Australia free trade agreement, and provides a set of policy recommendations regarding such an agreement. The article begins with a review of China,Australia trade relations, showing the widening gap in the importance of one country relative to the other. Next, the article examines the competitive advantages of China and Australia in trade by way of local revealed comparative advantage (RCA) and finds that trade between China and Australia is predicated on differences in their factor endowments. The study then investigates the main problems in the negotiations for the China,Australia Free Trade Agreement and concludes with a set of policy suggestions. [source] MANAGING EDUCATIONAL TRANSFORMATION IN THE GLOBALIZED WORLD: A DEWEYAN PERSPECTIVEEDUCATIONAL THEORY, Issue 4 2009Maura Striano In the globalization scenarios we currently face, educational systems are challenged by different and sometimes competing pressures and requests. These call for a deep transformation of the organization, role, and social function of educational systems. Within this context, the very concept of education has come to be understood in different ways, which sometimes distort its moral and social value. In this essay, Maura Striano contends that from a Deweyan perspective, educational transformation must be seen as strictly connected to social change, and education should be understood as a process that facilitates and supports social growth and development. In order to be effective and fruitful, Striano suggests, this transformation must occur from the inside of educational systems and can only be brought about by reflective and inquiry-based inner processes if it is to have a sound moral and social impact within the changing framework of the globalized world. That education shares in the confusion of transition, and in the demand for reorganization, is a source of encouragement and not of despair. It proves how integrally the school is bound up with the entire movement of modern life. ,John Dewey, The Educational Situation [source] THE SCHOOL AS AN EXCEPTIONAL SPACE: RETHINKING EDUCATION FROM THE PERSPECTIVE OF THE BIOPEDAGOGICALEDUCATIONAL THEORY, Issue 2 2006Tyson E. LewisArticle first published online: 3 MAY 200 Agamben's theory of the camp provides a challenging, critical vantage point for looking at the ambiguities that emerge from the complex field of disciplinary procedures now prevalent in inner-city, low-income, minority schools, and helps to clarify what exactly is at stake in the symbolic and sometimes physical violence of schooling. Key to understanding the primary relation between camp and classroom is Agamben's framework of the biopolitical, which paradoxically includes life as a political concern through its exclusion from the political sphere. Here Lewis appropriates Agamben's terminology in order to theorize the biopedagogical, wherein educational life is included in schooling through its abandonment. For Lewis, the theory of the camp is necessary to recognizing how schools function and, in turn, how they could function differently. [source] CONTROLLING GAMBLING: A POPULATION-BASED PERSPECTIVE TO MEASUREMENT AND MONITORING AS RESOURCE FOR EFFECTIVE INTERVENTIONSADDICTION, Issue 7 2009NORMAN GIESBRECHT No abstract is available for this article. [source] [Commentary] TIME FOR A CHANGE OF PERSPECTIVE ON BEHAVIOUR CHANGE INTERVENTIONS?ADDICTION, Issue 6 2009JEAN ADAMS No abstract is available for this article. [source] SOCIAL MARKETING AND PROBLEM GAMBLING: A CRITICAL PERSPECTIVEADDICTION, Issue 5 2009CRAWFORD MOODIE First page of article [source] PERSPECTIVE: SEVEN REASONS (NOT) TO NEGLECT NICHE CONSTRUCTIONEVOLUTION, Issue 9 2006Kevin N. Laland Abstract ,The niche-construction perspective within evolutionary biology places emphasis on the changes that organisms bring about in their selective environments. Advocates of this viewpoint argue that there is both accuracy and utility in treating niche construction as an evolutionary process in its own right, rather than merely as a product of evolution. Here we discuss and assess seven putative weaknesses of the niche-construction perspective. Niche construction has been neglected or rejected on the grounds that (1) it is not prevalent, (2) its study is not tractable, (3) it is not a process, (4) it is caused by natural selection, (5) it does not change our understanding of evolution in any fundamental way, (6) it does not bring about adaptation, and (7) it is not a single phenomenon. In each case, we critically evaluate the theoretical standing of these arguments and consider the empirical evidence that can be brought to bear on the debate. We conclude that none of these are strong criticisms of the niche-construction perspective and maintain that there are compelling reasons for treating niche construction as a major evolutionary process. [source] PERSPECTIVE: MATERNAL KIN GROUPS AND THE ORIGINS OF ASYMMETRIC GENETIC SYSTEMS,GENOMIC IMPRINTING, HAPLODIPLOIDY, AND PARTHENOGENESISEVOLUTION, Issue 4 2006Benjamin B. Normark Abstract The genetic systems of animals and plants are typically eumendelian. That is, an equal complement of autosomes is inherited from each of two parents, and at each locus, each parent's allele is equally likely to be expressed and equally likely to be transmitted. Genetic systems that violate any of these eumendelian symmetries are termed asymmetric and include parent-specific gene expression (PSGE), haplodiploidy, thelytoky, and related systems. Asymmetric genetic systems typically arise in lineages with close associations between kin (gregarious siblings, brooding, or viviparity). To date, different explanatory frameworks have been proposed to account for each of the different asymmetric genetic systems. Haig's kinship theory of genomic imprinting argues that PSGE arises when kinship asymmetries between interacting kin create conflicts between maternally and paternally derived alleles. Greater maternal than paternal relatedness within groups selects for more "abstemious" expression of maternally derived alleles and more "greedy" expression of paternally derived alleles. Here, I argue that this process may also underlie origins of haplodiploidy and many origins of thelytoky. The tendency for paternal alleles to be more "greedy" in maternal kin groups means that maternal-paternal conflict is not a zero-sum game: the maternal optimum will more closely correspond to the optimum for family groups and demes and for associated entities such as symbionts. Often in these circumstances, partial or complete suppression of paternal gene expression will evolve (haplodiploidy, thelytoky), or other features of the life cycle will evolve to minimize the conflict (monogamy, inbreeding). Maternally transmitted cytoplasmic elements and maternally imprinted nuclear alleles have a shared interest in minimizing agonistic interactions between female siblings and may cooperate to exclude the paternal genome. Eusociality is the most dramatic expression of the conflict-reducing effects of haplodiploidy, but its original and more widespread function may be suppression of intrafamilial cannibalism. In rare circumstances in which paternal gene products gain access to maternal physiology via a placenta, PSGE with greedy paternal gene expression can persist (e.g., in mammals). [source] PERSPECTIVE: SIGN EPISTASIS AND GENETIC COSTRAINT ON EVOLUTIONARY TRAJECTORIESEVOLUTION, Issue 6 2005Daniel M. Weinreich Abstract Epistasis for fitness means that the selective effect of a mutation is conditional on the genetic background in which it appears. Although epistasis is widely observed in nature, our understanding of its consequences for evolution by natural selection remains incomplete. In particular, much attention focuses only on its influence on the instantaneous rate of changes in frequency of selected alleles via epistatic contribution to the additive genetic variance for fitness. Thus, in this framework epistasis only has evolutionary importance if the interacting loci are simultaneously segregating in the population. However, the selective accessibility of mutational trajectories to high fitness genotypes may depend on the genetic background in which novel mutations appear, and this effect is independent of population polymorphism at other loci. Here we explore this second influence of epistasis on evolution by natural selection. We show that it is the consequence of a particular form of epistasis, which we designate sign epistasis. Sign epistasis means that the sign of the fitness effect of a mutation is under epistatic control; thus, such a mutation is beneficial on some genetic backgrounds and deleterious on others. Recent experimental innovations in microbial systems now permit assessment of the fitness effects of individual mutations on multiple genetic backgrounds. We review this literature and identify many examples of sign epistasis, and we suggest that the implications of these results may generalize to other organisms. These theoretical and empirical considerations imply that strong genetic constraint on the selective accessibility of trajectories to high fitness genotypes may exist and suggest specific areas of investigation for future research. [source] PERSPECTIVE: THE EVOLUTION OF WARNING COLORATION IS NOT PARADOXICALEVOLUTION, Issue 5 2005Nicola M. Marples Abstract Animals that are brightly colored have intrigued scientists since the time of Darwin, because it seems surprising that prey should have evolved to be clearly visible to predators. Often this self-advertisement is explained by the prey being unprofitable in some way, with the conspicuous warning coloration helping to protect the prey because it signals to potential predators that the prey is unprofitable. However, such signals only work in this way once predators have learned to associate the conspicuous color with the unprofitability of the prey. The evolution of warning coloration is still widely considered to be a paradox, because it has traditionally been assumed that the very first brightly colored individuals would be at an immediate selective disadvantage because of their greater conspicuousness to predators that are naive to the meaning of the signal. As a result, it has been difficult to understand how a novel conspicuous color morph could ever avoid extinction for long enough for predators to become educated about the signal. Thus, the traditional view that the evolution of warning coloration is difficult to explain rests entirely on assumptions about the foraging behavior of predators. However, we review recent evidence from a range of studies of predator foraging decisions, which refute these established assumptions. These studies show that: (1) Many predators are so conservative in their food preferences that even very conspicuous novel prey morphs are not necessarily at a selective disadvantage. (2) The survival and spread of novel color morphs can be simulated in field and aviary experiments using real predators (birds) foraging on successive generations of artificial prey populations. This work demostrates that the foraging preferences of predators can regularly (though not always) result in the increase to fixation of a novel morph appearing in a population of familiar-colored prey. Such fixation events occur even if both novel and familiar prey are fully palatable and despite the novel food being much more conspicuous than the familiar prey. These studies therefore provide strong empirical evidence that conspicuous coloration can evolve readily, and repeatedly, as a result of the conservative foraging decisions of predators. [source] PERSPECTIVE: THE SIZE-COMPLEXITY RULEEVOLUTION, Issue 9 2004J. T. Bonner Abstract It is widely accepted that bigger entities have a greater division of labor than smaller ones and this is reflected in the fact that larger multicellular organisms have a corresponding increase in the number of their cell types. This rule is examined in some detail from very small organisms to large animals, and plants, and societies. Compared to other size-related rules, the size-complexity rule is relatively rough and approximate, yet clearly it holds throughout the whole range of living organisms, as well as for societies. The relationship between size and complexity is analyzed by examining the effects of size increase and decrease: size increase requires an increase in complexity, whereas size decrease permits, and sometimes requires, a decrease in complexity. Conversely, an increase or decrease in complexity permits, but does not require changes in size. An especially compelling argument for the close relation between size and complexity can be found in size quorum sensing in very small multicellular organisms. [source] MACROECOLOGY FROM A SAWFLY'S PERSPECTIVE,EVOLUTION, Issue 8 2004Niklas Janz No abstract is available for this article. [source] PERSPECTIVE: MODELS OF SPECIATION: WHAT HAVE WE LEARNED IN 40 YEARS?EVOLUTION, Issue 10 2003Sergey Gavrilets Abstract Theoretical studies of speciation have been dominated by numerical simulations aiming to demonstrate that speciation in a certain scenario may occur. What is needed now is a shift in focus to identifying more general rules and patterns in the dynamics of speciation. The crucial step in achieving this goal is the development of simple and general dynamical models that can be studied not only numerically but analytically as well. I review some of the existing analytical results on speciation. I first show why the classical theories of speciation by peak shifts across adaptive valleys driven by random genetic drift run into trouble (and into what kind of trouble). Then I describe the Bateson-Dobzhansky-Muller (BDM) model of speciation that does not require overcoming selection. I describe exactly how the probability of speciation, the average waiting time to speciation, and the average duration of speciation depend on the mutation and migration rates, population size, and selection for local adaptation. The BDM model postulates a rather specific genetic architecture of reproductive isolation. I then show exactly why the genetic architecture required by the BDM model should be common in general. Next I consider the multilocus generalizations of the BDM model again concentrating on the qualitative characteristics of speciation such as the average waiting time to speciation and the average duration of speciation. Finally, I consider two models of sympatric speciation in which the conditions for sympatric speciation were found analytically. A number of important conclusions have emerged from analytical studies. Unless the population size is small and the adaptive valley is shallow, the waiting time to a stochastic transition between the adaptive peaks is extremely long. However, if transition does happen, it is very quick. Speciation can occur by mutation and random drift alone with no contribution from selection as different populations accumulate incompatible genes. The importance of mutations and drift in speciation is augmented by the general structure of adaptive landscapes. Speciation can be understood as the divergence along nearly neutral networks and holey adaptive landscapes (driven by mutation, drift, and selection for adaptation to a local biotic and/or abiotic environment) accompanied by the accumulation of reproductive isolation as a by-product. The waiting time to speciation driven by mutation and drift is typically very long. Selection for local adaptation (either acting directly on the loci underlying reproductive isolation via their pleiotropic effects or acting indirectly via establishing a genetic barrier to gene flow) can significantly decrease the waiting time to speciation. In the parapatric case the average actual duration of speciation is much shorter than the average waiting time to speciation. Speciation is expected to be triggered by changes in the environment. Once genetic changes underlying speciation start, they go to completion very rapidly. Sympatric speciation is possible if disruptive selection and/or assortativeness in mating are strong enough. Sympatric speciation is promoted if costs of being choosy are small (or absent) and if linkage between the loci experiencing disruptive selection and those controlling assortative mating is strong. [source] PERSPECTIVE: EVOLUTION AND DETECTION OF GENETIC ROBUSTNESSEVOLUTION, Issue 9 2003J. Arjan G. M. de Visser Abstract Robustness is the invariance of phenotypes in the face of perturbation. The robustness of phenotypes appears at various levels of biological organization, including gene expression, protein folding, metabolic flux, physiological homeostasis, development, and even organismal fitness. The mechanisms underlying robustness are diverse, ranging from thermodynamic stability at the RNA and protein level to behavior at the organismal level. Phenotypes can be robust either against heritable perturbations (e.g., mutations) or nonheritable perturbations (e.g., the weather). Here we primarily focus on the first kind of robustness,genetic robustness,and survey three growing avenues of research: (1) measuring genetic robustness in nature and in the laboratory; (2) understanding the evolution of genetic robustness; and (3) exploring the implications of genetic robustness for future evolution. [source] PERSPECTIVE: GENETIC ASSIMILATION AND A POSSIBLE EVOLUTIONARY PARADOX: CAN MACROEVOLUTION SOMETIMES BE SO FAST AS TO PASS US BY?EVOLUTION, Issue 7 2003Massimo Pigliucci Abstract., The idea of genetic assimilation, that environmentally induced phenotypes may become genetically fixed and no longer require the original environmental stimulus, has had varied success through time in evolutionary biology research. Proposed by Waddington in the 1940s, it became an area of active empirical research mostly thanks to the efforts of its inventor and his collaborators. It was then attacked as of minor importance during the "hardening" of the neo-Darwinian synthesis and was relegated to a secondary role for decades. Recently, several papers have appeared, mostly independently of each other, to explore the likelihood of genetic assimilation as a biological phenomenon and its potential importance to our understanding of evolution. In this article we briefly trace the history of the concept and then discuss theoretical models that have newly employed genetic assimilation in a variety of contexts. We propose a typical scenario of evolution of genetic assimilation via an intermediate stage of phenotypic plasticity and present potential examples of the same. We also discuss a conceptual map of current and future lines of research aimed at exploring the actual relevance of genetic assimilation for evolutionary biology. [source] PERSPECTIVE: SEXUAL CONFLICT AND SEXUAL SELECTION: CHASING AWAY PARADIGM SHIFTSEVOLUTION, Issue 6 2003TOMMASO PIZZARI Abstract., Traditional models of sexual selection propose that partner choice increases both average male and average female fitness in a population. Recent theoretical and empirical work, however, has stressed that sexual conflict may be a potent broker of sexual selection. When the fitness interests of males and females diverge, a reproductive strategy that increases the fitness of one sex may decrease the fitness of the other sex. The chase-away hypothesis proposes that sexual conflict promotes sexually antagonistic, rather than mutualistic, coevolution, whereby manipulative reproductive strategies in one sex are counteracted by the evolution of resistance to such strategies in the other sex. In this paper, we consider the criteria necessary to demonstrate the chase-away hypothesis. Specifically, we review sexual conflict with particular emphasis on the chase-away hypothesis; discuss the problems associated with testing the predictions of the chase-away hypothesis and the extent to which these predictions and the predictions of traditional models of sexual selection are mutually exclusive; discuss misconceptions and mismeasures of sexual conflict; and suggest an alternative approach to demonstrate sexual conflict, measure the intensity of sexually antagonistic selection in a population, and elucidate the coevolutionary trajectories of the sexes. [source] PERSPECTIVE: EMBEDDED MOLECULAR SWITCHES, ANTICANCER SELECTION, AND EFFECTS ON ONTOGENETIC RATES: A HYPOTHESIS OF DEVELOPMENTAL CONSTRAINT ON MORPHOGENESIS AND EVOLUTIONEVOLUTION, Issue 5 2003Kathryn D. Kavanagh Abstract The switch between the cell cycle and the progress of differentiation in developmental pathways is prevalent throughout the eukaryotes in all major cell lineages. Disruptions to the molecular signals regulating the switch between proliferative and differentiating states are severe, often resulting in cancer formation (uncontrolled proliferation) or major developmental disorders. Uncontrolled proliferation and developmental disorders are potentially lethal defects in the developing animal. Therefore, natural selection would likely favor a tightly controlled regulatory mechanism to help prevent these fundamental defects. Although selection is usually thought of as a consequence of environmental or ecological influences, in this case the selective force to maintain this molecular switch is internal, manifested as a potentially lethal developmental defect. The morphogenetic consequences of this prevalent, deeply embedded, and tightly controlled mechanistic switch are currently unexplored, however experimental and correlative evidence from several sources suggest that there are important consequences on the control of growth rates and developmental rates in organs and in the whole animal. These observations lead one to consider the possibility of a developmental constraint on ontogenetic rates and morphological evolution maintained by natural selection against cancer and other embryonic lethal defects. [source] PERSPECTIVE: PURGING THE GENETIC LOAD: A REVIEW OF THE EXPERIMENTAL EVIDENCEEVOLUTION, Issue 12 2002Peter Crnokrak Abstract., Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most important topics of research in evolutionary and conservation genetics. In the recent literature, much attention has been paid to the possibility of purging the genetic load. If inbreeding depression is due to deleterious alleles, whose effect on fitness are negative when in a homozygous state, then successive generations of inbreeding may result in a rebound in fitness due to the selective decrease in frequency of deleterious alleles. Here we examine the experimental evidence for purging of the genetic load by collating empirical tests of rebounds in fitness-related traits with inbreeding in animals and plants. We gathered data from 28 studies including five mammal, three insect, one mollusc, and 13 plant species. We tested for purging by examining three measures of fitness-component variation with serial generations of inbreeding: (1) changes in inbreeding depression, (2) changes in fitness components of inbred lines relative to the original outbred line, and (3) purged population (outcrossed inbred lines) trait means as a function of ancestral outbred trait means. Frequent and substantial purging was found using all three measures, but was particularly pronounced when tracking changes in inbreeding depression. Despite this, we found little correspondence between the three measures of purging within individual studies, indicating that the manner in which a researcher chooses to estimate purging will affect interpretation of the results obtained. The discrepancy suggests an alternative hypothesis: rebounds in fitness with inbreeding may have resulted from adaptation to laboratory conditions and not to purging when using outcrossed inbred lines. However, the pronounced reduction in inbreeding depression for a number of studies provides evidence for purging, as the measure is likely less affected by selection for laboratory conditions. Unlike other taxon-specific reviews on this topic, our results provide support for the purging hypothesis, but firm predictions about the situations in which purging is likely or the magnitude of fitness rebound possible when populations are inbred remain difficult. Further research is required to resolve the discrepancy between the results obtained using different experimental approaches. [source] A METAPOPULATION PERSPECTIVE ON GENETIC DIVERSITY AND DIFFERENTIATION IN PARTIALLY SELF-FERTILIZING PLANTSEVOLUTION, Issue 12 2002Pärk. 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] PERSPECTIVE: TEACHING EVOLUTION IN HIGHER EDUCATIONEVOLUTION, Issue 10 2002Brian J. Alters Abstract., In the past decade, the academic community has increased considerably its activity concerning the teaching and learning of evolution. Despite such beneficial activity, the state of public understanding of evolution is considered woefully lacking by most researchers and educators. This lack of understanding affects evolution/science literacy, research, and academia in general. Not only does the general public lack an understanding of evolution but so does a considerable proportion of college graduates. However, it is not just evolutionary concepts that students do not retain. In general, college students retain little of what they supposedly have learned. Worse yet, it is not just students who have avoided science and math who fail to retain fundamental science concepts. Students who have had extensive secondary-level and college courses in science have similar deficits. We examine these issues and explore what distinguishes effective pedagogy from ineffective pedagogy in higher education in general and evolution education in particular. The fundamental problem of students' prior conceptions is considered and why prior conceptions often underpin students' misunderstanding of the evolutionary concepts being taught. These conceptions can often be discovered and addressed. We also attend to concerns about coverage of course content and the influence of religious beliefs, and provide helpful strategies to improve college-level teaching of evolution. [source] PERSPECTIVE: FEMALE REMATING, OPERATIONAL SEX RATIO, AND THE ARENA OF SEXUAL SELECTION IN DROSOPHILA SPECIESEVOLUTION, Issue 9 2002Therese Ann Markow Abstract., As commonly observed among closely related species within a variety of taxa, Drosophila species differ considerably in whether they exhibit sexual dimorphism in coloration or morphology. Those Drosophila species in which male external sexual characters are minimal or absent tend, instead, to have exaggerated ejaculate traits such as sperm gigantism or seminal nutrient donations. Underlying explanations for the interspecific differences in the presence of external morphological sexual dimorphism versus exaggerated ejaculate traits are addressed here by examining the opportunity for sexual selection on males to occur before versus after mating in 21 species of Drosophila. Female remating frequency, an important component of the operational sex ratio, differs widely among Drosophila species and appears to dictate whether the arena of sexual selection is prior to, as opposed to after, copulation. Infrequent female mating results in fewer mating opportunities for males and thus stronger competition for receptive females that favors the evolution of male characters that maximize mating success. On the other hand, rapid female remating results in overlapping ejaculates in the female reproductive tract, such that ejaculate traits which enhance fertilization success are favored. The strong association between female remating frequency in a given species and the presence of sexually selected external versus internal male characters indicates that the relationship be examined in other taxa as well. [source] PERSPECTIVE: GENE DIVERGENCE, POPULATION DIVERGENCE, AND THE VARIANCE IN COALESCENCE TIME IN PHYLOGEOGRAPHIC STUDIESEVOLUTION, Issue 6 2000ScottV. Abstract Molecular methods as applied to the biogeography of single species (phylogeography) or multiple codistributed species (comparative phylogeography) have been productively and extensively used to elucidate common historical features in the diversification of the Earth's biota. However, only recently have methods for estimating population divergence times or their confidence limits while taking into account the critical effects of genetic polymorphism in ancestral species become available, and earlier methods for doing so are underutilized. We review models that address the crucial distinction between the gene divergence, the parameter that is typically recovered in molecular phylogeographic studies, and the population divergence, which is in most cases the parameter of interest and will almost always postdate the gene divergence. Assuming that population sizes of ancestral species are distributed similarly to those of extant species, we show that phylogeographic studies in vertebrates suggest that divergence of alleles in ancestral species can comprise from less than 10% to over 50% of the total divergence between sister species, suggesting that the problem of ancestral polymorphism in dating population divergence can be substantial. The variance in the number of substitutions (among loci for a given species or among species for a given gene) resulting from the stochastic nature of DNA change is generally smaller than the variance due to substitutions along allelic lines whose coalescence times vary due to genetic drift in the ancestral population. Whereas the former variance can be reduced by further DNA sequencing at a single locus, the latter cannot. Contrary to phylogeographic intuition, dating population divergence times when allelic lines have achieved reciprocal monophyly is in some ways more challenging than when allelic lines have not achieved monophyly, because in the former case critical data on ancestral population size provided by residual ancestral polymorphism is lost. In the former case differences in coalescence time between species pairs can in principle be explained entirely by differences in ancestral population size without resorting to explanations involving differences in divergence time. Furthermore, the confidence limits on population divergence times are severely underestimated when those for number of substitutions per site in the DNA sequences examined are used as a proxy. This uncertainty highlights the importance of multilocus data in estimating population divergence times; multilocus data can in principle distinguish differences in coalescence time (T) resulting from differences in population divergence time and differences in T due to differences in ancestral population sizes and will reduce the confidence limits on the estimates. We analyze the contribution of ancestral population size (,) to T and the effect of uncertainty in , on estimates of population divergence (,) for single loci under reciprocal monophyly using a simple Bayesian extension of Takahata and Satta's and Yang's recent coalescent methods. The confidence limits on , decrease when the range over which ancestral population size , is assumed to be distributed decreases and when increases; they generally exclude zero when /(4Ne) > 1. We also apply a maximum-likelihood method to several single and multilocus data sets. With multilocus data, the criterion for excluding = 0 is roughly that l/(4Ne)> 1, where l is the number of loci. Our analyses corroborate recent suggestions that increasing the number of loci is critical to decreasing the uncertainty in estimates of population divergence time. [source] PERSPECTIVE: EVOLUTIONARY DEVELOPMENTAL BIOLOGY AND THE PROBLEM OF VARIATIONEVOLUTION, Issue 4 2000David L. Stern Abstract. One of the oldest problems in evolutionary biology remains largely unsolved. Which mutations generate evolutionarily relevant phenotypic variation? What kinds of molecular changes do they entail? What are the phenotypic magnitudes, frequencies of origin, and pleiotropic effects of such mutations? How is the genome constructed to allow the observed abundance of phenotypic diversity? Historically, the neo-Darwinian synthesizers stressed the predominance of micromutations in evolution, whereas others noted the similarities between some dramatic mutations and evolutionary transitions to argue for macromutationism. Arguments on both sides have been biased by misconceptions of the developmental effects of mutations. For example, the traditional view that mutations of important developmental genes always have large pleiotropic effects can now be seen to be a conclusion drawn from observations of a small class of mutations with dramatic effects. It is possible that some mutations, for example, those in cis -regulatory DNA, have few or no pleiotropic effects and may be the predominant source of morphological evolution. In contrast, mutations causing dramatic phenotypic effects, although superficially similar to hypothesized evolutionary transitions, are unlikely to fairly represent the true path of evolution. Recent developmental studies of gene function provide a new way of conceptualizing and studying variation that contrasts with the traditional genetic view that was incorporated into neo-Darwinian theory and population genetics. This new approach in developmental biology is as important for micro-evolutionary studies as the actual results from recent evolutionary developmental studies. In particular, this approach will assist in the task of identifying the specific mutations generating phenotypic variation and elucidating how they alter gene function. These data will provide the current missing link between molecular and phenotypic variation in natural populations. [source] PERSPECTIVE: SEX, RECOMBINATION, AND THE EFFICACY OF SELECTION,WAS WEISMANN RIGHT?EVOLUTION, Issue 2 2000Austin Burt Abstract., The idea that sex functions to provide variation for natural selection to act upon was first advocated by August Weismann and it has dominated much discussion on the evolution of sex and recombination since then. The goal of this paper is to further extend this hypothesis and to assess its place in a larger body of theory on the evolution of sex and recombination. A simple generic model is developed to show how fitness variation and covariation interact with selection for recombination and illustrate some important implications of the hypothesis: (1) the advantage of sex and recombination can accrue both to reproductively isolated populations and to modifiers segregating within populations, but the former will be much larger than the latter; (2) forces of degradation that are correlated across loci within an individual can reduce or reverse selection for increased recombination; and (3) crossing-over (which can occur at different places in different meioses) will create more variability than having multiple chromosomes and so will have more influence on the efficacy of selection. Several long-term selection experiments support Weismann's hypothesis, including those showing a greater response to selection in populations with higher rates of recombination and higher rates of recombination evolving as a correlated response to selection for some other character. Weismann's hypothesis is also consistent with the sporadic distribution of obligate asexuality, which indicates that clones have a higher rate of extinction than sexuals. Weismann's hypothesis is then discussed in light of other patterns in the distribution of sexuality versus asexuality. To account for variation in the frequency of obligate asexuality in different taxa, a simple model is developed in which this frequency is a function of three parameters: the rate of clonal origin, the initial fitness of clones when they arise, and the rate at which that fitness declines over time. Variation in all three parameters is likely to be important in explaining the distribution of obligate asexuality. Facultative asexuality also exists, and for this to be stable it seems there must be ecological differences between the sexual and asexual propagules as well as genetic differences. Finally, the timing of sex in cyclical parthenogens is most likely set to minimize the opportunity costs of sex. None of these patterns contradict Weismann's hypothesis, but they do show that many additional principles unrelated to the function of sex are required to fully explain its distribution. Weismann's hypothesis is also consistent with what we know about the mechanics and molecular genetics of recombination, in particular the tendency for chromatids to recombine with a homolog rather than a sister chromatid at meiosis, which is opposite to what they do during mitosis. However, molecular genetic studies have shown that cis -acting sites at which recombination is initiated are lost by gene conversion as a result, a factor that can be expected to affect many fine details in the evolution of recombination. In summary, although Weismann's hypothesis must be considered the leading candidate for the function of sex and recombination, nevertheless, many additional principles are needed to fully account for their evolution. [source] | ||||||||||||||||||||||||||||||||||||||||