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Selection
Kinds of Selection Terms modified by Selection Selected AbstractsANTISOCIAL BEHAVIOR AND YOUTH GANG MEMBERSHIP: SELECTION AND SOCIALIZATIONCRIMINOLOGY, Issue 1 2004RACHEL A. GORDON We examine whether gang membership is associated with higher levels of delinquency because boys predisposed to delinquent activity are more likely than others to join. We use 10 years of longitudinal data from 858 participants of the Pittsburgh Youth Study to identify periods before, during and after gang membership. We build on prior research by controlling for ages and calendar time, by better accounting for gang memberships that occurred before the study began, and by using fixed effects statistical models. We find more evidence than has been found in prior studies that boys who join gangs are more delinquent before entering the gang than those who do not join. Even with such selective differences, however, we replicate research showing that drug selling, drug use, violent behaviors and vandalism of property increase significantly when a youth joins a gang. The delinquency of peers appears to be one mechanism of socialization. These findings are clearest in youth self-reports, but are also evident in reports from parents and teachers on boys' behavior and delinquency. Once we adjust for time trends, we find that the increase in delinquency is temporary, that delinquency falls to pre-gang levels when boys leave gangs. [source] TACIT COLLUSION IN AUCTIONS AND CONDITIONS FOR ITS FACILITATION AND PREVENTION: EQUILIBRIUM SELECTION IN LABORATORY EXPERIMENTAL MARKETSECONOMIC INQUIRY, Issue 3 2009JIN LI The paper studies bidder behavior in simultaneous, continuous, ascending price auctions. We design and implement a "collusion incubator" environment based on a type of public, symmetrically "folded" and "item-aligned" preferences. Tacit collusion develops quickly and reliably within the environment. Once tacit collusion developed, it proved remarkably robust to institutional changes that weakened it as an equilibrium of a game-theoretic model. The only successful remedy was a non-public change in the preference of participants that destroyed the symmetrically, "folded" and "item aligned" patterns of preferences, creating head-to-head competition between two agents reminiscent of the concept of a "maverick."(JEL L50, L94, D43) [source] POLITICAL SELECTION OF FIRMS INTO PRIVATIZATION PROGRAMS.ECONOMICS & POLITICS, Issue 3 2010EVIDENCE FROM ROMANIAN COMPREHENSIVE DATA Exploiting a unique institutional feature of early Romanian privatization, when a group of firms was explicitly barred from privatization and another was partially privatized by management,employee buyouts, we test how politicians select firms into privatization programs. Using comprehensive firm data, we estimate the relation between preprivatization firm characteristics , the information known to politicians at the time of decision-making , and the effect of privatization on employment, efficiency, and wages. With the estimated coefficients we simulate the effect of privatization on non-privatizable and privatizable firms. We find that politicians expected privatization to increase employment in the privatizable group by 7%,10%, while to decrease it in the non-privatizable group by 10%,30%, depending on the first-stage estimation method, ordinary least squares with or without matching. We do not find such discrepancies in the expected change in firm efficiency; the simulated efficiency effect of privatization is large and positive for both groups of firms, and it is 52%,65% for non-privatizable and 41%,43% for privatizable firms. The analysis does not support the hypothesis that wages played an important role in privatization decisions. Our study suggests that employment concerns played the key role in selecting firms for privatization, even if efficiency gains had to be sacrificed. [source] ON THE ROLE OF THE PRIMARY SYSTEM IN CANDIDATE SELECTIONECONOMICS & POLITICS, Issue 2 2006MANDAR P. OAK How does the type of the primary system affect political outcomes? We address this issue by constructing a simple model that accounts for intra-party as well as inter-party political competition. Our model suggests that allowing non-partisan voters to participate in the primaries (i.e. a semi-open primary system) indeed improves the chances of a moderate candidate getting elected. However, this need not necessarily happen in the case of a completely open primary system. Under such a system there arise multiple equilibria, some of which may lead to a greater degree of extremism than the closed primary system. Thus, our model contributes to the current debate on the choice of primary systems from an analytical perspective and helps explain some of the empirical findings. [source] EXTREME SELECTION ON SIZE IN THE EARLY LIVES OF FISHEVOLUTION, Issue 8 2010Kestrel O. Perez Although fitness typically increases with body size and selection gradients on size are generally positive, much of this information comes from terrestrial taxa. In the early life history of fish, there is evidence of selection both for and against larger size, leaving open the question of whether the general pattern for terrestrial taxa is valid for fish. We reviewed studies of size-dependent survival in the early life history of fish and obtained estimates of standardized selection differentials from 40 studies. We found that 77% of estimated selection differentials favored larger size and that the strength of selection was more than five times that seen in terrestrial taxa. Selection decreased with study period duration and initial length, and disruptive selection occurred significantly more frequently than stabilizing selection. Contrary to expectations from Bergmann's rule, selection on size did not increase with latitude. [source] CONFLICTING SELECTION FROM AN ANTAGONIST AND A MUTUALIST ENHANCES PHENOTYPIC VARIATION IN A PLANTEVOLUTION, Issue 4 2010Adam M. Siepielski The raw material for evolution is variation. Consequently, identifying the factors that generate, maintain, and erode phenotypic and genetic variation in ecologically important traits within and among populations is important. Although persistent directional or stabilizing selection can deplete variation, spatial variation in conflicting directional selection can enhance variation. Here, we present evidence that phenotypic variation in limber pine (Pinus flexilis) cone structure is enhanced by conflicting selection pressures exerted by its mutualistic seed disperser (Clark's nutcracker Nucifraga columbiana) and an antagonistic seed predator (pine squirrel Tamiasciurus spp.). Phenotypic variation in cone structure was bimodal and about two times greater where both agents of selection co-occurred than where one (the seed predator) was absent. Within the region where both agents of selection co-occurred, bimodality in cone structure was pronounced where there appears to be a mosaic of habitats with some persistent habitats supporting only the seed disperser. These results indicate that conflicting selection stemming from spatial variation in community diversity can enhance phenotypic variation in ecologically important traits. [source] GEOGRAPHIC VARIATION, FREQUENCY-DEPENDENT SELECTION, AND THE MAINTENANCE OF A FEMALE-LIMITED POLYMORPHISMEVOLUTION, Issue 1 2010Ryan Calsbeek A central problem in evolutionary biology is to understand how spatial and temporal variation in selection maintain genetic variation within and among populations. Brown anole lizards (Anolis sagrei) exhibit a dorsal pattern polymorphism that is expressed only in females, which occur in "diamond,""bar," and intermediate "diamond-bar" morphs. To understand the inheritance of this polymorphism, we conducted a captive breeding study that refuted several single-locus models and supported a two-locus mode of inheritance. To describe geographic variation in morph frequencies, we surveyed 13 populations from two major islands in The Bahamas. Morph frequencies differed substantially between major islands but were highly congruent within each island. Finally, we measured viability selection on each island to test two hypotheses regarding the maintenance of the polymorphism: (1) that spatial variation in selection maintains variation in morph frequencies between islands, and (2) that temporal variation in selection across years maintains variation within islands. Although bar females had relatively lower survival where they were rare, our data do not otherwise suggest that selection varies spatially between islands. However, diamond-bar females were subject to positive frequency-dependent selection across years, and the relative fitness of bar and diamond females alternated across years. We propose that this polymorphism is maintained by temporal variation in selection coupled with the sheltering of alleles via a two-locus inheritance pattern and sex-limited expression. [source] EVIDENCE FOR NEGATIVE FREQUENCY-DEPENDENT SELECTION DURING EXPERIMENTAL COEVOLUTION OF A FRESHWATER SNAIL AND A STERILIZING TREMATODEEVOLUTION, Issue 9 2009Britt Koskella Host,parasite coevolution is often suggested as a mechanism for maintaining genetic diversity, but finding direct evidence has proven difficult. In the present study, we examine the process of coevolution using a freshwater New Zealand snail (Potamopyrgus antipodarum) and its common parasite (the sterilizing trematode, Microphallus sp.) Specifically, we test for changes in genotypic composition of clonal host populations in experimental populations evolving either with or without parasites for six generations. As predicted under the Red Queen model of coevolution, the initially most common host genotype decreased in frequency in the presence, but not the absence, of parasitism. Furthermore, the initially most common host genotype became more susceptible to infection by the coevolving parasite populations over the course of the experiment. These results are consistent with parasite-meditated selection leading to a rare advantage, and they indicate rapid coevolution at the genotypic level between a host and its parasite. [source] ON THE OPPORTUNITY FOR SEXUAL SELECTION, THE BATEMAN GRADIENT AND THE MAXIMUM INTENSITY OF SEXUAL SELECTIONEVOLUTION, Issue 7 2009Adam G. Jones Bateman's classic paper on fly mating systems inspired quantitative study of sexual selection but also resulted in much debate and confusion. Here, I consider the meaning of Bateman's principles in the context of selection theory. Success in precopulatory sexual selection can be quantified as a "mating differential," which is the covariance between trait values and relative mating success. The mating differential is converted into a selection differential by the Bateman gradient, which is the least squares regression of relative reproductive success on relative mating success. Hence, a complete understanding of precopulatory sexual selection requires knowledge of two equally important aspects of mating patterns: the mating differential, which requires a focus on mechanisms generating covariance between trait values and mating success, and the Bateman gradient, which requires knowledge of the genetic mating system. An upper limit on the magnitude of the selection differential on any sexually selected trait is given by the product of the standard deviation in relative mating success and the Bateman gradient. This latter view of the maximum selection differential provides a clearer focus on the important aspects of precopulatory sexual selection than other methods and therefore should be an important part of future studies of sexual selection. [source] DOES VARIATION IN SELECTION IMPOSED BY BEARS DRIVE DIVERGENCE AMONG POPULATIONS IN THE SIZE AND SHAPE OF SOCKEYE SALMON?EVOLUTION, Issue 5 2009Stephanie M. Carlson Few studies have determined whether formal estimates of selection explain patterns of trait divergence among populations, yet this is one approach for evaluating whether the populations are in equilibria. If adaptive divergence is complete, directional selection should be absent and stabilizing selection should prevail. We estimated natural selection, due to bear predation, acting on the body size and shape of male salmon in three breeding populations that experience differing predation regimes. Our approach was to (1) estimate selection acting within each population on each trait based on an empirical estimate of reproductive activity, (2) test for trait divergence among populations, and (3) test whether selection coefficients were correlated with trait divergence among populations. Stabilizing selection was never significant, indicating that these populations have yet to attain equilibria. Directional selection varied among populations in a manner consistent with trait divergence, indicating ongoing population differentiation. Specifically, the rank order of the creeks in terms of patterns of selection paralleled the rank order in terms of size and shape. The shortest and least deep-bodied males had the highest reproductive activity in the creek with the most intense predation and longer and deeper-bodied males were favored in the creeks with lower predation risk. [source] RELATIVE ROLE OF GENETIC DETERMINATION AND PLASTIC RESPONSE DURING ONTOGENY FOR SHELL-SHAPE TRAITS SUBJECTED TO DIVERSIFYING SELECTIONEVOLUTION, Issue 5 2009Paula Conde-Padín We studied the relative role of genetic determination versus plastic response for traits involved in ecological adaptation of two ecotypes of Littorina saxatilis living at different shore levels. To investigate the magnitude of the plastic response across ontogeny, we compared morphological data from individuals grown in the laboratory and taken from the wild at three developmental stages: shelled embryos, juveniles, and adults. The results indicate that most shell shape variation (72,99%) in adaptive traits (globosity and aperture of the shell) is explained by the ecotype irrespective of the growth environment, suggesting that direct genetic determination is the main factor responsible for the process of adaptation in the wild. There was a tendency for the contribution of plasticity to increase over ontogeny but, in general, the direction of the plastic response did not suggest that this was adaptive. [source] EXPERIMENTAL MANIPULATION OF SEXUAL SELECTION PROMOTES GREATER MALE MATING CAPACITY BUT DOES NOT ALTER SPERM INVESTMENTEVOLUTION, Issue 4 2009Helen S. Crudgington Sexual selection theory makes clear predictions regarding male spermatogenic investment. To test these predictions we used experimental sexual selection in Drosophila pseudoobscura, a sperm heteromorphic species in which males produce both fertile and sterile sperm, the latter of which may function in postmating competition. Specifically, we determined whether the number and size of both sperm types, as well as relative testis mass and accessory gland size, increased with increased sperm competition risk and whether any fitness benefits could accrue from such changes. We found no effect of sexual selection history on either the number or size of either sperm morph, or on relative testis mass. However, males experiencing a greater opportunity for sexual selection evolved the largest accessory glands, had the greatest mating capacity, and sired the most progeny. These findings suggest that sterile sperm are not direct targets of sexual selection and that accessory gland size, rather than testis mass, appears to be an important determinant of male reproductive success. We briefly review the data from experimental sexual selection studies and find that testis mass may not be a frequent target of postcopulatory sexual selection and, even when it is, the resulting changes do not always improve fitness. [source] PURGING THE GENOME WITH SEXUAL SELECTION: REDUCING MUTATION LOAD THROUGH SELECTION ON MALESEVOLUTION, Issue 3 2009Michael C. Whitlock Healthy males are likely to have higher mating success than unhealthy males because of differential expression of condition-dependent traits such as mate searching intensity, fighting ability, display vigor, and some types of exaggerated morphological characters. We therefore expect that most new mutations that are deleterious for overall fitness may also be deleterious for male mating success. From this perspective, sexual selection is not limited to influencing those genes directly involved in exaggerated morphological traits but rather affects most, if not all, genes in the genome. If true, sexual selection can be an important force acting to reduce the frequency of deleterious mutations and, as a result, mutation load. We review the literature and find various forms of indirect evidence that sexual selection helps to eliminate deleterious mutations. However, direct evidence is scant, and there are almost no data available to address a key issue: is selection in males stronger than selection in females? In addition, the total effect of sexual selection on mutation load is complicated by possible increases in mutation rate that may be attributable to sexual selection. Finally, sexual selection affects population fitness not only through mutation load but also through sexual conflict, making it difficult to empirically measure how sexual selection affects load. Several lines of enquiry are suggested to better fill large gaps in our understanding of sexual selection and its effect on genetic load. [source] DRIFT PROMOTES SPECIATION BY SEXUAL SELECTIONEVOLUTION, Issue 3 2009Josef C. Uyeda Quantitative genetic models of sexual selection have generally failed to provide a direct connection to speciation and to explore the consequences of finite population size. The connection to speciation has been indirect because the models have treated only the evolution of male and female traits and have stopped short of modeling the evolution of sexual isolation. In this article we extend Lande's (1981) model of sexual selection to quantify predictions about the evolution of sexual isolation and speciation. Our results, based on computer simulations, support and extend Lande's claim that drift along a line of equilibria can rapidly lead to sexual isolation and speciation. Furthermore, we show that rapid speciation can occur by drift in populations of appreciable size (Ne, 1000). These results are in sharp contrast to the opinion of many researchers and textbook writers who have argued that drift does not play an important role in speciation. We argue that drift may be a powerful amplifier of speciation under a wide variety of modeling assumptions, even when selection acts directly on female mating preferences. [source] SELECTION IN HETEROGENEOUS ENVIRONMENTS MAINTAINS THE GENE ARRANGEMENT POLYMORPHISM OF DROSOPHILA PSEUDOOBSCURAEVOLUTION, Issue 12 2008Stephen W. Schaeffer Chromosomal rearrangements may play an important role in how populations adapt to a local environment. The gene arrangement polymorphism on the third chromosome of Drosophila pseudoobscura is a model system to help determine the role that inversions play in the evolution of this species. The gene arrangements are the likely target of strong selection because they form classical clines across diverse geographic habitats, they cycle in frequency over seasons, and they form stable equilibria in population cages. A numerical approach was developed to estimate the fitness sets for 15 gene arrangement karyotypes in six niches based on a model of selection,migration balance. Gene arrangement frequencies in the six different niches were able to reach a stable meta-population equilibrium that matched the observed gene arrangement frequencies when recursions used the estimated fitnesses with a variety of initial inversion frequencies. These analyses show that a complex pattern of selection is operating in the six niches to maintain the D. pseudoobscura gene arrangement polymorphism. Models of local adaptation predict that the new inversion mutations were able to invade populations because they held combinations of two to 13 local adaptation loci together. [source] EVOLUTIONARY REDUCTION IN TESTES SIZE AND COMPETITIVE FERTILIZATION SUCCESS IN RESPONSE TO THE EXPERIMENTAL REMOVAL OF SEXUAL SELECTION IN DUNG BEETLESEVOLUTION, Issue 10 2008Leigh W. Simmons Sexual selection is thought to favor the evolution of secondary sexual traits in males that contribute to mating success. In species where females mate with more than one male, sexual selection also continues after copulation in the form of sperm competition and cryptic female choice. Theory suggests that sperm competition should favor traits such as testes size and sperm production that increase a male's competitive fertilization success. Studies of experimental evolution offer a powerful approach for assessing evolutionary responses to variation in sexual selection pressures. Here we removed sexual selection by enforcing monogamy on replicate lines of a naturally polygamous horned beetle, Onthophagus taurus, and monitoring male investment in their testes for 21 generations. Testes size decreased in monogamous lines relative to lines in which sexual selection was allowed to continue. Differences in testes size were dependent on selection history and not breeding regime. Males from polygamous lines also had a competitive fertilization advantage when in sperm competition with males from monogamous lines. Females from polygamous lines produced sons in better condition, and those from monogamous lines increased their sons condition by mating polygamously. Rather than being costly for females, multiple mating appears to provide females with direct and/or indirect benefits. Neither body size nor horn size diverged between our monogamous and polygamous lines. Our data show that sperm competition does drive the evolution of testes size in onthophagine beetles, and provide general support for sperm competition theory. [source] EXPERIMENTAL EVIDENCE THAT SEXUAL CONFLICT INFLUENCES THE OPPORTUNITY, FORM AND INTENSITY OF SEXUAL SELECTIONEVOLUTION, Issue 9 2008Matthew D. Hall Sexual interactions are often rife with conflict. Conflict between members of the same sex over opportunities to mate has long been understood to effect evolution via sexual selection. Although conflict between males and females is now understood to be widespread, such conflict is seldom considered in the same light as a general agent of sexual selection. Any interaction between males or females that generates variation in fitness, whether due to conflict, competition or mate choice, can potentially influence sexual selection acting on a range of male traits. Here we seek to address a lack of direct experimental evidence for how sexual conflict influences sexual selection more broadly. We manipulate a major source of sexual conflict in the black field cricket, Teleogryllus commodus, and quantify the resulting changes in the nature of sexual selection using formal selection analysis to statistically compare multivariate fitness surfaces. In T. commodus, sexual conflict occurs over the attachment time of an external spermatophore. By experimentally manipulating the ability of males and females to influence spermatophore attachment, we found that sexual conflict significantly influences the opportunity, form, and intensity of sexual selection on male courtship call and body size. When males were able to harass females, the opportunity for selection was smaller, the form of selection changed, and sexual selection was weaker. We discuss the broader evolutionary implications of these findings, including the contributions of sexual conflict to fluctuating sexual selection and the maintenance of additive genetic variation. [source] NATURAL SELECTION ALONG AN ENVIRONMENTAL GRADIENT: A CLASSIC CLINE IN MOUSE PIGMENTATIONEVOLUTION, Issue 7 2008Lynne M. Mullen We revisited a classic study of morphological variation in the oldfield mouse (Peromyscus polionotus) to estimate the strength of selection acting on pigmentation patterns and to identify the underlying genes. We measured 215 specimens collected by Francis Sumner in the 1920s from eight populations across a 155-km, environmentally variable transect from the white sands of Florida's Gulf coast to the dark, loamy soil of southeastern Alabama. Like Sumner, we found significant variation among populations: mice inhabiting coastal sand dunes had larger feet, longer tails, and lighter pigmentation than inland populations. Most striking, all seven pigmentation traits examined showed a sharp decrease in reflectance about 55 km from the coast, with most of the phenotypic change occurring over less than 10 km. The largest change in soil reflectance occurred just south of this break in pigmentation. Geographic analysis of microsatellite markers shows little interpopulation differentiation, so the abrupt change in pigmentation is not associated with recent secondary contact or reduced gene flow between adjacent populations. Using these genetic data, we estimated that the strength of selection needed to maintain the observed distribution of pigment traits ranged from 0.0004 to 21%, depending on the trait and model used. We also examined changes in allele frequency of SNPs in two pigmentation genes, Mc1r and Agouti, and show that mutations in the cis -regulatory region of Agouti may contribute to this cline in pigmentation. The concordance between environmental variation and pigmentation in the face of high levels of interpopulation gene flow strongly implies that natural selection is maintaining a steep cline in pigmentation and the genes underlying it. [source] SEXUAL SELECTION AND INTERACTING PHENOTYPES IN EXPERIMENTAL EVOLUTION: A STUDY OF DROSOPHILA PSEUDOOBSCURA MATING BEHAVIOREVOLUTION, Issue 7 2008Leonardo D. Bacigalupe Sexual selection requires social interactions, particularly between the sexes. When trait expression is influenced by social interactions, such traits are called interacting phenotypes and only recently have the evolutionary consequences of interacting phenotypes been considered. Here we investigated how variation in relative fitness, or the opportunity for sexual selection, affected the evolutionary trajectories of interacting phenotypes. We used experimentally evolved populations of the naturally promiscuous Drosophila pseudoobscura, in which the numbers of potential interactions between the sexes, and therefore relative fitness, were manipulated by altering natural levels of female promiscuity. We considered two different mating interactions between the sexes: mating speed and copulation duration. We investigated the evolutionary trajectories of means and (co)variances (P) and also the influence of genetic drift on the evolutionary response of these interactions. Our sexual selection treatments did not affect the means of either mating speed or copulation duration, but they did affect P. We found that the means of both traits differed among replicates within each selection treatment whereas the Ps did not. Changes as a consequence of genetic drift were excluded. Our results show that although variable potential strengths of sexual interactions influence the evolution of interacting phenotypes, the influence may be nonlinear. [source] EVOLUTIONARY DYNAMICS OF A SEXUAL ORNAMENT IN THE HOUSE SPARROW (PASSER DOMESTICUS): THE ROLE OF INDIRECT SELECTION WITHIN AND BETWEEN SEXESEVOLUTION, Issue 6 2008Henrik Jensen The relative contribution of sexual and natural selection to evolution of sexual ornaments has rarely been quantified under natural conditions. In this study we used a long-term dataset of house sparrows in which parents and offspring were matched genetically to estimate the within- and across-sex genetic basis for variation and covariation among morphological traits. By applying two-sex multivariate "animal models" to estimate genetic parameters, we estimated evolutionary changes in a male sexual ornament, badge size, from the contribution of direct and indirect selection on correlated traits within males and females, after accounting for overlapping generations and age-structure. Indirect natural selection on genetically correlated traits in males and females was the major force causing evolutionary change in the male ornament. Thus, natural selection on female morphology may cause indirect evolutionary changes in male ornaments. We observed however no directional phenotypic change in the ornament size of one-year-old males during the study period. On the other hand, changes were recorded in other morphological characters of both sexes. Our analyses of evolutionary dynamics in sexual characters require application of appropriate two-sex models to account for how selection on correlated traits in both sexes affects the evolutionary outcome of sexual selection. [source] INCREASED POSTCOPULATORY SEXUAL SELECTION REDUCES THE INTRAMALE VARIATION IN SPERM DESIGNEVOLUTION, Issue 6 2008Simone Immler Sperm competition is an important force driving the evolution of sperm design and function. Inter- and intraspecific variation in sperm design are strongly influenced by the risk of sperm competition in many taxa. In contrast, the variation among sperm of one male (intramale variation) is less well understood. We investigated intramale variation in sperm design in passerine birds and found that risk of sperm competition is negatively associated with intramale variation. This result is the first clear evidence that variation among sperm within an individual male is influenced by postcopulatory sexual selection. Our finding has important implications for male traits under pre- and postcopulatory sexual selection. [source] POSTCOPULATORY FERTILIZATION BIAS AS A FORM OF CRYPTIC SEXUAL SELECTIONEVOLUTION, Issue 5 2008Ryan Calsbeek Males and females share most of their genetic material yet often experience very different selection pressures. Some traits that are adaptive when expressed in males may therefore be maladaptive when expressed in females. Recent studies demonstrating negative correlations in fitness between parents and their opposite-sex progeny suggest that natural selection may favor a reduction in trait correlations between the sexes to partially mitigate intralocus sexual conflict. We studied sex-specific forms of selection acting in Anolis lizards in the Greater Antilles, a group for which the importance of natural selection has been well documented in species-level diversification, but for which less is known about sexual selection. Using the brown anole (Anolis sagrei), we measured fitness-related variation in morphology (body size), and variation in two traits reflecting whole animal physiological condition: running endurance and immune function. Correlations between body size and physiological traits were opposite between males and females and the form of natural selection acting on physiological traits significantly differed between the sexes. Moreover, physiological traits in progeny were correlated with the body-size of their sires, but correlations were null or even negative between parents and their opposite-sex progeny. Although results based on phenotypic and genetic correlations, as well as the action of natural selection, suggest the potential for intralocus sexual conflict, females used sire body size as a cue to sort sperm for the production of either sons or daughters. Our results suggest that intralocus sexual conflict may be at least partly resolved through post-copulatory sperm choice in A. sagrei. [source] MATING DENSITY AND THE STRENGTH OF SEXUAL SELECTION AGAINST DELETERIOUS ALLELES IN DROSOPHILA MELANOGASTEREVOLUTION, Issue 4 2008Nathaniel P. Sharp Deleterious alleles constantly enter populations via mutation. Their presence reduces mean fitness and may threaten population persistence. It has been suggested that sexual selection may be an efficient way by which deleterious alleles are removed from populations but there is little direct experimental evidence. Because of its potential role in mutational meltdowns, there is particular interest in whether the strength of sexual selection changes with density. For each of eight visible markers in Drosophila melanogaster we have compared the strength of sexual selection at two densities. We find evidence of strong sexual selection against most but not all of these alleles. There is no evidence that sexual selection tends to be stronger (or weaker) at high density relative to low density. In addition, we also measure the effects of these mutations on two key parameters relevant to population productivity,juvenile viability and female fecundity. In most cases, sexual selection is as strong or stronger than these other forms of selection. [source] SEXUAL SELECTION AND IMMUNE FUNCTION IN DROSOPHILA MELANOGASTEREVOLUTION, Issue 2 2008Kurt A. McKean The evolution of immune function depends not only on variation in genes contributing directly to the immune response, but also on genetic variation in other traits indirectly affecting immunocompetence. In particular, sexual selection is predicted to trade-off with immunocompetence because the extra investment of resources needed to increase sexual competitiveness reduces investment in immune function. Additional possible immunological consequences of intensifying sexual selection include an exaggeration of immunological sexual dimorphism, and the reduction of condition-dependent immunological costs due to selection of ,good genes' (the immunocompetence handicap hypothesis, ICHH). We tested for these evolutionary possibilities by increasing sexual selection in laboratory populations of Drosophila melanogaster for 58 generations by reestablishing a male-biased sex ratio at the start of each generation. Sexually selected flies were larger, took longer to develop, and the males were more sexually competitive than males from control (equal sex ratio) lines. We found support for the trade-off hypothesis: sexually selected males were found to have reduced immune function compared to control males. However, we found no evidence that sexual selection promoted immunological sexual dimorphism because females showed a similar reduction in immune function. We found no evidence of evolutionary changes in the condition-dependent expression of immunocompetence contrary to the expectations of the ICHH. Lastly, we compared males from the unselected base population that were either successful (IS) or unsuccessful (IU) in a competitive mating experiment. IS males showed reduced immune function relative to IU males, suggesting that patterns of phenotypic correlation largely mirror patterns of genetic correlation revealed by the selection experiment. Our results suggest increased disease susceptibility could be an important cost limiting increases in sexual competitiveness in populations experiencing intense sexual selection. Such costs may be particularly important given the high intersex correlation, because this represents an apparent genetic conflict, preventing males from reaching their sexually selected optimum. [source] A REAPPRAISAL OF BATEMAN'S CLASSIC STUDY OF INTRASEXUAL SELECTIONEVOLUTION, Issue 11 2007Brian F. Snyder Bateman's (1948) study showing greater variances in number of mates and reproductive success in male than female Drosophila melanogaster is a foundational paper in sexual selection. Here we show for the first time that his methods had flaws, including the elimination of genetic variance, sampling biases, miscalculations of fitness variances, statistical pseudo-replication, and selective presentation of data. We conclude that Bateman's results are unreliable, his conclusions are questionable, and his observed variances are similar to those expected under random mating. Despite our analysis, we do not intend this article as a criticism of Bateman; he accomplished his work without modern computational tools, and his approach was groundbreaking emphasizing the significance of fitness variance for sexual selection. However, this reanalysis has implications for what counts as evidence for sexual selection and we believe that our concerns should be of interest to contemporary students of sexual selection. We call for repetitions of Bateman's study using modern statistical and molecular methods. [source] PROMISCUITY DRIVES SEXUAL SELECTION IN A SOCIALLY MONOGAMOUS BIRDEVOLUTION, Issue 9 2007Michael S. Webster Many socially monogamous species paradoxically show signs of strong sexual selection, suggesting cryptic sources of sexual competition among males. Darwin argued that sexual selection could operate in monogamous systems if breeding sex ratios are biased or if some males attract highly fecund females. Alternatively, sexual selection might result from promiscuous copulations outside the pair bond, although several recent studies have cast doubt on this possibility, in particular by showing that variance in apparent male reproductive success (number of social young) differs little from variance in actual male reproductive success (number of young sired). Our results from a long-term study of the socially monogamous splendid fairy-wren (Malurus splendens) demonstrate that such comparisons are misleading and do not adequately assess the effects of extra-pair paternity (EPP). By partitioning the opportunity for selection and calculating Bateman gradients, we show that EPP has a strong effect on male annual and lifetime fitness, whereas other proposed mechanisms of sexual selection do not. Thus, EPP drives sexual selection in this, and possibly other, socially monogamous species. [source] DIRECT AND CORRELATED RESPONSES TO SELECTION IN A HOST,PARASITE SYSTEM: TESTING FOR THE EMERGENCE OF GENOTYPE SPECIFICITYEVOLUTION, Issue 8 2007Thibault Nidelet Genotype × environment interactions can facilitate coexistence of locally adapted specialists. Interactions evolve if adaptation to one environment trades off with performance in others. We investigated whether evolution on one host genotype traded off with performance on others in long-term experimental populations of different genotypes of the protozoan Paramecium caudatum, infected with the bacterial parasite Holospora undulata. A total of nine parasite selection lines evolving on three host genotypes and the ancestral parasite were tested in a cross-infection experiment. We found that evolved parasites produced more infections than did the ancestral parasites, both on host genotypes they had evolved on (positive direct response to selection) and on genotypes they had not evolved on (positive correlated response to selection). On two host genotypes, a negative relationship between direct and correlated responses indicated pleiotropic costs of adaptation. On the third, a positive relationship suggested cost-free adaptation. Nonetheless, on all three hosts, resident parasites tended to be superior to the average nonresident parasite. Thus genotype specificity (i.e., patterns of local adaptation) may evolve without costs of adaptation, as long as direct responses to selection exceed correlated responses. [source] HAMILTON'S FORCES OF NATURAL SELECTION AFTER FORTY YEARSEVOLUTION, Issue 6 2007Michael R. Rose In 1966, William D. Hamilton published a landmark paper in evolutionary biology: "The Moulding of Senescence by Natural Selection." It is now apparent that this article is as important as his better-known 1964 articles on kin selection. Not only did the 1966 article explain aging, it also supplied the basic scaling forces for natural selection over the entire life history. Like the Lorentz transformations of relativistic physics, Hamilton's Forces of Natural Selection provide an overarching framework for understanding the power of natural selection at early ages, the existence of aging, the timing of aging, the cessation of aging, and the timing of the cessation of aging. His twin Forces show that natural selection shapes survival and fecundity in different ways, so their evolution can be somewhat distinct. Hamilton's Forces also define the context in which genetic variation is shaped. The Forces of Natural Selection are readily manipulable using experimental evolution, allowing the deceleration or acceleration of aging, and the shifting of the transition ages between development, aging, and late life. For these reasons, evolutionary research on the demographic features of life history should be referred to as "Hamiltonian." [source] NATURAL SELECTION ON A POLYMORPHIC DISEASE-RESISTANCE LOCUS IN IPOMOEA PURPUREAEVOLUTION, Issue 2 2007Joel M. Kniskern Although disease-resistance polymorphisms are common in natural plant populations, the mechanisms responsible for this variation are not well understood. Theoretical models predict that balancing selection can maintain polymorphism within a population if the fitness effects of a resistance allele vary from a net cost to a net benefit, depending upon the extent of pathogen damage. However, there have been a few attempts to determine how commonly this mechanism operates in natural plant,pathogen interactions. Ipomoea purpurea populations are often polymorphic for resistance and susceptibility alleles at a locus that influences resistance to the fungal pathogen, Coleosporium ipomoeae. We measured the fitness effects of resistance over three consecutive years at natural and manipulated levels of damage to characterize the type of selection acting on this locus. Costs of resistance varied in magnitude from undetectable to 15.5%, whereas benefits of resistance sometimes equaled, but never exceeded, these costs. In the absence of net benefits of resistance at natural or elevated levels of disease, we conclude that selection within individual populations of I. purpurea probably does not account completely for maintenance of this polymorphism. Rather, the persistence of this polymorphism is probably best explained by a combination of variable selection and meta-population processes. [source] BALANCING SELECTION, RANDOM GENETIC DRIFT, AND GENETIC VARIATION AT THE MAJOR HISTOCOMPATIBILITY COMPLEX IN TWO WILD POPULATIONS OF GUPPIES (POECILIA RETICULATA)EVOLUTION, Issue 12 2006Cock van Oosterhout Abstract Our understanding of the evolution of genes of the major histocompatibility complex (MHC) is rapidly increasing, but there are still enigmatic questions remaining, particularly regarding the maintenance of high levels of MHC polymorphisms in small, isolated populations. Here, we analyze the genetic variation at eight microsatellite loci and sequence variation at exon 2 of the MHC class IIB (DAB) genes in two wild populations of the Trinidadian guppy, Poecilia reticulata. We compare the genetic variation of a small (Ne, 100) and relatively isolated upland population to that of its much larger (Ne, 2400) downstream counterpart. As predicted, microsatellite diversity in the upland population is significantly lower and highly differentiated from the population further downstream. Surprisingly, however, these guppy populations are not differentiated by MHC genetic variation and show very similar levels of allelic richness. Computer simulations indicate that the observed level of genetic variation can be maintained with overdominant selection acting at three DAB loci. The selection coefficients differ dramatically between the upland (s 0.2) and lowland (s, 0.01) populations. Parasitological analysis on wild-caught fish shows that parasite load is significantly higher on upland than on lowland fish, which suggests that large differences in selection intensity may indeed exist between populations. Based on the infection intensity, a substantial proportion of the upland fish would have suffered direct or indirect fitness consequences as a result of their high parasite loads. Selection by parasites plays a particularly important role in the evolution of guppies in the upland habitat, which has resulted in high levels of MHC diversity being maintained in this population despite considerable genetic drift. [source] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||