Net Selection Research Articles

Does Selection in Insurance Markets Always Favor Buyers?

This paper provides empirical evidence of advantageous selection in insurance markets. By using a novel insurance setting where moral hazard is not a concern, I am able to overcome an important obstacle in most studies of selection: the inability to distinguish moral hazard from selection. In the US market for area yield crop insurance, payouts are based on average county yields. Moreover, area yield insurance is only offered in counties where no farmer is large enough to affect the mean yield. I find that area yield insurance takeup is higher when average yields in the county are higher and show that this effect is not being driven by prices. This suggests that the net selection into these plans thus favors insurance providers, not buyers. One possible mechanism is that providers have better information about aggregate yields. Another is that the desirability of other, non-area yield, insurance options changes, a potentially important but previously overlooked driver of selection.

Estimation of zooplankton secondary production in estuarine waters: Comparison between the enzymatic (chitobiase) method and mathematical models using crustaceans

Sampling was seasonally performed in the Patos Lagoon estuary (Rio Grande, RS, Southern Brazil) to estimate zooplankton biomass and production comparing values obtained using an enzymatic (chitobiase) method and the traditional mathematical models based on growth. Comparison of data obtained from zooplankton samples collected with 90 and 200-μm mesh nets showed that net selectivity influences the estimation of zooplankton biomass and production. Furthermore, it showed differential results for dominance of taxa and proportions of developmental stages in samples. Differences among samples collected at the different sites in the same season were observed using either the mathematical models or the enzymatic method. The two different approaches were also able to detect the seasonal variation in production. In a broad view, data obtained using the chitobiase method showed a similar pattern of zooplankton production compared to those obtained with the traditional mathematical models based on growth. However, values estimated using the enzymatic method were systematically higher than those obtained with the mathematical models. Maximum total production values were estimated as 12.5, 9.2 and 7.9mgCm−3day−1 for the “chitobiase method”, “Huntley model”, and “Hirst model”, respectively. Considering all sampling sites and seasons, the magnitude of this difference corresponded to 1.95 and 2.49mgCm−3day−1 for the “Huntley model” and the “Hirst model”, respectively. These findings indicate the reliability of the enzymatic method in estimating crustacean production also in estuarine environments of changing salinity, as previously demonstrated for marine waters. In addition, the use of this method is more practical and comparatively less time-consuming and cheaper than the use of the mathematical models based on growth.

Gill net selectivity in the White Nile fisheries, Khartoum State, Sudan

Gill-net selectivity is a size and type of fish that caught depending upon specific mesh size of a used gill-net. In this study, four different mesh sizes (4, 6, 8 and 15 cm) were used in al-Kalakla Fishery (KF) and Jabel Awlia Dam Fishery (JADF) in the White Nile for studying their selectivity. The results of a catch per unit effort (CPUE) study showed that meshes 4 and 6 cm had higher productivity in fishing in KF than that in JADF; whereas, meshes 8 and 15 cm caught more fishes in JADF than in KF for the entire year. In KF, autumn season was the best for fishing with meshes 6 and 8 cm. In JADF, summer and autumn were the best for fishing by meshes 4 and 6 cm. The gill-nets of the study were highly selective for fish species according to their body size. Five fishes of small sizes (body depth at around 4 cm) as Tilapias (Oreochromis niloticus: Linnaeus, 1758; Tilapia zilli: Gervais, 1848;Sarotherodon galilaeus: Linnaeus, 1758), Kas (Hydrocynus forskali: Cuvier, 1819;Hydrocynus vittatus: Castelnau, 1861; Hydrocynus brevis: Gunther, 1864), Nile Perch (Lates niloticus: Linnaeus, 1758), Dabis (Labeo vulgaris: Heckel, 1847) and Bayad (Bagrus bayad: Forsskal, 1775) were caught intensively by mesh of 4 cm during all seasons in both KF and JADF; whereas, meshes of 8 and 15 cm caught occasionally bigger fishes ranged between 8 to15 cm as Nile Perch and Bayad. The selectivity of these nets reflected over-fishing caused in the White Nile. Key words: Fisheries, freshwater fish, River Nile, fishing gear, fishing nets.

Species richness and nitrogen supply regulate the productivity and respiration of ectomycorrhizal fungi in pure culture

Abstract The effects of biodiversity of aboveground organisms have been widely investigated in a range of ecosystems, yet whether similar responses are also seen in belowground microbial communities, such as ectomycorrhizal (EM) fungi, are little understood. We investigated, in vitro , the effects of a gradient of 1–8 species of EM fungi interacting with substratum carbon:nitrogen (C:N) ratio on biomass production and CO 2 efflux. The model experimental systems enabled us to recover and measure biomass of individuals within communities and calculate net selection and complementarity effects. Both biomass and CO 2 efflux increased with species richness particularly under high N concentrations. Moreover, net biodiversity effects were largely positive, driven by both selection and complementarity effects. Our results reveal, in pure culture, the implications of EM species richness on community productivity and C cycling, particularly under high N conditions, and constitute the basis for future experiments under natural conditions.

Finding the key transition states and intermediates controlling net reaction rates and selectivity

AbstractIn this paper Campbell's degree of rate control is extended to introduce the concepts of degree of kinetic rate control, degree of kinetic selectivity control, degree of thermodynamic rate control and degree of thermodynamic selectivity control. It is demonstrated by applying hypothetical but realistic kinetic models of varying complexity that the new methods offers a rigorous framework to analyze the importance of kinetic and thermodynamic parameters i.e. establishing the critical parameters of the kinetic model. The methods are general and can be applied to complex reaction networks with multiple overall reactions not only in heterogeneous catalysis but for all sorts of chemical kinetic models.

Finding the key transition states and intermediates controlling net reaction rates and selectivity

AbstractIn this paper Campbell's degree of rate control is extended to introduce the concepts of degree of kinetic rate control, degree of kinetic selectivity control, degree of thermodynamic rate control and degree of thermodynamic selectivity control. It is demonstrated by applying hypothetical but realistic kinetic models of varying complexity that the new methods offers a rigorous framework to analyze the importance of kinetic and thermodynamic parameters i.e. establishing the critical parameters of the kinetic model. The methods are general and can be applied to complex reaction networks with multiple overall reactions not only in heterogeneous catalysis but for all sorts of chemical kinetic models.

Faith-Based Ethical Investing: The Case of Dow Jones Islamic Indexes

This paper examines the performance of seven indexes chosen from the Dow Jones Islamic Market Index (DJIM) vis-a-vis their non-Islamic counterparts using a variety of measures such as Sharpe, Treynor, Jensen and Fama’s selectivity, net selectivity and diversification. Second, we examine the persistence of performance using Carhart’s (1997) four factor pricing models. Third, we use cointegration to examine how the Islamic indexes compare to their non-Islamic counterparts. The sample period is from January 1996 to December 2005 (120 data points). It is further broken down into two sub-periods: January 1996 to December 2000 (60 data points) and January 2001 to December 2006 (60 data points). We find no difference between Islamic and non-Islamic indexes. The Dow Jones Islamic indexes outperform their conventional counterparts from 1996 to 2000 and underperform them from 2001 to 2005. Overall, similar reward to risk and diversification benefits exist for both the Islamic and conventional indexes.

Elastic Net Grouping Variable Selection Combined with Partial Least Squares Regression (EN-PLSR) for the Analysis of Strongly Multi-collinear Spectroscopic Data

In this paper a novel wavelength region selection algorithm, called elastic net grouping variable selection combined with partial least squares regression (EN-PLSR), is proposed for multi-component spectral data analysis. The EN-PLSR algorithm can automatically select successive strongly correlated prediction variable groups related to the response variable using two steps. First, a portion of the correlated predictors are selected and divided into subgroups by means of the grouping effect of elastic net estimation. Then, a recursive leave-one-group-out strategy is employed to further shrink the variable groups in terms of the root mean square error of cross-validation (RMSECV) criterion. The performance of the algorithm with real near-infrared (NIR) spectroscopic data sets shows that the EN-PLSR algorithm is competitive with full-spectrum PLS and moving window partial least squares (MWPLS) regression methods and it is suitable for use with strongly correlated spectroscopic data.

Phenotypic Selection in Natural Populations: What Limits Directional Selection?

Studies of phenotypic selection document directional selection in many natural populations. What factors reduce total directional selection and the cumulative evolutionary responses to selection? We combine two data sets for phenotypic selection, representing more than 4,600 distinct estimates of selection from 143 studies, to evaluate the potential roles of fitness trade-offs, indirect (correlated) selection, temporally varying selection, and stabilizing selection for reducing net directional selection and cumulative responses to selection. We detected little evidence that trade-offs among different fitness components reduced total directional selection in most study systems. Comparisons of selection gradients and selection differentials suggest that correlated selection frequently reduced total selection on size but not on other types of traits. The direction of selection on a trait often changes over time in many temporally replicated studies, but these fluctuations have limited impact in reducing cumulative directional selection in most study systems. Analyses of quadratic selection gradients indicated stabilizing selection on body size in at least some studies but provided little evidence that stabilizing selection is more common than disruptive selection for most traits or study systems. Our analyses provide little evidence that fitness trade-offs, correlated selection, or stabilizing selection strongly constrains the directional selection reported for most quantitative traits.

Inbreeding reveals stronger net selection on Drosophila melanogaster males: implications for mutation load and the fitness of sexual females

Stronger selection on males has the potential to lower the deleterious mutation load of females, reducing the cost of sex. However, few studies have directly quantified the strength of selection for both sexes. As the magnitude of inbreeding depression (ID) is related to the strength of selection, we measured the cost of inbreeding for both males and females in a laboratory population of Drosophila melanogaster. Using a novel technique for inbreeding, we found significant ID for both juvenile viability and adult fitness in both sexes. The genetic variation responsible for this depression in fitness appeared to be recessive for adult fitness (h=0.11) and partially additive for juvenile viability (h=0.29). ID was identical across the sexes in terms of juvenile viability but was significantly more deleterious for males than females as adults, even though female X-chromosome homogamety should predispose them to a higher inbreeding load. We estimated the strength of selection on adult males to be 1.24 greater than on adult females, and this appears to be a consequence of selection arising from competition for mates. Combined with the generally positive intersexual genetic correlation for inbred lines, our results suggest that the mutation load of sexual females could be meaningfully reduced by stronger selection acting on males.

Size selectivity of Korean flounder ( Glyptocephalus stelleri) by gillnets and trammel nets using an extension of SELECT for experiments with differing mesh sizes

In 2008, Korean flounder ( Glyptocephalus stelleri) was designated by the Korean Ministry for Food, Agriculture, Forestry and Fisheries as a species for restoration in the coastal waters of Korea. However, there are currently no regulations regarding the minimum landed size of flounder, or the minimum legal mesh size used by the coastal gillnet and trammel net fisheries. Selectivity experiments for gillnets and trammel nets were conducted using commercial vessels in several areas off the eastern coast of Korea between October 2006 and June 2009. The selectivity analysis required an adaption of the SELECT method because trammel net mesh sizes were altered part-way through the experiment. Gillnet and trammel net selectivity was best described using a bi-modal selection curve. The two gears were found to have different selection curves for a given mesh size. The length of maximum retention was similar for the two gears, but the selection curve of the trammel net was substantially wider than that of the gillnet. For the largest size of mesh used in the commercial fishery (8.5 cm stretched), 50% relative retention was estimated to occur at a length of 20.4 cm for the trammel net, and 22.5 cm for the gillnet. In comparison, the length of 50% maturity for this species is approximately 26 cm. As there are currently no size restrictions on landings of Korean flounder, these results suggest that use of these gears may pose a serious threat to local stocks of this species.

Is faith‐based investing rewarding? The case for Malaysian Islamic unit trust funds

PurposeThe growing demand for alternative investment vehicle which adheres to shari'a principles has prompted other measures to boost the Islamic capital market. Unit trust funds in Malaysia have been growing exponentially and their existence signifies the extent of development in the Malaysian financial market. For foreign and domestic investors who have low risk tolerance and wish to diversify, unit trust funds offer the opportunity to invest. The increasing relevance of unit trust funds as an investment instrument has driven us to analyze the fund's performance. This paper addresses these issues.Design/methodology/approachThe paper examines the comparative performance of Malaysian unit trust funds vis‐à‐vis their non‐Islamic counterparts using a variety of measures, such as Sharpe, Treynor, Jenson and Fama's selectivity, net selectivity and diversification. The paper also examines the persistence of performance using Carhart's four‐factor pricing models. Lastly, the paper employs an analysis of cointegration to examine how the Islamic unit trust funds are related in long term with their non‐Islamic counterparts, as well as their respective market portfolios.FindingsThe paper finds no convincing performance differences between Islamic and non‐Islamic Malaysian unit trust funds. Controlling performance for style differences, the paper finds that non‐Islamic unit trust funds in Malaysia are value‐focused while Islamic unit trust funds are small cap oriented. In addition, similar reward to risk and diversification benefits exist only between Islamic and non‐Islamic Malaysian unit trust funds.Research limitations/implicationsThe Worldscope data are used to construct four‐factor models as opposed to Malaysian‐based data – given that Malaysia is an open economy that attracts global investors. Also, US T‐Bill rate is used rather than Malaysian risk‐free rate because no other securities are as riskless as US Treasury Bills.Practical implicationsThe paper observes a significant long‐term relationship between Islamic unit trust funds portfolio and non‐Islamic unit trust funds portfolio. The implication here suggests that investors in Malaysian unit trust funds will most likely benefit from international diversification of financial risks. They do not, however, stand a good chance to gain from portfolio diversification in the local unit trust funds market.Originality/valueThe study contributes to the existing Islamic investment literature by pursuing an empirical analysis on the performance of both Islamic and non‐Islamic Malaysian unit trust funds by using more recent data and further investigating the long‐run relationship between Islamic and non‐Islamic unit trust funds.

Selectivity, Style, Sentiment and Skill in Mutual Fund Trades

Fund managers can only exhibit selectivity through purchasing (selling) stocks that appreciate (depreciate) more frequently than expected from random occurrence, if stocks are incorrectly priced. We develop a method that can statistically identify fund managers that exhibit net, buy, and sell selectivity in their trades, as well as distinguish manager skill from fortuitous stock selection. Stock investor sentiment betas are calculated from the recently developed investor sentiment index, and used to indicate stock mispricing. We find that superior stock selection is concentrated in funds that hold high sentiment beta stocks; the major constituent of funds with the aggressive growth objective.

Selectivity of plankton nets over mesozooplankton taxa: implications for abundance, biomass and diversity estimation

The importance of the smaller copepod species is often underestimated as a result of the use of standard 200 μm mesh size nets, even though the small copepods probably represent the dominant component of the mesozooplankton community. Although the inadequacy of relatively coarse nets in providing reliable descriptions of the mesozooplankton assemblage is increasingly welldocumented, such nets continue to be commonly used. A major reason for this unwillingness to break with tradition is the belief that samples remain comparable even though the absolute values are biased. A one-year study of the abundance and size distribution of zooplankton collected in the Venice Lagoon using an 80 μm mesh size net showed an overwhelming abundance of small taxa. These data were used to derive estimates of the mesh selection effects of standard WP-2 nets on zooplankton abundance and biomass. Only 11% of numbers and 54% of biomass of lagoon zooplankton are likely to be caught with standard WP-2 nets. A comparison of seasonal changes in diversity, estimated from the fine and coarse datasets, confirmed that retention efficiency is seasonally dependent, which results in serious implications when extrapolating temporal patterns in community structure from WP-2 mesozooplankton counts.

Effects of Hanging Ratio on Gill Net Selectivity for Annular Sea Bream (Diplodus annularis) in the Northern Aegean Sea, Turkey

Effects of Hanging Ratio on Gill Net Selectivity for Annular Sea Bream (Diplodus annularis) in the Northern Aegean Sea, Turkey

Coexistence mechanisms and the paradox of the plankton: quantifying selection from noisy data

Many species of phytoplankton typically co-occur within a single lake, as do many zooplankton species (the "paradox of the plankton"). Long-term co-occurrence suggests stable coexistence. Coexistence requires that species be equally "fit" on average. Coexistence mechanisms can equalize species' long-term average fitnesses by reducing fitness differences to low levels at all times, and by causing species' relative fitness to fluctuate over time, thereby reducing differences in time-averaged fitness. We use recently developed time series analysis techniques drawn from population genetics to estimate the strength of net selection (time-averaged selection over a year) and fluctuating selection (an index of the variation in selection throughout the year) in natural plankton communities. Analysis of 99 annual time series of zooplankton species dynamics and 49 algal time series reveals that within-year net selection generally is statistically significant but ecologically weak. Rates of net selection are -10 times faster in laboratory competition experiments than in nature, indicating that natural coexistence mechanisms are strong. Most species experience significant fluctuating selection, indicating that fluctuation-dependent mechanisms may contribute to coexistence. Within-year net selection increases with enrichment, implying that among-year coexistence mechanisms such as trade-offs between competitive ability and resting egg production are especially important at high enrichment. Fluctuating selection also increases with enrichment but is independent of the temporal variance of key abiotic factors, suggesting that fluctuating selection does not emerge solely from variation in abiotic conditions, as hypothesized by Hutchinson. Nor does fluctuating selection vary among lake-years because more variable abiotic conditions comprise stronger perturbations to which species exhibit frequency-dependent responses, since models of this mechanism fail to reproduce observed patterns of fluctuating selection. Instead, fluctuating selection may arise from internally generated fluctuations in relative fitness, as predicted by models of fluctuation-dependent coexistence mechanisms. Our results place novel constraints on hypotheses proposed to explain the paradox of the plankton.

Cuticular hydrocarbons influence female attractiveness to males in the Australian field cricket, Teleogryllus oceanicus

Sexual dimorphism is thought to result from directional sexual selection acting on male signal traits, with female signal traits given little, if any, attention. Here, we examine male mating preferences in the Australian field cricket, Teleogryllus oceanicus. Using a multivariate selection analysis approach, we found that male preferences have the potential to exert selection on female cuticular hydrocarbons, chemical compounds widely used as sexual signals in insects. In addition to finding both stabilizing and disruptive preference gradients, we also found weak negative directional preference for female cuticular hydrocarbons. We contrast our results with a recent study examining sexual selection via female choice on male T. oceanicus cuticular hydrocarbons and suggest that differences in the form and intensity of sexual selection between the genders may provide part of the net selection differential necessary for the evolution of sexual dimorphism in this species.

Spatial pattern affects diversity–productivity relationships in experimental meadow communities

Plant species create aggregations of conspecifics as a consequence of limited seed dispersal, clonal growth and heterogeneous environment. Such intraspecific aggregation increases the importance of intraspecific competition relative to interspecific competition which may slow down competitive exclusion and promote species coexistence. To examine how spatial aggregation impacts the functioning of experimental assemblages of varying species richness, eight perennial grassland species of different growth form were grown in random and aggregated patterns in monocultures, two-, four-, and eight-species mixtures. In mixtures with an aggregated pattern, monospecific clumps were interspecifically segregated. Mixed model ANOVA was used to test (i) how the total productivity and productivity of individual species is affected by the number of species in a mixture, and (ii) how these relationships are affected by spatial pattern of sown plants. The main patterns of productivity response to species richness conform to other studies: non-transgressive overyielding is omnipresent (the productivity of mixtures is higher than the average of its constituent species so that the net diversity, selection and complementarity effects are positive), whereas transgressive overyielding is found only in a minority of cases (average of log(overyielding) being close to zero or negative). The theoretical prediction that plants in a random pattern should produce more than in an aggregated pattern (the distances to neighbours are smaller and consequently the competition among neighbours stronger) was confirmed in monocultures of all the eight species. The situation is more complicated in mixtures, probably as a consequence of complicated interplay between interspecific and intraspecific competition. The most productive species ( Achillea, Holcus, Plantago) were competitively superior and increased their relative productivity with mixture richness. The intraspecific competition of these species is stronger than that of most other species. The aggregated pattern in the full mixture increased the survival of subordinate species, and consequently, we conclude that an aggregated pattern can promote species coexistence (or at least postpone competitive exclusion), particularly in comparison with homogeneously sown mixtures.

Chemical decay of an antibiotic inverts selection for resistance

Antibiotics are often unstable, decaying into various compounds with potential biological activities. We found that as tetracycline degrades, the competitive advantage conferred to bacteria by resistance not only diminishes, but reverses to become a prolonged disadvantage due to the activities of more stable degradation products. Tetracycline decay can lead to net selection against resistance, which may help explain the puzzling coexistence of sensitive and resistant strains in natural environments.

The selectivity of monofilament trammel nets used in catch of gilt head bream (Sparus aurata, Linneaus, 1758) in İskenderun Bay-Turkey

In this study, the mesh selectivity of monofilament trammel nets used in catch of gilt head bream (Sparus aurata) was investigated using four different mesh sizes (28, 30, 32 and 34mm). Fishing trials were carried out in ?skenderun Bay from February 2005 to May 2006. Selectivity on Sparus aurata was assessed using the SELECT method (Millar, 1992), implemented in the GILLNET software. Five different functional models; normal location, normal scale, gamma, log-normal and bi-modal were used to fit the selectivity curve to the catch data. It was shown that the bi-modal function was the best to fit the data. For the selectivity curves on the bimodal, the optimum lengths (100% retention probability for the 28, 30, 32 and 34mm mesh sizes) were obtained as 17.49cm, 18.74cm, 19.99cm and 21.24cm, respectively. Thus, considering the minimum allowable landing size (17cm), 28, 30, 32 and 34mm mesh sizes do not cause an over fishing on gilt head bream stocks in Iskenderun Bay, Turkey.