The effect of directed and stabilizing selection on the population size of Drosophila melanogaster

Population size as a measure of population fitness was assayed in laboratory lines of Drosophila melanogaster subjected to directional selection for positive photoactivity and stabilizing selection for a complex of metric wing characters. Three experimental designs were used: directional and stabilizing selection (DS); directional selection (D); unselected controls (C). Population size increased under all three experimental designs. The population size of DS and C lines as well as the rate of increase of the DS lines were higher as compared to the D lines. The results suggest that stabilizing selection maintains population fitness at the higher level.     

Experimental excursions on adaptive landscapes: density-dependent selection on egg size

Abstract. Theories of density-dependent natural selection suggest that intraspecific competition will favor juveniles of high competitive ability. Empirical evidence has been provided from laboratory selection experiments, but field studies are lacking due to the logistical difficulties of experimentally manipulating population densities in natural settings. Here, we present data from a decade-long experimental field study of side-blotched lizards, Uta stansburiana that overcomes these difficulties. We tested the hypothesis that density-dependent natural selection causes egg size to increase from early to late clutches in this and many other species. Using a novel combination of environmental manipulations of hatchling density and phenotypic manipulations of egg size, we demonstrate that the nature of selection on egg size changes dramatically in the absence of older competitors. The strength of selection on egg size among later-clutch hatchlings released in areas without competitors from early clutches became almost doubled in magnitude, compared to that among hatchlings released in the presence of older competitors. These experimental findings demonstrate density-dependent natural selection on egg size; however, they contradict the classical idea that egg size increases during the reproductive season because of competition between early and late hatchlings. The results indicate that competitive age or size asymmetries between early and late hatchlings can override within-cohort asymmetries due to egg size. We suggest that competition could be an important mediator of oscillating selection pressures in this and other systems. Finally, we discuss the utility of "double-level," simultaneous experimental manipulation of both phenotypic traits that are targets of selection (e.g., egg size) as well the environmental agents of selection (e.g., population density).     

A probability model for the fusion of nucleoli in metabolic cells

The nucleoli are assumed to be originally of negligible size and located at random in the nucleus, considered as a sphere. They increase to a maximum size during interphase. Fusion occurs if the nucleoli touch during the size increase process. The probability of this event is calculated for various values of the maximum size of the nucleolus as compared with the size of the nucleus. A set of experimental data is analysed, using the results obtained. It turns out that the frequency of fusion is significantly higher than the assumption of initial random location would indicate. For further details on experimental material and methods, see Darvey &; Driscoll (1972).     

Experimental litter size reduction reveals costs of gestation and delayed effects on offspring in a viviparous lizard

Experimental studies have often been employed to study costs of reproduction, but rarely to study costs of gestation. Disentangling the relative importance of each stage of the reproductive cycle should help to assess the costs and benefits of different reproductive strategies. To that end, we experimentally reduced litter size during gestation in a viviparous lizard. We measured physiological and behavioural parameters during gestation and shortly after parturition, as well as survival and growth of females and their offspring. This study showed four major results. First, the experimental litter size reduction did not significantly affect the cellular immune response, the metabolism and the survival of adult females. Second, females with reduced litter size decreased their basking time. Third, these females also had an increased postpartum body condition. As postpartum body condition is positively related to future reproduction, this result indicates a gestation cost. Fourth, even though offspring from experimentally reduced litters had similar weight and size at birth as other offspring, their growth rate after birth was significantly increased. This shows the existence of a maternal effect during gestation with delayed consequences. This experimental study demonstrates that there are some costs to gestation, but it also suggests that some classical trade-offs associated with reproduction may not be explained by gestation costs.     

Studies of effects of macromolecular crowding and confinement on protein folding and protein stability

In cells, proteins execute specific tasks in crowded environments; these environments influence their stability and dynamics. Similarly, for an enzyme molecule encapsulated in an inorganic cavity as in biosensors or biocatalysts, confinement or excluded volume plays an important role in its stability and dynamics. In this article we present results of our experimental and theoretical investigations of the confinement and macromolecular crowding effects on protein. On the experimental side we study the stability of encapsulated cytochrome c against unfolding induced by the presence of denaturants, such as urea. Results show that, as the pore size in which protein is trapped is reduced, protein shows higher stability against denaturant-induced unfolding. On the theoretical side, after reviewing our previous study of the confinement effects on the equilibrium and dynamic properties of protein using a minimalist (two-dimensional lattice, Monte Carlo, Brownian dynamics) model, we have extended the model so that the effects of macromolecular crowding on such properties can be studied. Our simulations show that both folding and unfolding times increase with the number of crowders in solution, however, the equilibrium constant is affected such that the equilibrium is shifted towards the folded state. Furthermore, our results show that, for a fixed number of crowders as the size of crowder (or excluded volume) increases, the average size of protein at equilibrium decreases.     

Habitat patch size alters the importance of dispersal for species diversity in an experimental freshwater community

Increased dispersal of individuals among discrete habitat patches should increase the average number of species present in each local habitat patch. However, experimental studies have found variable effects of dispersal on local species richness. Priority effects, predators, and habitat heterogeneity have been proposed as mechanisms that limit the effect of dispersal on species richness. However, the size of a habitat patch could affect how dispersal regulates the number of species able to persist. We investigated whether habitat size interacted with dispersal rate to affect the number of species present in local habitats. We hypothesized that increased dispersal rates would positively affect local species richness more in small habitats than in large habitats, because rare species would be protected from demographic extinction. To test the interaction between dispersal rate and habitat size, we factorially manipulated the size of experimental ponds and dispersal rates, using a model community of freshwater zooplankton. We found that high‐dispersal rates enhanced local species richness in small experimental ponds, but had no effect in large experimental ponds. Our results suggest that there is a trade‐off between patch connectivity (a mediator of dispersal rates) and patch size, providing context for understanding the variability observed in dispersal effects among natural communities, as well as for developing conservation and management plans in an increasingly fragmented world.     

Cost of reproduction, T-lymphocyte mediated immunocompetence and health status in female and nestling barn swallows Hirundo rustica

We investigate the trade-off between reproductive effort, health status and T-lymphocyte acquired immunity in female and nestling barn swallows Hirundo rustica using a brood size manipulation experiment. Maternal and total feeding effort increased with experimental brood size. Parents did not fully compensate for the increased food demand of the enlarged broods and as a consequence the per capita feeding rate of nestlings decreased with increasing experimental brood size. Body mass and a measure of T-cell mediated immunity in 12 days old nestlings also decreased with increasing experimental brood size. Different leucocyte concentrations and the heterophile/lymphocyte ratio – an index of stress – of nestlings did not change in relation to experimental brood size, suggesting that within brood competition did not affect stress to nestlings. The brood size manipulation had a significant effect on maternal T-cell mediated immunity, measured by the phytohemagglutinin skin test, but not on maternal body mass, haematocrit or differential or total white blood cell counts. Our results seem to support the prediction that under mild work stress females respond first by reducing the energetically expensive acquired immunity. Different leucocyte types and the heterophile/lymphocyte ratio appear less sensitive to parental workload.     

Effects of flower size and number on pollinator visitation to wild radish,Raphanus raphanistrum

Plant traits that increase pollinator visitation should be under strong selection. However, few studies have demonstrated a causal link between natural variation in attractive traits and natural variation in visitation to whole plants. Here we examine the effects of flower number and size on visitation to wild radish by two taxa of pollinators over 3 years, using a combination of multiple regression and experimental reductions in both traits. We found strong, consistent evidence that increases in both flower number and size cause increased visitation by syrphid flies. The results for small bees were harder to interpret, because the multiple regression and experimental manipulation results did not agree. It is likely that increased flower size causes a weak increase in small-bee visitation, but strong relationships between flower number and small-bee visitation seen in 2 years of observational studies were not corroborated by experimental manipulation of this trait. Small bees may actually have responded to an unmeasured trait correlated with flower number, or lower small-bee abundances when the flower number manipulation was conducted may have reduced our ability to detect a causal relationship. We conclude that studies using only 1 year, one method, or measuring only one trait may not provide an adequate understanding of the effects of plant traits on pollinator attraction.     

The Impact of Psychological Stress on Men's Judgements of Female Body Size

Background

Previous work has suggested that the experience of psychological stress may influence physical attractiveness ideals, but most evidence in favour of this hypothesis remains archival. The objective of this study was to experimentally investigate the impact of stress on men''s judgements of female body size.

Methods

Men were randomly assigned to either an experimental group, in which they took part in a task that heightened stress (experimental group, n = 41) or in which they did not take part in such a task (control group, n = 40). Both groups rated the attractiveness of female bodies varying in size from emaciated to obese, completed a measure of appetite sensation, and had their body mass indices (BMIs) measured.

Results

Between-groups analyses showed that the experimental group was matched with the control group in terms of mean age, BMI, and appetite sensation. Further analyses showed that men in the experimental group rated a significantly heavier female body size as maximally attractive than the control group. Men in the experimental group also rated heavier female bodies as more attractive and idealised a wider range of female figures than did the control group.

Conclusion

This study found that the experience of stress was associated with a preference among men for heavier female body sizes. These results indicate that human attractiveness judgements are sensitive to variations in local ecologies and reflect adaptive strategies for dealing with changing environmental conditions.     

Estimation of population profiles of two strains of the fly Megaselia scalaris (Diptera: Phoridae) by bootstrap simulation

Based on experimental population profiles of strains of the fly Megaselia scalaris (Phoridae), the minimal number of sample profiles was determined that should be repeated by bootstrap simulation process in order to obtain a confident estimation of the mean population profile and present estimations of the standard error as a precise measure of the simulations made. The original data are from experimental populations founded with SR and R4 strains, with three replicates, which were kept for 33 weeks by serial transfer technique in a constant temperature room (25 +/- 1.0 degrees C). The variable used was population size and the model adopted for each profile was a stationary stochastic process. By these simulations, the three experimental population profiles were enlarged so as to determine minimum sample size. After sample size was determined, bootstrap simulations were made in order to calculate confidence intervals and to compare the mean population profiles of these two strains. The results show that with a minimum sample size of 50, stabilization of means begins.     

Determinants of size at metamorphosis in an endangered amphibian and their projected effects on population stability

Roughly 80% of animal species have complex life cycles spanning a major habitat shift, and delayed life history effects play an important role in their population dynamics. Through their effect on size at metamorphosis, factors in the pre‐ metamorphic environment often have profound effects upon survival and fecundity in the post‐metamorphic environment. Here, we adopted a combined experimental and field observational approach to investigate the factors that determine size at metamorphosis in pond‐breeding amphibians, and to predict some of their downstream effects on population stability. We set up ecologically realistic mesocosm communities for the endangered California tiger salamander Ambystoma californiense to test the effects of larval density, prey density and hydroperiod on mean size at metamorphosis. We found significant effects for all three factors, with mean size at metamorphosis negatively correlated with larval density and positively correlated with prey density and hydroperiod. We also used six years of field survey data to identify the most informative model explaining mean size at metamorphosis and thus validate our mesocosm results. The optimal three‐term model identified terms that were roughly analogous to each of the mesocosm treatments and with similar effect sizes, providing strong field confirmation of our experimental results. The field data also provide correlations between each factor and the number of metamorphs recruited to the population, allowing us to predict the effect of each factor on population stability. Finally, we show that these populations of the endangered A. californiense are strongly resource limited, which has important implications for their management and recovery as an endangered taxon.     

In vivo effect of methylcobalamin on the peripheral nerve structure in streptozotocin diabetic rats

To study in vivo effect of methylcobalamin (CH3-B12) on the peripheral nerve structures, rats with experimental diabetes induced by streptozotocin were administered with daily intramuscular injection of CH3-B12 (500 microgram/kg) for 16 weeks. By isolated nerve fiber studies, CH3-B12-treated diabetic rats showed less incidence of paranodal demyelination as an early sign of segmental demyelination than non-treated diabetic rats. From morphometrical analysis on sural nerves, the reduction in the density of myelinated nerve fibers, nerve fiber size and axon size of myelinated fibers was definitely protected in treated diabetic rats. The results suggested that continuous treatment with CH3-B12 had an ameliorative effect on the peripheral nerve lesions in experimental diabetic neuropathy.     

Effects of paranasal sinus ostia and volume on acoustic rhinometry measurements: a model study.

We used pipe models to investigate the effects of paranasal sinus ostium size and paranasal sinus volume on the area-distance curves derived by acoustic rhinometry (AR). Each model had a Helmholtz resonator or a short neck as a side branch that simulated the paranasal sinus and sinus ostium. The AR-derived cross-sectional areas posterior to the ostium were significantly overestimated. Sinus volume affected the AR measurements only when the sinus was connected via a relatively large ostium. The experimental area-distance curve posterior to the side branch showed pronounced oscillations in association with low-frequency acoustic resonances in this distal part of the pipe. The experimental results are discussed in terms of theoretically calculated "sound-power reflection coefficients" for the pipe models used. The results indicate that the effects of paranasal sinuses and low-frequency acoustic resonances in the posterior part of the nasal cavity are not accounted for in the current AR algorithms. AR does not provide reliable information about sinus ostium size, sinus volume, or cross-sectional area in the distal parts of nasal cavity.     

Seasonal variation on size and chemical constituents of Sargassum sinicola Setchell et Gardner from Bahía de La Paz, Baja California Sur, Mexico

Investigation on seasonal variation in size and chemical constituents of Sargassum sinicola Setchell et Gardner from Bahía de La Paz, Baja California Sur, Mexico, was carried out from a control bed and compared with an experimental bed with artificial nutrients added. No significant differences were found between the control and experimental thalli for size or chemical composition, except for iodine and raw fiber. For control thalli the results were: size 7.5–56.0 cm, alginate yield 7.2–13.7%, viscosity 58.7–191.7 millipascal seconds (mPa s), mannitol 2.9–8.1%, raw fiber 5.5–7.5% and iodine 0.020–0.141%; while in the experimental thalli the size ranged from 7.5 to 80.3 cm and the alginate yield was 7.8–10.4%, viscosity 41.4–163.4 mPa s, mannitol 2.9–8.3%, raw fiber 5.9–10.7% and iodine 0.021–0.098%. These variations were related to its natural growth cycle, and showed reductions during the senescence period. Results suggest that S. sinicola is not affected by relatively low nutrient concentrations, and could be considered as raw material for alginate production.     

On the cavity size distribution in water and n-hexane

The cavity size distribution functions in water and n-hexane were determined by Pohorille and Pratt, in a series of important works, from molecular dynamics simulations. These functions are considered as experimental data. In the present investigation the ability of scaled particle theory in reproducing such distributions is tested. In the case of water the scaled particle theory results compare favorably with the experimental distribution if a proper choice of the size to be assigned to water molecules is performed. Specifically, a slight size increase from 2.70 to 2.80 A is necessary to reach agreement for the largest cavities detected by Pohorille and Pratt. In the case of n-hexane the scaled particle theory results do not agree with the experimental distribution especially in the region of small cavities. This deficiency is because a n-hexane molecule cannot be realistically treated as a single spherical exclusion volume. The implications of such findings are analyzed and discussed in depth.     

Size-specific predation on marine invertebrate larvae

Predation on planktonic larval stages is frequently a major source of mortality for the offspring of benthic marine invertebrates. Mortality rate likely varies with larval size and developmental stage, but few experiments have measured how these factors affect predation rates. I used experimental reductions in egg size to test how variation in larval size affects the likelihood of predation during planktonic development. Blastomeres of the sand dollar Dendraster excentricus were separated at the two-cell stage to produce half-sized zygotes. Larvae resulting from this manipulation were tested for their susceptibility to predation relative to whole-sized siblings at four ages. Individuals from each size class were simultaneously presented as prey items to five predators (crab zoeae, crab megalopae, chaetognaths, solitary tunicates, and postlarval fish) in the laboratory. Four predators consumed significantly more half-sized larvae than whole-sized larvae, but one predator type (postlarval fish) consumed more whole-sized larvae. Predators that consumed more half-sized larvae also preferentially consumed younger larvae. In contrast, postlarval fish showed no significant prey preference based on larval age. These results suggest that assumptions of constant mortality rates during development should be modified to account for the effects of larval size and age.     

Sexual dimorphism in Gammarus pulex: the effect of current flow on pre-copula pair formation

SUMMARY. 1. Male Gammarus pulex (L.) are typically twice as heavy as females in field caught pre-copula pairs and mating is positively size assortative. Earlier work had shown that relatively large males can swim in pre-copula at higher current speeds than males that are relatively small compared with their mates. We describe experiments on pair formation in the laboratory, in static water and in an artificial stream.
2. Pairs formed in the artificial stream had a significantly higher weight ratio than either those formed in static water or those caught in the field. In both experimental conditions, males in pairs were significantly larger than singles. In the stream, the weight ratio of pairs increased with formation sequence due to a significant decrease in the size of selected females.
3. It is concluded that these results are consistent with the hypothesis that the relatively large size of paired male G. pulex is a product of mechanical constraints rather than male–male competition.     

Effects of habitat quality and size on extinction in experimental populations

Stochastic population theory makes clear predictions about the effects of reproductive potential and carrying capacity on characteristic time-scales of extinction. At the same time, the effects of habitat size and quality on reproduction and regulation have been hotly debated. To trace the causal relationships among these factors, we looked at the effects of habitat size and quality on extinction time in experimental populations of Daphnia magna. Replicate model systems representative of a broad-spectrum consumer foraging on a continuously supplied resource were established under crossed treatments of habitat size (two levels) and habitat quality (three levels) and monitored until eventual extinction of all populations. Using statistically derived estimates of key parameters, we related experimental treatments to persistence time through their effect on carrying capacity and the population growth rate. We found that carrying capacity and the intrinsic rate of increase were each influenced similarly by habitat size and quality, and that carrying capacity and the intrinsic rate of increase were in turn both correlated with time to population extinction. We expected habitat quality to have a greater influence on extinction. However, owing to an unexpected effect of habitat size on reproductive potential, habitat size and quality were similarly important for population persistence. These results support the idea that improving the population growth rate or carrying capacity will reduce extinction risk and demonstrate that both are possible by improving habitat quality or increasing habitat size.     

Experimental and model analyses of the effects of competition on individual size variation in wood frog (Rana sylvatica) tadpoles

1. Size variation is a ubiquitous feature of animal populations and is predicted to strongly influence species abundance and dynamics; however, the factors that determine size variation are not well understood. 2. In a mesocosm experiment, we found that the relationship between mean and variation in wood frog (Rana sylvatica) tadpole size is qualitatively different at different levels of competition created by manipulating resource supply rates or tadpole density. At low competition, relative size variation (as measured by the coefficient of variation) decreased as a function of mean size, while at high competition, relative size variation increased. Therefore, increased competition magnified differences in individual performance as measured by growth rate. 3. A model was developed to estimate the contribution of size-dependent factors (i.e. based on size alone) and size-independent factors (i.e. resulting from persistent inherent phenotypic differences other than size that affect growth) on the empirical patterns. 4. Model analysis of the low competition treatment indicated that size-dependent factors alone can describe the relationship between mean size and size variation. To fit the data, the size scaling exponent that describes the dependence of growth rate on size was determined. The estimated value, 0-83, is in the range of that derived from physiological studies. 5. At high competition, the model analysis indicated that individual differences in foraging ability, either size-based or due to inherent phenotypic differences (size-independent factors), were much more pronounced than at low competition. The model was used to quantify the changes in size-dependent or size-independent factors that underlie the effect of competition on size-variation. In contrast to results at low competition, parameters derived from physiological studies could not be used to describe the observed relationships. 6. Our experimental and model results elucidate the role of size-dependent and size-independent factors in the development of size variation, and highlight and quantify the context dependence of individual (intrapopulation) differences in competitive abilities.     

Increased body size confers greater fitness at lower experimental temperature in male Drosophila melanogaster

Genetic variation of body size along latitudinal clines is found globally in Drosophila melanogaster, with larger individuals encountered at higher latitudes. Temperature has been implicated as a selective agent for these clines, because the body size of laboratory populations allowed to evolve in culture at lower temperatures is larger. In this study, we investigated the hypothesis that larger size is favoured at lower temperature through natural selection on adult males. We measured life‐span and age‐specific fertility of males from lines of flies artificially selected for body size at two different experimental temperatures. There was an interaction between experimental temperature and body size selection for male fitness; large‐line males were fitter than controls at both temperatures, but the difference in fitness was greater at the lower experimental temperature. Smaller males did not perform significantly differently from control males at either experimental temperature. The results imply that thermal selection for larger adult males is at least in part responsible for the evolution of larger body size at lower temperatures in this species. The responsible mechanisms require further investigation.