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PubMed2001

Selection for growth rate or against back fat thickness in pigs is associated with changes in growth hormone axis plasma protein concentration and mRNA level.

Selection for increased growth rate or decreased back fat thickness results in concomitant changes in endocrine and metabolic status. Growth hormone (GH) changes in blood plasma concentration related to selection for growth rate and fat deposition were reported in pigs. The molecular mechanisms regulating selection-induced changes in GH plasma concentration remain largely unknown. We investigated selection-associated changes in GH axis parameters in 2 pig lines selected for increased growth rate (F-line), or decreased back fat thickness (L-line), respectively. First, we investigated selection-associated changes in GH pulse parameters. In both selection lines we found each generation a declining GH peak maximum concentration and area under the GH curve. GH pulse width was not associated with generation number. In both lines generation number was associated with a declined pulse interval, indicating that the number of pulses per day increased on average with 1 pulse per 24 h per generation. Second, plasma concentration of GH axis related Insulin-like growth factor-I (IGF-I) and insulin were investigated. Plasma IGF-I concentration was not associated with generation number in the F-line. Mean plasma insulin concentration declined each generation in both lines. Third, we investigated changes in GH and Pit-1 mRNA levels. In both selection lines GH and Pit-1 mRNA levels increased approximately 50% each generation. The high SD of the GH mRNA levels in both lines may suggest that the GH mRNA levels are pulsatile in vivo. We postulate a molecular mechanism that may explain how selection is associated with increased GH mRNA levels and GH pulse numbers, while lowering GH release per pulse.

Adipose Tissue↗
PubMed1999

Stepwise selection in small data sets: a simulation study of bias in logistic regression analysis.

Stepwise selection methods are widely applied to identify covariables for inclusion in regression models. One of the problems of stepwise selection is biased estimation of the regression coefficients. We illustrate this "selection bias" with logistic regression in the GUSTO-I trial (40,830 patients with an acute myocardial infarction). Random samples were drawn that included 3, 5, 10, 20, or 40 events per variable (EPV). Backward stepwise selection was applied in models containing 8 or 16 pre-specified predictors of 30-day mortality. We found a considerable overestimation of regression coefficients of selected covariables. The selection bias decreased with increasing EPV. For EPV 3, 10, or 40, the bias exceeded 25% for 7, 3, and 1 in the 8-predictor model respectively, when a conventional selection criterion was used (alpha = 0.05). For these EPV values, the bias was less than 20% for all covariables when no selection was applied. We conclude that stepwise selection may result in a substantial bias of estimated regression coefficients.

Aged↗
PubMed1995

Dynamics of unconditionally deleterious mutations: Gaussian approximation and soft selection.

This paper studies the influence of two opposite forces, unidirectional unconditionally deleterious mutations and directional selection against them, on an amphimictic population. Mutant alleles are assumed to be equally deleterious and rare, so that homozygous mutations can be ignored. Thus, a genotype is completely described by its value with respect to a quantitative trait chi, the number of mutations it carries, while a population is described by its distribution p(chi) with mean M[p] and variance V[p] = sigma(2)[p]. When mutations are only slightly deleterious, so that M > 1, before selection p(chi) is close to Gaussian with any mode of selection. I assume that selection is soft in the sense that the fitness of a genotype depends on the difference between its value of chi and M, in units of sigma. This leads to a simple system of equations connecting the values of M and V in successive generations. This system has a unique and stable equilibrium, M = U/delta)2(2--rho) and V = (U/delta)2, where U is the genomic deleterious mutation rate, delta is the selection differential for chi in units of sigma, and rho is the ratio of variances of p(chi) after and before selection. Both delta and rho are parameters of the mode of soft selection, and do not depend on M or V. In an equilibrium population, the selection coefficient against a mutant allele is s = delta2[U(2--rho)]-1. The mutation load can be tolerable only if the genome degradation rate v = U/sigma is below 2. Other features of mutation-selection equilibrium are also discussed.

Alleles↗
PubMed1996

The effect of recombination on background selection.

An approximate equation is derived, which predicts the effect on variability at a neutral locus of background selection due to a set of partly linked deleterious mutations. Random mating, multiplicative fitnesses, and sufficiently large population size that the selected loci are in mutation/selection equilibrium are assumed. Given these assumptions, the equation is valid for an arbitrary genetic map, and for an arbitrary distribution of selection coefficients across loci. Monte Carlo computer simulations show that the formula performs well for small population sizes under a wide range of conditions, and even seems to apply when there are epistatic fitness interactions among the selected loci. Failure occurred only with very weak selection and tight linkage. The formula is shown to imply that weakly selected mutations are more likely than strongly selected mutations to produce regional patterning of variability along a chromosome in response to local variation in recombination rates. Loci at the extreme tip of a chromosome experience a smaller effect of background selection than loci closer to the centre. It is shown that background selection can produce a considerable overall reduction in variation in organisms with small numbers of chromosomes and short maps, such as Drosophila. Large overall effects are less likely in species with higher levels of genetic recombination, such as mammals, although local reductions in regions of reduced recombination might be detectable.

Animals↗
PubMed2003

Detection of deleterious genotypes in multigenerational studies. III. Estimation of selection components in highly selfing populations.

New paradigms in genetics have increased the chance of finding genes that appear redundant but in fact may have been preserved due to a small level of positive selection potential acting during each generation. Monitoring changes in genotypic frequencies within and between generations allows the dissection of the fertility, viability and meiotic drive selection components acting on such genes in natural and experimental populations. Here, a formal maximum likelihood procedure is developed to identify and estimate these selection components in highly selfing populations by fitting the time-dependent solutions for genotypic frequencies to observed multigenerational counts. With adult census alone, we can not simultaneously estimate all three selection components considered. In such cases, we instead consider a hierarchy of 11 models with either fewer selection components, complete dominance, or multiplicative meiotic drive with a single parameter. We identify the best-fitting of these models by applying likelihood ratio tests to nested models and Akaike's Information Criterion (AIC) and the Bayesian Information Criterion (BIC) to non-nested models. With seed census, fertility and viability selection are not distinguishable and thus can only be estimated jointly. A combination of joint seed and adult census data allows us to estimate all three selection components simultaneously. Simulated data validate the estimation procedure and provide some practical guidelines for experimental design. An application to Arabidopsis data establishes that viability selection is the major selective force acting on the ACT2 actin gene in laboratory-grown Arabidopsis populations.

Arabidopsis↗
PubMed1999

Effect of selection for development rate on reproductive onset in female mice.

This research reports analyses of correlated response in reproductive onset in ICR mice after 23 generations of restricted index selection for divergent body weight gain, early (birth-10 days) or later (28-56 days) in life. Long-term selection altered growth trajectories and 56 day body weight of individuals under different selection regimes in this study. Mice in lines under early selection have the same percentage mature weight at vaginal opening as controls (63%). Vaginal opening is delayed in mice selected for slow early growth, which take longer to reach what appears to be a weight threshold. In contrast, individuals in lines selected for later slow growth undergo vaginal opening at the same age as controls, but at a lower weight and increased percentage mature weight. Pre-compensation or 'counter-balance growth' is observed in these lines, with mice selected for late enhanced growth reaching 52% of mature weight at vaginal opening while mice with late slow growth attain 71% of mature weight prior to vaginal opening. Only 42% of mice with late slow growth attain first oestrus by 56 days. We speculate this is a function of growth rate and fat/lean ratio. Mice with early slow growth show compensatory growth, reaching first oestrus at a similar time to controls. We conclude that selection for growth rate has asymmetrically affected reproductive onset, with lines selected for suppressed gains experiencing delays in the reproductive onset traits measured.

Age Factors↗
PubMed1998

Influence of nonionic surfactants on the potentiometric response of ion-selective polymeric membrane electrodes designed for blood electrolyte measurements.

The effect of the nonionic surfactants Brij 35 and Triton X-100 on the selectivity of neutral carrier-based ion-selective electrodes (ISEs) commonly used for measurements of electrolytes in whole blood is investigated. Studies are conducted with plasticized PVC membranes doped with several neutral ionophores commonly employed to prepare clinically useful potassium, calcium, and sodium ISEs. An observed increase in the electrodes' EMF values upon the addition of surfactant to the test solution suggests a change in the ion selectivity of the polymeric membranes in the presence of Brij 35 or Triton X-100. For membranes doped with K(+)-selective valinomycin, the effect of nonionic surfactants is relatively small. However, in the case of calcium-selective membranes prepared with ETH 1001 and ETH 129, nonionic surfactants, especially Triton X-100, decrease the selectivity for calcium over potassium cations by nearly 1 order of magnitude. Such behavior is even more dramatic for sodium-selective membranes, with the degree of surfactant-induced loss of ion selectivity dependent on the specific sodium ionophore employed, the lipophilic tetraphenylborate derivative content of the membrane, and the surfactant type. A detailed theoretical model is presented to explain the effect of nonionic surfactants on the EMF response function of cation-selective polymeric membrane electrodes. Experimental results are in good agreement with theoretical predictions based on known binding constants for ionophores and surfactants with given cations.

Calcium↗
PubMed1994

Increased recombination frequencies resulting from directional selection for geotaxis in Drosophila.

Several classes of models have been suggested to explain how natural selection can favour non-zero recombination. Directional and fluctuating selection, abiotic and biotic, and selection against harmful mutations seem to be the most plausible factors, but little has been done to test the problem experimentally. Here we show that long-term selection for positive or negative geotaxis in Drosophila melanogaster results in a dramatic increase in recombination rates in different genomic regions. The total increment in recombination for the genome portion considered is 78 cM for geo+ and 66 cM for geo-. Selection for negative geotaxis did not result in recombination changes in chromosome 2 whereas selection in the opposite direction caused nearly a four-fold increase in the b-cn segment and a significant, albeit not as high, increase in the adjacent regions, al-b and cn-vg. In chromosomes X and 3, a significant increase in recombination was found in both selected lines. In total, the increment in exchange frequency in chromosome X (y-cv-ct-v-car) was from 72.6 per cent (the control level) to 124.7 and 110.3 per cent geo- and geo+, respectively, whereas for the studied portion of chromosome 3 (ru-h-cu-sr-e) we obtained, correspondingly, 60.8, 76.4 and 73.8 per cent. Thus, in general, selection for geotaxis resulted in increased recombination frequencies regardless of the direction of selection.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2004

Detecting selection using a single genome sequence of M. tuberculosis and P. falciparum.

Selective pressures on proteins are usually measured by comparing nucleotide sequences. Here we introduce a method to detect selection on the basis of a single genome sequence. We catalogue the relative strength of selection on each gene in the entire genomes of Mycobacterium tuberculosis and Plasmodium falciparum. Our analysis confirms that most antigens are under strong selection for amino-acid substitutions, particularly the PE/PPE family of putative surface proteins in M. tuberculosis and the EMP1 family of cytoadhering surface proteins in P. falciparum. We also identify many uncharacterized proteins that are under strong selection in each pathogen. We provide a genome-wide analysis of natural selection acting on different stages of an organism's life cycle: genes expressed in the ring stage of P. falciparum are under stronger positive selection than those expressed in other stages of the parasite's life cycle. Our method of estimating selective pressures requires far fewer data than comparative sequence analysis, and it measures selection across an entire genome; the method can readily be applied to a large range of sequenced organisms.

Animals↗
PubMed2002

Correlational selection and the evolution of genomic architecture.

We review and discuss the importance of correlational selection (selection for optimal character combinations) in natural populations. If two or more traits subject to multivariate selection are heritable, correlational selection builds favourable genetic correlations through the formation of linkage disequilibrium at underlying loci governing the traits. However, linkage disequilibria built up by correlational selection are expected to decay rapidly (ie, within a few generations), unless correlational selection is strong and chronic. We argue that frequency-dependent biotic interactions that have 'Red Queen dynamics' (eg, host-parasite interactions, predator-prey relationships or intraspecific arms races) often fuel chronic correlational selection, which is strong enough to maintain adaptive genetic correlations of the kind we describe. We illustrate these processes and phenomena using empirical examples from various plant and animal systems, including our own recent work on the evolutionary dynamics of a heritable throat colour polymorphism in the side-blotched lizard Uta stansburiana. In particular, male and female colour morphs of side-blotched lizards cycle on five- and two-generation (year) timescales under the force of strong frequency-dependent selection. Each morph refines the other morph in a Red Queen dynamic. Strong correlational selection gradients among life history, immunological and morphological traits shape the genetic correlations of the side-blotched lizard polymorphism. We discuss the broader evolutionary consequences of the buildup of co-adapted trait complexes within species, such as the implications for speciation processes.

Animals↗
PubMed2002

Sexual selection on multivariate phenotype in wild and mass-reared Ceratitis capitata (Diptera: Tephritidae).

Males with a larger thorax and narrower face were found to be favoured by directional sexual selection in a previous field cage experiment where Ceratitis capitata males from the Seib 6-96 genetic sexing strain competed with wild males from Alto Valle (Patagonia) for the possession of wild females. Targets of sexual selection, however, might differ between wild and laboratory females as a response to adaptation to mass-rearing conditions. To evaluate possible divergences on the targets of sexual selection as a by-product of adaptation to mass-rearing conditions, field cage tests were performed with both wild and laboratory females. To avoid possible bias due to correlation among the measured traits (eye length [EL], face width [FW], head width [HW], and thorax length [TL]), a multivariate analysis was applied. Consistent with the previous experiment, the results indicated that TL and FW are probable targets of directional sexual selection independently of female strain. However, laboratory females were less selective than wild ones. Additionally, correlational sexual selection was detected acting on the multivariate phenotype. The effects of correlational selection overlap with those of directional selection on each single trait. The analysis of mating pair characteristics showed patterns that do not match the expectations for a random mating system. The current analysis indicates that during mating pair formation two processes overlap. On the one hand, sexual selection favours males with larger size (TL) and narrower faces (FW). This effect occurs in both wild and laboratory females. In addition, assortative mating based on both phenotype and origin was also observed.

Animals↗
PubMed1998

Efficiency of multistage marker-assisted selection in the improvement of multiple quantitative traits.

The application of marker-assisted selection (MAS) to breeding programmes depends on its relative cost and the expected economic return compared to conventional phenotypic selection. The relative efficiency of MAS can be increased through a two-stage selection scheme or through marker-based, multiple-trait improvement. However, the effectiveness of these alternatives has not been quantified. In this study, we evaluate the efficiency of MAS relative to conventional phenotypic selection and marker-only selection in multistage selection for the improvement of multiple traits. We further incorporate the costs of obtaining measurements on phenotypic characters and marker loci into the objective function to evaluate the efficiency of MAS with respect to the gain per unit cost. Deterministic analyses indicate that excluding costs, multiple-trait MAS can be used to increase the aggregate breeding values in quantitative characters and is expected to be more effective than conventional selection or single-trait MAS. Two-stage MAS has a slightly reduced gain because of culling in the first stage. If the objective function is to maximize the gain per unit cost, multiple-trait MAS is inferior to phenotypic selection in most of the selection schemes investigated when the cost ratio (r) of obtaining measurements on phenotypic characters to scoring marker loci is less than unity (r < or = 1.0) and the heritability (h2) is greater than 0.3. The efficiency of MAS increases as r increases and h2 decreases. For MAS to be more effective, it is necessary to decrease further the cost associated with molecular marker assays.

Chromosome Mapping↗
PubMed2002

Gender and viability selection on morphology in fledgling pied flycatchers.

Until recently, analyses of gender-dependent differences in viability selection and the ontogeny of sexual size dimorphism have been plagued by difficulties in determining the sex of nestling birds on the basis of morphology. Recently, this problem was overcome using molecular sex identification to report for the first time body-size-mediated antagonistic selection on the viability of male and female collared flycatchers. We used molecular sex identification to analyse natural selection on fledgling viability, sexual size dimorphism and effects of parasites in relation to gender in a Mediterranean population of the related pied flycatcher Ficedula hypoleuca. There was directional positive selection on fledgling weight but no selection on tarsus length. Fledgling weight was the most important determinant of fledgling survival, with heavier fledglings having increased viability. Although selective trends were of the same sign for both sexes, only among female fledglings were selection differentials and gradients statistically significant. Therefore, similar trends in selection were revealed in analyses of a data set where sex was ignored and in separate analyses using same-sex sibship trait means. Mite nest ectoparasites negatively affected fledgling weight, and the effects were stronger in female than male fledglings. There was no effect of parasitism on the tarsus length in males, as previously reported in retrospective analyses performed without knowledge of sex until recruitment. Overall, selection on fledgling viability on the basis of morphological traits and hatching date was not confounded by an individual's gender.

Age Factors↗
PubMed2006

Sympatric speciation under incompatibility selection.

The existing theory of sympatric speciation assumes that a local population splits into two species under one-dimensional disruptive selection, which favors both of the opposite extreme values of a quantitative trait. Here we model sympatric speciation under selection that favors high values of either of the two independently inherited traits, each required to efficiently consume one of the two available resources, but acts, because of a tradeoff, against those possessing high values of both traits. Such two-dimensional incompatibility selection is similar to that involved in allopatric speciation. Using a hypergeometric phenotypic model, we show that incompatibility selection readily leads to sympatric speciation. In contrast to disruptive selection, two distinct modes of sympatric speciation exist under incompatibility selection: under strong tradeoffs both of the new species are specialists, each consuming its own resource, but under moderate tradeoffs speciation may be asymmetric and involve the origin of a specialist and a generalist species. Also, incompatibility selection may lead to irreversible specialization: under strong tradeoffs, the population speciates if it consists mostly of unspecialized individuals, but remains undivided if most of the individuals are specialized to consume one of the resources. Incompatibility selection appears to be more realistic than disruptive selection, implying that incompatibility between individually adaptive alleles or trait states drives both allopatric and sympatric speciation.

Animals↗
PubMed2002

Directional selection is the primary cause of phenotypic diversification.

Selection is widely accepted as the principal force shaping phenotypic variation within populations. Its importance in speciation and macroevolution has been questioned, however, because phenotypic differences between species or higher taxa sometimes appear to be nonadaptive. Here, we use the quantitative trait locus (QTL) sign test to evaluate the importance of directional selection in phenotypic divergence. If a trait has a history of directional selection, QTL effects should be mostly in the same direction; otherwise QTLs with antagonistic effects should be common. Analysis of QTL effects for 572 traits from 86 studies revealed significantly fewer antagonistic QTLs than expected under neutrality, a result that validates Darwin's claim that phenotypic diversification is caused mainly by selection. Moreover, interspecific trait differences were more strongly or consistently selected than intraspecific differences, strengthening a growing consensus among students of speciation that directional selection is the primary cause of speciation. Contrary to studies of selection in contemporary populations, life history traits appear to be selected more strongly than morphological traits, but traits related to the timing of development are weakly selected relative to most other traits.

Animals↗
PubMed2003

Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine.

Few studies have shown both reciprocal selection and reciprocal adaptations for a coevolving system in the wild. The goal of our study was to determine whether the patterns of selection on Rocky Mountain lodgepole pine (Pinus contorta spp. latifolia) and red crossbills (Loxia curvirostra complex) were concordant with earlier published evidence of reciprocal adaptations in lodgepole pine and crossbills on isolated mountain ranges in the absence of red squirrels (Tamiasciurus hudsonicus). We found that selection (directional) by crossbills on lodgepole pine where Tamiasciurus are absent was divergent from the selection (directional) exerted by Tamiasciurus on lodgepole pine. This resulted in divergent selection between areas with and without Tamiasciurus that was congruent with the geographic patterns of cone variation. In the South Hills, Idaho, where Tamiasciurus are absent and red crossbills are thought to be coevolving with lodgepole pine, crossbills experienced stabilizing selection on bill size, with cone structure as the agent of selection. These results show that crossbills and lodgepole pine exhibit reciprocal adaptations in response to reciprocal selection, and they provide insight into the traits mediating and responding to selection in a coevolutionary arms race.

Adaptation, Physiological↗
PubMed2004

Orientation of the genetic variance-covariance matrix and the fitness surface for multiple male sexually selected traits.

Stabilizing selection has been predicted to change genetic variances and covariances so that the orientation of the genetic variance-covariance matrix (G) becomes aligned with the orientation of the fitness surface, but it is less clear how directional selection may change G. Here we develop statistical approaches to the comparison of G with vectors of linear and nonlinear selection. We apply these approaches to a set of male sexually selected cuticular hydrocarbons (CHCs) of Drosophila serrata. Even though male CHCs displayed substantial additive genetic variance, more than 99% of the genetic variance was orientated 74.9 degrees away from the vector of linear sexual selection, suggesting that open-ended female preferences may greatly reduce genetic variation in male display traits. Although the orientation of G and the fitness surface were found to differ significantly, the similarity present in eigenstructure was a consequence of traits under weak linear selection and strong nonlinear (convex) selection. Associating the eigenstructure of G with vectors of linear and nonlinear selection may provide a way of determining what long-term changes in G may be generated by the processes of natural and sexual selection.

Animals↗
PubMed1999

Levels of selection, altruism, and primate behavior.

Altruistic behaviors seem anomalous from a traditional view of Darwinian natural selection, and evolutionary explanations for them have generated much discussion. The debate centers around four major explanations: classic individual-level selection, reciprocity and game theory, kin selection, and trait-group selection. The historical context and defining criteria of each model must be reviewed before its validity can be assessed. Of these proposed mechanisms, group selection historically has been the most controversial. Although the extent to which empirical data support group selection hypotheses is uncertain, there is evidence for group-level selection among avirulent virus strains and foraging ant queens. Researchers studying mammalian behavior, particularly primatologists, have largely dismissed models of group-level selection. Most discussion of altruism among primates has focused on differences in fitness among individuals within a single group, but students of altruistic behaviors exhibited by primates also need to investigate intergroup variation with respect to these behaviors. Various altruistic behaviors are likely to have evolved through different forms of selection, and each example of apparent altruism therefore needs to be evaluated separately.

Altruism↗