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1.
For the triangle, duo‐trio, same‐different and 2‐AFC methods, using a model system, mean d′ values for the same subjects, discriminating between the same taste stimuli, were not significantly different. This confirmed the postulated cognitive strategies used for these methods in their respective Thurstonian/signal detection models. Introduction of perceptual variance as a result of the effects of sequences of tasting within a test, forgetting stimulus perceptions and τcriterion variation resulted in the 2‐AFC eliciting a significantly higher d′ than the other three methods. Yet, after a warm‐up procedure, which not only significantly increased values of d′ for all methods but also aligned subjects' τcriteria, the same‐different test had a d′ comparable to that of the 2‐AFC, while both d′ values were significantly higher than those of the triangle and duo‐trio. This suggested that effects of memory were more important those of sequence of tasting.  相似文献   

2.
The genetic and ecological effects of population subdividsion were investigated for two wild strains of Tribolium castaneum and two wild strains of T. confusum and compared with the effects of population subdivision on the synthetic laboratory strain of T. castaneum (c-SM), used extensively in earlier experiments. For the c-SM strain, it has been shown repeatedly, for a variety of different population structures (different combinations of effective numbers, Ne, and migration rates, m), that large heritable differences in population growth rate arise among demes during 10 to 15 generations of population subdivision. Because this laboratory strain was synthesized by mass mating several “inbred” strains in 1973 (80 to 100 generations ago), it is possible that it has genetic variation for fitness (measured as the heritable variance among demes in the rate of population increase) unusually large compared to natural populations of flour beetles. In this paper, I report that natural populations of flour beetle exhibit as much or more phenotypic and genetic variation in the effects of population structure on fitness than the laboratory strain, c-SM. The observation of substantial heritable variation for fitness in natural populations is unexpected under additive theory and may be indicative of nonadditive genetic variance.  相似文献   

3.
Fifty-two inbred populations of Drosophila melanogaster, each founded from a single pair, and a large number of control, outbred flies were measured for fitness and a set of six traits. A survey of the literature on the effects of inbreeding and population bottlenecks demonstrates that the commonly observed pattern of an apparent variance among characters and among species in changes of phenotypic variance may in fact be largely the result of sampling error, given the pattern of change that we demonstrate within a species for the same character. In our study, population bottlenecks on average decrease the amount of phenotypic variance for a suite of wing characteristics and size, but there is large and significant variation among lines in the amount of phenotypic variance. As a result, several lines increased in variance in spite of the average decrease. Interestingly, the changes in phenotypic variance for fitness are in sharp contrast to those seen for phenotypic variance for morphological traits. The amount of phenotypic variance for fitness varies highly significantly among lines but, on average, is increased by bottlenecks. The changes in phenotypic variance as a result of population bottlenecks are large enough to significantly affect the probability of peak shifts by the variance-induced peak shift model.  相似文献   

4.
The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks.  相似文献   

5.
Recent quantitative genetic studies have attempted to infer long-term selection responsible for differences in observed phenotypes. These analyses are greatly simplified by the assumption that the within-population genetic variance remains constant through time and over space, or for the multivariate case, that the matrix of additive genetic variances and covariances (G matrix) is constant. We examined differences in G matrices and the association of these differences with differences in multivariate means (Mahalanobis D2) among 11 populations of the California endemic annual plant, Clarkia dudleyana. Based on nine continuous morphological traits, the relationship between Mahalanobis D2 and a distance measure summarizing differences in G matrices reflected no concomitant change in (co)variances with changes in means. Based on both broad- and narrow-sense analyses, we found little evidence that G matrices differed between populations. These results suggest that both the additive and nonadditive (co)variances for traits have remained relatively constant despite changes in means.  相似文献   

6.
Additive genetic variance maintained by mutation in a selectively neutral quantitative character is analyzed for an ideal population distributed on n islands, each with local effective size N, that exchange migrants at a small rate, m. In a stable population structure, the expected genetic variance maintained within islands is identical to that in a panmictic population of the same total size, regardless of the migration rate (m > 0). This result contrasts with Wright's classical conclusion, based on inbreeding coefficients, that at least one immigrant per island every other generation (Nm > ½) is necessary for the genetic variance within local populations to approach that under panmixia. The expected genetic variance maintained among islands is inversely proportional to m and increases with the number of islands, but is independent of N. Local extinction and colonization diminish the genetic variance maintained within islands by reducing the effective size of island populations through the founder effect, although the expected genetic variance within islands is nearly as large as that in a panmictic population of the same total effective size. If the founders of new colonies originate from more than one island, rates of local extinction and colonization larger than about twice the migration rate will substantially reduce the genetic variance maintained among islands. These results indicate the importance of mutation and migration in maintaining quantitative genetic variance within small local populations.  相似文献   

7.
Interdemic selection by the differential migration of individuals out from demes of high fitness and into demes of low fitness (Phase III) is one of the most controversial aspects of Wright's Shifting Balance Theory. I derive a relationship between Phase III migration and the interdemic selection differential, S, and show its potential effect on FST. The relationship reveals a diversifying effect of interdemic selection by Phase III migration on the genetic structure of a metapopulation. Using experimental metapopulations, I explored the effect of Phase III migration on FST by comparing the genetic variance among demes for two different patterns of migration: (1) island model migration and (2) Wright's Phase III migration. Although mean migration rates were the same, I found that the variance among demes in migration rate was significantly higher with Phase III than with island model migration. As a result, FST for the frequency of a neutral marker locus was higher with Phase III than it was with island model migration. By increasing FST, Phase III enhanced the genetic differentiation among demes for traits not subject to interdemic selection. This feature makes Wright's process different from individual selection which, by reducing effective population size, decreases the genetic variance within demes for all other traits. I discussed this finding in relation to the efficacy of Phase III and random migration for effecting peak shifts, and the contribution of genes with indirect effects to among‐deme variation.  相似文献   

8.
Consumers rated a set of toothbrushes and a set of potato chips on 9‐point and 21‐point hedonic scales under two experimental protocols: a traditional approach and rank‐rating. The hedonic data were analyzed in the usual way by using anova and multiple comparisons and also by ranking the data and using an R‐index analysis. The hypothesis that the latter analysis would elicit fewer significant differences was not confirmed.  相似文献   

9.
We estimated mutational variance–covariance matrices, M , for wing shape and size in two genotypes of Drosophila melanogaster after 192 generations of mutation accumulation. We characterized 21 potentially independent aspects of wing shape and size using geometric morphometrics, and analyzed the data using a likelihood‐based factor‐analytic approach. We implement a previously unused analysis that describes those directions with the greatest difference in evolvability between pairs of matrices. There are significant mutational effects on 19 of 21 possible aspects of wing form, consistent with the high dimensionality of standing genetic variation for wing shape previously identified in D. melanogaster. Mutations have partially recessive effects, consistent with average dominance around 0.25. Sex‐specific matrices are relatively similar, although male‐specific matrices are slightly larger, as expected due to dosage compensation on the X chromosome. Genotype‐specific matrices are quite different. Matrices may differ both because of sampling error based on small samples of mutations with large phenotypic effects, and because of the mutational properties of the genotypes. Genotypic differences are likely to be involved, as the two genotypes have different molecular mutation rates and properties.  相似文献   

10.
The effects of inbreeding on the phenotypic variance within populations were measured in a set of 30 bottlenecked lines derived from a single source population of Drosophila melanogaster. Inbred lines had significant variance among lines in the amount of phenotypic variance within lines, for thorax length, and sternopleural bristle scores. When significance levels were corrected on an experimentwide basis, no line had significant increases in phenotypic variance for sternopleural bristle counts, although two lines had significant increases in thorax length variance. These results demonstrate that inbred lines cannot be treated as necessarily more uniform than outbred lines and that results on changes in variance due to inbreeding should be treated with caution unless there has been sufficient replication. These results also demonstrate the validity of an important assumption of models of evolution by variance-mediated mechanisms, such as the variance-induced peak-shift model.  相似文献   

11.
12.
Mayr (1963) proposed that small isolated propagules from a large panmictic population would occasionally undergo a genetic revolution due to loss of genetic variability. More recently Templeton (1980a) has suggested that founder events may be much more important in systems that have strong epistasis. Because of the work of these and other authors it becomes an interesting theoretical problem to study the distribution of epistatic variance in a population following a founder event. In the model presented here measures of coancestry (Cockerham, 1967, 1984; Cockerham and Weir, 1973; Weir and Cockerham, 1973, 1977; Tachida and Cockerham, unpubl.) are used to examine the effect of founder events on additive-by-additive epistasis. Using this approach, the coancestries, or intraclass correlations, within individuals and within demes, together with the genetic variance components in the ancestral population are used to obtain the variance within and among demes following a founder event. Examples are analyzed for single founder events of 1–25 individuals and multiple founder events of two individuals. Following a single founder event, the contribution of the additive variance to the variance within demes relative to the additive variance in the ancestral population is always less than one. However, the contribution of epistatic variance to the variance within demes relative to the epistatic variance in the ancestral population is always greater than one. Thus, while a founder event decreases the contribution of additive variance to the variance within demes, it increases the contribution of epistatic variance to the variance within demes. The contribution of epistatic variance to the variance among demes following a single founder event is not qualitatively different from the contribution of additive variance to the variance among demes. These results indicate that epistatic variance is less likely than additive variance to cause a genetic revolution following a single founder event. When populations undergo multiple founder events the situation changes considerably. Epistatic variance may contribute as much as four times its original value to the variance among demes, while additive variance can contribute maximally twice its original value to the variance among demes. Thus, epistasis, which is relatively unimportant following a single founder event, may have major evolutionary implications if drift is allowed to continue for several generations.  相似文献   

13.
Theory predicts that sex chromsome linkage should reduce intersexual genetic correlations thereby allowing the evolution of sexual dimorphism. Empirical evidence for sex linkage has come largely from crosses and few studies have examined how sexual dimorphism and sex linkage are related within outbred populations. Here, we use data on an array of different traits measured on over 10,000 individuals from two pedigreed populations of birds (collared flycatcher and zebra finch) to estimate the amount of sex‐linked genetic variance (h2z). Of 17 traits examined, eight showed a nonzero h2Z estimate but only four were significantly different from zero (wing patch size and tarsus length in collared flycatchers, wing length and beak color in zebra finches). We further tested how sexual dimorphism and the mode of selection operating on the trait relate to the proportion of sex‐linked genetic variance. Sexually selected traits did not show higher h2Z than morphological traits and there was only a weak positive relationship between h2Z and sexual dimorphism. However, given the relative scarcity of empirical studies, it is premature to make conclusions about the role of sex chromosome linkage in the evolution of sexual dimorphism.  相似文献   

14.
Traditional models of genetic drift predict a linear decrease in additive genetic variance for populations passing through a bottleneck. This perceived lack of heritable variance limits the scope of founder-effect models of speciation. We produced 55 replicate bottleneck populations maintained at two male-female pairs through four generations of inbreeding (average F = 0.39). These populations were formed from an F2 intercross of the LG/J and SM/J inbred mouse strains. Two contemporaneous control strains maintained with more than 60 mating pairs per generation were formed from this same source population. The average level of within-strain additive genetic variance for adult body weight was compared between the control and experimental lines. Additive genetic variance for adult body weight within experimental bottleneck strains was significantly higher than expected under an additive genetic model This enhancement of additive genetic variance under inbreeding is likely to be due to epistasis, which retards or reverses the loss of additive genetic variance under inbreeding for adult body weight in this population. Therefore, founder-effect speciation processes may not be constrained by a loss of heritable variance due to population bottlenecks.  相似文献   

15.
Maternal effects can dramatically influence the evolutionary process, in some cases facilitating and in others hindering adaptive evolution. Maternal effects have been incorporated into quantitative genetic models using two theoretical frameworks: the variance‐components approach, which partitions variance into direct and maternal components, and the trait‐based approach, which assumes that maternal effects are mediated by specific maternal traits. Here, we demonstrate parallels between these models and test their ability to predict evolutionary change. First, we show that the two approaches predict equivalent responses to selection in the absence of maternal effects mediated by traits that are themselves maternally influenced. We also introduce a correction factor that may be applied when such cascading maternal effects are present. Second, we use several maternal effect models, as well as the standard breeder's equation, to predict evolution in response to artificial selection on flowering time in American bellflower, Campanulastrum americanum. Models that included maternal effects made much more accurate predictions of selection response than the breeder's equation. Maternal effect models differed somewhat in their fit, with a version of the trait‐based model providing the best fit. We recommend fitting such trait‐based models when possible and appropriate to make the most accurate evolutionary predictions.  相似文献   

16.
A cornerstone of evolutionary theory is that the phenotypic variance of a population may be partitioned into genetic and environmental (nonheritable) components. The traditional motivation for this distinction is that the rate of evolution under natural selection depends on the (relative) magnitudes of certain genetic components of variance. The components of variation are also interesting from another perspective, as illustrated here. Phenotypic variation may be selectively maintained in a population according to its components: selection may favor the maintenance of only the environmental components, only the genetic components, or be indifferent to the composition of the variance. Even when selection is shown to favor phenotypic variation regardless of its components, the possibility exists that environmental variance will evolve to displace the genetic components or vice versa. Environmental and genetic factors may thus compete to produce a given selected level of phenotypic variance. A test of some of these models is provided from the example of seed dormancy: the prediction that variation in seed germination time should be purely environmental is supported by the demonstration of low heritability of germination time in the two available studies.  相似文献   

17.
Epistatic genetic variance for quantitative traits may play an important role in evolution, but detecting epistasis in diploid organisms is difficult and requires complex breeding programs and very large sample sizes. We develop a model for detecting epistasis in organisms with a free-living haploid stage in their life cycles. We show that epistasis is indicated by greater variance among families of haploid progeny derived from individual diploids than among clonally replicated haploid sibs from the same sporophyte. Simulations show that the power to detect epistasis is linearly related to the number of sporophytes and the number of haploids per sporophyte in the dataset. We illustrate the model with data from growth variation among gametophytes of the moss, Ceratodon purpureus. The experiment failed to detect epistatic variance for biomass production, although there was evidence of additive variance.  相似文献   

18.
Directional selection is prevalent in nature, yet phenotypes tend to remain relatively constant, suggesting a limit to trait evolution. However, the genetic basis of this limit is unresolved. Given widespread pleiotropy, opposing selection on a trait may arise from the effects of the underlying alleles on other traits under selection, generating net stabilizing selection on trait genetic variance. These pleiotropic costs of trait exaggeration may arise through any number of other traits, making them hard to detect in phenotypic analyses. Stabilizing selection can be inferred, however, if genetic variance is greater among low‐ compared to high‐fitness individuals. We extend a recently suggested approach to provide a direct test of a difference in genetic variance for a suite of cuticular hydrocarbons (CHCs) in Drosophila serrata. Despite strong directional sexual selection on these traits, genetic variance differed between high‐ and low‐fitness individuals and was greater among the low‐fitness males for seven of eight CHCs, significantly more than expected by chance. Univariate tests of a difference in genetic variance were nonsignificant but likely have low power. Our results suggest that further CHC exaggeration in D. serrata in response to sexual selection is limited by pleiotropic costs mediated through other traits.  相似文献   

19.
Four types of rating scales, nine-point category scales, line marking, magnitude estimation, and a hybrid of the category and line scale, were employed to assess visual, tactile and olfactory characteristics of products by consumers. The scales were compared for their ability to discriminate differences among products, for variability, for reliability, and for ease of use. All methods were able to generate highly significant differences among products. However, a modest advantage for category scales was observed in almost all comparisons.  相似文献   

20.
Events that follow pollination, such as pollen-tube growth and seed maturation, comprise an important phase of angiosperm reproduction. Differential success during this “postpollination” phase may represent phenotypic selection, including sexual selection, or interaction between parents caused, for example, by their genetic similarity. By providing a detailed partitioning of variance in success, diallel crossing designs offer great potential to determine which processes are occurring and their relative magnitudes. We performed three partial diallels with the montane herb Ipomopsis aggregata, using a large sample of parental plants (69 total). Embedded in the designs were crossing-distance treatments of 1 m, 10 m, and 100 m, reflecting a range of parental genetic similarity. We partitioned phenotypic variance in seed set per fruit into six components using restricted maximum-likelihood (REML) analysis. For one diallel, we also partitioned variance in seed mass into five components, and estimated two components of covariance between seed set and mass. Variance caused by maternal effects (Vmat) comprised 12%–35% of total variance in seed set and 62% of variance in seed mass, and there was a significant negative environmental covariance between seed set and seed mass. Parental interaction made no detectable contribution to phenotypic variance in either of our measures of postpollination success, although crossing distance did contribute slightly but significantly to fit of the model in some cases. Finally, there was no detectable paternal variance (Vpat) in seed set or seed mass. These results are in keeping with reports from other studies of natural plant populations. The finding of little or no paternal variance in particular suggests little scope for postpollination sexual selection through the male function of cosexual plants such as I. aggregata.  相似文献   

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