Jordi Ocaña

Distances between populations ofDrosophila subobscura, based on chromosome arrangement frequencies

SummaryDistances between populations ofDrosophila subobscura, based on differences in the frequencies of chromosomal arrangements have been estimated using data from about 65 populations. The distances have been calculated using the formula:

EVOLUTIONARY HISTORY OF DROSOPHILA BUZZATII. II. HOW MUCH HAS CHROMOSOMAL POLYMORPHISM CHANGED IN COLONIZATION

D = \frac{1}{{2r}}{\text{ }}\sum\limits_{j = 1}^r {\sum\limits_{k = 1}^{Sj} {|p_{1jk} - } p_{2jk} |} {\text{ }},

Effect of larval crowding on adult mating behaviour in Drosophila melanogaster.

wherer is the number of loci or chromosomes (in the case of chromosomal polymorphism) considered,p1jk the frequency of the allele or chromosomal arrangementk in the locus or chromosomej in the first population, andpzjk the corrsponding value in the second population.The main conclusion drawn from this analysis is that historical as well as adaptive factors are important in explaining the geographical distribution of chromosomal arrangements inD. subobscura. In general, isolated populations maintain primitive features in their chromosomal polymorphism. This is reflected in a tendency to similarity between these populations. Also, a very sharp effect of geographical barriers is detected in the distribution of the chromosomal arrangements.Two main factors are considered responsible for the strong effect that isolation has on geographical distribution of chromosome arrangements. These factors are the non-recurrence of rearrangements and the difficulty of establishing in one population the supergenes originated in another area, because of lack of coadaptation with the gene pool of the recipient population.

Computer Generation and Estimation in a One-Parameter System Of Bivariate Distributions with Specified Marginals

Most species arise in a center of origin from where they disperse to larger areas. The ability to colonize varies from species to species and must be related to its genetic endowment. Yet, the relationship between genotype and colonizing ability is not well defined. Chromosomal polymorphism has been related to ecogeographical distribution by several authors and there is ample evidence (Carson, 1958; Da Cunha et al., 1959; Carson and Heed, 1964; Sperlich, 1964) that in many widespread species of Drosophila central populations are more polymorphic for inversions than marginal populations. The adaptive significance of this chromosomal differentiation is still subject to debate and it is often related to environmental heterogeneity (Dobzhansky, 1951) and/or to selective pressure for new genic recombinants (Carson, 1959). Carson (1965) has advanced the idea that much original chromosomal polymorphism is lost when endemic species become widespread. Genetic drift and selection of homozygotes (homoselection) may be contributing factors for this phenomenon. However, there are several widespread species which have maintained polymorphism for certain inversions. throughout their entire distribution. This polymorphism is generally termed as rigid in opposition to flexible polymorphism, which is typical of endemic species (Dobzhansky, 1962). One way to become a widespread species is to fix a genetic novelty highly adapted to a narrow niche, such as a host plant, which exists in many parts of the

Variance reduction for Bernoulli response variables in simulation

The correlations often found between chromosomal polymorphism distribution and ecogeographical conditions have interested many earlier workers in microevolution (Carson, 1958; Da Cunha et al., 1959; Carson and Heed, 1964; Sperlich, 1964). It is widely recognized that, at least in Drosophila, central populations are more polymorphic for inversions than marginal populations, although the adaptive causes of this differentiation (Dobzhansky, 1951; Carson, 1959) are still subjected to debate. Most species arise in a single geographic region from which they may colonize new areas. Colonization stirred the attention of biologists at a symposium (Baker and Stebbins, 1965) in 1964, but since then very little experimental work (Martins and Jain, 1979) has been done to study the genetic structure of colonizing species. A few historically documented cases of colonizations may illustrate the types of genetic changes which generate the present inversion polymorphism distributions of these colonizing species and thus lead to a better understanding of their adaptive strategies. Drosophila buzzatii, a South American species which colonized the Old World in historical times, is one of these cases. Carson (1965) has explained the low levels of chromosomal polymorphism found in colonizing populations of some species of Drosophila in terms of his theory of homoselection in marginal environments

Sampling Theory, Estimation, and Significance Testing for Prevosti's Estimate of Genetic Distance

BackgroundIn longitudinal studies where subjects experience recurrent incidents over a period of time, such as respiratory infections, fever or diarrhea, statistical methods are required to take into account the within-subject correlation.MethodsFor repeated events data with censored failure, the independent increment (AG), marginal (WLW) and conditional (PWP) models are three multiple failure models that generalize Cox’s proportional hazard model. In this paper, we revise the efficiency, accuracy and robustness of all three models under simulated scenarios with varying degrees of within-subject correlation, censoring levels, maximum number of possible recurrences and sample size. We also study the methods performance on a real dataset from a cohort study with bronchial obstruction.ResultsWe find substantial differences between methods and there is not an optimal method. AG and PWP seem to be preferable to WLW for low correlation levels but the situation reverts for high correlations.ConclusionsAll methods are stable in front of censoring, worsen with increasing recurrence levels and share a bias problem which, among other consequences, makes asymptotic normal confidence intervals not fully reliable, although they are well developed theoretically.

Comparison of lists of genes based on functional profiles

The effect of larval density on male mating success has been investigated with two strains of Drosophila melanogaster, a wild strain and a mutant strain, under low and high larval competition, and four different genotypic frequencies. The results show a strong sexual selection against mutant males when flies have been raised under low larval competition. Under high larval competition, there is a reduction in mating disadvantage of mutant males. In both instances, a frequency-dependent sexual selection exists. These results explain adequately the evolution of experimental populations where egg to adult viability and male mating success are the most important components of fitness.

An Environment for Montecarlo Simulation Studies (EMSS)

A method for generating samples from any member of a one-parameter family of bivariate distributions is presented. Using this algorithm, a simulation study for six estimators of the dependence parameter (based on Kendall’s tau, Spearman’s rho and maximum likelihood) is also presented. Maximum likelihood estimators are clearly the best, but are dependent on the marginals. Among the nonparametric estimators, the one based on Kendall’s tau is preferable to the estimator based on Spearman’s rho. Jackknifing does not provide any significative improvement.