TY - JOUR TI - Genotypic distribution at the limits to natural and artificial selection with mutation AU - Zeng, Zhao-Bang T2 - Theoretical Population Biology AB - A general procedure for analysing the change of genotypic distributions under stabilizing and truncation selection is described here and used to investigate the genotypic distribution at the limits to selection. For comparison, a simple approximate procedure using a normal distribution is also presented. It is clear that in the long term truncation introduces departures from normality mainly through gene frequency change, rather than through the generation of linkage disequilibrium under random mating. The Gaussian approximation performs reasonably well for additive gene effects unless the mean gene frequency is very extreme (say, outside the range of 0.05 to 0.95) and the number of loci is small (say, less then 50) regardless of the type of selection in operation. The genotypic distribution at the limits to selection largely depends on the type of limit reached. If a limit is obtained due to the action of natural selection before the exhaustion of existing variation, the distribution will normally not be very skew, but if a limit is reached at which mutation plays a central role in the maintenance of genetic variability, it could have high coefficients of skewness and kurtosis. The role of mutation on the long-term response is also discussed. DA - 1987/8// PY - 1987/8// DO - 10.1016/0040-5809(87)90042-6 VL - 32 IS - 1 SP - 90-113 J2 - Theoretical Population Biology LA - en OP - SN - 0040-5809 UR - http://dx.doi.org/10.1016/0040-5809(87)90042-6 DB - Crossref ER - TY - JOUR TI - NONLINEARITY AND THE EFFECTS OF MICROCLIMATIC VARIABILITY ON A CODLING MOTH POPULATION (CYDIA-POMONELLA) A SENSITIVITY SIMULATION AU - GOLD, HJ AU - KENDALL, WL AU - SHAFFER, PL T2 - ECOLOGICAL MODELLING AB - Using a recently developed simulation model, we have examined the sensitivity of the dynamics of a population of codling moth to temperature variations of the magnitude that might reasonably occur between weather station measurements and tree canopy temperatures. Base temperatures used were from NOAA Weather Service records for 1976 and for 1978. Effects of temperature variation showed different patterns for the 2 years. Examination of the temperature dependence of rates of development and of mortality for the developmental life stages, indicates that the difference arises from the nonlinear temperature dependence of the underlying life stage processes. Implications are discussed concerning both the qualitative nature of effects of temperature variations, and the nature of the experimental and monitoring data needed if simulation models are to be useable for predicting and controlling codling moth populations. DA - 1987/7// PY - 1987/7// DO - 10.1016/0304-3800(87)90022-6 VL - 37 IS - 3-4 SP - 139-154 SN - 0304-3800 ER - TY - JOUR TI - CONVERGENCE TO EQUILIBRIUM IN A GENETIC MODEL WITH DIFFERENTIAL VIABILITY BETWEEN THE SEXES AU - SELGRADE, JF AU - ZIEHE, M T2 - JOURNAL OF MATHEMATICAL BIOLOGY DA - 1987/// PY - 1987/// DO - 10.1007/BF00276194 VL - 25 IS - 5 SP - 477-490 SN - 1432-1416 ER - TY - JOUR TI - A STAGE-BASED POPULATION-MODEL FOR LOGGERHEAD SEA-TURTLES AND IMPLICATIONS FOR CONSERVATION AU - CROUSE, DT AU - CROWDER, LB AU - CASWELL, H T2 - ECOLOGY AB - Management of many species is currently based on an inadequate understanding of their population dynamics. Lack of age—specific demographic information, particularly for long—lived iteroparous species, has impeded development of useful models. We use Lefkovitch stage class matrix model, based on preliminary life table developed by Frazer (1983), to point to interim management measures and to identify those data most critical to refining our knowledge about the population dynamics of threatened loggerhead sea turtles (Caretta caretta). Population projections are used to examine the sensitivity of Frazer's life table to variations in parameter estimates as well as the likely response of the population to various management alternatives. Current management practices appear to be focused on the least responsive life stage, eggs on nesting—beaches. Alternative protection efforts for juvenile loggerheads,such as using turtle excluder devices (TEDs), may be far more effective. DA - 1987/10// PY - 1987/10// DO - 10.2307/1939225 VL - 68 IS - 5 SP - 1412-1423 SN - 1939-9170 ER - TY - JOUR TI - VARIANCE FUNCTION ESTIMATION AU - DAVIDIAN, M AU - CARROLL, RJ T2 - JOURNAL OF THE AMERICAN STATISTICAL ASSOCIATION AB - Abstract Heteroscedastic regression models are used in fields including economics, engineering, and the biological and physical sciences. Often, the heteroscedasticity is modeled as a function of the covariates or the regression and other structural parameters. Standard asymptotic theory implies that how one estimates the variance function, in particular the structural parameters, has no effect on the first-order properties of the regression parameter estimates; there is evidence, however, both in practice and higher-order theory to suggest that how one estimates the variance function does matter. Further, in some settings, estimation of the variance function is of independent interest or plays an important role in estimation of other quantities. In this article, we study variance function estimation in a unified way, focusing on common methods proposed in the statistical and other literature, to make both general observations and compare different estimation schemes. We show that there are significant differences in both efficiency and robustness for many common methods. We develop a general theory for variance function estimation, focusing on estimation of the structural parameters and including most methods in common use in our development. The general qualitative conclusions are these. First, most variance function estimation procedures can be looked upon as regressions with “responses” being transformations of absolute residuals from a preliminary fit or sample standard deviations from replicates at a design point. Our conclusion is that the former is typically more efficient, but not uniformly so. Second, for variance function estimates based on transformations of absolute residuals, we show that efficiency is a monotone function of the efficiency of the fit from which the residuals are formed, at least for symmetric errors. Our conclusion is that one should iterate so that residuals are based on generalized least squares. Finally, robustness issues are of even more importance here than in estimation of a regression function for the mean. The loss of efficiency of the standard method away from the normal distribution is much more rapid than in the regression problem. As an example of the type of model and estimation methods we consider, for observation-covariate pairs (Yi, xi ), one may model the variance as proportional to a power of the mean response, for example, Where f(xi , β) is the possibly nonlinear mean function and θ is the structural parameter of interest. “Regression methods” for estimation of θ and σ based on residuals for some regression fit involve minimizing a sum of squares where some function T of the ‖ri ‖ plays the role of the “responses” and an appropriate function of the variance plays the role of the “regression function.” For example, if T(x) = x 2, the responses would be ri 2, and the form of the regression function would be suggested by the aproximate fact . One could weight the sum of squares appropriately by considering the approximate variance of ri 2. For the case of replication at each xi , some methods suggest replacing the ri , in the function T by sample standard deviations at each xi . Other functions T, such as T(x) = x or log x have also been proposed. DA - 1987/12// PY - 1987/12// DO - 10.2307/2289384 VL - 82 IS - 400 SP - 1079-1091 SN - 0162-1459 ER - TY - BOOK TI - Virginia's forest and wildlife resources: Present and future AU - Hall, O. F. AU - McElwee, R. L. Wisdom AU - H. W., Lamb AU - F. M., Nielsen AU - Nielsen, L. A. DA - 1987/// PY - 1987/// PB - Blacksburg: School of Forestry and Wildlife Resources, Virginia Polytechnic Institute & State University ER -