Selection In Heterogeneous Environments Research Articles

Reduced within-population quantitative genetic variation is associated with climate harshness in maritime pine.

How evolutionary forces interact to maintain genetic variation within populations has been a matter of extensive theoretical debates. While mutation and exogenous gene flow increase genetic variation, stabilizing selection and genetic drift are expected to deplete it. To date, levels of genetic variation observed in natural populations are hard to predict without accounting for other processes, such as balancing selection in heterogeneous environments. We aimed to empirically test three hypotheses: (i) admixed populations have higher quantitative genetic variation due to introgression from other gene pools, (ii) quantitative genetic variation is lower in populations from harsher environments (i.e., experiencing stronger selection), and (iii) quantitative genetic variation is higher in populations from heterogeneous environments. Using growth, phenological and functional trait data from three clonal common gardens and 33 populations (522 clones) of maritime pine (Pinus pinaster Aiton), we estimated the association between the population-specific total genetic variances (i.e., among-clone variances) for these traits and ten population-specific indices related to admixture levels (estimated based on 5165 SNPs), environmental temporal and spatial heterogeneity and climate harshness. Populations experiencing colder winters showed consistently lower genetic variation for early height growth (a fitness-related trait in forest trees) in the three common gardens. Within-population quantitative genetic variation was not associated with environmental heterogeneity or population admixture for any trait. Our results provide empirical support for the potential role of natural selection in reducing genetic variation for early height growth within populations, which indirectly gives insight into the adaptive potential of populations to changing environments.

High-elevation plants have reduced plasticity in flowering time in response to warming compared to low-elevation congeners

Global warming has caused shifts in the flowering time of many plant species. In alpine regions the temperature rise has been especially pronounced and together with decreasing winter precipitation has led to earlier snowmelt. The close association between time of snowmelt and plant growth at high elevations makes climate change for alpine plants particularly threatening. Here we transplanted eleven congeneric pairs of high- and low-elevation herbaceous species to common gardens differing c. 800m in elevation, and c. 4°C in mean growing season temperature to test whether reproductive phenologies of high- and low-elevation plants differ in their respective responses to temperature. Results indicate that high-elevation plants were less plastic in response to transplantation than their low-elevation congeners as the onsets of phenophases on average shifted 7days less than in low-elevation plants. Plasticity of phenophase durations was overall weaker than that of phenophase onsets, and slightly stronger in high-elevation species compared to low-elevation congeners. We suggest that weaker plasticity in the onsets of early stages of reproductive phenology of high-elevation plants is related to spring frost, which constitutes a strong selective agent against early loss of winter hardiness. Some of the plastic responses of both low- and high-elevation species might potentially be adaptive under predicted climate change. However, the observed plasticity can be largely explained as a passive response to temperature and not as the result of natural selection in heterogeneous environments. The strong temperature-sensitivity of low-elevation species might promote their upward range expansion, but only to a certain threshold after which it becomes limited by the short growing season.

Bayesian inference of selection in a heterogeneous environment from genetic time‐series data

Evolutionary geneticists have sought to characterize the causes and molecular targets of selection in natural populations for many years. Although this research programme has been somewhat successful, most statistical methods employed were designed to detect consistent, weak to moderate selection. In contrast, phenotypic studies in nature show that selection varies in time and that individual bouts of selection can be strong. Measurements of the genomic consequences of such fluctuating selection could help test and refine hypotheses concerning the causes of ecological specialization and the maintenance of genetic variation in populations. Herein, I proposed a Bayesian nonhomogeneous hidden Markov model to estimate effective population sizes and quantify variable selection in heterogeneous environments from genetic time-series data. The model is described and then evaluated using a series of simulated data, including cases where selection occurs on a trait with a simple or polygenic molecular basis. The proposed method accurately distinguished neutral loci from non-neutral loci under strong selection, but not from those under weak selection. Selection coefficients were accurately estimated when selection was constant or when the fitness values of genotypes varied linearly with the environment, but these estimates were less accurate when fitness was polygenic or the relationship between the environment and the fitness of genotypes was nonlinear. Past studies of temporal evolutionary dynamics in laboratory populations have been remarkably successful. The proposed method makes similar analyses of genetic time-series data from natural populations more feasible and thereby could help answer fundamental questions about the causes and consequences of evolution in the wild.

Evolution of Quantitative Traits under a Migration-Selection Balance: When Does Skew Matter?

Quantitative-genetic models of differentiation under migration-selection balance often rely on the assumption of normally distributed genotypic and phenotypic values. When a population is subdivided into demes with selection toward different local optima, migration between demes may result in asymmetric, or skewed, local distributions. Using a simplified two-habitat model, we derive formulas without a priori assuming a Gaussian distribution of genotypic values, and we find expressions that naturally incorporate higher moments, such as skew. These formulas yield predictions of the expected divergence under migration-selection balance that are more accurate than models assuming Gaussian distributions, which illustrates the importance of incorporating these higher moments to assess the response to selection in heterogeneous environments. We further show with simulations that traits with loci of large effect display the largest skew in their distribution at migration-selection balance.

Within-Niche Constant Selection ofDrosophilaInversions?

Extensive and fruitful work is being devoted for more than 70 years to elucidate the fine points of the maintenance of inversion polymorphisms of the genus Drosophila. Recent studies have resumed selection in heterogeneous environments (or niches) as a major underlying mechanism for these balanced polymorphisms. In those studies, constant selection within niches is assumed throughout although this assumption is since long known not to hold. In the present communication it is sustained that the results in those studies are robust in the face of this fact. To that end, this communication deals with a particular long-lasting question within this topic—whether the minimal model of constant viability selection (MCV, assuming frequency-, sex-, and stage-independent adaptive values) suffices to reproduce the trajectories of frequencies of Drosophila chromosomal arrangements observed in experimental populations along generations under homogeneous environments. Fitness estimates are here obtained from published trajectories of frequencies using a maximum likelihood approach, and relevant literature is revised in the light of these new analyses, pointing to an affirmative answer to that question.

Multivariate selection shapes environment-dependent variation in the clonal morphology of a red seaweed

Within-individual strategies of variation (e.g., phenotypic plasticity) are particularly relevant to modular organisms, in which ramets of the same genetic individual may encounter diverse environments imposing diverse patterns of selection. Hence, measuring selection in heterogeneous environments is essential to understanding whether environment-dependent phenotypic change enhances the fitness of modular individuals. In sublittoral marine habitats, competition for light and space among modular taxa generates extreme patchiness in resource availability. Little is known, however, of the potential for plasticity within individuals to arise from spatially-variable selection in such systems. We tested whether plasticity enhances genet-level fitness in Asparagopsis armata, a clonal seaweed in which correlated traits mediate morphological responses to variation in light. Using the capacity for rapid, clonal growth to measure fitness, we identified aspects of ramet morphology targeted by selection in two contrasting light environments and compared patterns of selection across environments. We found that directional selection on single traits, coupled with linear and nonlinear selection on multi-trait interactions, shape ramet morphology within environments and favor different phenotypes in each. Evidence of environment-dependent, multivariate selection on correlated traits is novel for any marine modular organism and demonstrates that seaweeds, such as A. armata, may potentially adapt to environmental heterogeneity via plasticity in clonal morphology.

On the experimental evolution of specialization and diversity in heterogeneous environments

Environmental variance can be decomposed into two components: the contrast among patches in the optimal phenotype and the variance in productivity among patches. Both components can influence the outcome of selection in heterogeneous environments. In general genetic diversity within a population should increase as the contrast among patches increases. As the variance in productivity increases, on the other hand, individual niche breadth should decrease since the response to selection becomes dominated by adaptation to the most productive patch. We tested this interpretation of diversity in heterogeneous environments by selecting the bacterium Pseudomonas fluorescens over several hundred generations in pair-wise mixtures of four carbon substrates. We then assessed the response to selection in the mixtures in terms of performance on each component substrate. The results provide general support for this interpretation of the environmental variance and suggest further that competition among genotypes promotes diversification in heterogeneous environments.

CLIMATE VS. SOIL FACTORS IN LOCAL ADAPTATION OF TWO COMMON PLANT SPECIES

Evolutionary theory suggests that divergent natural selection in heterogeneous environments can result in locally adapted plant genotypes. To understand local adaptation it is important to study the ecological factors responsible for divergent selection. At a continental scale, variation in climate can be important while at a local scale soil properties could also play a role. We designed an experiment aimed to disentangle the role of climate and (abiotic and biotic) soil properties in local adaptation of two common plant species. A grass (Holcus lanatus) and a legume (Lotus corniculatus), as well as their local soils, were reciprocally transplanted between three sites across an Atlantic-Continental gradient in Europe and grown in common gardens in either their home soil or foreign soils. Growth and reproductive traits were measured over two growing seasons. In both species, we found significant environmental and genetic effects on most of the growth and reproductive traits and a significant interaction between the two environmental effects of soil and climate. The grass species showed significant home site advantage in most of the fitness components, which indicated adaptation to climate. We found no indication that the grass was adapted to local soil conditions. The legume showed a significant home soil advantage for number of fruits only and thus a weak indication of adaptation to soil and no adaptation to climate. Our results show that the importance of climate and soil factors as drivers of local adaptation is species-dependent. This could be related to differences in interactions between plant species and soil biota.

Terminal Management and Intelligent Access Selection in Heterogeneous Environments

This paper presents a mobile terminal architecture for devices operating in heterogeneous environments, which incorporates intelligence for supporting mobility and roaming across legacy access netw...

Genetic Polymorphism in Heterogeneous Environments: The Age of Genomics

The selective mechanisms for maintaining polymorphism in natural populations has been the subject of theory, experiments, and review over the past half century. Advances in molecular genetic techniques have provided new insight into many examples of balancing selection. In addition, new theoretical developments demonstrate how diversifying selection over environments may maintain polymorphism. Tests for balancing selection in the current generation, the recent past, and the distant past provide a comprehensive approach for evaluating selective impacts. In particular, sequenced-based tests provide new ways to evaluate the long-term impact of selection on particular genes and the overall genome in natural populations. Overall, there appear to be many loci exhibiting the signal of adaptive directional selection from genomic scans, but the present evidence suggests that the proportion of loci where polymorphism is maintained by environmental heterogeneity is low. However, as more molecular genetic details become available, more examples of polymorphism maintained by selection in heterogeneous environments may be found.

Rapid Evolutionary Responses to Selection in Heterogeneous Environments among Agricultural and Nonagricultural Weeds

Agricultural settings are hypothesized to impose strong selection on invading weed species, and the direction of selection can be consistent across many generations, depending on crop‐rotation schedules. Because the history of selection is often known, agricultural environments provide a useful experimental setting for studies testing mechanisms of adaptation and rates of evolutionary response. Here, two populations of Abutilon theophrasti with known evolutionary histories of competing either in agricultural or nonagricultural settings were reciprocally transplanted across experimental treatments simulating the site of origin (rotating corn and soybean cultivation, local weeds, or no‐competition control). Consistent with hypotheses of local adaptation, the agricultural population had higher fitness than the nonagricultural population in the cornfield treatment. The nonagricultural population had higher fitness in the no‐competition control. Because weedy, nonagricultural sites are typically patchy and consist of both high‐ and low‐density (i.e., noncompetitive) microsites, the observed population differences in fitness in the no‐competition control also indicate local adaptation. In addition to fitness, the two populations differed in the expression of life‐history traits such that the life‐history of the agricultural population was accelerated relative to the nonagricultural population. The observed life‐history differences probably reflect adaptive synchronization of life‐history events to the shorter growth season duration of agricultural relative to nonagricultural sites. Because the two experimental populations have histories of only 10–50 yr in the site of origin, the results provide evidence of rapid differentiation.

Evolution in heterogeneous environments and the potential of maintenance of genetic variation in traits of adaptive significance

The maintenance of genetic variation in traits of adaptive significance has been a major dilemma of evolutionary biology. Considering the pattern of increased genetic variation associated with environmental clines and heterogeneous environments, selection in heterogeneous environments has been proposed to facilitate the maintenance of genetic variation. Some models examining whether genetic variation can be maintained, in heterogeneous environments are reviewed. Genetic mechanisms that constrain evolution in quantitative genetic traits indicate that genetic variation can be maintained but when is not clear. Furthermore, no comprehensive models have been developed, likely due to the genetic and environmental complexity of this issue. Therefore, I have suggested two empirical approaches to provide insight for future theoretical and empirical research. Traditional path analysis has been a very powerful approach for understanding phenotypic selection. However, it requires substantial information on the biology of the study system to construct a causal model and alternatives. Exploratory path analysis is a data driven approach that uses the statistical relationships in the data to construct a set of models. For example, it can be used for understanding phenotypic selection in different environments, where there is no prior information to develop path models in the different environments. Data from Brassica rapa grown in different nutrients indicated that selection changed in the different environments. Experimental evolutionary studies will provide direct tests as to when genetic variation is maintained.

The maintenance (or not) of polygenic variation by soft selection in heterogeneous environments.

On the basis of single-locus models, spatial heterogeneity of the environment coupled with strong population regulation within each habitat (soft selection) is considered an important mechanism maintaining genetic variation. We studied the capacity of soft selection to maintain polygenic variation for a trait determined by several additive loci, selected in opposite directions in two habitats connected by dispersal. We found three main types of stable equilibria. Extreme equilibria are characterized by extreme specialization to one habitat and loss of polymorphism. They are analogous to monomorphic equilibria in singe-locus models and are favored by similar factors: high dispersal, weak selection, and low marginal average fitness of intermediate genotypes. At the remaining two types of equilibria the population mean is intermediate but variance is very different. At fully polymorphic equilibria all loci are polymorphic, whereas at low-variance equilibria at most one locus remains polymorphic. For most parameters only one type of equilibrium is stable; the transition between the domains of fully polymorphic and low-variance equilibria is typically sharp. Low-variance equilibria are favored by high marginal average fitness of intermediate genotypes, in contrast to single-locus models, in which marginal overdominance is particularly favorable for maintenance of polymorphism. The capacity of soft selection to maintain polygenic variation is thus more limited than extrapolation from single-locus models would suggest, in particular if dispersal is high and selection weak. This is because in a polygenic model, variance can evolve independently of the mean, whereas in the single-locus two-allele case, selection for an intermediate mean automatically leads to maintenance of polymorphism.

Environmental heterogeneity and the evolution of foraging behaviour in long ranging greater albatrosses

Habitat selection in heterogeneous environments is assumed to allow diversification. Wide‐ranging species like pelagic seabirds present a paradox, in that their diversity appears difficult to reconcile with a frequent lack of geographical isolation between populations. We studied the foraging strategies of three closely related species of greater albatrosses, wandering albatross, Diomedea exulans, Amsterdam albatrosses D. amsterdamensis and royal albatross, D. epomophora, in relation to environmental heterogeneity at coarse‐grained and fine‐grained scales. During the incubation period the three species foraged at long distances from their colonies. We observed significant differences between the species in the duration of foraging trips and the distance travelled per day. There were significant differences in preference for habitat types in relation to bathymetric features, and in chlorophyll a concentrations in the waters traversed. Royal albatross preferred shallower waters (<1500 m depth), which were rich in chlorophyll (>0.5 mg/m3), while the other species spent on average 80% of their time in waters deeper than this, where chlorophyll levels were lower. Wandering albatrosses foraged in colder waters than Amsterdam albatrosses. Patterns of activity divided the species into two groups: those exploiting oceanic habitats (wandering and Amsterdam albatrosses) spent high proportions of time on the water (49%), and had on average 1.35 takeoffs and landings per hour, while royal albatross, which foraged mainly over neritic waters spent only 35% of their time sitting on the water, and made on average 2.6 takeoff per hour. Further, royal albatross showed a similar pattern of activity during all periods of the day, while wandering and Amsterdam albatrosses were mostly inactive during the night. We link these differences in activity to prey patch availability in two contrasting habitats – continental shelf areas compared to open ocean habitats. The divergent styles of foraging observed in this study suggest that these closely‐related and wide‐ranging species could effectively co‐exist by dividing the resources available to them by different modes of exploitation.

Canopy gaps to climate change - extreme events, ecology and evolution.

Canopy gaps to climate change - extreme events, ecology and evolution.

Ecogenetic models, competition, and heteropatry

We develop a system of equations to analyse the existence of genetic polymorphisms under disruptive selection in heterogeneous environments. These equations have both a genetic and a population density regulation component. In the absence of the genetic component, the equations reduce to a discrete time description of competition between interacting clonal lines or species. We use these equations to demonstrate that different populations, competing along a resource spectrum, are able to dynamically coexist, as asymptotically periodic or chaotic solutions to our system of equations, despite the fact that a coexistence equilibrium—stable or unstable—does not exist. We then extend these results to environments in which several niches are explicitly defined. Our analysis of the ecological component of our model establishes that the answer to questions of coexistence among groups of individuals cannot rely on analyses of the existence of equilibria and their stability properties. In the most general model presented here, we allow for an assortative mating structure that is induced by the spatial heterogeneity of the environment. The level of assortative mating is controlled by a parameter so that at one extreme mating is panmictic, while at the other extreme individuals mate within their natal niches before dispersing to oviposit in other niches. We refer to this spatial mating structure as heteropatry. We investigate, through numerical studies, the properties of a heteropatric model containing both ecological and genetic components. First we address the question of the existence of protected genetic polymorphisms (i.e., the different allelles at a particular locus all increase in frequency when rare) under a wide range of model parameter values in a diallelic one-locus version of our model, assuming panmixis and partial dominance selection of varying direction. We make the point that establishing the instability of monomorphic population equilibria is insufficient to guarantee the existence of a protected polymorphism, since the instability may be in the population size component rather than the genetic frequency component of the monomorphic equilibrium solution. The results indicate that density dependence serves to decrease the likelihood that a protected polymorphism exists, while the degree of selection of natal habitat for oviposition purposes serves to increase this likelihood. The situation is complicated, though, since conditions which promote the existence of protected polymorphisms may actually reduce the possibility that a stable polymorphism exists. Further, the way we introduce density dependence (either by scaling the population interaction coefficients or by altering the shape of the response function) differentially affects the results. Finally, our results suggest that if individuals prefer to oviposit in their best niche (where they are most fit), rather than their natal niche (where they mature), then allele fixation very probably occurs.

NATURAL SELECTION IN EXPERIMENTAL POPULATIONS OF REEF-BUILDING CORALS (SCLERACTINIA).

Correlations between genetic diversity and environmental heterogeneity are well documented (e.g., Levins and MacArthur, 1966; Levins, 1968; Hedrick et al., 1976; Nevo, 1978). Most recent investigations of selection in heterogeneous environments have involved one (or a very few) loci in models or experimental laboratory populations. Many such studies have concentrated on disruptive selection, in which more than one phenotype is favored (e.g., Mather, 1953, 1955; Maynard Smith, 1962; Thoday, 1972; Powell and Wistrand, 1978; Mackay, 1980, 1981; Halliburton and Gall, 1981; Mark, 1982) or more specifically, on the protection of polymorphisms by prevention of allelic fixation in subdivided populations (e.g., Levene, 1953; Dempster, 1955; Christiansen and Feldman, 1975; Felsenstein, 1976; Hedrick et al., 1976; Karlin, 1976; Ewing, 1979; Spieth, 1979; Karlin and Campbell, 1981). Temporal and/or spatial variation in selective regimes is always required for protection, and is often necessary for disruptive selection. Although selection by single environmental factors (e.g., predation, temperature) acting on single phenotypic characters has now been demonstrated in numerous natural populations, there are still few descriptions for any natural population of the array of environmental factors likely to cause selection, of the temporal and spatial variation in the selective regime, or of the array of phenotypic and genotypic responses. In what is probably the best known field system, Jones et al. (1977) concluded that shell polymorphisms in the landsnail Cepaea are maintained by complex, often highly localized, selective regimes that vary in time and involve climate, topography, predators, competitors, frequencyand density-dependence, linkage disequilibrium, gene flow, and founder effects. Nevertheless, this is not a description of a single selective regime but, instead, is a composite based on data from numerous populations in many parts of Europe and North America. The primary purpose of this paper is to provide, as a first approximation, a description of the regime of natural selection affecting the long-lived reefbuilding corals Acropora palifera and A. cuneata (Cnidaria: Anthozoa: Scleractinia), and to assess smaller scale temporal (<10 yr) and spatial (<10 km) patterns of variation in the selective regime. These corals are abundant and important framebuilders on many Indo-Pacific reefs (e.g., Wells, 1954; Veron and Hudson, 1978; Crame, 1980). Secondary objectives are to determine the relative favorability of several habitats for corals, and to contribute to the understanding of environmental stability on tropical reefs. I consider only two components of fitness, survival and growth, and present data from five habitats, spanning up to eight years. This is much less than the potential lifespans and generation times of corals, since many corals live for decades and some can live for centuries (Connell, 1973; Nozaki et al., 1978; Hughes and Jackson, 1980; Druffel, 1982). Therefore, this paper only indicates some of the ways in which selective regimes vary; it cannot

RESOURCE PARTITIONING AMONG GENOTYPES OF DROSOPHILA PSEUDOOBSCURA.

It is appealing to think that competition among genotypes within a population is essentially like that among species within a community. Important for the community are interspecific resource specialization and partitioning, which contribute to its species richness and stability. It is possible that resource specialization and partitioning among genotypes also contribute to the richness and stability of polymorphisms. Evolutionists have long believed that habitat specialization by the variants in a population maintains variability (e.g., Darwin, 1872; Mayr, 1945; daCunhaetal., 1950). A wide variety of models for such selection in heterogeneous environments has now been studied (see Felsenstein, 1976; Hedrick et al., 1976; and Powell and Taylor, 1979, for reviews). The consequences predicted by these models are diverse and frequently seem in conflict. For example, some models predict that environmental heterogeneity should increase polymorphism (Gillespie, 1976; Taylor, 1976), others that it should have little or no effect (Fisher, 1958; Maynard Smith and Hoekstra, 1980), and others that it should decrease variability (Templeton, 1977). Consequences for loads and net biomass are equally diverse. Just what is expected from environmental diversity rests on the assumptions that go into the model. It has proven difficult to determine which of these models is appropriate for natural populations. This has usually been because genotypes were difficult to identify (e.g., Van Valen, 1965) or because fitnesses in the different microhabitats could not be measured (e.g., Taylor and Powell, 1977). In the study reported below we attempted to construct a model situation of heterogeneous environments in the laboratory; these were small vials that contained two different media. We then tried to determine what set of assumptions were most appropriate for describing competition and natural selection in such an environment. We examined: (1) whether there were genotype x environment (G x E) interactions for fitness components (Do some genotypes fare better in one environment while others do better elsewhere?); (2) whether larvae preferred the microenvironments in which they were more fit or if they moved at random; and (3) if there was resource specialization and partitioning by genotypes within the species. Our observations were then compared to the assumptions used in models of selection in heterogeneous environments. The mathematical models alluded to above all make one or another assumption that affects the conclusions in important ways and that is inconsistent with our laboratory measurements. It therefore seemed appropriate to develop a model of natural selection in such a system. This is described and some of its consequences deduced. It leads to the conclusions that: (1) there is no necessary relation between variation and environmental diversity; (2) loads are not necessarily less in heterogeneous environments; (3) additional variation does not necessarily lead to greater mean fitness (i.e., there is no genetic facilitation or overcompensation); but (4) all else being equal,

Microgeographic differentiation of chromosomal and enzyme polymorphisms in Drosophila persimilis.

We studied microgeographic and temporal genetic differentiation in natural populations of Drosophila persimilis with respect to chromosome inversion and enzyme polymorphisms. Both inversion frequencies and allozyme frequencies varied significantly over short distances. Neither differed significantly between morning and evening collections. Because several studies of the dispersal behavior of this species have been performed, we attempt to fit the observed data to mathematical models which relate dispersion to random genetic drift and to spatially varying selection coefficients. We conclude that the observations are due at least partly to behavioral differences among genotypes. i.e., habitat preferences. These results have implications for genetic load theory and models of selection in heterogeneous environments.

GENETIC VARIABILITY IN EDAPHICALLY RESTRICTED AND WIDESPREAD PLANT SPECIES.

The intuitively appealing notion that there are adaptive relationships between the genetic variability and ecological amplitude of populations is a persistent theme in evolutionary biology (Van Valen, 1965; Dobzhansky, 1970; Sammeta and Levins, 1970; Thoday, 1972). Adaptation to spatial and temporal heterogeneity of the environment can be achieved in several ways (Levins, 1969), including the employment of multiple genotypes specialized to exploit different subniches and the maintenance of purpose genotypes (Baker, 1965) providing phenotypic flexibility (Thoday, 1959) and plasticity (Bradshaw, 1965). Genetic aspects of the latter mode of adaptation are poorly understood (Cook and Johnson, 1968; Marshall and Jain, 1968), but it is well established theoretically (Levins, 1968; Maynard Smith, 1970) and experimentally (Beardmore, 1970; Powell, 1971) that disruptive (diversifying) selection in heterogeneous environments can maintain genetic polymorphism. One method of detecting possible ecogenetic relationships is to compare levels of genetic variability in closely related species differing in ecological versatility. For animals it is generally difficult to specify variation in ecological amplitude in an unequivocal manner. Although animal parasites differing in degree of host specificity and certain groups of monoand polyphytophagous insects may prove to be useful in the analysis ofthe problem, plants in general provide better material because the spatial limits of species and races often can be described in terms of the distribution of specific environmental factors. The