A countrywide survey of colour morph frequencies in the spider Enoplognatha ovata (Glerck) (Araneae: Theridiidae): evidence for natural selection

Data on the frequencies of the main colour morphs ( lineata, redimita and ovata ) of Enoplognatha ovata have been collected from a total of 454 Ordnance Survey 10 km squares distributed throughout Great Britain. Only c. 0.5% of samples were monomorphic, for lineata in each case. Multiple regression analysis has been used to assess possible associations between morph frequencies and principal components derived from nine environmental variables. The distribution of morph frequencies is not random but shows weak clines associated with certain climatic factors. These large scale clines indicate the action of natural selection although very local variations in morph frequencies may result from selection and/or drift.     

Trans-continental visible morph-frequency variation at homologous loci in two species of spider, Enoplognatha ovata s.s. & E. latimana

The spiders Enoplognatha ovata s.s. and E. latimana are sibling species which share a number of visible genetic polymorphisms. Data on colour and black-spotting morph frequencies in these species have been collected from 67 sites in western Europe. Sixty nine percent of the collections contained both species. In all adequately-sized samples, both species were polymorphic for colour and, in general, exhibited the same rank order of morphs lineata and redimita. (The top dominant morph, ovata, has not been found in E. latimana). Colour-morph frequencies are not correlated between species in sympatric populations from mainland Europe and from a previously studied area in Pembrokeshire, South Wales. Although associations with certain climatic variables are evident in E. ovata they are not consistent between transects, making their biological significance unclear. For black spotting, E. ovata s.s. is nearly fixed for spotting throughout mainland Europe but is highly variable in the Pembrokeshire populations. E. latimana is polymorphic in both areas. In Europe, spotting frequencies in E. latimana show significant associations with climatic factors but, again, their biological significance is not obvious. In E. ovata s.s. the variance in both colour and spotting frequencies among populations in Pembrokeshire is significantly greater than that in the whole of mainland Europe. The implications of these and previous results are considered in the context of the persistence of visible polymorphisms across species and the forces which determine morph frequencies in local populations.     

Genetic drift within a protected polymorphism: enigmatic variation in color-morph frequencies in the candy-stripe spider, Enoplognatha ovata

The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.     

Demographic Genetics of Brown Trout (Salmo Trutta) and Estimation of Effective Population Size from Temporal Change of Allele Frequencies

We studied temporal allele frequency shifts over 15 years and estimated the genetically effective size of four natural populations of brown trout (Salmo trutta L.) on the basis of the variation at 14 polymorphic allozyme loci. The allele frequency differences between consecutive cohorts were significant in all four populations. There were no indications of natural selection, and we conclude that random genetic drift is the most likely cause of temporal allele frequency shifts at the loci examined. Effective population sizes were estimated from observed allele frequency shifts among cohorts, taking into consideration the demographic characteristics of each population. The estimated effective sizes of the four populations range from 52 to 480 individuals, and we conclude that the effective size of natural brown trout populations may differ considerably among lakes that are similar in size and other apparent characteristics. In spite of their different effective sizes all four populations have similar levels of genetic variation (average heterozygosity) indicating that excessive loss of genetic variability has been retarded, most likely because of gene flow among neighboring populations.     

Frequency and fitness cost of resistance to Bacillus thuringiensis in Chrysomela tremulae (Coleoptera: Chrysomelidae)

The "high dose-refuge" (HDR) strategy is commonly recommended and currently used for delaying or preventing pest adaptation to transgenic plants producing Bacillus thuringiensis (Bt) toxins. The efficiency of this strategy depends, among other factors, on the initial frequency of Bt resistance alleles and on the fitness costs associated with these alleles. Two years ago, an allele conferring resistance to Bt poplar was detected in a French population of the poplar pest Chrysomela tremulae F. Although this pest had never been subjected to Bt selection pressure due to human activities, the frequency of this allele was estimated at 0.0037, with a 95% credible (CI) interval of 0.00045-0.0080. We investigated the frequency of this allele in a second sample of C. tremulae collected more than 500 km from the site of the initial population. The estimated frequency in this sample was 0.0113 (95% CI 0.0031-0.0247), reinforcing the conclusion that resistance to Bt plants may be present at detectable frequencies in pest populations before selection resulting from pest management by humans. The frequency of the Bt resistance allele over the two samples was 0.0049 (95% CI 0.0020-0.0091). We also followed five laboratory lines in which the frequency of this allele was initially fixed at 0.500. After five generations maintained on non-Bt poplar leaves, the frequency of this allele decreased in all lines, whereas allelic frequencies at a neutral locus were unaffected. Thus, the Bt resistance allele detected in French populations of C. tremulae is probably associated with a fitness cost.     

The influence of fluctuating population densities on evolutionary dynamics

The causes and consequences of fluctuating population densities are an important topic in ecological literature. Yet, the effects of such fluctuations on maintenance of variation in spatially structured populations have received little analytic treatment. We analyze what happens when two habitats coupled by migration not only differ in their trade‐offs in selection but also in their demographic stability—and show that equilibrium allele frequencies can change significantly due to ecological feedback arising from locally fluctuating population sizes. When an ecological niche exhibits such fluctuations, these drive an asymmetry in the relative impact of gene flow, and therefore, the equilibrium frequency of the locally adapted type decreases. Our results extend the classic conditions on maintenance of diversity under selection and migration by including the effect of fluctuating population densities. We find simple analytic conditions in terms of the strength of selection, immigration, and the extent of fluctuations between generations in a continent‐island model. Although weak fluctuations hardly affect coexistence, strong recurrent fluctuations lead to extinction of the type better adapted to the fluctuating niche—even if the invader is locally maladapted. There is a disadvantage to specialization to an unstable habitat, as it makes the population vulnerable to swamping from more stable habitats.     

Ecological genetics of adaptive color polymorphism in pocket mice: geographic variation in selected and neutral genes

Patterns of geographic variation in phenotype or genotype may provide evidence for natural selection. Here, we compare phenotypic variation in color, allele frequencies of a pigmentation gene (the melanocortin-1 receptor, Mc1r), and patterns of neutral mitochondrial DNA (mtDNA) variation in rock pocket mice (Chaetodipus intermedius) across a habitat gradient in southern Arizona. Pocket mice inhabiting volcanic lava have dark coats with unbanded, uniformly melanic hairs, whereas mice from nearby light-colored granitic rocks have light coats with banded hairs. This color polymorphism is a presumed adaptation to avoid predation. Previous work has demonstrated that two Mc1r alleles, D and d, differ by four amino acids, and are responsible for the color polymorphism: DD and Dd genotypes are melanic whereas dd genotypes are light colored. To determine the frequency of the two Mc1r allelic classes across the dark-colored lava and neighboring light-colored granite, we sequenced the Mc1r gene in 175 individuals from a 35-km transect in the Pinacate lava region. We also sequenced two neutral mtDNA genes, COIII and ND3, in the same individuals. We found a strong correlation between Mc1r allele frequency and habitat color and no correlation between mtDNA markers and habitat color. Using estimates of migration from mtDNA haplotypes between dark- and light-colored sampling sites and Mc1r allele frequencies at each site, we estimated selection coefficients against mismatched Mc1r alleles, assuming a simple model of migration-selection balance. Habitat-dependent selection appears strong but asymmetric: selection is stronger against light mice on dark rock than against melanic mice on light rock. Together these results suggest that natural selection acts to match pocket mouse coat color to substrate color, despite high levels of gene flow between light and melanic populations.     

Estimation of migration from a perturbation experiment in natural populations of Drosophila buzzatii Patterson & Wheeler

In order to estimate migration and gene flow, allele frequencies in populations at two sites separated by 120 m were differentially perturbed by the continuous release over 413 days of flies homozygous at particular allozyme loci. The effects of perturbation were determined by genotype assay at two collections prior to, thirteen during and nine after the perturbation period. Maximum likelihood methods were developed to estimate migration into the two populations from the homozygous releases, and migration between the two populations. The successful perturbation of allele frequencies in a natural population is demonstrated. A plateau in allele frequencies during perturbation and a return to original frequencies following cessation of perturbation was most likely due to selection during development against recessive alleles concurrently introduced into the populations by the released flies. There is unequivocal evidence for short distance gene flow between the two populations. The migration rates estimated at ten times over a nine month period were extremely variable, but with higher population density at one site positively related with migration from that site to the other.     

Ecological and developmental dynamics of a host-parasite system involving a sea anemone and two ctenophores

The lined sea anemone Edwardsiella lineata has evolved a derived parasitic life history that includes a novel body plan adapted for life inside its ctenophore hosts. Reputedly its sole host is the sea walnut, Mnemiopsis leidyi, a voracious planktivore and a seasonally abundant member of many pelagic ecosystems. However, we have observed substantially higher E. lineata prevalence in a second ctenophore species, the ctenophore predator Bero? ovata. The interplay among these 3 species has important conservation consequences as M. leidyi introductions are thought to be responsible for the severe depletion of numerous commercial fisheries in the Mediterranean basin, and both E. lineata and B. ovata have been proposed as biological controls for invasive M. leidyi. Over a 3-yr period (2004-2006), we collected 8,253 ctenophores from Woods Hole, Massachusetts, including M. leidyi, B. ovata, and a third ctenophore, Pleurobrachia pileus, and we recorded E. lineata infection frequencies, parasite load, and parasite location. We also conducted laboratory experiments to determine the likely mechanisms for parasite introduction and the effect of each host on parasite development. We observed peak E. lineata infection frequencies of 0% in P. pileus, 59% in M. leidyi, and 100% in B. ovata, suggesting that B. ovata could be an important natural host for E. lineata. However, in laboratory experiments, E. lineata larvae proved far more successful at infecting M. leidyi than B. ovata, and E. lineata parasites excised from M. leidyi exhibited greater developmental competence than parasites excised from B. ovata. Although we show that E. lineata is efficiently transferred from M. leidyi to B. ovata when the latter preys upon the former, we conclude that E. lineata larvae are not well adapted for parasitizing the latter species and that the E. lineata parasite is not well adapted for feeding in B. ovata; these developmental and ecological factors underlie the host specificity of this recently evolved parasite.     

Apparent Selection of Enzyme Alleles in Laboratory Populations of Drosophila

Twelve laboratory populations of recently collected Drosophila willistoni were begun with different frequencies of alleles at three enzyme loci, six populations at 25 degrees and six at 19 degrees . Periodic sampling of the populations allowed monitoring of the frequency changes in allozymes over time.-At Lap-5 (a locus coding for leucine amino peptidase), three alleles converged to the same frequencies in all populations at both temperatures. The apparent equilibrium frequency of the major allele was about.75; this is different from the frequency (.57) found in the natural population from which the experimental populations were begun. Allele frequency changes at the esterase-5 locus (Est-5) were slower but consistent in all cages. It is difficult to determine if an equilibrium has been reached. However, the frequency of the rare allele in all cages is about the same as in wild populations, 5%. Alleles at both Lap-5 and Est-5 are non-randomly associated with inversions in the chromosomes onto which they map. Because of these associations, it is impossible to unambiguously attribute the change in allele frequencies to selection at the loci being observed.-After one year, no significant gene frequency changes were detected at Est-7, the third locus studied.     

Sexual Antagonism,Temporally Fluctuating Selection,and Variable Dominance Affect a Regulatory Polymorphism in Drosophila melanogaster

Understanding how genetic variation is maintained within species is a major goal of evolutionary genetics that can shed light on the preservation of biodiversity. Here, we examined the maintenance of a regulatory single-nucleotide polymorphism (SNP) of the X-linked Drosophila melanogaster gene fezzik. The derived variant at this site is at intermediate frequency in many worldwide populations but absent in populations from the ancestral species range in sub-Saharan Africa. We collected and genotyped wild-caught individuals from a single European population biannually over a period of 5 years, which revealed an overall difference in allele frequency between the sexes and a consistent change in allele frequency across seasons in females but not in males. Modeling based on the observed allele and genotype frequencies suggested that both sexually antagonistic and temporally fluctuating selection may help maintain variation at this site. The derived variant is predicted to be female-beneficial and mostly recessive; however, there was uncertainty surrounding our dominance estimates and long-term modeling projections suggest that it is more likely to be dominant. By examining gene expression phenotypes, we found that phenotypic dominance was variable and dependent upon developmental stage and genetic background, suggesting that dominance may be variable at this locus. We further determined that fezzik expression and genotype are associated with starvation resistance in a sex-dependent manner, suggesting a potential phenotypic target of selection. By characterizing the mechanisms of selection acting on this SNP, our results improve our understanding of how selection maintains genetic and phenotypic variation in natural populations.     

The mode of evolution of molecular markers in populations of house mice under artificial selection for locomotor behavior

A complete understanding of the mode of evolution of molecular markers is important for making inferences about different population genetic parameters, especially because a number of studies have reported patterns of allelic variation at molecular markers that are not in agreement with neutral evolutionary expectations. In the present study, house mice (Mus domesticus) from the fourteenth generation of a selection experiment for increased voluntary wheel-running activity were used to test how selection on a complex behavior affects the distribution of allelic variation by examining patterns of variation at six microsatellite and four allozyme loci. This population had a hierarchical structure that allowed for simultaneous testing of the effects of selection and genetic drift on the distribution of allelic variation by comparing observed patterns of allele frequencies and estimates of genetic divergence at multiple hierarchical levels to expectations under models of neutral evolution. The levels of genetic divergence among replicate lines and between selection groups, estimated from microsatellite data or pooled microsatellite and allozyme data, were not significantly different from expectations under neutral evolution. Furthermore, the pattern of change of allele frequencies between the base population and generation 14 was largely in agreement with expectations under neutral evolution (although the PGM locus exhibited a pattern of change within populations that was difficult to explain under neutral evolution). Overall the results generally provide support for the neutral evolution of molecular markers.     

Patterns of differentiation in a colour polymorphism and in neutral markers reveal rapid genetic changes in natural damselfly populations

The existence and mode of selection operating on heritable adaptive traits can be inferred by comparing population differentiation in neutral genetic variation between populations (often using F(ST) values) with the corresponding estimates for adaptive traits. Such comparisons indicate if selection acts in a diversifying way between populations, in which case differentiation in selected traits is expected to exceed differentiation in neutral markers [F(ST )(selected) > F(ST )(neutral)], or if negative frequency-dependent selection maintains genetic polymorphisms and pulls populations towards a common stable equilibrium [F(ST) (selected) < F(ST) (neutral)]. Here, we compared F(ST) values for putatively neutral data (obtained using amplified fragment length polymorphism) with estimates of differentiation in morph frequencies in the colour-polymorphic damselfly Ischnura elegans. We found that in the first year (2000), population differentiation in morph frequencies was significantly greater than differentiation in neutral loci, while in 2002 (only 2 years and 2 generations later), population differentiation in morph frequencies had decreased to a level significantly lower than differentiation in neutral loci. Genetic drift as an explanation for population differentiation in morph frequencies could thus be rejected in both years. These results indicate that the type and/or strength of selection on morph frequencies in this system can change substantially between years. We suggest that an approach to a common equilibrium morph frequency across all populations, driven by negative frequency-dependent selection, is the cause of these temporal changes. We conclude that inferences about selection obtained by comparing F(ST) values from neutral and adaptive genetic variation are most useful when spatial and temporal data are available from several populations and time points and when such information is combined with other ecological sources of data.     

Assessing population genetic structure and variability with RAPD data: Application to Vaccinium macrocarpon (American Cranberry)

A method for estimating and comparing population genetic variation using random amplified polymorphic DNA (RAPD) profiling is presented. An analysis of molecular variance (AMOVA) is extended to accomodate phenotypic molecular data in diploid populations in Hardy-Weinberg equilibrium or with an assumed degree of selfing. We present a two step strategy: 1) Estimate RAPD site frequencies without preliminary assumptions on the unknown population structure, then perform significance testing for population substructuring. 2) If population structure is evident from the first step, use this data to calculate better estimates for RAPD site frequencies and sub-population variance components. A nonparametric test for the homogeneity of molecular variance (HOMOVA) is also presented. This test was designed to statistically test for differences in intrapopulational molecular variances (heteroscedasticity among populations). These theoretical developments are applied to a RAPD data set in Vaccinium macrocarpon (American cranberry) using small sample sizes, where a gradient of molecular diversity is found between central and marginal populations. The AMOVA and HOMOVA methods provide flexible population analysis tools when using data from RAPD or other DNA methods that provide many polymorphic markers with or without direct allelic data.     

Selective pressures on MHC class II genes in the guppy (Poecilia reticulata) as inferred by hierarchical analysis of population structure

Genes of the major histocompatibility complex, which are the most polymorphic of all vertebrate genes, are a pre‐eminent system for the study of selective pressures that arise from host–pathogen interactions. Balancing selection capable of maintaining high polymorphism should lead to the homogenization of MHC allele frequencies among populations, but there is some evidence to suggest that diversifying selection also operates on the MHC. However, the pattern of population structure observed at MHC loci is likely to depend on the spatial and/or temporal scale examined. Here, we investigated selection acting on MHC genes at different geographic scales using Venezuelan guppy populations inhabiting four regions. We found a significant correlation between MHC and microsatellite allelic richness across populations, which suggests the role of genetic drift in shaping MHC diversity. However, compared to microsatellites, more MHC variation was explained by differences between populations within larger geographic regions and less by the differences between the regions. Furthermore, among proximate populations, variation in MHC allele frequencies was significantly higher compared to microsatellites, indicating that selection acting on MHC may increase population structure at small spatial scales. However, in populations that have significantly diverged at neutral markers, the population‐genetic signature of diversifying selection may be eradicated in the long term by that of balancing selection, which acts to preserve rare alleles and thus maintain a common pool of MHC alleles.     

A General Model of Negative Frequency Dependent Selection Explains Global Patterns of Human ABO Polymorphism

The ABO locus in humans is characterized by elevated heterozygosity and very similar allele frequencies among populations scattered across the globe. Using knowledge of ABO protein function, we generated a simple model of asymmetric negative frequency dependent selection and genetic drift to explain the maintenance of ABO polymorphism and its loss in human populations. In our models, regardless of the strength of selection, models with large effective population sizes result in ABO allele frequencies that closely match those observed in most continental populations. Populations must be moderately small to fall out of equilibrium and lose either the A or B allele (N_e ≤ 50) and much smaller (N_e ≤ 25) for the complete loss of diversity, which nearly always involved the fixation of the O allele. A pattern of low heterozygosity at the ABO locus where loss of polymorphism occurs in our model is consistent with small populations, such as Native American populations. This study provides a general evolutionary model to explain the observed global patterns of polymorphism at the ABO locus and the pattern of allele loss in small populations. Moreover, these results inform the range of population sizes associated with the recent human colonization of the Americas.     

Biosystematic studies onCapsella bursa-pastoris (Brassicaceae): Enzyme polymorphism in natural populations

The genetic variability of natural populations ofCapsella bursapastoris in North- and Middle-Europe has been estimated by means of enzyme assays. Zymograms of 81 populations have been developed. 17 loci could be identified, and 8 of them can be heterozygous. Genetic variability is greater between populations than within. No correlation between actual population sizes and genetic heterogeneity could be detected. Some electromorphs shift their frequencies proportionally to increasing adversity of climatic conditions, some appear to be constant over the whole area, and others are characterized by an apparently irregular variation pattern. Marginal populations comprise a significantly higher proportion of heterozygous plants than central ones. Apart from this clinal variation pattern, a mosaic pattern, strongly related to habitat conditions, was observed: genetic heterogeneity is greater in more intensively disturbed sites. The pattern of genetic variation in natural populations ofCapsella bursa-pastoris is rather highly influenced by the breeding system.     

Temporal variation in the genetic structure of host-associated populations of the small ermine moth Yponomeuta padellus (Lepidoptera, Yponomeutidae)

Temporal changes in allele frequencies were studied in host-associated populations of the small ermine moth Yponomeuta padellus. At one site, populations from three host plants (Sorbus aucuparia, Amelanchier larnarckii , and Crataegus spp.) were sampled annually during a four-year-period and analysed with 20 polymorphic allozyme markers. At two other sites, allele frequencies at 5- 6 enzyme loci of Y. padellus populations from two different host plants were also tested for consistency over a 13-year-pcriod. Significant allele frequency changes occurred in the short-term analysis, whereas allele frequencies remained relatively stable through time in the long-term analyses. Furthermore, allele frequencies of Y. padellus populations from Crataegus spp. were relatively stable compared to the other host populations. The role of the agents responsible for the observed patterns is discussed.     

Empirical Distributions of F ST from Large-Scale Human Polymorphism Data

Studies of the apportionment of human genetic variation have long established that most human variation is within population groups and that the additional variation between population groups is small but greatest when comparing different continental populations. These studies often used Wright’s F _ST that apportions the standardized variance in allele frequencies within and between population groups. Because local adaptations increase population differentiation, high-F _ST may be found at closely linked loci under selection and used to identify genes undergoing directional or heterotic selection. We re-examined these processes using HapMap data. We analyzed 3 million SNPs on 602 samples from eight worldwide populations and a consensus subset of 1 million SNPs found in all populations. We identified four major features of the data: First, a hierarchically F _ST analysis showed that only a paucity (12%) of the total genetic variation is distributed between continental populations and even a lesser genetic variation (1%) is found between intra-continental populations. Second, the global F _ST distribution closely follows an exponential distribution. Third, although the overall F _ST distribution is similarly shaped (inverse J), F _ST distributions varies markedly by allele frequency when divided into non-overlapping groups by allele frequency range. Because the mean allele frequency is a crude indicator of allele age, these distributions mark the time-dependent change in genetic differentiation. Finally, the change in mean-F _ST of these groups is linear in allele frequency. These results suggest that investigating the extremes of the F _ST distribution for each allele frequency group is more efficient for detecting selection. Consequently, we demonstrate that such extreme SNPs are more clustered along the chromosomes than expected from linkage disequilibrium for each allele frequency group. These genomic regions are therefore likely candidates for natural selection.     

Temporal and Microgeographic Variation in Allozyme Frequencies in a Natural Population of DROSOPHILA BUZZATII

Temporal variation in allozyme frequencies at six loci was studied by making monthly collections over 4 yr in one population of the cactophilic species Drosophila buzzatii. Ten sites were defined within the study locality, and for all temporal samples, separate collections were made at each of these sites. Population structure over microgeographic space and changes in population structure over time were analyzed using F-statistic estimators, and multivariate analyses of allele and genotype frequencies with environmental variables were carried out. Allele frequencies showed significant variation over time, although there were no clear cyclical or seasonal patterns. A biplot analysis of allele frequencies over seasons within years and over years showed clear discrimination among years by alleles at four loci. During the 4 yr, three alleles showed directional changes which were associated with directional changes in environmental variables. Significant associations with one or more environmental variables were found for allele frequencies at every locus and for both expected and observed heterozygosities (except those for Est-1 and Est-2). Thus, variation in allele frequencies over time cannot be attributed solely to drift. Significant linkage disequilibria were detected among three loci (Est-2, Hex and Aldox), but there was no evidence for spatial or temporal patterns. The F-statistic analyses showed significant differentiation among months within years for all loci, but the statistic used (coancestry) was heterogeneous among loci. Estimates of F (inbreeding) for all loci were significantly different from zero, with the loci in four groups, Adh-1 (negative), Pgm(small positive), Est-2 and Hex (intermediate) and Est-1 and Aldox (high positive). The correlation of genes within individuals within populations (f) for each locus in each month by site sample differed among loci, as did the (f) for each locus in each month by site sample differed among loci, as did the patterns of change in f over time (seasons). Heterogeneity in the F-statistic estimates indicates that natural selection is directly or indirectly affecting allele and genotype frequencies at some loci. However, the F-statistic analyses showed essentially no microgeographic structure (i.e., among sites), although there was significant heterogeneity in allele frequencies among flies emerging from individual rots. Thus, microspatial heterogeneity probably is most important at the level of individual rots, and coupled with habitat selection, it could be a major factor promoting diversifying selection and the maintenance of polymorphism.(ABSTRACT TRUNCATED AT 400 WORDS)