COMPARATIVE POPULATION STRUCTURE AND GENE FLOW OF A BROOD PARASITE,THE GREAT SPOTTED CUCKOO (CLAMATOR GLANDARIUS), AND ITS PRIMARY HOST,THE MAGPIE (PICA PICA)

The amount of gene flow is an important determinant of population structure and therefore of central importance for understanding coevolutionary processes. We used microsatellite markers to estimate population structure and gene flow rates of the great spotted cuckoo (Clamator glandarius) and its main host in Europe, the magpie (Pica pica), in a number of populations (seven and 15, respectively) across their distribution range in Europe. The genetic analysis shows that there exists a pattern of isolation by distance in both species, although the cuckoo data are only indicative due to a small sample size. Gene flow seems to be extensive between nearby populations, higher for magpies than cuckoos, and especially high for magpie populations within the area of distribution of the great spotted cuckoo. There is no correlation between genetic distances between magpie populations and genetic distances between cuckoo populations. We discuss the implications of extensive gene flow between magpie populations in sympatry with cuckoos for the population dynamics of hosts, in particular for the occurrence of egg rejection behavior in host populations and how the different rates of migration for both species can affect the dynamics of coevolutionary processes.     

Antagonistic antiparasite defenses: nest defense and egg rejection in the magpie host of the great spotted cuckoo

Brood parasites dramatically reduce the reproductive successof their hosts, which therefore have developed defenses againstbrood parasites. The first line of defense is protecting thenest against adult parasites. When the parasite has successfullyparasitized a host nest, some hosts are able to recognize andreject the eggs of the brood parasite, which constitutes the secondline of defense. Both defense tactics are costly and would be counteractedby brood parasites. While a failure in nest defense implies successfulparasitism and therefore great reduction of reproductive successof hosts, a host that recognizes parasitic eggs has the opportunityto reduce the effect of parasitism by removing the parasiticegg. We hypothesized that, when nest defense is counteractedby the brood parasite, hosts that recognize cuckoo eggs shoulddefend their nests at a lower level than nonrecognizers becausethe former also recognize adult cuckoos. Magpie (Pica pica) hoststhat rejected model eggs of the brood parasitic great spottedcuckoo (Clamator glandarius) showed lower levels of nest defensewhen exposed to a great spotted cuckoo than when exposed toa nest predator (a carrion crow Corvus corone). Moreover, magpiesrejecting cuckoo eggs showed lower levels of nest defense againstgreat spotted cuckoos than nonrecognizer magpies, whereas differencesin levels of defense disappeared when exposed to a carrion crow.These results suggest that hosts specialize in antiparasitedefense and that different kinds of defense are antagonistically expressed.We suggest that nest-defense mechanisms are ancestral, whereasegg recognition and rejection is a subsequent stage in the coevolutionaryprocess. However, host recognition ability will not be expressedwhen brood parasites break this second line of defense.     

MAGPIE HOST MANIPULATION BY GREAT SPOTTED CUCKOOS: EVIDENCE FOR AN AVIAN MAFIA?

Why should the hosts of brood parasites accept and raise parasitic offspring that differ dramatically in appearance from their own? There are two solutions to this evolutionary enigma. (1) Hosts may not yet have evolved the capability to discriminate against the parasite, or (2) parasite-host systems have reached an evolutionary equilibrium. Avian brood parasites may either gain renesting opportunities or force their hosts to raise parasitic offspring by destroying or preying upon host eggs or nestlings following host ejection of parasite offspring. These hypotheses may explain why hosts do not remove parasite offspring because only then will hosts avoid clutch destruction by the cuckoo. Here we show experimentally that if the egg of the parasitic great spotted cuckoo Clamator glandarius is removed from nests of its magpie Pica pica host, nests suffer significantly higher predation rates than control nests in which parasite eggs have not been removed. Using plasticine model eggs resembling those of magpies and observations of parasites, we also confirm that great spotted cuckoos that have laid an ejected egg are indeed responsible for destruction of magpie nests with experimentally ejected parasite eggs. Cuckoos benefit from destroying host offspring because they thereby induce some magpies to renest and subsequently accept a cuckoo egg.     

Change in host rejection behavior mediated by the predatory behavior of its brood parasite

Passerine hosts of parasitic cuckoos usually vary in their abilityto discriminate and reject cuckoo eggs. Costs of discriminationand rejection errors have been invoked to explain the maintenanceof this within-population variability. Recently, enforcementof acceptance by parasites has been identified as a rejectioncost in the magpie (Pica pica) and its brood parasite, the greatspotted cuckoo (Clamator glandarius). Previous experimentalwork has shown that rejecter magpies suffer from increased nestpredation by the great spotted cuckoo. Cuckoo predatory behavioris supposed to confer a selective advantage to the parasitebecause magpies experiencing a reproductive failure may providea second opportunity for the cuckoo to parasitize a replacementclutch. This hypothesis implicitly assumes that magpies modulatetheir propensity to reject parasite eggs as a function of previousexperience. We tested this hypothesis in a magpie populationbreeding in study plots varying in parasitism rate. Magpie pairs thatwere experimentally parasitized and had their nests depredated,after their rejection behavior had been assessed, changed theirbehavior from rejection to acceptance. The change in host behaviorwas prominent in study plots with high levels of parasitism,but not in plots with rare or no cuckoo parasitism. We discussthree possible explanations for these differences, concludingthat in study plots with a high density of cuckoos, the probability fora rejecter magpie nest of being revisited and depredated bya cuckoo is high, particularly for replacement clutches, and,therefore, the cost for magpies of rejecting a cuckoo egg ina replacement clutch is increased. Moreover, in areas with highlevels of host defense (low parasitism rate), the probabilityof parasitism and predation of rejecter-magpie nests by thecuckoo is reduced in both first and replacement clutches. Therefore,rejecter magpies in such areas should not change their rejectionbehavior in replacement clutches.     

GENETIC MODELS OF ADAPTATION AND GENE FLOW IN PERIPHERAL POPULATIONS

We investigate the interplay between gene flow and adaptation in peripheral populations of a widespread species. Models are developed for the evolution of a quantitative trait under clinally varying selection in a species whose density decreases from the center of the range to its periphery. Two major results emerge. First, gene flow from populations at the range center can be a strong force that inhibits peripheral populations from evolving to their local ecological optima. As a result, peripheral populations experience persistent directional selection. Second, response to local selection pressures can cause rapid and substantial evolution when a peripheral population is isolated from gene flow. The amount of evolutionary change depends on gene flow, selection, the ecological gradient, and the trait's heritability. Rapid divergence can also occur between the two halves of a formerly continuous population that is divided by a vicariant event. A general conclusion is that disruption of gene flow can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random genetic drift.     

GENETIC AND MORPHOMETRIC DIVERGENCE IN ANCESTRAL EUROPEAN AND DESCENDENT NEW ZEALAND POPULATIONS OF CHAFFINCHES (FRINGILLA COELEBS)

Descendent populations of chaffinches (Fringilla coelebs) introduced to New Zealand about 120 years ago were compared with “ancestral” populations in northern Europe and with those in a broader region of Europe (including Iberia) using protein electrophoresis at 42 loci and 12 skeletal measurements. The New Zealand populations exhibit very small scale differentiation in genetics (F_st = 0.040) and morphometrics, and the haphazard pattern of among-population variation does not align with environmental variation nor is it predicted by the geographic proximity of populations. Thus random drift is implicated in the differentiation among the descendent populations. The New Zealand chaffinches have diverged only slightly in morphometrics from an extant population in southern England, and constant heritability rate tests suggest that random drift alone could account for this small shift. In sharp contrast, the European populations are subdivided genetically (F_st = 0.222) and morphometrically, and this subdivision coincides with the Pyrenees mountains between Iberia and northern Europe which act as a barrier to gene flow between these regions. Iberian populations have smaller skulls and longer wings on average than northern European populations and are characterized by high frequencies of alternative common alleles at Ada and Np. Within both the Iberian and northern European regions, however, populations are effectively panmictic in protein-encoding genes, indicating that homogenizing gene flow is apparently extensive enough to prevent among-population differentiation in allozymes by drift. Variation in body size as represented by PC I is related to environmental productivity across Europe, unlike in New Zealand. These observations jointly suggest that longer term adaptive differentiation via selection for optimal body size has evolved in Europe. Because multilocus evolution is expected to proceed slowly in populations subject to the opposing forces of selection and homogenizing gene flow, I argue that local adaptation within “ancestral” populations in northern Europe may still be evolving.     

MAINTENANCE OF POLYGENIC VARIATION IN SPATIALLY STRUCTURED POPULATIONS: ROLES FOR LOCAL MATING AND GENETIC REDUNDANCY

Although heritable genetic variation is critical to the evolutionary process, we know little about how it is maintained. Obviously, mutation-selection balance must play a role, but there is considerable doubt over whether it can account for heritabilities as high as 0.5, which are commonly found in natural populations. Most models of mutation-selection balance assume panmictic populations. In this paper we use Monte Carlo simulations to examine the effect of isolation by distance on the variation maintained by mutation in a polygenic trait subject to optimizing selection. We show that isolation by distance can substantially increase the total variation maintained in continuous populations over a wide range of dispersal patterns, but only if more than one genotype produces the optimal phenotype (genetic redundancy). Isolation by distance alone has only a slight effect on the variation maintained in the total population for neutral alleles. The combined effect of isolation by distance and genetic redundancy, however, allows the maintenance of substantial variation despite strong stabilizing selection. The mechanism is straightforward. Isolation by distance allows mutation and drift to operate independently in different parts of the population. Because of their independent evolutionary histories, different parts of the population independently draw from the available set of redundant genotypes. Because the genotypes are redundant, selection does not discriminate among them, and they will persist until eliminated by drift. The population as a whole maintains many distinct genotypes. We show that this process allows mutation to maintain high levels of variation, even under strong stabilizing selection, and that over a moderate range of dispersal patterns the amount of variation maintained in the entire population is independent of both the strength of selection and the variance of the dispersal distance. Furthermore, we show that individual heterozygosity is increased in locally mating populations when selection is strong. Finally, our simulations provide a rough picture of how selection and the dispersal pattern influence the spatial distribution of genetic and phenotypic variation.     

GENE FLOW,REFUGIA, AND EVOLUTION OF GEOGRAPHIC VARIATION IN THE SONG SPARROW (MELOSPIZA MELODIA)

We surveyed mtDNA restriction-site variation in song sparrows taken from across their continental range. Despite marked geographic variation in size and plumage color, mtDNA variation was not geographically structured. Subspecies were not identifiable by mtDNA analysis. We suggest that postglaciation dispersal scattered mtDNA haplotypes across the continent, explaining the lack of mtDNA geographic patterns. Evolution of size and plumage coloration has probably proceeded faster than mtDNA evolution, leading to the well-structured continental pattern of morphological variation. We suggest that the nonordered geographic distribution of haplotypes reflects the recency of population establishment following completion of range expansion. Dispersal distance was estimated from the mtDNA data at 6.1 km per generation, an order of magnitude greater than that (0.3 km) estimated from demographic data. Island samples were not especially different from continental ones. Rooting the haplotype cladogram with a putative primitive haplotype identified Newfoundland and the Queen Charlotte Islands as potential sites of recent refugia. We question whether study of geographic variation in song sparrows leads to insights concerning speciation.     

BEHAVIORAL DEFENSES AGAINST AVIAN BROOD PARASITISM IN SYMPATRIC AND ALLOPATRIC HOST POPULATIONS

The brown-headed cowbird (Molothrus ater) is a widespread, obligate brood parasite of North American passerine birds. In southern Manitoba, where hosts are sympatric with cowbirds, American robins (Turdus migratorius) ejected parasitic eggs from all experimentally parasitized clutches (N = 25) and no eggs were accepted for more than four days. In contrast, robins in northern Manitoba, an area where cowbirds do not breed, accepted parasitic eggs in 33% of nests (N = 18) for at least five days. Acceptance of experimental cowbird eggs by a second host, the yellow warbler (Dendroica petechia), was similar in allopatric (100% of 20 nests) and sympatric (88.6% of 35 nests) populations, but models of a female cowbird elicited greater nest defense by warblers in the area of sympatry. Neither host rejected eggs of conspecifics, thus, rejection of cowbird eggs was not an epiphenomenon of conspecific brood parasitism. These results support the hypothesis that recognition of cowbirds and their eggs evolved as adaptations to counter cowbird parasitism and not some other selection pressure. The expression of anti-parasite defenses by some individuals within allopatric populations further suggests these traits may be controlled genetically but persist in such areas either through the continued introgression of rejecter genes from sympatric populations or because of the low cost of rejection behavior when parasitism is absent or rare.     

CORRELATION OF PAIRWISE GENETIC AND GEOGRAPHIC DISTANCE MEASURES: INFERRING THE RELATIVE INFLUENCES OF GENE FLOW AND DRIFT ON THE DISTRIBUTION OF GENETIC VARIABILITY

Attempts to relate estimates of regional F_ST to gene flow and drift via Wright's (1931) equation F_ST ≈ 1/ (4Nm + 1) are often inappropriate because most natural sets of populations probably are not at equilibrium (McCauley 1993), as assumed by the island model upon which the equation is based, or ineffective because the influences of gene flow and drift are confounded in the product Nm. Evaluations of the association between genetic (F_ST) and geographic distances separating all pairwise populations combinations in a region allows one to test for regional equilibrium, to evaluate the relative influences of gene flow and drift on population structure both within and between regions, and to visualize the behavior of the association across all degrees of geographic separation. Tests of the model using microsatellite data from 51 populations of eastern collared lizards (Crotaphytus collaris collaris) collected from four distinct geographical regions gave results highly consistent with predicted patterns of association based on regional differences in various historical and ecological factors that affect the amount of drift and gene flow. The model provides a prerequisite for and an alternative to regional F_ST analyses, which often simply assume regional equilibrium, thus potentially leading to erroneous and misleading inferences regarding regional population structure.     

SPATIAL SCALE OF GENETIC STRUCTURE AND AN INDIRECT ESTIMATE OF GENE FLOW IN EELGRASS,ZOSTERA MARINA

In this study, the first investigation of population structure in an aquatic angiosperm, I show that populations of a marine angiosperm (eelgrass, Zostera marina) are genetically differentiated at a number of spatial scales. I find also that there is no correspondence between geographic and genetic distances separating subpopulations, an increasingly common result in spatially stratified studies of genetic structure in marine invertebrates. F-statistics, calculated for two years from electrophoretic variation at five polymorphic allozyme loci, indicate significant genetic differentiation among sampling quadrats within each of two bays (θ = 0.064-0.208), between tide zones within a bay (θ = 0.025-0.157) and between bays (θ = 0.079). Spatial autocorrelation analysis was used to explore genetic differentiation at smaller spatial scales; estimated patch sizes (within which genetic individuals are randomly associated) indicated no appeciable genetic structure at scales less than 20 m × 20 m. Calculated values of F-statistics were a function of the spatial scale from which samples were drawn: increasing the size of the “subpopulation” included in calculation of fixation indices for the same “total” sample resulted in an increase in the magnitude of f (e.g., from 0.092 to 0.181) and a decrease in θ (e.g., from 0.186 to 0.025). On the basis of the best estimate of the spatial scale of subpopulations, the effective number of migrants per generation (N_em) ranges from 1.1 to 2.8. Genetic consequences of the disturbance regime in the eelgrass habitat sampled were extreme variation between years in the allele richness and proportion of heterozygotes in a sample and a positive relationship between the extinction probability of patches and the genetic variance among them. The changes in F-statistics as a function of sampling scale and the observation that θ among sampled quadrats was positively associated with the probability of extinction among quadrats indicated that indirect estimates of gene flow (N_em) calculated from θ should be cautiously interpreted in populations that may not yet be in drift-migration equilibrium.     

THE EFFECTS OF THE RELATIVE GEOGRAPHIC SCALES OF GENE FLOW AND SELECTION ON MORPH FREQUENCIES IN THE WALKING-STICK TIMEMA CRISTINAE

Gene frequencies in large populations are determined by a balance between selection and gene flow between neighborhoods of different selection regimes. This balance is affected by the area of the patches of a given selection regime relative to the gene-flow distance. If patches are small relative to gene-flow distance, similarity in the total area occupied by different patch types is a crucial condition for the stability of polymorphisms. However, if patches are larger than the gene-flow distance, then the relative area of different patch types is less important because of reduced gene flow resulting from isolation by distance. Two morphs (striped and unstriped) of the walking-stick Timema cristinae were each strongly associated with patches of distinct species of food plants on which they are most cryptic. The frequency of a morph was high on the plant on which it is most cryptic when either: (1) the area occupied by the food plant (patch) was very large; (2) the patch was completely isolated from other patches; or (3) the patch was larger than adjacent patches. Results (1) and (2) are consistent with isolation-by-distance models, and result (3) is consistent with Levene's multiple-niche polymorphism model.     

ESTERASE VARIATION AND GENETIC STRUCTURE IN THREE GEOGRAPHIC POPULATIONS OF SITODIPLOSIS MOSELLANA (GEHIN) (DIPTERA: CECIDOMYIIDAE) IN WESTERN CHINA*

Abstract This paper investigates the esterase variation and genetic structure in three geographic populations of Sitodiplosis mosellana (Géhin) in western China by PAGE. The localities surveyed are Gaolan (36.3°N, 103.9°E) and Wuwei (37.9°N, 102.6°E) in spring wheat region and Chang'an (34. 1°N, 108.9° E) in winter wheat region. The results suggest that the esterase is coded by two loci: Est‐1 and Est‐2. Est‐1 is coded by a plastogene producing only one band that is the fastest on the gel among all bands. The Est‐2 is duplicated loci with 8 alleles, namely, a, b, c, d, e, f, g, h, which produce altogether 8 bands in all the populations and 1–4 bands in individual samples. There are 19 zymogram types observed in the three geographic populations. Seventeen zymogram types emerge in Chang'an population, but 5 and 4 zymogram types are found respectively in Gaolan and Wuwei populations. II₂ zymogram type is the commonest in all the populations. The alleles that had the highest frequencies in all the populations are d, e, g. All 8 alleles at the Est2 were observed in Chang'an population, but only total 3 alleles‐d, e, g at the Est‐2 appeared in Gaolan and Wuwei populations. The analysis of genetic identity and cluster (UPGMA) on the alloenzyme indicates that the relationship between the two populations of spring wheat region seems to be closer, as compared with the relationship between spring wheat population and winter wheat population. It is evident that there exists some infraspecific variation caused mainly by genetic drift in S. mosellana and the gene flow among the populations possibly took place to some extent.     

INDIRECT METHODS TO ESTIMATE GENE FLOW IN CAVE AND SURFACE POPULATIONS OF ANDRONISCUS DENTIGER (ISOPODA: ONISCIDEA)

We estimateci gene flow among several populations of the troglophilic woodlouse Androniscus dentiger from central Italy using allozyme data. Estimates of gene flow were obtained from G_ST, θ, and the private alleles method, after being tested whether the assumption of the population genetic model and the assumption of neutrality of alleles had been met. Hierarchical analysis of gene flow has been used to investigate the geographic scale at which gene exchange can actually occur. Results showed that, independent of the methods, no ongoing gene flow can be detected among populations, even among geographically proximate ones. Genetic drift is likely the main agent shaping the pattern of genetic divergence among these populations. Patterns of past and ongoing gene flow were considered, as surface populations have become genetically isolated more recently than cave populations. In general, all three methods to estimate gene flow provided Nm estimates of the same magnitude.     

GENETIC DRIFT AND FOUNDER EFFECT IN NATIVE VERSUS INTRODUCED POPULATIONS OF AN INVADING PLANT,LYTHRUM SALICARIA (LYTHRACEAE)

There are few convincing examples of genetic drift at loci under selection in natural populations. The plant sexual polymorphism tristyly provides an opportunity to investigate genetic drift because stochastic processes interacting with frequency-dependent selection give rise to a diagnostic pattern of morph-frequency variation. A previous study of 102 Ontario populations of the introduced tristylous wetland herb Lythrum salicaria provided evidence for the role of stochastic processes during colonization. However, whether stochastic effects are greater in these recently introduced populations compared to native Eurasian populations remains unclear. The propensity of this species to invade disturbed habitats suggests that episodes of colonization and periods of small population size must also occur in the native range. A survey of 102 populations in southwestern France indicated reduced stochastic effects in native populations. Populations exhibited significantly lower morph loss than in Ontario (5% vs. 23%) and significantly higher values of morph evenness. The greater incidence of trimorphism in French populations was not associated with larger population sizes; populations were significantly smaller than those in Ontario (means: 266 vs. 487). Morph evenness was positively correlated with population size among French but not Ontario populations, providing further evidence of nonequilibrium conditions in introduced compared to native populations. The incidence of trimorphism was unexpectedly high in small native populations (N ≤ 25; 22 of 27 populations trimorphic). Computer simulations indicated that levels of gene flow on the order of m ≥ 0.05 can account for the maintenance of tristyly in small populations. The high connectivity of populations within the agricultural landscape typical of southwestern France may facilitate levels of gene flow sufficient to maintain trimorphism in small populations.     

PHYLOGEOGRAPHY OF SPOTTED OWL (STRIX OCCIDENTALIS) POPULATIONS BASED ON MITOCHONDRIAL DNA SEQUENCES: GENE FLOW,GENETIC STRUCTURE,AND A NOVEL BIOGEOGRAPHIC PATTERN

Mitochondrial DNA control region sequences of spotted owls (Strix occidentalis) allowed us to investigate gene flow, genetic structure, and biogeographic relationships among these forest-dwelling birds of western North America Estimates of gene flow based on genetic partitioning and the phylogeography of haplotypes indicate substantial dispersal within three long-recognized subspecies. However, patterns of individual phyletic relationships indicate a historical absence of gene flow among the subspecies, which are essentially monophyletic. The pattern of haplotype coalescence enabled us to identify the approximate timing and direction of a recent episode of gene flow from the Sierra Nevada to the northern coastal ranges. The three subspecies comprise phylogenetic species, and the northern spotted owl (S. o. caurina) is sister to a clade of California (S. o. occidentalis) plus Mexican spotted owls (S o lucida); this represents a novel biogeographic pattern within birds. The California spotted owl had substantially lower nucleotide diversity than the other two subspecies; this result is inconsistent with present patterns of population density A causal explanation requires postulating a severe bottleneck or a selective sweep, either of which was confined to only one geographic region.     

THE INFLUENCE OF SELF-FERTILIZATION AND POPULATION DYNAMICS ON THE GENETIC STRUCTURE OF SUBDIVIDED POPULATIONS: A CASE STUDY USING MICROSATELLITE MARKERS IN THE FRESHWATER SNAIL BULINUS TRUNCATUS

The distribution of neutral genetic variability within and among sets of populations results from the combined actions of genetic drift, migration, extinction and recolonization processes, mutation, and the mating system. We here analyzed these factors in 38 populations of the hermaphroditic snail Bulinus truncatus. The sampling area covered a large part of the species range. The variability was analyzed using four polymorphic microsatellite loci. A very large number of alleles (up to 55) was found at the level of the whole study. Observed heterozygote deficiencies within populations are consistent with very high selfing rates, generally above 0.80, in all populations. These should depress the variability within populations, because of low effective size, genetic hitchhiking, and background selection, whatever the model of mutation assumed. However, that some populations exhibit much more variability than others suggests that historical demographic processes (e.g., population size variation, bottlenecks, or founding events) may play a significant role. A hierarchical analysis of the distribution of the variability across populations indicates a strong pattern of isolation by distance, whatever the geographical scale considered. Our analysis also illustrates how the mutation rate may affect population differentiation, as different mutation rates result in different levels of homoplasy at microsatellite loci. The effects of both genetic drift and gene flow vary with the temporal and spatial scales considered in B. truncatus populations.     

GENETIC STRUCTURE OF THE BLUE RIDGE DUSKY SALAMANDER (DESMOGNATHUS ORESTES): INFERENCES FROM ALLOZYMES, MITOCHONDRIAL DNA, AND BEHAVIOR

Abstract The plethodontid salamander Desmognathus orestes, a member of the D. ochrophaeus species complex, is distributed in southwestern Virginia, eastern Tennessee, and western North Carolina. Previous allozyme analyses indicate that D. orestes consists of two distinct groups of populations (D. orestes‘B’ and D. orestes‘C’) with extensive intergradation and probable gene flow between these two groups. Spatially varying allele frequencies can reflect historical associations, current gene flow, or a combination of population‐level processes. To differentiate among these processes, we use multiple markers to further characterize divergence among populations of D. orestes and assess the degree of intergradation between D. orestes‘B’ and D. orestes‘C’, specifically investigating variation in allozymes, mitochondrial DNA (mtDNA), and reproductive behavior among populations. On a broad scale, the mtDNA genealogies reconstruct haplotype clades that correspond to the species identified from previous allozyme analyses. However, at a finer geographic scale, the distributions of the allozyme and mtDNA markers for D. orestes‘B’ and D. orestes‘C’ are discordant. MtDNA haplotypes corresponding to D. orestes‘B’ are more broadly distributed across western North Carolina than predicted by allozyme data, and the region of intergradation with D. orestes‘C’ indicates asymmetric gene flow of these markers. Asymmetric mating may contribute to observed discordance in nuclear versus cytoplasmic markers. Results support describing D. orestes as a single species and emphasize the importance of using multiple markers to examine fine‐scale patterns and elucidate evolutionary processes affecting gene flow when making species‐level taxonomic decisions.     

LOCAL ADAPTATION IN TWO SUBSPECIES OF AN ANNUAL PLANT: IMPLICATIONS FOR MIGRATION AND GENE FLOW

Plant populations often adapt to local environmental conditions. Here we demonstrate local adaptation in two subspecies of the California native annual Gilia capitata using standard reciprocal transplant techniques in two sites (coastal and inland) over three consecutive years. Subspecies performance in each site was measured in four ways: probability of seedling emergence, early vegetative size (length of longest leaf), probability of flowering, and total number of inflorescences produced per plant. Analysis of three of the four variables demonstrated local adaptation through site-by-subspecies interactions in which natives outperformed immigrants. The disparity between natives and immigrants in their probability of emergence and probability of flowering was greater at the coastal site than at the inland site. Treated in isolation, these two fitness components suggest that migration from the coast to the inland site may be less restricted by selection than migration in the opposite direction. Two measurements of individual size (leaf length and number of inflorescences), suggest (though not strongly) that immigrants may be subject to weaker selection at the coastal site than at the inland site. A standard cohort life table is used to compare replacement rates (R₀) for each subspecies at each site. Comparisons of R₀s suggest that immigrants are under a severe demographic disadvantage at the coastal site, but only a small disadvantage at the inland site. The results point out the importance of integrating over several fitness components when documenting the magnitude of local adaptation.     

GENETIC POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE STREAM DWELLING WATERSTRIDER,AQUARIUS (=GERRIS) REMIGIS (HEMIPTERA: GERRIDAE)

Gene flow, in combination with selection and drift, determines levels of differentiation among local populations. In this study we estimate gene flow in a stream dwelling, flightless waterstrider, Aquarius remigis. Twenty-eight Aquarius remigis populations from Quebec, Ontario, New Brunswick, Iowa, North Carolina, and California were genetically characterized at 15 loci using starch gel electrophoresis. Sampling over two years was designed for a hierarchical analysis of population structure incorporating variation among sites within streams, streams within watersheds, watersheds within regions, and regions within North America. Hierarchical F statistics indicated that only sites within streams maintained enough gene flow to prevent differentiation through drift (Nm = 27.5). Above the level of sites within streams gene flow is highly restricted (Nm ≤ 0.5) and no correlation is found between genetic and geographic distances. This agrees well with direct estimates of gene flow based on mark and recapture data, yielding an N_e of approximately 170 individuals. Previous assignment of subspecific status to Californian A. remigis is not supported by genetic distances between those populations and other populations in North America. Previous suggestion of specific status for south-eastern A. remigis is supported by genetic distances between North Carolina populations and other populations in North America, and a high proportion of region specific alleles in the North Carolina populations. However, because of the high degree of morphological and genetic variability throughout the range of this species, the assignment of specific or subspecific status to parts of the range may be premature.