GENETICS OF MIMICRY IN THE TIGER SWALLOWTAIL BUTTERFLIES,PAPILIO GLAUCUS AND P. CANADENSIS (LEPIDOPTERA: PAPILIONIDAE)

The tiger swallowtail butterfly, Papilio glaucus, exhibits a female-limited polymorphism for Batesian mimicry; the Canadian tiger swallowtail, Papilio canadensis, lacks the mimetic (dark) form entirely. The species hybridize to a limited extent where their ranges overlap. Field collections and censuses indicate that mimetic females occur throughout the range of P. glaucus but at lowest frequencies in populations at the latitudinal edges of its geographic range such as the southernmost part of Florida and along the entire northern edge of its distribution from Massachusetts to Minnesota. Frequencies of mimetic females have remained relatively stable over time. Inheritance of the mimetic form is controlled primarily by two interacting sex-linked loci. The typical matrilineal pattern of inheritance in P. glaucus can be explained by polymorphism at a Y-linked locus, b. Analysis of P. glaucus × P. canadensis crosses has also revealed an X-linked locus, s, which controls the expression of the mimetic phenotype. The P. canadensis allele, s^can, suppresses the mimetic phenotype in hybrid and backcross females. Results from more than 12 yr of rearing tiger swallowtails, including interspecies hybrids, indicate that the absence of mimetic P. canadensis females is due to both a high frequency of the “suppressing” allele s^can and low frequency of the black-pigment-determining b + allele. The frequency of s^can (or other suppressing alleles of s) in P. glaucus populations outside the hybrid zone is low. Some males heterozygous at the s locus and some suppressed mimetic females occur within the hybrid zone. A simple genetic model predicts the frequency of daughters that differ in phenotype from their mothers.     

PREFERENCE HIERARCHIES AND THE ORIGIN OF GEOGRAPHIC SPECIALIZATION IN HOST USE IN SWALLOWTAIL BUTTERFLIES

Four allopatric populations of the widely distributed western anise swallowtail butterfly, Papilio zelicaon, use different plant genera as hosts, but simultaneous choice experiments showed that these populations have diverged only slightly in oviposition preference. Of the four populations—two from southeastern Washington State, one from coastal southwestern Washington State, and one from central California—three use hosts that are not available to any of the others. Although variation for the degree of preference for particular plant species occurred within and among populations, all four populations ranked hosts in the same overall order. Monophagy on a local, low-ranking host outside the range of high-ranking hosts did not necessarily lead to the loss of preference for those high-ranking hosts, thereby indicating that the high-ranking hosts would still be accepted, and in some cases even preferred, if a population encountered them again. Hence, the overall preference hierarchy among P. zelicaon populations appears to be evolutionarily conservative. Analyses of differences among families within the California population indicated that increased preference for some hosts is inversely correlated, whereas preference for other hosts may be uncorrelated. Positive correlations may also occur but were not observed among the plant species tested. Overall, the results indicate local monophagy on different plant species in P. zelicaon has not involved major reorganizations in the preference hierarchy of ovipositing females, even in populations that may have fed on a low-ranking host for many generations. Instead, small increases in preference for local hosts have occurred within an evolutionarily conservative preference hierarchy.     

GENE FLOW AND SELECTION ON PHENOTYPIC PLASTICITY IN AN ISLAND SYSTEM OF RANA TEMPORARIA

Gene flow is often considered to be one of the main factors that constrains local adaptation in a heterogeneous environment. However, gene flow may also lead to the evolution of phenotypic plasticity. We investigated the effect of gene flow on local adaptation and phenotypic plasticity in development time in island populations of the common frog Rana temporaria which breed in pools that differ in drying regimes. This was done by investigating associations between traits (measured in a common garden experiment) and selective factors (pool drying regimes and gene flow from other populations inhabiting different environments) by regression analyses and by comparing pairwise F_ST values (obtained from microsatellite analyses) with pairwise Q_ST values. We found that the degree of phenotypic plasticity was positively correlated with gene flow from other populations inhabiting different environments (among‐island environmental heterogeneity), as well as with local environmental heterogeneity within each population. Furthermore, local adaptation, manifested in the correlation between development time and the degree of pool drying on the islands, appears to have been caused by divergent selection pressures. The local adaptation in development time and phenotypic plasticity is quite remarkable, because the populations are young (less than 300 generations) and substantial gene flow is present among islands.     

SEXUAL CONFLICT AND THE MAINTENANCE OF MULTIVARIATE GENETIC VARIATION

Mate choice should erode additive genetic variation in sexual displays, yet these traits often harbor substantial genetic variation. Nevertheless, recent developments in quantitative genetics have suggested that multivariate genetic variation in the combinations of traits under selection may still be depleted. Accordingly, the erosion and maintenance of variation may only be detectable by studying whole suites of traits. One potential process favoring the maintenance of genetic variance in multiple trait combinations is the modification of sexual selection via sexually antagonistic interactions between males and females. Here we consider how interlocus sexual conflict can shape the genetic architecture of male sexual traits in the cricket, Teleogryllus commodus. In this species, the ability of each sex to manipulate insemination success significantly alters the selection acting on male courtship call properties. Using a quantitative genetic breeding design we estimated the additive genetic variation in these traits and then predicted the change in variation due to previously documented patterns of sexual selection. Our results indicate that female choice should indeed deplete multivariate genetic variance, but that sexual conflict over insemination success may oppose this loss of variance. We suggest that changes in the direction of selection due to sexually antagonistic interactions will be an important and potentially widespread factor in maintaining multivariate genetic variation.     

A QUANTITATIVE GENETIC ANALYSIS OF OVIPOSITION PREFERENCE AND LARVAL PERFORMANCE ON TWO HOSTS IN THE BRUCHID BEETLE,CALLOSOBRUCHUS MACULATUS

The presence of positive genetic correlations between oviposition or feeding preference for hosts, and performance on those hosts, is of fundamental importance to models of host race formation, sympatric speciation, and the maintenance of genetic variation within phytophagous insect populations. In this paper, I estimate the amount of genetic variation in oviposition preference and larval performance present in two California populations of a cosmopolitan pest of stored legumes, Callosobruchus maculatus (Bruchidae: Coleoptera), and examine whether positive genetic correlations exist between preference and performance. High levels of genetic variation in both preference and performance were detected in one population (Bay Area population, h² = 0.73 for oviposition preference), but not in another population (Davis population). A second estimate of the amount of genetic variation for oviposition preference in the Bay Area population, after three generations of laboratory rearing, supports the hypothesis that the absence of significantly nonzero heritabilities in the Davis population is probably due to the three generations of laboratory rearing prior to the start of the experiment. No positive genetic correlations were detected between preference and any performance character measured. Data are also presented on the genetic correlations between performance on azuki (Vigna angularis) and cowpea (Vigna unguiculata). Genetic correlations were found to be positive for all characters in both populations of C. maculatus (range 0.132 to 0.542).     

PARALLEL EVOLUTION OF LOCAL ADAPTATION AND REPRODUCTIVE ISOLATION IN THE FACE OF GENE FLOW

Parallel evolution of similar phenotypes provides strong evidence for the operation of natural selection. Where these phenotypes contribute to reproductive isolation, they further support a role for divergent, habitat‐associated selection in speciation. However, the observation of pairs of divergent ecotypes currently occupying contrasting habitats in distinct geographical regions is not sufficient to infer parallel origins. Here we show striking parallel phenotypic divergence between populations of the rocky‐shore gastropod, Littorina saxatilis, occupying contrasting habitats exposed to either wave action or crab predation. This divergence is associated with barriers to gene exchange but, nevertheless, genetic variation is more strongly structured by geography than by ecotype. Using approximate Bayesian analysis of sequence data and amplified fragment length polymorphism markers, we show that the ecotypes are likely to have arisen in the face of continuous gene flow and that the demographic separation of ecotypes has occurred in parallel at both regional and local scales. Parameter estimates suggest a long delay between colonization of a locality and ecotype formation, perhaps because the postglacial spread of crab populations was slower than the spread of snails. Adaptive differentiation may not be fully genetically independent despite being demographically parallel. These results provide new insight into a major model of ecologically driven speciation.     

GENE FLOW AND MITOCHONDRIAL DNA VARIATION IN THE KILLIFISH,FUNDULUS HETEROCLITUS

Two subspecies of killifish, Fundulus heteroclitus, inhabit the Atlantic coast of the United States. The contact zones between them are typified by morphological, physiological, nuclear gene, and mitochondrial DNA clines. Considerable debate exists about the importance of restricted gene flow and natural selection in maintaining these clines. To evaluate the relative importance of these two evolutionary forces we employed analysis of mtDNA as an independent measure of gene flow. Solutions of equilibrium diffusion equations suggested that killifish dispersal must be less than 2 km to maintain previously observed allozyme frequency clines in the absence of strong selection. To determine whether populations separated by distances greater than 2 km accumulate significant genetic differences we examined a total of N = 480 individuals from five killifish populations spanning a continuous shoreline distance of 8.4 km. Distribution of the 25 detected haplotypes indicated that most of the variation was within rather than between sampling locations. No evidence of geographic structure was detected, nor were there any significant genetic differences between killifish populations. With these data, gene flow was evaluated by estimating effective migration rate (N_em) between the populations. N_em was estimated from G_st(N_em = 49.4), from F_st (N_em = 24.1), by the private alleles approach (N_em = 18.5) and via four phylogenetic analyses (N_em ranged from 11.4 to 16.9). Regardless of the analysis, N_em was greater than one; the threshold level at which gene flow is strong enough to prevent differentiation due to genetic drift. This suggests that while the characteristics that distinguish Fundulus subspecies may have arisen in isolation, the current clinal distributions exhibited along the Atlantic coast are most likely maintained by selection.     

GEOGRAPHIC VARIATION, FREQUENCY-DEPENDENT SELECTION, AND THE MAINTENANCE OF A FEMALE-LIMITED POLYMORPHISM

A central problem in evolutionary biology is to understand how spatial and temporal variation in selection maintain genetic variation within and among populations. Brown anole lizards ( Anolis sagrei ) exhibit a dorsal pattern polymorphism that is expressed only in females, which occur in diamond,bar, and intermediate diamond-bar morphs. To understand the inheritance of this polymorphism, we conducted a captive breeding study that refuted several single-locus models and supported a two-locus mode of inheritance. To describe geographic variation in morph frequencies, we surveyed 13 populations from two major islands in The Bahamas. Morph frequencies differed substantially between major islands but were highly congruent within each island. Finally, we measured viability selection on each island to test two hypotheses regarding the maintenance of the polymorphism: (1) that spatial variation in selection maintains variation in morph frequencies between islands, and (2) that temporal variation in selection across years maintains variation within islands. Although bar females had relatively lower survival where they were rare, our data do not otherwise suggest that selection varies spatially between islands. However, diamond-bar females were subject to positive frequency-dependent selection across years, and the relative fitness of bar and diamond females alternated across years. We propose that this polymorphism is maintained by temporal variation in selection coupled with the sheltering of alleles via a two-locus inheritance pattern and sex-limited expression.     

THE MAINTENANCE OF GENETIC VARIATION FOR OVIPOSITION RATE IN TWO‐SPOTTED SPIDER MITES: INFERENCES FROM ARTIFICIAL SELECTION

Despite the directional selection acting on life‐history traits, substantial amounts of standing variation for these traits have frequently been found. This variation may result from balancing selection (e.g., through genetic trade‐offs) or from mutation‐selection balance. These mechanisms affect allele frequencies in different ways: Under balancing selection alleles are maintained at intermediate frequencies, whereas under mutation‐selection balance variation is generated by deleterious mutations and removed by directional selection, which leads to asymmetry in the distribution of allele frequencies. To investigate the importance of these two mechanisms in maintaining heritable variation in oviposition rate of the two‐spotted spider mite, we analyzed the response to artificial selection. In three replicate experiments, we selected for higher and lower oviposition rate, compared to control lines. A response to selection only occurred in the downward direction. Selection for lower oviposition rate did not lead to an increase in any other component of fitness, but led to a decline in female juvenile survival. The results suggest standing variation for oviposition rate in this population consists largely of deleterious alleles, as in a mutation‐selection balance. Consequently, the standing variation for this trait does not appear to be indicative of its adaptive potential.     

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color-pattern variation in Lake Erie island water snake (Nerodia sipedon) populations, rates of gene flow among island and mainland populations were estimated from patterns of allozymic variation detected using electrophoresis. Rates of gene flow were high with Nm, the number of migrants per generation, averaging 25.5 among island sites, 9.2 between the Ontario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark-recapture work and of past population size extrapolated from the extent of shoreline habitat, values of m between island and mainland populations ranged from 0.0008–0.01. Synthesis of estimates of the rate of gene flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color-pattern variation in island populations results from a balance between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as drift may have been important in the initial stages of differentiation between island and mainland populations.     

SPATIAL VARIATION IN SELECTIVE REGIMES: SEXUAL SELECTION IN THE WATER STRIDER,GERRIS ODONTOGASTER

Studies of phenotypic selection in natural populations are often concerned with simply detecting selection. In adopting a more mechanistic approach, this study compares the sexual selection regimes in natural populations of the water strider Gerris odontogaster with a priori predictions of selection, based on a number of previous field and laboratory studies of the behavioral mechanisms of selection. In this species, a general reluctance of females to mate allows for intersexual selection for ability to subdue reluctant females in males. Female reluctance to mate has been shown to decrease with increasing population density, suggesting that sexual selection should be weaker in high density populations. Three different populations with large differences in population density were studied. A number of traits including parasite load, body mass, body size and male abdominal process length were found to experience significant sexual selection. The investigated populations differed considerably with regard to the total strength of selection on the measured traits and the form of selection on single traits. In general, males in the population with the highest density experienced the weakest selection for grasping ability. This pattern is ascribed to density-related alterations of female mating behavior. Selection for male grasping ability, as reflected by selection on male abdominal process length, is reduced in high-density populations where reluctant females are more easily subdued. Further, the studied populations differed significantly in mean phenotype and phenotypic variance for male abdominal process length. It is suggested that interpopulational differences in selective regimes may generate local adaptations with respect to male abdominal process length, and that gene flow may contribute to the maintenance of the high genetic variation in this trait. It is further suggested that more empirical effort should be made in quantifying and understanding spatial and temporal variation in selection in natural populations, since this may provide information on the prevalence of local adaptations in metric traits and on the mechanisms of selection.     

NO EFFECT OF ENVIRONMENTAL HETEROGENEITY ON THE MAINTENANCE OF GENETIC VARIATION IN WING SHAPE IN DROSOPHILA MELANOGASTER

Theory suggests that heterogeneous environments should maintain more genetic variation within populations than homogeneous environments, yet experimental evidence for this effect in quantitative traits has been inconsistent. To examine the effect of heterogeneity on quantitative genetic variation, we maintained replicate populations of Drosophila melanogaster under treatments with constant temperatures, temporally variable temperature, or spatially variable temperature with either panmictic or limited migration. Despite observing differences in fitness and divergence in several wing traits between the environments, we did not find any differences in the additive genetic variance for any wing traits among any of the treatments. Although we found an effect of gene flow constraining adaptive divergence between cages in the limited migration treatment, it did not tend to increase within‐population genetic variance relative to any of the other treatments. The lack of any clear and repeatable patterns of response to heterogeneous versus homogeneous environments across several empirical studies suggests that a single general mechanism for the maintenance of standing genetic variation is unlikely; rather, the relative importance of putative mechanisms likely varies considerably from one trait and ecological context to another.     

MUTATION,SELECTION, AND THE MAINTENANCE OF LIFE-HISTORY VARIATION IN A NATURAL POPULATION

In an effort to provide insight into the role of mutation in the maintenance of genetic variance for life-history traits, we accumulated spontaneous mutations in 10 sets of clonal replicates of Daphnia pulex for approximately 30 generations and compared the variance generated by mutation with the standing level of variation in the wild population. Mutations for quantitative traits appear to arise at a fairly high rate in this species, on the order of at least 0.6 per character per generation, but have relatively small heterozygous effects, changing the phenotype by less than 2.5% of the mean. The mean persistence time of a new mutation affecting life-history/body-size traits is approximately 40 generations in the natural population, which requires an average selection coefficient against new mutations of approximately 3% in the heterozygous state. These data are consistent with the idea that the vast majority of standing genetic variance for life-history characters may be largely a consequence of the recurrent introduction of transient cohorts of mutations that are at least conditionally deleterious and raise issues about the meaning of conventional measures of standing levels of variation for fitness-related traits.     

VARIATION IN THE STRENGTH AND SOFTNESS OF SELECTION ON DELETERIOUS MUTATIONS

Deleterious alleles constantly enter populations through mutation. Understanding the nature of selection against such alleles is required to assess their impact on populations. In a subdivided population, two distinct aspects of selection are important: the strength and softness of selection. Using Drosophila melanogaster, we estimated both aspects of selection for each of eight loci across two environments. These data allow us to test conflicting predictions about the factors affecting the softness of selection. First, we show that the softness of selection is not determined by ecological conditions alone. Second, we find that resource limitation makes selection stronger but does not make it softer. Third, we find that wild‐type individuals tend to benefit more than mutants from being reared with competitors of low genetic quality. This means that selection is effectively “harder” on mutants than wild types. A model is presented showing that the sensitivities of mutants and wild types to local competitors differentially affect equilibrium mutation frequency and measures of load.     

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.     

THE EVOLUTIONARY GENETICS AND DEVELOPMENTAL BASIS OF WING PATTERN VARIATION IN THE BUTTERFLY BICYCLUS ANYNANA

We have studied interactions between developmental processes and genetic variation for the eyespot color pattern on the adult dorsal forewing of the nymphalid butterfly, Bicyclus anynana. Truncation selection was applied in both an upward and a downward direction to the size of a single eyespot consisting of rings with wing scales of differing color pigments. High heritabilities resulted in rapid responses to selection yielding divergent lines with very large or very small eyespots. Strong correlated responses occurred in most of the other eyespots on each wing surface. The cells at the center of a presumptive eyespot (the “focus”) act in the early pupal stage to establish the adult wing pattern. The developmental fate of the scale cells within an eyespot is specified by the “signaling” properties of the focus and the “response” thresholds of the epidermis. The individual eyespots can be envisaged as developmental homologues. Grafting experiments performed with the eyespot foci of the selected lines showed that additive genetic variance exists for both the response and, in particular, the signaling components of the developmental system. The results are discussed in the context of how constraints on the evolution of this wing pattern may be related to the developmental organization.     

GENETIC HITCHHIKING AND THE DYNAMIC BUILDUP OF GENOMIC DIVERGENCE DURING SPECIATION WITH GENE FLOW

A major issue in evolutionary biology is explaining patterns of differentiation observed in population genomic data, as divergence can be due to both direct selection on a locus and genetic hitchhiking. “Divergence hitchhiking” (DH) theory postulates that divergent selection on a locus reduces gene flow at physically linked sites, facilitating the formation of localized clusters of tightly linked, diverged loci. “Genome hitchhiking” (GH) theory emphasizes genome‐wide effects of divergent selection. Past theoretical investigations of DH and GH focused on static snapshots of divergence. Here, we used simulations assessing a variety of strengths of selection, migration rates, population sizes, and mutation rates to investigate the relative importance of direct selection, GH, and DH in facilitating the dynamic buildup of genomic divergence as speciation proceeds through time. When divergently selected mutations were limiting, GH promoted divergence, but DH had little measurable effect. When populations were small and divergently selected mutations were common, DH enhanced the accumulation of weakly selected mutations, but this contributed little to reproductive isolation. In general, GH promoted reproductive isolation by reducing effective migration rates below that due to direct selection alone, and was important for genome‐wide “congealing” or “coupling” of differentiation (F_ST) across loci as speciation progressed.     

DIFFERENT SPATIAL SCALES OF ADAPTATION IN THE CLIMBING BEHAVIOR OF PEROMYSCUS MANICULATUS: GEOGRAPHIC VARIATION,NATURAL SELECTION,AND GENE FLOW

Patterns of geographic variation in tree-climbing ability of Peromyscus maniculatus were used to examine the influence of spatial variation in natural selection and gene flow on the genetic divergence of climbing behavior among populations. Offspring of adults of two subspecies sampled from 10 localities in montane conifer forest, conifer woodland, and desert scrub/grassland habitats were raised in the laboratory and tested to determine their tree-climbing ability (the maximum diameter artificial rod that a mouse could climb). Comparisons of mean rod-climbing scores revealed that individuals of P. m. rufinus sampled from montane conifer forest and conifer woodland in Arizona were better climbers than P. m. sonoriensis sampled from conifer woodland and desert habitats in Nevada. This result was consistent with the hypothesis that natural selection has produced large-scale adaptation in climbing behavior. However, the climbing ability of P. m. sonoriensis sampled from conifer woodland habitats on isolated mountaintops in Nevada has not evolved in response to natural selection to the degree expected. In addition, populations sampled from desert grassland habitat, adjacent to woodland P. m. rufinus in Arizona, have climbing abilities that are not significantly different from conifer woodland populations. These observations indicate that local adaptation was constrained. An estimate of the heritability of climbing ability (h² = 0.352 ± 0.077) indicates that lack of a response to selection was not due to the absence of additive genetic variation. In addition, regressions of interpopulation differences on the degree of geographic isolation between pairs of populations do not support the hypothesis that gene flow between habitats has constrained evolution. Instead, a combination of historical events and insufficient time to respond to selection appears to have influenced geographic variation and the spatial scale of adaptation in climbing ability.     

THE GENETIC BASIS OF DIFFERENCES IN GROWTH AND BEHAVIOR OF SPECIALIST AND GENERALIST HERBIVORE SPECIES: SELECTION ON HYBRIDS OF HELIOTHIS VIRESCENS AND HELIOTHIS SUBFLEXA (LEPIDOPTERA)

Two species of moths and their hybrids were studied to determine the genetic basis of host range. One species, Heliothis virescens (HV), has a broad host range and is an agricultural pest on cotton (Malvaceae), soybean (Fabaceae), and tobacco (Solanaceae). The other species, Heliothis subflexa (HS), has a narrow host range, feeding on plants in the genus Physalis (Solanaceae). Experiments were done to determine whether the abilities to feed on cotton, soybean, and tobacco were each under separate genetic control in HV (genetically uncorrelated) or whether feeding on all three hosts was genetically correlated. By repeatedly backcrossing hybrids to HS while selecting for high larval survival and weight on soybean, loci conferring the ability to feed on soybean were moved from HV into the genetic background of the specialist, HS. After six generations of selection on soybean we tested the selected line on soybean, cotton, Physalis, and tobacco to determine if ability to feed on soybean was correlated with feeding ability on these other hosts. We found no evidence to support the hypothesis that feeding on all three hosts was correlated and controlled by a single locus. Compared to HS, the survival and weight of the selected line were significantly higher on soybean, similar on tobacco and Physalis, and slightly, but not significantly, higher on cotton. We also conducted a behavioral choice test to determine if larval preference for soybean was correlated with preference for other hosts. The selected line larvae had higher preference for soybean and cotton than HS. Preference for tobacco was equal in the selected line and in HS. These results support the trends seen in survival and growth tests where performances on soybean and cotton appeared partially correlated and performances on soybean and tobacco were uncorrelated. In conclusion, the generalist, HV, did not have a single set of loci that governed feeding on soybean, cotton and tobacco. It is likely that some of the loci governing performance and preference for soybean also contributed to performance and preference for cotton and that the soybean loci examined did not interact negatively to lower performance and preference for Physalis, the specialist's host. This approach can be applied more widely to address evolutionary questions about host range and other ecological traits.