Contrasting levels of evolutionary potential in populations of the invasive plant Polygonum cespitosum

The amount of quantitative genetic variation within an invasive species influences its ability to adapt to conditions in the new range and its long-term persistence. Consequently, this aspect of genetic diversity (or evolutionary potential) can be a key factor in the success of species invasions. Previous studies have compared the evolutionary potential of populations in introduced versus native ranges of invasive species, but to date no study has examined differences among introduced-range populations of such species in levels of quantitative genetic variation expressed in ecologically relevant environments. We assessed quantitative variation of fitness, life-history, and functional traits in six geographically separate introduced-range populations of the invasive annual Polygonum cespitosum, by comparing norms of reaction for a large sample of genotypes (16–19 per population) expressed in response to two glasshouse environments simulating contrasting habitats in this new range. Patterns of reaction norm diversity varied considerably among the 6 populations studied. Two populations showed very little quantitative genetic variation in both environments. In contrast, two other populations contained significant genetic variation for fitness and life-history traits in the form of genotypes with low performance in both habitats. Finally, two populations showed significant norm of reaction diversity in the form of cross-over interaction: genotypes that performed relatively well in one environment did poorly in the other. Differences among populations in potential selective response are likely to affect the dynamics and future spread of P. cespitosum, since specific populations will likely contribute differently to the invasion process. More generally, our results suggest that the evolutionary component of long-term invasion success may depend on population rather than on species-level processes.     

The role of environment and core‐margin effects on range‐wide phenotypic variation in a montane grasshopper

The integration of genetic information with ecological and phenotypic data constitutes an effective approach to gain insight into the mechanisms determining interpopulation variability and the evolutionary processes underlying local adaptation and incipient speciation. Here, we use the Pyrenean Morales grasshopper (Chorthippus saulcyi moralesi) as study system to (i) analyse the relative role of genetic drift and selection in range‐wide patterns of phenotypic differentiation and (ii) identify the potential selective agents (environment, elevation) responsible for variation. We also test the hypothesis that (iii) the development of dispersal‐related traits is associated with different parameters related to population persistence/turnover, including habitat suitability stability over the last 120 000 years, distance to the species distribution core and population genetic variability. Our results indicate that selection shaped phenotypic differentiation across all the studied morphological traits (body size, forewing length and shape). Subsequent analyses revealed that among‐population differentiation in forewing length was significantly explained by a temperature gradient, suggesting an adaptive response to thermoregulation or flight performance under contrasting temperature regimes. We found support for our hypothesis predicting a positive association between the distance to the species distribution core and the development of dispersal‐related morphology, which suggests an increased dispersal capability in populations located at range edges that, in turn, exhibit lower levels of genetic variability. Overall, our results indicate that range‐wide patterns of phenotypic variation are partially explained by adaptation in response to local environmental conditions and differences in habitat persistence between core and peripheral populations.     

Demographic and genetic invasion history of a 9-year-old roadside population of Bunias orientalis L. (Brassicaceae)

The population history of a 9-year-old roadside population of the invasive plant Bunias orientalis was reconstructed by demographic analysis including size, position, age (determined by herbchronology) and RAPD-PCR patterns of individual plants. We evaluated emerging patterns of population growth and genetic structure during a full period of population development under typical site conditions (anthropogenic disturbance) and their possible consequences for the invasion potential of the species. The population has grown rapidly and continuously (though with slowing geometric population increase) during the 9 years since its foundation, filling the space available in the study area. Genetic variation (RAPD markers) was already high in the founder cohorts and remained at the same level throughout population development (variance fluctuations <15%). Both results may be related to the mowing management at the site which seems to promote population growth of B. orientalis relative to other co-occuring species and to prevent the genetic drift and the development of spatial genetic structure that would be expected under isolation-by-distance models. Large founder plants had comparatively low genetic variance and were more closely related to younger cohorts than were small founder plants, indicating that selection acted during population development. Overall, the current anthropogenic disturbance regimes may contribute to high genetic variability by artificially increasing gene flow and thereby promoting the adaptability of invasive species to the often unpredictable conditions at disturbed sites. Our approach using retrospective demographic investigation allows the detection of spatio-temporal microscale patterns in genetic and phenotypic variation. Thus it allows a thorough understanding of local invasions of perennial herbaceous plants. Received: 23 November 1998 / Accepted: 14 April 1999     

Development time and size-related traits in the oriental blowfly, Chrysomya megacephala along a latitudinal gradient from China

Organisms can respond to variation in temperature through the direct effect of temperature on phenotypes (phenotypic plasticity), or through long-term adaptation to temperature (and thus evolution of either mean size or thermal reaction norm). We examined the effects of various temperatures (of 20 and 30 °C) on development time, adult body size (body length and body width) and pre-adult survivorship in six populations of Chrysomya megacephala, collected at different latitudes. We found that temperature changes induced substantial plasticity in terms of development time, body size and pre-adult survivorship, indicating that developmental temperature significantly affects growth and life history traits of C. megacephala. We also detected genetic differences among populations for body size and development time, and these two traits exhibited highly significant variations in the responses of different populations to various temperature conditions, indicating genetic differences among populations in terms of thermal reaction norms. The latitude of origin of the different populations (and hence mean temperature regimes in the environments from where the populations originated) did not appear to fully explain these genetic differences. In short, changes in development time and body size in C. megacephala can be regarded as adaptations to changing thermal regimes.     

Population divergence along lines of genetic variance and covariance in the invasive plant Lythrum salicaria in eastern North America

Evolution during biological invasion may occur over contemporary timescales, but the rate of evolutionary change may be inhibited by a lack of standing genetic variation for ecologically relevant traits and by fitness trade-offs among them. The extent to which these genetic constraints limit the evolution of local adaptation during biological invasion has rarely been examined. To investigate genetic constraints on life-history traits, we measured standing genetic variance and covariance in 20 populations of the invasive plant purple loosestrife (Lythrum salicaria) sampled along a latitudinal climatic gradient in eastern North America and grown under uniform conditions in a glasshouse. Genetic variances within and among populations were significant for all traits; however, strong intercorrelations among measurements of seedling growth rate, time to reproductive maturity and adult size suggested that fitness trade-offs have constrained population divergence. Evidence to support this hypothesis was obtained from the genetic variance-covariance matrix (G) and the matrix of (co)variance among population means (D), which were 79.8% (95% C.I. 77.7-82.9%) similar. These results suggest that population divergence during invasive spread of L. salicaria in eastern North America has been constrained by strong genetic correlations among life-history traits, despite large amounts of standing genetic variation for individual traits.     

Plant reproductive systems and evolution during biological invasion

Recent biological invasions provide opportunities to investigate microevolution during contemporary timescales. The tempo and scope of local adaptation will be determined by the intensity of natural selection and the amounts and kinds of genetic variation within populations. In flowering plants, genetic diversity is strongly affected by interactions between reproductive systems and stochastic forces associated with immigration history and range expansion. Here, we explore the significance of reproductive system diversity for contemporary evolution during plant invasion. We focus in particular on how reproductive modes influence the genetic consequences of long-distance colonization and determine the likelihood of adaptive responses during invasion. In many clonal invaders, strong founder effects and restrictions on sexual reproduction limit opportunities for local adaptation. In contrast, adaptive changes to life-history traits should be a general expectation in both outbreeding and inbreeding species. We provide evidence that evolutionary modifications to reproductive systems promote the colonizing ability of invading populations and that reproductive timing is an important target of selection during range expansion. Knowledge of the likelihood and speed at which local adaptation evolves in invasive plants will be particularly important for management practices when evolutionary changes enhance ecological opportunities and invasive spread.     

A critical review of adaptive genetic variation in Atlantic salmon: implications for conservation

Here we critically review the scale and extent of adaptive genetic variation in Atlantic salmon (Salmo salar L.), an important model system in evolutionary and conservation biology that provides fundamental insights into population persistence, adaptive response and the effects of anthropogenic change. We consider the process of adaptation as the end product of natural selection, one that can best be viewed as the degree of matching between phenotype and environment. We recognise three potential sources of adaptive variation: heritable variation in phenotypic traits related to fitness, variation at the molecular level in genes influenced by selection, and variation in the way genes interact with the environment to produce phenotypes of varying plasticity. Of all phenotypic traits examined, variation in body size (or in correlated characters such as growth rates, age of seaward migration or age at sexual maturity) generally shows the highest heritability, as well as a strong effect on fitness. Thus, body size in Atlantic salmon tends to be positively correlated with freshwater and marine survival, as well as with fecundity, egg size, reproductive success, and offspring survival. By contrast, the fitness implications of variation in behavioural traits such as aggression, sheltering behaviour, or timing of migration are largely unknown. The adaptive significance of molecular variation in salmonids is also scant and largely circumstantial, despite extensive molecular screening on these species. Adaptive variation can result in local adaptations (LA) when, among other necessary conditions, populations live in patchy environments, exchange few or no migrants, and are subjected to differential selective pressures. Evidence for LA in Atlantic salmon is indirect and comes mostly from ecological correlates in fitness-related traits, the failure of many translocations, the poor performance of domesticated stocks, results of a few common-garden experiments (where different populations were raised in a common environment in an attempt to dissociate heritable from environmentally induced phenotypic variation), and the pattern of inherited resistance to some parasites and diseases. Genotype x environment interactions occurr for many fitness traits, suggesting that LA might be important. However, the scale and extent of adaptive variation remains poorly understood and probably varies, depending on habitat heterogeneity, environmental stability and the relative roles of selection and drift. As maladaptation often results from phenotype-environment mismatch, we argue that acting as if populations are not locally adapted carries a much greater risk of mismanagement than acting under the assumption for local adaptations when there are none. As such, an evolutionary approach to salmon conservation is required, aimed at maintaining the conditions necessary for natural selection to operate most efficiently and unhindered. This may require minimising alterations to native genotypes and habitats to which populations have likely become adapted, but also allowing for population size to reach or extend beyond carrying capacity to encourage competition and other sources of natural mortality.     

Biological invasions and phenotypic evolution: a quantitative genetic perspective

Among the many different components of global environmental change, biological invasions represent the one with the most long-term ecological and evolutionary consequences, as effects are irreversible. Although the ecological impact of invasive species has been under great scrutiny, its evolutionary aspects and consequences have remained less explored. Once established, an important part of the success of an invasive species will depend on the presence of genetic variation in populations at the geographic boundaries upon which natural selection can act. This information is integrated in G, the matrix of additive genetic variances and covariances for a suite of traits. The G-matrix shows the restrictions and potentialities of adaptive evolution and, together with natural selection determine the direction and rate of phenotypic evolution. Here I propose that a geographic analysis of G in populations of the introduced and native range becomes essential to understand critical evolutionary issues associated with invasion success.     

Molecular genetic approaches to elucidate the ecological and evolutionary issues associated with biological invasions

Biological invasions may cause serious damage to the native environments and threaten the native biodiversity. Molecular genetic approaches have been found to be powerful tools for investigating the ecological and evolutionary aspects of biological invasions because the genetic structure and level of genetic variation of an invasive species are changed following its invasion. The present article reviews the use of molecular markers in addressing various aspects of invasive species. The application of these techniques has shown that many invasive species are actually "cryptic" species – species whose uniqueness is only recognizable at the genetic level. An estimation of the actual number of invasive species is essential when evaluating its ecological and economic impacts. Molecular genetic approaches have also enabled the source populations of invasive species to be identified. Reconstructions of invasion histories are crucial to preventing future invasions and conserving the native biodiversity, while comparisons of genetic variations between the native and introduced populations provide valuable opportunities to elucidate the mechanisms of rapid adaptation demonstrated by many invasive species.     

Introduced plants of Lupinus polyphyllus are larger but flower less frequently than conspecifics from the native range: Results of the first year

Introduced species, which establish in novel environments, provide an opportunity to explore trait evolution and how it may contribute to the distribution and spread of species. Here, we explore trait changes of the perennial herb Lupinus polyphyllus based on 11 native populations in the western USA and 17 introduced populations in Finland. More specifically, we investigated whether introduced populations outperformed native populations in traits measured in situ (seed mass) and under common garden conditions during their first year (plant size, flowering probability, and number of flowering shoots). We also explored whether climate of origin (temperature) influenced plant traits and quantified the degree to which trait variability was explained collectively by country and temperature as compared to other population‐level differences. Three out of four plant traits differed between the native and introduced populations; only seed mass was similar between countries, with most of its variation attributed to other sources of intraspecific variation not accounted for by country and temperature. Under common garden conditions, plants originating from introduced populations were larger than those originating from native populations. However, plants from the introduced range flowered less frequently and had fewer flowering shoots than their native‐range counterparts. Temperature of a population''s origin influenced plant size in the common garden, with plant size increasing with increasing mean annual temperature in both native and introduced populations. Our results of the first year reveal genetic basis for phenotypic differences in some fitness‐related traits between the native and introduced populations of L. polyphyllus. However, not all of these trait differences necessarily contribute to the invasion success of the species and thus may not be adaptive, which raises a question how persistent the trait differences observed in the first year are later in individuals’ life for perennial herbs.     

Invasion success and genetic diversity of introduced populations of guppies Poecilia reticulata in Australia

High genetic diversity is thought to characterize successful invasive species, as the potential to adapt to new environments is enhanced and inbreeding is reduced. In the last century, guppies, Poecilia reticulata, repeatedly invaded streams in Australia and elsewhere. Quantitative genetic studies of one Australian guppy population have demonstrated high additive genetic variation for autosomal and Y-linked morphological traits. The combination of colonization success, high heritability of morphological traits, and the possibility of multiple introductions to Australia raised the prediction that neutral genetic diversity is high in introduced populations of guppies. In this study we examine genetic diversity at nine microsatellite and one mitochondrial locus for seven Australian populations. We used mtDNA haplotypes from the natural range of guppies and from domesticated varieties to identify source populations. There were a minimum of two introductions, but there was no haplotype diversity within Australian populations, suggesting a founder effect. This was supported by microsatellite markers, as allelic diversity and heterozygosity were severely reduced compared to one wild source population, and evidence of recent bottlenecks was found. Between Australian populations little differentiation of microsatellite allele frequencies was detected, suggesting that population admixture has occurred historically, perhaps due to male-biased gene flow followed by bottlenecks. Thus success of invasion of Australia and high additive genetic variance in Australian guppies are not associated with high levels of diversity at molecular loci. This finding is consistent with the release of additive genetic variation by dominance and epistasis following inbreeding, and with disruptive and negative frequency-dependent selection on fitness traits.     

Investigating the genetic load of an emblematic invasive species: the case of the invasive harlequin ladybird Harmonia axyridis

Introduction events can lead to admixture between genetically differentiated populations and bottlenecks in population size. These processes can alter the adaptive potential of invasive species by shaping genetic variation, but more importantly, they can also directly affect mean population fitness either increasing it or decreasing it. Which outcome is observed depends on the structure of the genetic load of the species. The ladybird Harmonia axyridis is a good example of invasive species where introduced populations have gone through admixture and bottleneck events. We used laboratory experiments to manipulate the relatedness among H. axyridis parental individuals to assess the possibility for heterosis or outbreeding depression in F₁ generation offspring for two traits related to fitness (lifetime performance and generation time). We found that inter‐populations crosses had no major impact on the lifetime performance of the offspring produced by individuals from either native or invasive populations. Significant outbreeding depression was observed only for crosses between native populations for generation time. The absence of observed heterosis is indicative of a low occurrence of fixed deleterious mutations within both the native and invasive populations of H. axyridis. The observed deterioration of fitness in native inter‐population crosses most likely results from genetic incompatibilities between native genomic backgrounds. We discuss the implications of these results for the structure of genetic load in H. axyridis in the light of the available information regarding the introduction history of this species.     

Rapid differentiation of plasticity in life history and morphology during invasive range expansion and concurrent local adaptation in the horned beetle Onthophagus taurus

Understanding the interplay between genetic differentiation, ancestral plasticity, and the evolution of plasticity during adaptation to environmental variation is critical to predict populations’ responses to environmental change. However, the role of plasticity in rapid adaptation in nature remains poorly understood. We here use the invasion of the horned beetle Onthophagus taurus in the United States during the last half century to study the contribution of ancestral plasticity and post-invasion evolution of plastic responses in rapid population differentiation. We document latitudinal variation in life history and morphology, including genetic compensation in development time and body size, likely adaptive responses to seasonal constraints in the North. However, clinal variation in development time and size was strongly dependent on rearing temperature, suggesting that population differentiation in plasticity played a critical role in successful adaptation on ecological timescales. Clinal variation in wing shape was independent of ancestral plasticity, but correlated with derived plasticity, consistent with evolutionary interdependence. In contrast, clinal variation in tibia shape aligned poorly with thermal plasticity. Overall, this study suggests that post-invasion evolution of plasticity contributed to range expansions and concurrent adaptation to novel climatic conditions.     

Variations in life history traits and flight capacity among populations of the light brown apple moth Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae)

Abstract The empirical study of interpopulation variation in life history and other fitness traits has been an important approach to understanding the ecology and evolution of organisms and gaining insight into possible sources of variation. We report a quantitative analysis for variations of five life history traits (larval developmental time, adult body weight, adult lifespan, age at first reproduction, total fecundity) and flight capacity among populations of Epiphyas postvittana originating from four localities in Australia and one in New Zealand. These populations were compared at two temperatures (15° and 25°C) after being maintained under uniform laboratory conditions for 1.5 generations, so that the relative role of genetic divergence and phenotypic plasticity in determining interpopulation variation could be disentangled. Genetic differentiation between populations was shown in all measured traits, with the greatest divergence occurring in developmental time, fecundity and adult body size. However, these traits were highly sensitive to changes in environmental temperatures; and furthermore, significant interactions between population and temperature occurred in all traits except for flight capacity of female moths. Thus, phenotypic plasticity may be another cause of interpopulation variation. The interpopulation variation for some measured traits was apparently related to climatic differences found where the populations originated. Individuals of the populations from the warmer climates tended to develop more slowly at immature stages, producing smaller and less fecund moths but with stronger flight capacity, in comparison to those from the cooler regions. It seems, therefore, that natural populations of E. postvittana have evolved different strategies to cope with local environmental conditions.     

Contrasting patterns of genetic diversity and spatial structure in an invasive symbiont-host association

Do host invaders and their associated symbiont co-invaders have different genetic responses to the same invasion process? To answer this question, we compared genetic patterns of native and exotic populations of an invasive symbiont-host association. This is an approach applied by very few studies, of which most are based on parasites with complex life cycles. We used the mitochondrial genetic marker cytochrome oxidase subunit I (COI) to investigate a non-parasitic freshwater ectosymbiont with direct life-cycle, low host specificity and well-documented invasion history. The study system was the crayfish Procambarus clarkii and its commensal ostracod Ankylocythere sinuosa, sampled in native (N American) and exotic (European) ranges. Results of analyses indicated: (1) higher genetic diversity in the symbiont than its host; (2) genetic diversity loss in the exotic range for both species, but less pronounced in the symbiont; (3) native populations genetically structured in space, with stronger patterns in the symbiont and (4) loss of spatial genetic structure in the exotic range in both species. The combination of historical, demographic and genetic data supports a higher genetic diversity of source populations and a higher propagule size that allowed the symbiont to overcome founder effects better than its host co-invader. Thus, the symbiont might be endowed with a higher adaptive potential to new hosts or off-host environmental pressures expected in the invasive range. We highlight the usefulness of this relatively unexplored kind of symbiont-host systems in the invasion context to test important ecological and evolutionary questions.     

Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions

Invasive species are predicted to suffer from reductions in genetic diversity during founding events, reducing adaptive potential. Integrating evidence from two literature reviews and two case studies, we address the following questions: How much genetic diversity is lost in invasions? Do multiple introductions ameliorate this loss? Is there evidence for loss of diversity in quantitative traits? Do invaders that have experienced strong bottlenecks show adaptive evolution? How do multiple introductions influence adaptation on a landscape scale? We reviewed studies of 80 species of animals, plants, and fungi that quantified nuclear molecular diversity within introduced and source populations. Overall, there were significant losses of both allelic richness and heterozygosity in introduced populations, and large gains in diversity were rare. Evidence for multiple introductions was associated with increased diversity, and allelic variation appeared to increase over long timescales (~100 years), suggesting a role for gene flow in augmenting diversity over the long‐term. We then reviewed the literature on quantitative trait diversity and found that broad‐sense variation rarely declines in introductions, but direct comparisons of additive variance were lacking. Our studies of Hypericum canariense invasions illustrate how populations with diminished diversity may still evolve rapidly. Given the prevalence of genetic bottlenecks in successful invading populations and the potential for adaptive evolution in quantitative traits, we suggest that the disadvantages associated with founding events may have been overstated. However, our work on the successful invader Verbascum thapsus illustrates how multiple introductions may take time to commingle, instead persisting as a ‘mosaic of maladaptation’ where traits are not distributed in a pattern consistent with adaptation. We conclude that management limiting gene flow among introduced populations may reduce adaptive potential but is unlikely to prevent expansion or the evolution of novel invasive behaviour.     

Stability of genetic variance and covariance for reproductive characters in the face of climate change in a wild bird population

Global warming has had numerous effects on populations of animals and plants, with many species in temperate regions experiencing environmental change at unprecedented rates. Populations with low potential for adaptive evolutionary change and plasticity will have little chance of persistence in the face of environmental change. Assessment of the potential for adaptive evolution requires the estimation of quantitative genetic parameters, but it is as yet unclear what impact, if any, global warming will have on the expression of genetic variances and covariances. Here we assess the impact of a changing climate on the genetic architecture underlying three reproductive traits in a wild bird population. We use a large, long-term, data set collected on great tits (Parus major) in Wytham Woods, Oxford, and an 'animal model' approach to quantify the heritability of, and genetic correlations among, laying date, clutch size and egg mass during two periods with contrasting temperature conditions over a 40-year period (1965-1988 [cooler] vs. 1989-2004 [warmer]). We found significant additive genetic variance and heritability for all traits under both temperature regimes. We also found significant negative genetic covariances and correlations between clutch size and egg weight during both periods, and among laying date and clutch size in the colder years only. The overall G matrix comparison among periods, however, showed only a minor difference among periods, thus suggesting that genotype by environment interactions are negligible in this context. Our results therefore suggest that despite substantial changes in temperature and in mean laying date phenotype over the last decades, and despite the large sample sizes available, we are unable to detect any significant change in the genetic architecture of the reproductive traits studied.     

Living in isolation – population structure,reproduction, and genetic variation of the endangered plant species Dianthus gratianopolitanus (Cheddar pink)

The endangered plant species Dianthus gratianopolitanus exhibits a highly fragmented distribution range comprising many isolated populations. Based upon this pattern of distribution, we selected a study region in Switzerland with a lower magnitude of isolation (Swiss Jura) and another study region in Germany with a higher degree of isolation (Franconian Jura). In each region, we chose ten populations to analyze population structure, reproduction, and genetic variation in a comparative approach. Therefore, we determined population density, cushion size, and cushion density to analyze population structure, investigated reproductive traits, including number of flowers, capsules, and germination rate, and analyzed amplified fragment length polymorphisms to study genetic variation. Population and cushion density were credibly higher in German than in Swiss populations, whereas reproductive traits and genetic variation within populations were similar in both study regions. However, genetic variation among populations and isolation by distance were stronger in Germany than in Switzerland. Generally, cushion size and density as well as flower and capsule production increased with population size and density, whereas genetic variation decreased with population density. In contrast to our assumptions, we observed denser populations and cushions in the region with the higher magnitude of isolation, whereas reproductive traits and genetic variation within populations were comparable in both regions. This corroborates the assumption that stronger isolation must not necessarily result in the loss of fitness and genetic variation. Furthermore, it supports our conclusion that the protection of strongly isolated populations contributes essentially to the conservation of a species' full evolutionary potential.     

Genetic architecture and phenotypic plasticity of thermally-regulated traits in an eruptive species, <Emphasis Type="Italic">Dendroctonus ponderosae</Emphasis>

Phenotypic plasticity in thermally-regulated traits enables close tracking of changing environmental conditions, and can thereby enhance the potential for rapid population increase, a hallmark of outbreak insect species. In a changing climate, exposure to conditions that exceed the capacity of existing phenotypic plasticity may occur. Combining information on genetic architecture and trait plasticity among populations that are distributed along a latitudinal cline can provide insight into how thermally-regulated traits evolve in divergent environments and the potential for adaptation. Dendroctonus ponderosae feed on Pinus species in diverse climatic regimes throughout western North America, and show eruptive population dynamics. We describe geographical patterns of plasticity in D. ponderosae development time and adult size by examining reaction norms of populations from multiple latitudes. The relative influence of additive and non-additive genetic effects on population differences in the two phenotypic traits at a single temperature is quantified using line-cross experiments and joint-scaling tests. We found significant genetic and phenotypic variation among D. ponderosae populations. Simple additive genetic variance was not the primary source of the observed variation, and dominance and epistasis contributed greatly to the genetic divergence of the two thermally-regulated traits. Hybrid breakdown was also observed in F₂ hybrid crosses between northern and southern populations, further indication of substantial genetic differences among clinal populations and potential reproductive isolation within D. ponderosae. Although it is unclear what maintains variation in the life-history traits, observed plasticity in thermally-regulated traits that are directly linked to rapid numerical change may contribute to the outbreak nature of D. ponderosae, particularly in a changing climate.