SEASONALLY VARYING DIET QUALITY AND THE QUANTITATIVE GENETICS OF DEVELOPMENT TIME AND BODY SIZE IN BIRCH FEEDING INSECTS

Abstract Genetic variance‐covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life‐history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch‐insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet‐imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early‐summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild‐wide patterns in the genetic architectures brought about by seasonal variation in resource quality.     

ADAPTIVE GENETIC VARIATION IN GROWTH AND DEVELOPMENT OF TADPOLES OF THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX

The distribution and proportion of the sexual species Rana lessonae to the hemiclonal hybrid R. esculenta among natural habitats suggests that these anurans may differ in adaptive abilities. I used a half-sib design to partition phenotypic and quantitative genetic variation in tadpole responses at two food levels into causal variance components. Rana lessonae displays strong phenotypic variation across food levels. Growth rate is strictly determined by environmental factors and includes weak maternal effects. Larval period and body size at metamorphosis both contain moderate levels of additive genetic variance. The sire x food interactions and the lack of environmental correlations indicate that adaptive phenotypic plasticity is present in both of these traits. In contrast, R. esculenta displays less phenotypic variation across food levels, especially for larval period. Variation in body size at metamorphosis is underlain by genetic variation as shown by high levels of additive genetic variance, yet growth rate and larval period are not. Significant environmental correlations between larval period at high food level and growth, larval period, and body size at low food, indicate phenotypic plasticity is absent. A positive phenotypic correlation between body size at metamorphosis and larval period for R. lessonae at both food levels suggests a trade-off between growing large and metamorphosing quickly to escape predation or pond drying. The lack of a similar correlation for R. esculenta at the high food level suggests it may be less constrained. Different levels of adaptive genetic variation among larval traits suggest that the sexual species and the hybridogenetic hemiclone differ in their abilities to cope with temporally and spatially heterogeneous environments.     

Photoperiod affects compensating developmental rate across latitudes in the damselfly Lestes sponsa

1. Although there is a great deal of theoretical and empirical data about the life history responses of time constraints in organisms, little is known about the latitude‐compensating mechanism that enables northern populations' developmental rates to compensate for latitude. To investigate the importance of photoperiod on development, offspring of the obligatory univoltine damselfly Lestes sponsa from two populations at different latitudes (53°N and 63°N) were raised in a common laboratory environment at both northern and southern photoperiods that corresponded to the sites of collection. 2. Egg development time was shorter under northern photoperiod regimes for both populations. However, the northern latitude population showed a higher phenotypic plasticity response to photoperiod compared with the southern latitude population, suggesting a genetic difference in egg development time in response to photoperiod. 3. Larvae from both latitudes expressed shorter larval development time and faster growth rates under northern photoperiod regimes. There was no difference in phenotypic plastic response between northern and southern latitude populations with regard to development time. 4. Data on field collected adults showed that adult sizes decreased with an increase in latitude. This adult size difference was a genetically fixed trait, as the same size difference between populations was also found when larvae were reared in the laboratory. 5. The results suggest phenotypic plasticity responses in life history traits to photoperiod, but also genetic differences between north and south latitude populations in response to photoperiod, which indicates the presence of a latitudinal compensating mechanism that is triggered by a photoperiod.     

Trade‐offs between growth and reproduction: an analysis of the quantitative genetic evidence

The assumption of a trade‐off between development time and fecundity, resulting from a positive correlation between body size and fecundity and between body size and development time, is a common feature of life history models. The present paper examines the evidence for such a trade‐off as indicated by genetic correlations between traits. The genetic covariances between traits are derived using a model in which maturation occurs when the organism achieves a genetically variable size threshold, and fecundity is an allometric function of body size with one genetically variable parameter (excluding body size itself). This model predicts that the heritabilities of the life history traits (growth rate, development time, fecundity) will not necessarily be less than the heritability of adult size (i.e. morphological traits). It is shown that if growth rate is genetically correlated with adult size then it is not possible, in general, to predict the sign of the genetic correlation between development time and fecundity. For particular cases the signs of the covariances between traits can be predicted. These predictions are tested using data drawn from the literature.     

An integrated analysis of phenotypic selection on insect body size and development time

Most studies of phenotypic selection do not estimate selection or fitness surfaces for multiple components of fitness within a unified statistical framework. This makes it difficult or impossible to assess how selection operates on traits through variation in multiple components of fitness. We describe a new generation of aster models that can evaluate phenotypic selection by accounting for timing of life‐history transitions and their effect on population growth rate, in addition to survival and reproductive output. We use this approach to estimate selection on body size and development time for a field population of the herbivorous insect, Manduca sexta (Lepidoptera: Sphingidae). Estimated fitness surfaces revealed strong and significant directional selection favoring both larger adult size (via effects on egg counts) and more rapid rates of early larval development (via effects on larval survival). Incorporating the timing of reproduction and its influence on population growth rate into the analysis resulted in larger values for size in early larval development at which fitness is maximized, and weaker selection on size in early larval development. These results illustrate how the interplay of different components of fitness can influence selection on size and development time. This integrated modeling framework can be readily applied to studies of phenotypic selection via multiple fitness components in other systems.     

Environment-dependent genetic correlations between development time and body mass in a scorpionfly

Development time and body mass at maturation are two important fitness traits fundamental for our understanding of life history theory. Generally, fast development is associated with small adult body mass, as it will take longer to grow large. However, the strength of this trade-off may depend on average food availability, as the potential benefit of long development will depend on the rate of food intake. Here, I report results of a food manipulation experiment during larval development of the scorpionfly Panorpa cognata (Insecta, Mecoptera). Development time showed considerable genetic variation, yet food level had no influence and there was a strong genetic correlation in development time across environments. As expected, larval and adult body weight was significantly affected by food availability. Furthermore, body mass was influenced by a highly significant genotype-by-environment interaction. The reaction norm for body mass in response to food treatment was much stronger in families with long development time compared with rapidly developing genotypes. This effect was accompanied by a shift in the genetic correlation between development time and body size when comparing the two food levels. Specifically, the genetic correlation between body mass and development time changed from being positive at high food levels to a negative genetic correlation at low food levels. These results are consistent with other empirical findings demonstrating a similar shift in genetic correlations between body mass and development time when comparing favourable and unfavourable environmental conditions.     

Genetic variation in the life‐history traits of Epiphyas postvittana: population structure and local adaptation

The light brown apple moth, Epiphyas postvittana (Walker) shows high intraspecific variability in morphological, physiological, demographic and behavioural characters. To gain insight into the extent of adaptation and evolutionary changes in response to environmental heterogeneity in this species, quantitative genetic analyses of life‐history variation were conducted for two natural populations under two thermal conditions (23°C and 28°C). Paternal half‐sib heritability and genetic correlation in six life‐history traits (i.e. development time, adult body weight, adult lifespan, age at first reproduction, the number of eggs laid during the first 5 days after emergence, and total fecundity) were compared. Significant heritabilities were shown consistently in development time; this is also true for adult body weight, except for the Canberra population at 23°C. However, neither population differences nor the effect of temperature were statistically detectable for any of these heritabilities, confirming the genetically determined flexibility. Positive genetic correlations between development time and adult body weight, and negative genetic correlations between the number of eggs laid during the first 5 days and adult lifespan were present for these populations at both temperatures, indicating the presence of genetic constraints. Pairwise comparisons of genetic correlations revealed the heterogeneity of the two populations and across temperatures. These results suggest that the structure of genetic covariance might have changed significantly during the divergence of natural populations and in response to the alteration of environmental conditions in E. postvittana.     

Diet and a developmental time constraint alter life‐history trade‐offs in a caddis fly (Trichoptera: Limnephilidae)

Environmental factors influence variation in life histories by affecting growth, development, and reproduction. We conducted an experiment in outdoor mesocosms to examine how diet and a time constraint on juvenile development (pond‐drying) influence life‐history trade‐offs (growth, development, adult body mass) in the caddis fly Limnephilus externus (Trichoptera: Limnephilidae). We predicted that: (1) diet supplementation would accelerate larval growth and development, and enhance survival to adulthood; (2) pond‐drying would accelerate development and increase larval mortality; and (3) the relationship between adult mass and age at maturity would be negative. Diet supplementation did lead to larger adult mass under nondrying conditions, but did not significantly alter growth or development rates. Contrary to predictions, pond‐drying reduced growth rates and delayed development. The slope (positive or negative) of the female mass–age at maturity relationship depended on interactions with diet or pond‐drying, but the male mass–age relationship was negative and independent of treatment. Our results suggest that pond‐drying can have negative effects on the future fitness of individuals by increasing the risk of desiccation‐induced, pre‐reproductive mortality and decreasing adult body size at maturity. These negative effects on life history cannot be overcome with additional nutritional resources in this species. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 495–504.     

Rapid evolution of larval development time during mass-rearing in the melon fly, Bactrocera cucurbitae

A quantitative genetic analysis of rapid evolution of a life history trait has been conducted on the first 24 generations of mass-rearing in the melon fly Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae). The phenotypic variance of larval development time in each generation was divided into genetic and residual components. Mean and phenotypic coefficients of variation of larval development time decreased gradually as generations proceeded as a result of artificial selection for shorter larval period in the mass-rearing procedure. There was a trend that additive genetic coefficients of variation in larval development time decreased with generations. These changes are entirely attributed to genetic responses to laboratory selection under the mass-rearing environment because the population was maintained at a very large size so as to exclude random genetic drift and inbreeding depression, which would be other factors responsible for the observed genetic changes. The residual coefficients of variation in larval development time did not change with generations. Realized heritability of larval development time was low. The heritabilities for larval development time estimated from parent–offspring regression at generations 60 and 70, when the evolutionary plateau was asymptotically reached, were not significantly larger than 0. Received: April 22, 1999 / Accepted: September 20, 1999     

How to become large quickly: quantitative genetics of growth and foraging in a flush feeding lepidopteran larva

Rapid larval growth in insects may be selected for by rapid ephemeral phenological changes in food resources modifying the structure of phenotypic and genetic (co)variation in and among individual traits. We studied the relative effects of three processes which can modify expression of additive genetic and nongenetic variation in traits. First, natural selection tends to erode genetic variation in fitness-related traits. Second, there may be high variance even in traits closely coupled with fitness, if these traits are themselves products of variable lower level traits. Third, traits may be canalized by developmental processes which reduce phenotypic variation. Moreover, we investigated the phenotypic and genetic role played by the underlying traits in attaining simultaneously both large size and short development time. We measured phenotypic and genetic (co)variation in several pre- and post-ingestive foraging traits, growth, development rate, development time and size, together forming a hierarchical network of traits, in the larvae of a flush feeding geometrid, Epirrita autumnata. Rapid larval growth rate and high pupal mass are closely related to fitness in E. autumnata. Traits closely associated with larval growth displayed low levels of additive genetic variation, indicating that genetic variability may have been exhausted by selection for rapid growth. The body size of E. autumnata, in spite of its close correlation with fitness, exhibited a significant additive genetic variation, possiblye because caterpillar size is the outcome of many underlying heritable traits. The low level traits in the hierarchical net, number (indicating larval movements) and size of feeding bouts in leaves, relative consumption rate and efficiency of conversion of ingested food, displayed high levels of residual variation. High residual variation in consumption and physiological ability to handle leaf material resulted from their flexibility which reduced variation in growth rate, i.e. growth rate was canalized. We did not detect a trade-off between development time and final size. On the contrary, large pupal masses were attained by short larval periods, and this relationship was strongly genetically determined, suggesting that both developmental time and final size are expressions of the same developmental process (vigorous growth) and the same genes (or linkage disequilibrium).     

DENSITY-DEPENDENT NATURAL SELECTION IN DROSOPHILA: EVOLUTION OF GROWTH RATE AND BODY SIZE

Drosophila melanogaster populations subjected to extreme larval crowding (CU lines) in our laboratory have evolved higher larval feeding rates than their corresponding controls (UU lines). It has been suggested that this genetically based behavior may involve an energetic cost, which precludes natural selection in a density-regulated population to simultaneously maximize food acquisition and food conversion into biomass. If true, this stands against some basic predictions of the general theory of density-dependent natural selection. Here we investigate the evolutionary consequences of density-dependent natural selection on growth rate and body size in D. melanogaster. The CU populations showed a higher growth rate during the postcritical period of larval life than UU populations, but the sustained differences in weight did not translate into the adult stage. The simplest explanation for these findings (that natural selection in a crowded larval environment favors a faster food acquisition for the individual to attain the same final body size in a shorter period of time) was tested and rejected by looking at the larva-to-adult development times. Larvae of CU populations starved for different periods of time develop into comparatively smaller adults, suggesting that food seeking behavior in a food depleted environment carries a higher cost to these larvae than to their UU counterparts. The results have important implications for understanding the evolution of body size in natural populations of Drosophila, and stand against some widespread beliefs that body size may represent a compromise between the conflicting effects of genetic variation in larval and adult performance.     

Genetic covariance structure of growth in the salamander Ambystoma macrodactylum

The size of an organism at any point during ontogeny often has fitness consequences through either direct selection on size or through selection on size-related morphological, performance, or life history traits. However, the evolutionary response to selection on size across ontogeny (a growth trajectory) may be limited by genetic correlations across ages. Here we characterize the phenotypic and genetic covariance structure of length and mass growth trajectories in a natural population of larval Ambystoma macrodactylum using function-valued quantitative genetic analyses and principal component decomposition. Most of the phenotypic and genetic variation in both growth trajectories appears to be confined to a single principal component describing a pattern of positive covariation among sizes across all ages. Higher order principal components with no significant associated genetic variation were identified for both trajectories, suggesting that evolution towards certain patterns of negative covariation between sizes across ages is constrained. The well-characterized positive relationship between size at metamorphosis and fitness in pond-breeding amphibians predicts that the across-age covariance structure will strongly limit evolution only if there is negative selection on size prior to metamorphosis. The pattern of genetic covariation observed in this study is similar to that observed in other vertebrate taxa, indicating that size may often be highly genetically and phenotypically integrated across ontogeny. Additionally, we find that phenotypic and genetic analyses of growth trajectories can yield qualitatively similar patterns of covariance structure.     

Grasshopper ontogeny in relation to time constraints: adaptive divergence and stasis

1. Life history theory generally predicts a trade-off between shortjuvenile development and large adult size, assuming invariant growth rates within species. This pivotal assumption has been explicitly tested in few organisms. 2. We studied ontogeny in 13 populations of Omocestus viridulus grasshoppers under common garden conditions. High-altitude populations, facing short growing seasons and thus seasonal time constraints, were found to grow at a similar rate to low altitude conspecifics. 3. Instead, high-altitude grasshoppers evolved faster development, and the correlated change in body size led to an altitudinal size cline mediating a trade-off with female fecundity. 4. An additional juvenile stage occurred in low- but not high-altitude females. This difference is probably due to the evolution of lowered critical size thresholds in high-altitude grasshoppers to accelerate development. 5. We found a strikingly lower growth rate in males than females that we interpret as the outcome of concurrent selection for protandry and small male size. 6. Within populations, large individuals developed faster than small individuals, suggesting within-population genetic variation in growth rates. 7. We provide evidence that different time constraints (seasonal, protandry selection) can lead to different evolutionary responses in intrinsic growth, and that correlations among ontogenetic traits within populations cannot generally be used to predict life history adaptation among populations. Moreover, our study illustrates that comparisons of ontogenetic patterns can shed light on the developmental basis underlying phenotypic evolution.     

ANTAGONISTIC PLEIOTROPIC EFFECT OF SECOND-CHROMOSOME INVERSIONS ON BODY SIZE AND EARLY LIFE-HISTORY TRAITS IN DROSOPHILA BUZZATII

A simple way to think of evolutionary trade-offs is to suppose genetic effects of opposed direction that give rise to antagonistic pleiotropy. Maintenance of additive genetic variability for fitness related characters, in association with negative correlations between these characters, may result. In the cactophilic species Drosophila buzzatii, there is evidence that second-chromosome polymorphic inversions affect size-related traits. Because a trade-off between body size and larval developmental time has been reported in Drosophila, we study here whether or not these inversions also affect larva-adult viability and developmental time. In particular, we expect that polymorphic inversions make a statistically significant contribution to the genetic correlation between body size (as measured by thorax length) and larval developmental time. This contribution is expected to be in the direction predicted by the trade-off, namely, those flies whose karyotypes cause them to be genetically larger should also have a longer developmental time than flies with other karyotypes. Using two different experimental approaches, a statistically significant contribution of the second-chromosome inversions to the phenotypic variances of body size and developmental time in D. buzzatii was found. Further, these inversions make a positive contribution to the total genetic correlation between the traits, as expected by the suggested trade-off. The data do not provide evidence as to whether the genetic correlation is due to antagonistic pleiotropic gene action or to gametic disequilibrium of linked genes that affect one or both traits. The results do suggest, however, a possible explanation for the maintenance of inversion polymorphism in this species.     

Genetic correlation between temperature-induced plasticity of life-history traits in a soil arthropod

Temperature is considered one of the most important mediators of phenotypic plasticity in ectotherms. However, the costs and benefits shaping the evolution of different thermal responses are poorly elucidated. One of the possible constraints to phenotypic plasticity is its intrinsic genetic cost, such as genetic linkage or pleiotropy. Genetic coupling of the thermal response curves for different life history traits may significantly affect the evolution of thermal sensitivity in thermally fluctuating environments. We used the collembolan Orchesella cincta to study if there is genetic variation in temperature-induced phenotypic plasticity in life history traits, and if the degree of temperature-induced plasticity is correlated across traits. Egg development rate, juvenile growth rate and egg size of 19 inbred isofemale lines were measured at two temperatures. Our results show that temperature was a highly significant factor for all three traits. Egg development rate and juvenile growth rate increased with increasing temperature, while egg size decreased. Line by temperature interaction was significant for all traits tested; indicating that genetic variation for temperature-induced plasticity existed. The degree of plasticity was significantly positively correlated between egg development rate and growth rate, but plasticity in egg size was not correlated to the other two plasticity traits. The findings suggest that the thermal plasticities of egg development rate and growth rate are partly under the control of the same genes or genetic regions. Hence, evolution of the thermal plasticity of traits cannot be understood in isolation of the response of other traits. If traits have similar and additive effects on fitness, genetic coupling between these traits may well facilitate the evolution of optimal phenotypes. However, for this we need to know the selective forces under field conditions.     

Realized correlated responses to artificial selection on pre-adult life-history traits in a butterfly

We use artificial selection experiments targeted on egg size, development time or pupal mass within a single butterfly population followed by a common-garden experiment to explore the interactions among these life-history traits. Relationships were predicted to be negative between egg size and development time, but to be positive between development time and body size and between egg size and body size. Correlated responses to selection were in part inconsistent with these predictions. Although there was evidence for a positive genetic correlation between egg and body size, there was no support for genetic correlations between larval development time and either egg size or pupal mass. Phenotypic correlations among the three target traits of selection gave comparable results for the relationships between egg mass and development time (no association) as well as between egg mass and pupal mass (positive association), but not for the relation between development time and pupal mass (negative phenotypic correlation). In summary, correlated responses to selection as well as phenotypic correlations were rather unpredictable. The impact of variation in acquisition and allocation of energy as well as of the benign conditions used deserve further investigation.     

Evolutionary responses of Drosophila melanogaster life history to differences in larval density

The study examined the effects of evolution at two different larval densities on pre-adult and adult fitness traits. Five replicate selection lines each were cultured at either 50 or 150 larvae per vial, avoiding selection on development time, age at breeding or for adaptation to adult density, one or more of which factors has been a confounding variable in previous studies. Low density selection lines evolved extended development times at both growth densities. The extended development times were associated with greater adult body size at the lower growth density only, and particularly in females. The lines did not differ significantly in larval competitive ability at either growth density. At neither growth density did the early adult fertility of females or the lifespan of either sex differ between the lines from the two selection regimes, but at the lower growth density the late fertility of low density line females was significantly enhanced. The results suggest that larval density does have important effects on the expression and resolution of life history trade-offs in Drosophila melanogaster, but that these may be somewhat different from those reported in previous studies.     

Temperature‐ and sex‐related effects of serine protease alleles on larval development in the Glanville fritillary butterfly

The body reserves of adult Lepidoptera are accumulated during larval development. In the Glanville fritillary butterfly, larger body size increases female fecundity, but in males fast larval development and early eclosion, rather than large body size, increase mating success and hence fitness. Larval growth rate is highly heritable, but genetic variation associated with larval development is largely unknown. By comparing the Glanville fritillary population living in the Åland Islands in northern Europe with a population in Nantaizi in China, within the source of the post‐glacial range expansion, we identified candidate genes with reduced variation in Åland, potentially affected by selection under cooler climatic conditions than in Nantaizi. We conducted an association study of larval growth traits by genotyping the extremes of phenotypic trait distributions for 23 SNPs in 10 genes. Three genes in clip‐domain serine protease family were associated with larval growth rate, development time and pupal weight. Additive effects of two SNPs in the prophenoloxidase‐activating proteinase‐3 (ProPO3) gene, related to melanization, showed elevated growth rate in high temperature but reduced growth rate in moderate temperature. The allelic effects of the vitellin‐degrading protease precursor gene on development time were opposite in the two sexes, one genotype being associated with long development time and heavy larvae in females but short development time in males. Sexually antagonistic selection is here evident in spite of sexual size dimorphism.     

ADAPTIVE PHENOTYPIC PLASTICITY IN GROWTH,DEVELOPMENT, AND BODY SIZE IN THE YELLOW DUNG FLY

Life-history theory predicts that age and size at maturity of organisms should be influenced by time and food constraints on development. This study investigated phenotypic plasticity in growth, development, body size, and diapause in the yellow dung fly, Scathophaga stercoraria. Full-sib families were allowed to develop under predator-free field conditions. The time before the onset of winter was varied and each brood was split into three environments differing in the amount of dung a set number of larvae had as a resource. When resources were abundant and competition was minimal, individuals of both sexes grew to larger body sizes, took longer time to mature, and were able to increase their growth rates to attain large body sizes despite shorter effective development periods later in the season. In contrast, limited larval resources and strong competition constrained individuals to mature earlier at a smaller adult size, and growth rates could not be increased but were at least maintained. This outcome is predicted by only two life-history optimality models, which treat mortality due to long development periods and mortality due to fast growth as independent. Elevated preadult mortality indicated physiological costs of fast growth independent of predation. When larval resources were limited, mortality increased with heritable variation in development time for males, and toward the end of the season mortality increased as larval resources became more abundant because this induced longer development periods. Sexual and fecundity selection favoring large body size in this species is thus opposed by larval viability selection favoring slower growth in general and shorter development periods when time and resources are limited; this overall combination of selective pressures is presumably shaping the reaction norms obtained here. Flexible growth rates are facilitated by low genetic correlations between development time and body size, a possible consequence of selection for plasticity. Heritable variation was evident in all traits investigated, as well as in phenotypic plasticity of these traits (genotype X interactions). This is possibly maintained by unpredictable spatiotemporal variation in dung abundance, competition, and hence selection.     

THE HERITABLE BASIS OF VARIATION IN LARVAL DEVELOPMENTAL PATTERNS WITHIN POPULATIONS OF THE WOOD FROG (RANA SYLVATICA)

This study compares the heritable basis of variation in larval developmental patterns of mountain and lowland populations of the wood frog, Rana sylvatica. Additive genetic variances, heritabilities, and genetic correlations for larval developmental time and size at metamorphosis are estimated from half-sib and full-sib crosses. Considerable additive-genetic variances and high heritabilities are revealed for developmental time in both the mountain and the lowland population. There was a high level of additive-genetic variance and high heritability for body size at metamorphosis in the mountain population, but these were very low in the lowland population. The genetic correlations between developmental rate and larval body size are negative for the mountain population and near zero for the lowland population. It is argued that the differences in genetic structure between these two populations reflect differences in the selective regimes of their respective environments.