Differences in the Phenotypic Mean and Variance Between Two Geographically Separated Populations of Wood Frog (Lithobates sylvaticus)

Intraspecific phenotypic variation between populations separated by large geographic distances is common. Differences in the mean and variance of traits among populations can be used to infer the relative strength, direction, and type of selection on traits. Patterns in the mean provide information on the type of selection, and patterns in variance provide information on the strength of selection. However, interpretation of mean/variance patterns is difficult when two traits are linked and strongly correlated to fitness because it is unlikely that each trait will reach phenotypic optima. In amphibians time to metamorphosis and size at metamorphosis are positively related both phenotypically and genetically. Using a common-garden experiment we investigated whether selection favours shorter time to metamorphosis or increased mass at metamorphosis between two populations which differ in the length of the post-metamorphic growing season by 2–4 weeks. Animals from the population a shorter growing season took longer to reach and metamorphosed at a greater mass, while animals from the population with a longer period for post metamorphic growth reached metamorphosis faster, but at a smaller mass. Greater phenotypic variance was observed in both traits in the population with the shorter growing season. These data suggest that animals from the population with a restricted growth period maximise mass at metamorphosis at the expense of longer larval periods while animals from population with the longer post-metamorphic growth period sacrifice mass at metamorphosis to shorten the larval period and maximise larval survival. Differences in phenotypic variance among populations suggest either directional or diversifying selection has acted on both traits.     

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.     

Variable shrimp in variable environments: reproductive investment within Palaemon varians

The maternal environment may influence the quantity and quality of resources invested in offspring (per-offspring provisioning) and this trait, in turn, affects larval fitness and may carry-over into early juvenile life. Here, per-offspring investment was measured across three consecutive breeding seasons for the caridean shrimp, Palaemon varians. Egg and hatchling larval dry weight as well as hatchling larval biochemical composition were measured. Results indicate that egg volume is positively correlated with dry weight, but egg volume as a proxy for dry weight should be used with caution. Correlations were identified between per-offspring investment and average breeding season temperature, but the period over which temperature was averaged was important to whether these correlations were positive or negative, highlighting the complexity of identifying environmental influence on phenotypic traits. The most important factor determining offspring size and per-offspring investment was maternal size, which may be influenced by the environment as carapace length varied significantly between breeding seasons. These data demonstrate variation in egg and larval composition, related to maternal size, which will affect early life traits and survivorship for P. varians.

     

GENETIC CORRELATIONS AMONG MORPHOMETRIC TRAITS AND RATES OF GROWTH AND DIFFERENTIATION IN THE GREEN TREE FROG,HYLA CINEREA

It is often proposed that the morphometric shape of animals often evolves as a correlated response to selection on life-history traits such as whole-body growth and differentiation rates. However, there exists little empirical information on whether selection on rates of growth or differentiation in animals could generate correlated response in morphometric shape beyond that owing to the correlation between these rates and body size. In this study genetic correlations were estimated among growth rate, differentiation rate, and body-size-adjusted head width in the green tree frog, Hyla cinerea. Head width was adjusted for size by using the residuals from log-log regressions of head width on snout-vent length. Size-adjusted head width at metamorphosis was positively genetically correlated with larval period length. Thus, size-independent shape might evolve as a correlated response to selection on a larval life-history trait. Larval growth rate was not significantly genetically correlated with size-adjusted head width. An additional morphometric trait, size-adjusted tibiofibula length, had a nonnormal distribution of breeding values, and so was not included in the analysis of genetic correlations (offspring from one sire had unusually short legs). This result is interesting because, although using genetic covariance matrices to predict long-term multivariate response to selection depends on the assumption that all loci follow a multivariate Gaussian distribution of allelic effects, few data are available on the distribution of breeding values for traits in wild populations. Size at metamorphosis was positively genetically correlated with larval period and larval growth rate. Quickly growing larvae that delay metamorphosis therefore emerge at a large size. The genetic correlation between larval growth rate and juvenile (postmetamorphic) growth rate was near zero. Growth rate may therefore be an example of a fitness-related trait that is free to evolve in one stage of a complex life cycle without pleiotropic constraints on the same trait expressed in the other stage.     

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.     

Genetic polymorphism and trade-offs in the early life-history strategy of the Pacific oyster, Crassostrea gigas (Thunberg, 1795): a quantitative genetic study

We investigated genetic variability and genetic correlations in early life-history traits of Crassostrea gigas. Larval survival, larval development rate, size at settlement and metamorphosis success were found to be substantially heritable, whereas larval growth rate and juvenile traits were not. We identified a strong positive genetic correlation between larval development rate and size at settlement, and argue that selection could optimize both age and size at settlement. However, trade-offs, resulting in costs of metamorphosing early and large, were suggested by negative genetic correlations or covariances between larval development rate/size at settlement and both metamorphosis success and juvenile survival. Moreover, size advantage at settlement disappeared with time during the juvenile stage. Finally, we observed no genetic correlations between larval and juvenile stages, implying genetic independence of life-history traits between life-stages. We suggest two possible scenarios for the maintenance of genetic polymorphism in the early life-history strategy of C. gigas.     

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).     

Genetic and environmental sources of egg size, fecundity and body size in the migrant skipper, Parnara guttata guttata (Lepidoptera: Hesperiidae)

Genetic and environmental sources of egg size, fecundity and body size (forewing length) were examined in the butterfly, Parnara guttata guttata. Phenotypic and genetic correlation and heritability were estimated for these traits under different day-length and temperature conditions. Egg size and fecundity had relatively high heritabilities, and body sizes in males and females had moderate and high heritability, respectively. Negative phenotypic and genetic correlations between egg size and fecundity were estimated in treatments corresponding to the natural conditions during larval development of the first and second generations. Phenotypic and genetic correlations between body size and egg size differed considerably between insects reared under long and short day-lengths. Next, genotype–environment interactions were estimated by comparing reaction norms to day-length or temperature of these traits among families. ANOVA analysis revealed significant genotype–environment interactions in egg size and forewing length in both sexes for day-length and temperature. These results suggested that a large additive genetic variance for egg size might have been maintained by a genetic trade-off and/or by genotype–environment interactions in P. g. guttata.     

Fitness consequences of larval traits persist across the metamorphic boundary

Metamorphosis is thought to provide an adaptive decoupling between traits specialized for each life-history stage in species with complex life cycles. However, an increasing number of studies are finding that larval traits can carry-over to influence postmetamorphic performance, suggesting that these life-history stages may not be free to evolve independently of each other. We used a phenotypic selection framework to compare the relative and interactive effects of larval size, time to hatching, and time to settlement on postmetamorphic survival and growth in a marine invertebrate, Styela plicata. Time to hatching was the only larval trait found to be under directional selection, individuals that took more time to hatch into larvae survived better after metamorphosis but grew more slowly. Nonlinear selection was found to act on multivariate trait combinations, once again acting in opposite directions for selection acting via survival and growth. Individuals with above average values of larval traits were most likely to survive, but surviving individuals with intermediate larval traits grew to the largest size. These results demonstrate that larval traits can have multiple, complex fitness consequences that persist across the metamorphic boundary; and thus postmetamorphic selection pressures may constrain the evolution of larval traits.     

HABITAT DURATION,LENGTH OF LARVAL PERIOD,AND THE EVOLUTION OF A COMPLEX LIFE CYCLE OF A SALAMANDER,AMBYSTOMA TEXANUM

In many organisms, genotypic selection may be a less effective means of adapting to unpredictable environments than is selection for phenotypic plasticity. To determine whether genotypic selection is important in the evolution of complex life cycles of amphibians that breed in seasonally ephemeral habitats, we examined whether mortality risk from habitat drying in natural populations of small-mouthed salamanders (Ambystoma texanum) corresponded to length of larval period when larvae from the same populations were grown in a common laboratory environment. Comparisons were made at two levels of organization within the species: 1) among geographic races that are under strongly divergent selection regimes associated with the use of pond and stream habitats and 2) among populations within races that use the same types of breeding habitats. Morphological evidence indicates that stream-breeding A. texanum evolved from pond-breeding populations that recently colonized streams. Larvae in streams incur heavy mortality from stream drying, so the upper bound on length of larval period is currently set by the seasonal duration of breeding sites. We hypothesized that selection would reduce length of larval period of pond-breeders that colonize streams if their larval periods are inherently longer than those of stream-breeders. The results of laboratory experiments support this hypothesis. When grown individually in a common environment, larvae from stream populations had significantly shorter larval periods than larvae from pond populations. Within races, however, length of larval period did not correlate significantly with seasonal duration of breeding sites. When males of both races were crossed to a single pond female, offspring of stream males had significantly shorter larval periods than offspring of pond males. Collectively, these data suggest that differences in complex life cycles among pond and stream-breeders are due to genotypic selection related to mortality from habitat drying. Stream larvae in the common-environment experiment were significantly smaller at metamorphosis than pond larvae. Yet, the evolution of metamorphic size cannot be explained readily by direct selection: there are no intuitively obvious advantages of being relatively small at metamorphosis in streams. A positive phenotypic correlation was observed between size at metamorphosis and length of larval period in most laboratory populations. A positive additive genetic correlation between these traits was demonstrated recently in another amphibian. Thus, we suspect that metamorphic size of stream-breeders evolved indirectly as a consequence of selection to shorten length of larval period.     

Life-history consequences of egg size in Drosophila melanogaster

We used a novel approach to study the effects of egg size on offspring fitness components in Drosophila melanogaster. Populations that differed genetically in egg size were crossed, and the female offspring from these reciprocal crosses were examined for life-history traits. These flies expressed effects of egg size, because they developed from eggs of different sizes as a result of maternal genetic effects, but displayed an equivalent range of nuclear genetic variation. The crosses used four independent pairs of outbred populations that differed in the pattern of covariation between egg size and life-history traits, so that the maternal genetic effects of egg size on offspring characters could be contrasted to the associations present among the parental populations. Egg size showed positive maternal genetic effects on embryonic viability and development rate, hatchling weight and feeding rate, and egg-larva and egg-adult development rate but no consistent effects on larval competitive ability, adult weight, or egg size in the offspring. Our method revealed a pattern of causality that could not be deduced from interpopulation comparisons and therefore provides a good way of disentangling the causes and consequences of variation in egg size while controlling for zygotic genetic effects.     

THE GENETIC BASIS OF LIFE-HISTORY CHARACTERS IN A POLYCHAETE EXHIBITING PLANKTOTROPHY AND LECITHOTROPHY

The polychaete Streblospio benedicti is unusual in that several field populations consist of individuals that exhibit either planktotrophic or lecithotrophic larval development. Planktotrophy in this species involves production of many small ova that develop into feeding larvae with a two- to three-week planktonic period. Lecithotrophy involves production of fewer, larger ova that develop into nonfeeding larvae that are brooded longer and have a brief planktonic stage. Reciprocal matings were performed to investigate genetic variance components and the correlation structure of life-history traits associated with planktotrophy and lecithotrophy. Our objective was to better understand persistence of this developmental dichotomy in Streblospio benedicti, and among marine invertebrates in general. Substantial additive genetic variation (75–98% of total) was detected for the following characters at first reproduction: female length; position of the first gametogenic setiger and first brood pouch; ovum diameter; three traits related to fecundity (numbers of ova per ovary, larvae per brood pouch, and larvae per brood); median larval planktonic period and the presence of larval swimming setae; but not for total number of brood pouches; larval length; larval feeding; and larval survivorship. Based on the unusual geographic distribution of development modes in this species, we hypothesize that the developmental traits have evolved in allopatry and have only recently come into contact in North Carolina. The high additive contribution to variance observed for many traits may be inflated due to (a) nonrandom breeding in nature, and (b) examination of only one component of an age-structured population at one time. Nuclear interaction variance and maternal variance accounted for 84% of the total variation in larval survivorship. This observation supports other empirical studies and theoretical predictions that nonadditive components of variance will increase in importance in individual traits that are most closely tied to fitness. A network of life-history trait correlations was observed that defines distinct planktotrophic and lecithotrophic trait complexes. Negative genetic correlations were present between fecundity and egg size, between fecundity and position of the first gametes, and between larval survivorship and median planktonic period. Positive genetic correlations were detected between fecundity and female size at first reproduction and between planktonic period and the presence of swimming setae. Intergenerational product-moment correlations were negative for larval length and fecundity, planktonic period and egg size, female size and larval survivorship, and fecundity and larval survivorship. If the genetic correlation structure observed in the laboratory persists in the field, it may constrain responses of individual characters to directional selection, and indirectly perpetuate the dichotomies associated with planktotrophy and lecithotrophy.     

The genetic covariance between life cycle stages separated by metamorphosis

Metamorphosis is common in animals, yet the genetic associations between life cycle stages are poorly understood. Given the radical changes that occur at metamorphosis, selection may differ before and after metamorphosis, and the extent that genetic associations between pre- and post-metamorphic traits constrain evolutionary change is a subject of considerable interest. In some instances, metamorphosis may allow the genetic decoupling of life cycle stages, whereas in others, metamorphosis could allow complementary responses to selection across the life cycle. Using a diallel breeding design, we measured viability at four ontogenetic stages (embryo, larval, juvenile and adult viability), in the ascidian Ciona intestinalis and examined the orientation of additive genetic variation with respect to the metamorphic boundary. We found support for one eigenvector of G (g_obsmax), which contrasted larval viability against embryo viability and juvenile viability. Target matrix rotation confirmed that while g_obsmax shows genetic associations can extend beyond metamorphosis, there is still considerable scope for decoupled phenotypic evolution. Therefore, although genetic associations across metamorphosis could limit that range of phenotypes that are attainable, traits on either side of the metamorphic boundary are capable of some independent evolutionary change in response to the divergent conditions encountered during each life cycle stage.     

Dominance Genetic Variance for Traits Under Directional Selection in Drosophila serrata

In contrast to our growing understanding of patterns of additive genetic variance in single- and multi-trait combinations, the relative contribution of nonadditive genetic variance, particularly dominance variance, to multivariate phenotypes is largely unknown. While mechanisms for the evolution of dominance genetic variance have been, and to some degree remain, subject to debate, the pervasiveness of dominance is widely recognized and may play a key role in several evolutionary processes. Theoretical and empirical evidence suggests that the contribution of dominance variance to phenotypic variance may increase with the correlation between a trait and fitness; however, direct tests of this hypothesis are few. Using a multigenerational breeding design in an unmanipulated population of Drosophila serrata, we estimated additive and dominance genetic covariance matrices for multivariate wing-shape phenotypes, together with a comprehensive measure of fitness, to determine whether there is an association between directional selection and dominance variance. Fitness, a trait unequivocally under directional selection, had no detectable additive genetic variance, but significant dominance genetic variance contributing 32% of the phenotypic variance. For single and multivariate morphological traits, however, no relationship was observed between trait–fitness correlations and dominance variance. A similar proportion of additive and dominance variance was found to contribute to phenotypic variance for single traits, and double the amount of additive compared to dominance variance was found for the multivariate trait combination under directional selection. These data suggest that for many fitness components a positive association between directional selection and dominance genetic variance may not be expected.     

Ontogenetic patterns in heritable variation for body size: using random regression models in a wild ungulate population

Body size is an important determinant of fitness in many organisms. While size will typically change over the lifetime of an individual, heritable components of phenotypic variance may also show ontogenetic variation. We estimated genetic (additive and maternal) and environmental covariance structures for a size trait (June weight) measured over the first 5 years of life in a natural population of bighorn sheep Ovis canadensis. We also assessed the utility of random regression models for estimating these structures. Additive genetic variance was found for June weight, with heritability increasing over ontogeny because of declining environmental variance. This pattern, mirrored at the phenotypic level, likely reflects viability selection acting on early size traits. Maternal genetic effects were significant at ages 0 and 1, having important evolutionary implications for early weight, but declined with age being negligible by age 2. Strong positive genetic correlations between age-specific traits suggest that selection on June weight at any age will likely induce positively correlated responses across ontogeny. Random regression modeling yielded similar results to traditional methods. However, by facilitating more efficient data use where phenotypic sampling is incomplete, random regression should allow better estimation of genetic (co)variances for size and growth traits in natural populations.     

EVALUATING A HYPOTHESIS ABOUT HETEROCHRONY: LARVAL LIFE-HISTORY TRAITS AND JUVENILE HIND-LIMB MORPHOLOGY IN HYLA CRUCIFER

This paper reports the results of an investigation into whether selection on genetically based differences in the timing or rate of development (heterochrony) can give rise to nonadaptive morphological differences among individual frogs. We used a quantitative-genetics approach to examine the relationships among the life-history characters time to metamorphosis and larval-growth rate and a functionally significant morphological features, relative hind-limb length, in the spring peeper, Hyla crucifer. Time to metamorphosis and growth rate had low heritabilities in our population. Morphological traits had moderate heritabilities. There were positive genetic correlations between the life-history traits and the components of relative hind-limb length but no significant correlations with the shape variable itself. We used field observations of pond-drying time and experimental results of selection on growth rate to simulate the correlated responses of hind-limb shape to four reasonable selection regimes on the life-history traits. We found little evidence to suggest that relative hind-limb length would display much of a correlated response to such selection. The differences in relative hind-limb length seen among closely related species or among populations of a single species that appear to be unrelated to performance differences are not obviously explicable as neutral correlated responses to selection on larval traits.     

GENETIC VARIATION FOR LARVAL ANURAN (SCAPHIOPUS COUCHII) DEVELOPMENT TIME IN AN UNCERTAIN ENVIRONMENT

Couch's spadefoot toads (Scaphiopus couchii) breed in ponds of uncertain duration. In natural ponds, larvae exhibit considerable variation in growth, size at metamorphosis, and development time. Phenotypic differences in development time may dramatically affect survivorship in these ponds. A quantitative-genetic analysis of larval traits was undertaken to determine the potential evolutionary relevance of phenotypic variation observed in the field. Additive genetic variance was detected for development time, but not for any other trait. Some variation in early growth was attributed to maternal effects; maternal effects, however, were not apparent for size at metamorphosis or development time. Nonadditive genetic effects were not statistically significant for any trait, but the nonsignificance of these effects must be interpreted cautiously, as the sample of females was relatively small and the mating design used is not very effective for detecting interactions. Genetic variation in development time in this population is most likely maintained by variability in the direction of selection as a consequence of variation in pond duration.     

Quantitative genetics of ontogeny of sexual dimorphism in red junglefowl (Gallus gallus)

We studied phenotypic patterns and underlying quantitative genetics of development of sexual size dimorphism in red junglefowl (Gallus gallus). Using a multigenerational pedigree and the 'animal model' technique, we found significant heritability for many of the size and growth-related traits we examined, as well as significant genetic correlations among them. Despite sexual size dimorphism throughout posthatching ontogeny, the genetic correlation between males and females for all size measurements and growth parameters remained high. Significant positive phenotypic and genetic correlations between the fastest rate of growth and mass at week 26 (near asymptote) indicate that faster growth when young promotes larger adult size. However, age at which peak growth is reached does not appear to be phenotypically or genetically correlated with adult size. Positive genetic correlations within traits among ages were common, demonstrating that the genetic variance important to growth is relatively consistent among ages. However, male mass and tarsus length showed no genetic correlation between week 0 values and those from later ages. The body size traits of mass and tarsus length were genetically correlated with each other in females, but this pattern was not significant in males. Thus, despite striking sexual dimorphism in size and growth trajectories, size dimorphic traits in junglefowl show, with some exceptions, genetic integration between the sexes, among ages, and between traits.     

GENETIC VARIATION FOR PHENOTYPIC PLASTICITY IN THE LARVAL LIFE HISTORY OF SPADEFOOT TOADS (SCAPHIOPUS COUCHII)

Phenotypic plasticity in life-history traits is common. The relationship between phenotype and environment, or reaction norm, associated with life-history plasticity can evolve by natural selection if there is genetic variation within a population for the reaction norm and if the traits involved affect fitness. As with other traits, selection on plasticity in a particular trait or in response to a particular environmental factor may be constrained by trade-offs with other traits that affect fitness. In this paper, I experimentally evaluated broad-sense genetic variation in the reaction norms of age and size at metamorphosis in response to two environmental factors, food level and temperature. Differences among full-sib families in one or both traits were evident in all treatments. However, variation among families in their responses to each treatment (genotype-environment interaction) resulted in variation among treatments in estimated heritabilities and genetic correlations. Age at metamorphosis was equally sensitive to temperature in all families, but size at metamorphosis was more sensitive to temperature in some families than in others. Size at metamorphosis was equally sensitive to food level in all families, but age at metamorphosis was sensitive to food in some families but not in others. At high temperature or low food, the genetic correlation between age and size at metamorphosis was positive, generating a potential trade-off between metamorphosing early to attain higher larval survival and metamorphosing later to achieve larger size. This trade-off extends across treatments: families with the largest average size at metamorphosis achieved larger size with the longest average and greatest plasticity in age at metamorphosis. Other families achieved shorter average larval periods by exhibiting greater plasticity in size at metamorphosis but had the smallest average size at metamorphosis. This trade-off may reflect an underlying functional constraint on the ability to respond optimally to all environments, resulting in persistent genetic variation in reaction norms.     

Genetic variance and covariance patterns of larval development in the small white butterflyPieris rapae crucivora Boisduval

Quantitative genetic theory indicates that genetic covariance patterns among life history characters should have played an important role as genetic constraint in life history evolution. Highly positve (and negative) genetic correlations between larval development time (or larval growth rate) and adult size characters were detected by means of sib analysis for the small white butterfly Pieris rapae crucivora. The genetic associations suggested that evolution of developmental characteristics and adult phenotypic traits were constrained by pleiotropy. The positive genetic correlations between development time and adult body size may be compatible with the trade-off between them, but the negative genetic correlations between larval growth rate and adult body size are not predicted from theories of optimal energy allocation. That phenotypic correlations drastically differed from the genetic correlations indicates limitations of evolutionary inferences based only on phenotypic variation.