Age-specific Traits Research Articles

Natural Selection and the Evolution of Asynchronous Aging.

AbstractUnderstanding within-population variation in aging rates across different phenotypic traits is a central focus of biogerontological studies. Early evolutionary models predict that natural selection acts to cause all traits to deteriorate simultaneously. However, observations of aging rates provide evidence for widespread patterns of asynchronous aging in laboratory and natural populations. Recent verbal models put forth to explain such observations argue that because senescence is costly to fitness, selection should cause phenotypic traits that are most important to fitness to senesce slower than traits that are less related to fitness. Here, we show that formal evolutionary theory supports neither prediction. Instead, we find that selection will favor the evolution of the most rapid rates of aging in those traits that are under the strongest selection at early ages because selection for these traits erodes the fastest. This reinforces the expectation that natural selection should play a role in the evolution of among-trait variation in aging, but in a contradictory way to that suggested previously. We demonstrate how to quantify age-specific sources of selection for age-specific traits and how these estimates can be used to understand how well patterns of age-related changes in selection can explain observed patterns of among-trait variation in aging rates.

Food Creatine and DXA-Derived Body Composition in Boys and Girls Aged 8 to 19 Years.

Several small-scale trials indicate a positive correlation between dietary creatine intake and fat-free mass in the pediatric population; whether this connection occurs at the population-wide level remains currently unknown. The main purpose of this cross-sectional study was to calculate the amount of creatine consumed through a regular diet among U.S. boys and girls aged 8 to 19 years, and investigate the link between creatine consumption and dual-energy X-ray absorptiometry (DXA)-derived body composition indices in this population. Data were obtained from the National Health and Nutrition Examination Survey 2017-2018 round, with dietary information and whole-body DXA body composition measures extracted for respondents aged 8 to 19 years (1273 participants, 649 boys and 624 girls). Individual values for total grams of creatine consumed per day for each participant were computed using the average amount of creatine (3.88 g/kg) across all creatine-containing foods. The primary exposure was the mean daily intake of creatine; the primary and secondary outcomes comprised lean mass excluding bone mineral content (BMC), and bone mineral density, BMC, lean mass including BMC, fat mass, and percent body fat, respectively. The average intake of creatine across the sample was 0.65 ± 0.72 g/day (95% CI, from 0.61 to 0.69). Creatine positively correlated with lean mass (excluding BMC) and BMC across the whole sample (r = .18 and .20, respectively; P < .001); a significant negative correlation was found between creatine intake and percent body fat (r = −.09; P = .001). The higher intake of creatine was associated with higher lean mass in girls and higher BMC in boys, while taking more creatine corresponded to less body fat for both genders (P < .05). Our findings indicate a significant correlation between dietary creatine and DXA-derived body composition biomarkers in a nationally representative cohort of U.S. youth. These results justify further research of creatine’s role in modifying body morphology in the pediatric population, taking into account the age and sex specific traits.

Seasonal grouping dynamics in a territorial vulture: ecological drivers and social consequences

Despite widespread occurrence of seasonal sociality among animals, little is still known about the social drivers and population-level social implications of seasonal grouping behaviours, especially in birds. Here, we studied the combined effects of ecological and social factors on seasonal grouping patterns in a sedentary population of Egyptian vultures living on the Eastern Canary Islands. We focussed on the social significance of large-scale gatherings taking place outside the breeding season at a highly preferred feeding station and a nearby temporary roost. Group sizes at this feeding patch followed a strong seasonal pattern characterized by distinct monthly changes in group composition, according to sex, age and territorial status. In between reproductive periods, vulture numbers at the feeding station may reach up 50% of the total population on a single day. GPS-tracking showed that this increase in vulture numbers was in part due to a shift in foraging range towards the centre of Fuerteventura by low-ranked territorial birds breeding in remote areas. During this period, vultures may spend on average 30% of their monthly time in a social gatherings context, depending on social status. We show that seasonal grouping patterns are shaped by the complex interplay between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific traits and social competitive processes, while social attraction may be an important additional component. We propose that for facultative social foragers living in highly despotic territorial systems, collective foraging may be of particular relevance regarding the development of hierarchical social relations and maintenance of population-level social cohesion. We show that seasonal grouping patterns in a sedentary island population of a territorial vulture are shaped by the complex interplay between ecological factors (reproductive constraints, resource seasonality, food predictability), age-specific traits and social competitive processes. We argue that vultures visit large gatherings also for social purposes. Group foraging events may be of special interest to vultures, given opportunities for rank maintenance/improvement, but also for conspecific evaluation and mate-seeking, and play an important role in the social structuring of populations. Vultures may serve as important model species to test the role of scavenging and fission–fusion social dynamics in driving the evolution of avian social cognition, or avian social complexity in general.

Age-Specific Traits and a Post-Maturation Molt in Black Widow Spiders Latrodectus tredecimguttatus (Aranei, Theridiidae)

Age-specific traits in the black widow spiders Latrodectus tredecimguttatus from Crimea were studied. A post-maturation molt in black widow females was recorded for the first time. The somatic traits (body size and cuticle structure) in some adult females were found to be identical to those of juvenile specimens. Some females in the studied spider population may become mature 1 or 2 instars earlier than others, continue their growth, and molt when mature.

How Long Does a Short-Lived Perennial Live? A Modeling Approach

We have composed a “scale of ontogenesis,” i.e., the sequence of ontogenetic stages, and the life cycle graph (LCG) for Androsace albana, a monocarpic plant species that is considered a short-lived perennial, according to annual observations on permanent sample plots in an alpine lichen heath during the 2009–2016 period. There is only one reproduction event in the LCG, which eliminates any reproductive uncertainty from the data of “identified individuals” type so typical for the former projects with polycarpic species, while the monocarpic cycle excludes any returns from the generative stage to a non-flowering status. The LCG describes the ontogeny through five successive stages: seedlings, juvenile, immature, adult virginal, and generative plants, the generative plants perishing after they have flowered and produced seeds. We have constructed a matrix model of the stage-structured population that corresponds to the LCG; its calibration has reduced to calculating the vital rates directly from the data of one time step, i.e., from the censuses at two successive years of observation. Therefore, the nonautonomous model represents a set of 7 annual matrices, each giving a quantitative measure of how the local population is adapted to its environment as the dominant eigenvalue of the model matrix. Its variations have turned out quite significant from year to year, signifying either a growth or decline of the local population and, in general, its vulnerability to stress factors of the environment. Averaging the annual matrices geometrically over the whole observation period has revealed the tendency to decline and enabled us to extract certain age-specific traits (in years) from the stage-structured model using the technique of virtual absorbing Markov chains and their fundamental matrices. The mean life expectancy at various stages of the A. albana ontogeny has turned out maximal in the adult virginal plants, while the mean age at flowering equal to 13 years has exceeded the horizon of the observation time series, thus proving the technique to be efficient. The model indicates that A. albana plants spend most of their life spans as virginal adults, which characterizes the space holder strategy by Korner (2003), or the delayed-development strategy by Zhukova (1995). The model outcome gives another evidence that the A. albana population is endangered.

Statistical tests for detecting variance effects in quantitative trait studies.

MotivationIdentifying variants, both discrete and continuous, that are associated with quantitative traits, or QTs, is the primary focus of quantitative genetics. Most current methods are limited to identifying mean effects, or associations between genotype or covariates and the mean value of a quantitative trait. It is possible, however, that a variant may affect the variance of the quantitative trait in lieu of, or in addition to, affecting the trait mean. Here, we develop a general methodology to identify covariates with variance effects on a quantitative trait using a Bayesian heteroskedastic linear regression model (BTH). We compare BTH with existing methods to detect variance effects across a large range of simulations drawn from scenarios common to the analysis of quantitative traits.ResultsWe find that BTH and a double generalized linear model (dglm) outperform classical tests used for detecting variance effects in recent genomic studies. We show BTH and dglm are less likely to generate spurious discoveries through simulations and application to identifying methylation variance QTs and expression variance QTs. We identify four variance effects of sex in the Cardiovascular and Pharmacogenetics study. Our work is the first to offer a comprehensive view of variance identifying methodology. We identify shortcomings in previously used methodology and provide a more conservative and robust alternative. We extend variance effect analysis to a wide array of covariates that enables a new statistical dimension in the study of sex and age specific quantitative trait effects.Availability and implementation https://github.com/b2du/bth.Supplementary information Supplementary data are available at Bioinformatics online.

Crown structure of 25-year-old Siberian stone pine grafts from mother trees of different ages

Inheritance of the age-specific traits of the Siberian stone pine crown structure by vegetative progeny was analyzed. Studies were carried out on 25-year-old grafts onto the same species seedlings as rootstocks. Scions for grafting were sampled from two groups of trees: young-mature (YM) and old growth (OG) trees in the southeast of the West Siberian Plain. The analysis of graft development showed that the YM trees had a significantly higher and thicker trunk, and their crowns were broader and denser due to a larger number of shoots. They multiplied by means of the branching feature due to the predominance of axes 2-3 branching orders. The OG trees formed small and transparent crowns that visually resembled separate branches from the upper parts of the crown of ontogenetically old trees. It was suggested that in the vegetative progeny of trees of a different ontogenetic state, the growth potential was epigenetically inherited.

Averaging the population projection matrices: Heuristics against uncertainty and nonexistence

In matrix population models, the process of population “projection” through a number of time steps is fundamentally multiplicative, hence the arithmetic mean of the consecutive matrices is of doubtful meaning, while the geometric mean quite corresponds to the multiplication principle. The geometric mean of positive numbers does not bear any problem, but that of matrices does. The “population projection matrices” (PPMs) are rather nonnegative than positive, with the allocation of non-zeros that is predetermined by the life cycle graph (LCG) reflecting the development biology of a given species, and this graph is principally incomplete. The average matrix A should logically have the same fixed pattern of zeros as those to be averaged, and this causes the averaging matrix equation to be overdetermined as a system of element-wise algebraic equations for the unknown positive elements. Therefore, the exact solution to the problem of pattern-geometric averaging does generally not exist, while the classical (least-squares) approximation leads to a significant error. My heuristic approach to finding a better approximate A for the PPMs in the form of L = T(transition) + F(fertility) is to solve the problem in a combined way: the pattern-geometric approximation for the T part and the exact arithmetic mean for F (as population recruitment is an additive process). As a result, the approximation error decreases drastically due to matrix T being always substochastic, while the combined, TF-averaging, method turns out efficient even under ‘reproductive uncertainty’ in data, i.e., for the whole families of feasible matrices F in the sum L = T + {F}. I illustrate the method of TF-averaging with 5 matrices L(t) calibrated for each pair of consecutive years from a 6-year period of observation in a case study of Eritrichium caucasicum, a short-lived perennial herbaceous species. The approximate TF-average enables gaining the ‘age-specific traits from stage-specific models’ (Caswell, 2001, p. 116) that are characteristic of the entire period, and I discuss other motivations/advantages for/of pattern-geometric means in matrix population models.

Effects of incorporating age-specific traits of zooplankton into a marine ecosystem model

Morphological, physiological, behavioral, and life-history traits of zooplankton are vital when assessing their roles in marine ecosystem. A recent study has shown that certain traits of marine copepod change with life stage, where fast growing young nauplii have low N:P ratio, selecting for P rich-food and excreting at high N:P; while older, slow growing copepodites are characterized by higher N:P, selecting for N-rich food and excreting at low N:P. We developed a simple model for incorporating these age-specific traits within the European Regional Seas Ecosystem Model framework to test the importance of these traits in the marine ecosystem. Our results suggest that copepod’s community growth is limited by different factors in different life stages: nauplii population is limited by prey’s food quality; copepodites population is limited by food quality as well as aging-related reduction in assimilation rate. We also show that age-specific traits of zooplankton have impacts on nutrient cycling especially on the elemental stoichiometry of large particulate organic matter.

Simultaneous age-dependent and age-independent sexual selection in the lekking black grouse (Lyrurus tetrix).

Individuals' reproductive success is often strongly associated with their age, with typical patterns of early-life reproductive improvement and late-life senescence. These age-related patterns are due to the inherent trade-offs between life-history traits competing for a limited amount of resources available to the organisms. In males, such trade-offs are exacerbated by the resource requirements associated with the expression of costly sexual traits, leading to dynamic changes in trait expression throughout their life span. Due to the age dependency of male phenotypes, the relationship between the expression of male traits and mating success can also vary with male age. Hence, using longitudinal data in a lekking species with strong sexual selection - the black grouse Lyrurus tetrix - we quantified the effects of age, life span and age of first lek attendance (AFL) on male annual mating success (AMS) to separate the effects of within-individual improvement and senescence on AMS from selective (dis)appearance of certain phenotypes. Then, we used male AMS to quantify univariate and multivariate sexual selection gradients on male morphological and behavioural traits with and without accounting for age and age-related effects of other traits. Male AMS increased with age, and there was no significant reproductive senescence. Most males never copulated, and of the ones that did, the majority had only one successful year. Life span was unrelated to AMS, but early AFL tended to lead to higher AMS at ages 1-3. AMS was related to most morphological and behavioural traits when male age was ignored. Accounting for age and age-specific trait effects (i.e. the interaction between a trait and age) reduced the magnitude of the selection gradients and revealed that behavioural traits are under consistent sexual selection, while sexual selection on morphological traits is stronger in old males. Therefore, sexual selection in black grouse operates primarily on male behaviour and morphological traits may act as additional cues to supplement female choice. These results demonstrate the multifaceted influence of age on both fitness and sexual traits and highlight the importance of accounting for such effects when quantifying sexual selection.

Asynchrony of senescence among phenotypic traits in a wild mammal population

The degree to which changes in lifespan are coupled to changes in senescence in different physiological systems and phenotypic traits is a central question in biogerontology. It is underpinned by deeper biological questions about whether or not senescence is a synchronised process, or whether levels of synchrony depend on species or environmental context. Understanding how natural selection shapes patterns of synchrony in senescence across physiological systems and phenotypic traits demands the longitudinal study of many phenotypes under natural conditions. Here, we examine the patterns of age-related variation in late adulthood in a wild population of Soay sheep (Ovis aries) that have been the subject of individual-based monitoring for thirty years. We examined twenty different phenotypic traits in both males and females, encompassing vital rates (survival and fecundity), maternal reproductive performance (offspring birth weight, birth date and survival), male rutting behaviour, home range measures, parasite burdens, and body mass. We initially quantified age-related variation in each trait having controlled for annual variation in the environment, among-individual variation and selective disappearance effects. We then standardised our age-specific trait means and tested whether age trajectories could be meaningfully grouped according to sex or the type of trait. Whilst most traits showed age-related declines in later life, we found striking levels of asynchrony both within and between the sexes. Of particular note, female fecundity and reproductive performance declined with age, but male annual reproductive success did not. We also discovered that whilst home range size and quality decline with age in females, home range size increases with age in males. Our findings highlight the complexity of phenotypic ageing under natural conditions and, along with emerging data from other wild populations and laboratory models, suggest that the long-standing hypothesis within evolutionary biology that fitness-related traits should senesce in a synchronous manner is seriously flawed.

Developmental and genetic origins of murine long bone length variation.

If we wish to understand whether development influences the rate or direction of morphological evolution, we must first understand the developmental bases of morphological variation within species. However, quantitative variation in adult morphology is the product of molecular and cellular processes unfolding from embryonic development through juvenile growth to maturity. The Atchley-Hall model provides a useful framework for dissecting complex morphologies into their component parts as a way of determining which developmental processes contribute to variation in adult form. We have examined differences in postnatal allometry and the patterns of genetic correlation between age-specific traits for ten recombinant inbred strains of mice generated from an intercross of LG/J and SM/J. Long bone length is closely tied to body size, but variation in adult morphology is more closely tied to differences in growth rate between 3 and 5 weeks of age. These analyses show that variation generated during early development is overridden by variation generated later in life. To more precisely determine the cellular processes generating this variation we then examined the cellular dynamics of long bone growth plates at the time of maximum elongation rate differences in the parent strains. Our analyses revealed that variation in long bone length is the result of faster elongation rates of the LG/J stain. The developmental bases for these differences in growth rate involve the rate of cell division and chondrocyte hypertrophy in the growth plate.

Evolutionary genetics of ageing in the wild: empirical patterns and future perspectives

Summary 1 Classical evolutionary theory states that senescence should arise as a consequence of the declining force of selection late in life. Although the quantitative genetic predictions of hypotheses derived from this theory have been extensively tested in laboratory studies of invertebrate systems, relatively little is known about the genetics of ageing in the wild. 2 Data from long-term ecological studies is increasingly allowing quantitative genetic approaches to be used in studies of senescence in free-living populations of vertebrates. We review work to date and argue that the patterns are broadly consistent with theoretical predictions, although there is also a clear need for more empirical work. 3 We argue that further advances in this field of research might be facilitated by increased use of reaction norm models, and a decreased emphasis on attempting to discriminate between mutation accumulation and antagonistic pleiotropy models of senescence. We also suggest a framework for the better integration of environmental and genetic effects on ageing. 4 Finally, we discuss some of the difficulties in applying quantitative genetic models to studies of senescence outside the laboratory. In particular we highlight the problems that viability selection can cause for an accurate estimation of parameters used in the prediction of age-trajectory evolution.

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.

Age-Specific Changes in Epistatic Effects on Mortality Rate in Drosophila melanogaster

Models for the evolution of senescence assume that genes with age-specific effects act independently of one another. Although recent empirical data show that longevity is influenced in part by interactions between genes, there are currently few data on whether epistasis influences age-specific components of mortality. To gauge if and how interactions affect age-specific traits, we incorporated the Drosophila visible marker mutations ebony, forked, and purple into seven wild-caught strains of D. melanogaster to examine gene x genetic background interactions. We found significant natural genetic variation for longevity and baseline mortality rates. Gene x genetic background interactions were prevalent not only for longevity but also for baseline mortality rates and age-specific mortality rates. We conclude that gene x genetic background epistasis is prevalent for aging-related traits and could play a significant role in the evolution of aging. These results suggest that future genetic models for the evolution of aging should incorporate the effects of epistasis.

Dynamic Latent Trait Models for Multidimensional Longitudinal Data

This article presents a new approach for analysis of multidimensional longitudinal data, motivated by studies using an item response battery to measure traits of an individual repeatedly over time. A general modeling framework is proposed that allows mixtures of count, categorical, and continuous response variables. Each response is related to age-specific latent traits through a generalized linear model that accommodates item-specific measurement errors. A transition model allows the latent traits at a given age to depend on observed predictors and on previous latent traits for that individual. Following a Bayesian approach to inference, a Markov chain Monte Carlo algorithm is proposed for posterior computation. The methods are applied to data from a neurotoxicity study of the pesticide methoxychlor, and evidence of a dose-dependent increase in motor activity is presented.

Testing hypotheses of aging in long-lived painted turtles ( Chrysemys picta)

For 38 of the past 50 years, Painted Turtles were studied on the University of Michigan's E.S. George Reserve in southeastern Michigan. We compared age specific body sizes, reproductive traits and survival of Painted Turtles ranging from 9 to 61 years of age to test contrasting predictions of the Relative Reproductive Rate and Senescence Hypotheses of aging. Indeterminate growth (i.e. continued body growth of adults) was important in increasing reproductive output of older turtles; however, growth rate of the oldest age-group was reduced compared to that of younger adults. Although clutch size and among year reproductive frequency did not increase with age, within year reproductive frequency (production of second clutches), egg size, and hatchling size did. Nest predation rates and the proportion of surviving nests that produced hatchlings were similar among age groups, and embryo mortality in nests was not related to age. Survivorship of males was less than that of females, and survivorship of the oldest group of females was not statistically different from that of a younger group of females. No decline in reproductive output or survivorship was detected in the oldest females as predicted by the Senescence Hypothesis. Thus, the majority of data on reproductive traits and survivorship support the Relative Reproductive Rate Hypothesis. We also compared Painted Turtles to Blanding's Turtles, another species studied on the E.S. George Reserve. That Painted Turtles exhibit indeterminate growth whereas Blanding's Turtles do not, appears to be a primary mechanism for some differences between species in the relationships between reproductive traits and age. An important mechanism for increasing reproductive output in both species was increased reproductive frequency in older females. Painted Turtles also increased offspring quality (egg and hatchling size) with age, whereas Blanding's Turtles did not. Compared to younger individuals, there was no reduction in survivorship in the oldest Painted Turtles and survivorship increased in the oldest Blanding's Turtles.

Genetic analysis of growth curves for a woody perennial species, Pinus taeda L.

Inheritance of growth curves is critical for understanding evolutionary change and formulating efficient breeding plans, yet has received limited attention. Growth curves, like other characters that change in concert with development, often have higher heritability than age-specific traits. This study compared genetic parameters of height-growth curves with those of age-specific heights for a conifer, Pinus taeda L. Growth curves were fitted with: (1) a linear regression model, and (2) a non-linear model based on Richards' function using two sources of height data: two six-parent diallel tests assessed at age 2 to 10 years and two tests from a nested mating design with 222 parents assessed at 1 to 25 years. Additive genetic control of growth-curve parameters was moderate (h(2) = 0.06 to 0.26) and slightly lower than that for age-specific heights. Additive variance exceeded dominance variance for rate and shape parameters, but not for the asymptote. Genetic correlations among growth-curve parameters were high. Early selection on height was as efficient as selection on growth-curve parameters.

QUANTITATIVE GENETICS OF DEVELOPMENT: GENETIC CORRELATIONS AMONG AGE-SPECIFIC TRAIT VALUES AND THE EVOLUTION OF ONTOGENY.

It has recently been re-emphasized, by Gould (1977) and others, that evolution of adult forms takes place through the evolution of ontogenies. Direct selection at any age results in a correlated response at all other life history stages. This selection alters growth curves, or causes the evolution of ontogeny. Lande (1982) has presented a quantitative genetic model for the evolution of ontogenetically varying traits which clearly defines the parameters required for an analysis of ontogeny and evolution: (1) the genetic variance/covariance matrix; (2) the phentoypic variance/ covariance matrix; and (3) the vector of selection differentials. Genetic covariances or correlations among age-specific trait values quantitatively describe the genetic link between expressions of the same trait at different points in ontogeny. These genetic links between age-specific trait values are the result of pleiotropy, in this case the effects of one gene on the phenotype as expressed at more than one age, and linkage disequilibrium. The general effects of genetic variation on growth curves may be summarized using two parameters, curve height and curve shape (see Fig. 1). The average height of the curve can be defined as the average value of a trait over the ontogenetic period under consideration or the area under the growth curve divided by the length of the growth period. When there are only height differences between two growthi curves, the two curves will be parallel. Differences in curve shape, or the rate of growth at all ages, can be quantified independent of height, by standardizing the curves so that they have the same average height, or enclose the same area, and then calculating the absolute value of the area remaining between curves and dividing by the length of the growth period (see Fig. 1). This curve shape measure does not specify particular shapes but rather gives a general measure of differences in growth rates to compare with the independent measure of curve height differences. Specific curve shapes are presented without summarization. When heritabilities and phenotypic variances are the same at all ages, one can discern the independent effects of genetic correlations among age-specific trait values on ontogenetic variation and evolution. For example, in Figure 2 the ancestral ontogeny is displayed as a horizontal line (A) for purposes of illustration. Selection on the adult, or any other age, when the genetic correlations between ages are all 1.000, results in the descendant ontogeny, curve B. Note that the average height of the curve has changed but not its shape. When all the genetic correlations are one, the spectral decomposition of the age-specific traits correlation matrix includes only one non-zero eigenvalue equal to 'n,' the number of traits represented in the matrix, and an associated isometric eigenvector with loadings of 1/\Vn for each trait. This vector measures variation in growth curve height independent of variation in curve shape. Any deviation of the correlations

Quantitative Genetics of Development: Genetic Correlations Among Age-Specific Trait Values and the Evolution of Ontogeny

Quantitative Genetics of Development: Genetic Correlations Among Age-Specific Trait Values and the Evolution of Ontogeny