ONTOGENETIC VARIATION IN PATTERNS OF PHENOTYPIC INTEGRATION IN THE LABORATORY RAT

I used confirmatory factor analysis to evaluate the ability of causal developmental models to predict observed phenotypic integration in limb and skull measures at five stages of postnatal ontogeny in the laboratory rat. To analyze the dynamics of phenotypic integration, I fit successive age-classes simultaneously to a common model. Growth was the principal developmental explanation of observed phenotypic covariation in the limb and skull. No complex morphogenetic model more adequately reconstructed observed covariance structure. Models that could not be interpreted in embryological terms, coupled with a growth component, provide the best models for observed phenotypic integration. During postnatal growth, some aspects of integration vary in both the skull and limb. The covariance between factors and the proportion of variance unique to each character differ between some sequential age-classes. The factor-pattern is invariant in the limb; however, repatterning in the skull occurs in the interval between eye-opening and weaning. The temporal variation in the structure of covariation suggests that functional interactions among characters may create observed patterns of phenotypic integration. The developmental constraints responsible for evolutionary modification of phenotypes might be equally dynamic and responsive to embryonic functional interactions.     

Ontogenetic integration of the hominoid face

By investigating similarity in cranial covariation patterns, it is possible to locate underlying functional and developmental causes for the patterning, and to make inferences about the evolutionary forces that have acted to produce the patterns. Furthermore, establishing where these covariation patterns may diverge in ontogeny can offer insight into when selection may have acted on development. Here, covariation patterns are compared among adult and non-adult members of the African ape/human clade, in order to address three questions. First, are integration patterns constant among adult African apes and humans? Second, are they are constant in non-adults--i.e. throughout ontogeny? Third, if they are not constant, when do they diverge? Measurements are obtained from 677 crania of adult and non-adult African apes and humans. In order to address the first two questions, correlation matrices and theoretical integration matrices are compared using matrix correlation methods. The third question is evaluated by comparing correlation and variance/covariance patterns, using matrix correlation and random skewers methods, respectively, between adjacent age categories within each species, and between equivalent age categories among the four species. Results show that the hominoids share a similar pattern of ontogenetic integration, suggesting that common developmental/functional integrative processes may play an important role in keeping covariance structure stable across this lineage. However, there are some important differences in the magnitude of integration and in phenotypic covariance structure among the species, which may provide some insight into how selection acted to differentiate humans from the great apes.     

Individual variation in growth trajectories: phenotypic and genetic correlations in ontogeny of the house finch (Carpodacus mexicanus)

We studied patterns of growth in a recently established natural population of the house finch (Carpodacus mexicanus) to examine whether phenotypic and genetic covariation among age‐specific trait values is likely to constrain morphological change favoured by selection acting on adults. We found variable patterns of allometric relationships during ontogeny, and documented relatively weak covariations among ages or among traits in individual growth trajectories. Frequent compensatory growth largely cancelled out the initial differences among nestlings, potentially enabling house finches to raise offspring under diverse and unpredictable environmental conditions. Moderate levels of additive genetic variance in morphological traits throughout ontogeny, and relatively low and fluctuating phenotypic and genetic covariation among ages imply strong potential for evolutionary change in morphological traits under selection. This conclusion is consistent with the profound population‐level divergence in morphological patterns that accompanied very successful colonization of most of North America by the house finch over the last 50 years.     

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.     

Deciphering the Palimpsest: Studying the Relationship Between Morphological Integration and Phenotypic Covariation

Organisms represent a complex arrangement of anatomical structures and individuated parts that must maintain functional associations through development. This integration of variation between functionally related body parts and the modular organization of development are fundamental determinants of their evolvability. This is because integration results in the expression of coordinated variation that can create preferred directions for evolutionary change, while modularity enables variation in a group of traits or regions to accumulate without deleterious effects on other aspects of the organism. Using our own work on both model systems (e.g., lab mice, avians) and natural populations of rodents and primates, we explore in this paper the relationship between patterns of phenotypic covariation and the developmental determinants of integration that those patterns are assumed to reflect. We show that integration cannot be reliably studied through phenotypic covariance patterns alone and argue that the relationship between phenotypic covariation and integration is obscured in two ways. One is the superimposition of multiple determinants of covariance in complex systems and the other is the dependence of covariation structure on variances in covariance-generating processes. As a consequence, we argue that the direct study of the developmental determinants of integration in model systems is necessary to fully interpret patterns of covariation in natural populations, to link covariation patterns to the processes that generate them, and to understand their significance for evolutionary explanation.     

Morphological integration and natural selection in the postcranium of wild verreaux's sifaka (Propithecus verreauxi verreauxi)

Morphological integration manifests as strong phenotypic covariation among interacting traits. In this study, a graph-theory approach is used to analyze patterns of morphological integration in a wild population of Verreaux's sifaka (Propithecus verreauxi verreauxi). The motivation for this study is to determine the relative roles of development versus function in shaping patterns of morphological integration in the sifaka postcranium. A developmental and a functional hypothesis of integration are compared with the observed pattern of integration and the fit of these hypotheses is assessed using information theoretic statistics. Correlational selection is also estimated on limb elements. Information theoretic statistics indicate that the developmental hypothesis fits the observed pattern of integration slightly better than the functional hypothesis. Only two pairs of traits experience correlational selection but neither of the traits within each pair are morphologically integrated. The observed pattern of integration contains several trait-trait associations that are specified by both the functional and developmental hypotheses. These results likely reflect the nested covariation structure in which a novel locomotor mode, vertical clinging and leaping, is derived from a primitive quadrupedal morphotype.     

QUANTITATIVE GENETICS OF SHAPE IN CRICKET WINGS: DEVELOPMENTAL INTEGRATION IN A FUNCTIONAL STRUCTURE

The role of developmental and genetic integration for evolution is contentious. One hypothesis states that integration acts as a constraint on evolution, whereas an alternative is that developmental and genetic systems evolve to match the functional modularity of organisms. This study examined a morphological structure, the cricket wing, where developmental and functional modules are discordant, making it possible to distinguish the two alternatives. Wing shape was characterized with geometric morphometrics, quantitative genetic information was extracted using a full‐sibling breeding design, and patterns of developmental integration were inferred from fluctuating asymmetry of wing shape. The patterns of genetic, phenotypic, and developmental integration were clearly similar, but not identical. Heritabilities for different shape variables varied widely, but no shape variables were devoid of genetic variation. Simulated selection for specific shape changes produced predicted responses with marked deflections due to the genetic covariance structure. Three hypotheses of modularity according to the wing structures involved in sound production were inconsistent with the genetic, phenotypic, or developmental covariance structure. Instead, there appears to be strong integration throughout the wing. The hypothesis that genetic and developmental integration evolve to match functional modularity can therefore be rejected for this example.     

Constraints on evolution and postcopulatory sexual selection: trade-offs among ejaculate characteristics

Ejaculates function as an integrated unit to ensure male fertility and paternity, can have a complex structure, and can experience multiple episodes of selection. Current studies on the evolution of ejaculates typically focus on phenotypic variation in sperm number, size, or related traits such as testes size as adaptations to postcopulatory male-male competition. However, the evolution of the integrated nature of ejaculate structure and function depends on genetic variation in and covariation between the component parts. Here we report a quantitative genetic study of the components of the ejaculate of the cockroach Nauphoeta cinerea, including those we know to experience postcopulatory sexual selection, in the context of functional integration of ejaculate characters. We use the patterns of genetic variation and covariation to infer how the integration of the functions of the ejaculate constrain and shape its evolution. Ejaculate components were highly variable, showed significant additive genetic variance, and moderate to high evolvability. The level of genetic variation in these characters, despite strong directional or truncating selection, may reflect the integration of multiple episodes of selection that occur in N. cinerea. There were few significant phenotypic correlations, but all the genetic correlations among ejaculate characters were significantly different from zero. The patterns of genetic variation and covariation suggest that there are important trade-offs among individual traits of the ejaculate and that evolution of ejaculate characteristics will not proceed unconstrained. Fully describing the genetic relationships among traits that perform as an integrated unit helps us understand how functional relationships constrain or facilitate the evolution of the complex structure that is the ejaculate.     

DISSECTING CORRELATED CHARACTERS: ADAPTIVE ASPECTS OF PHENOTYPIC COVARIATION IN MELANIZATION PATTERN OF PIERIS BUTTERFLIES

In this study we address the question of how much of the covariation among phenotypic characters observed in natural populations is adaptive. We examine covariation among a set of phenotypic characters that describe the wing-melanization pattern of Pieris butterflies. Previous functional analyses of thermoregulatory performance allow us to predict a priori whether and how different wing melanic characters should be correlated. We quantify and analyze the variation in the wing-melanization pattern within species for a series of Pieris populations from relatively cool environments in North America and compare these results with the predictions based on our adaptive hypothesis. We consider adaptive covariation both for biogeographic variation among populations and for seasonal polyphenism (phenotypic plasticity) within populations. Our hypothesis correctly predicts many of the qualitative features of covariation in melanization among major regions of the wings, at the level of biogeographic variation among populations, for both males and females of Pieris occidentalis. When within-population variation is considered, agreement with the adaptive predictions varies considerably in different populations for both P. occidentalis and P. napi males and females. Agreement for P. napi, particularly the females, is generally poorer than for P. occidentalis. In both species, there is a consistent difference in melanization pattern between alpine and arctic sites; this difference is discussed in relation to the differences in the radiative environment between these two types of “cold” habitats. Our results suggest that some important aspects of phenotypic correlation among wing melanic characters in Pieris are adaptive. We emphasize the important distinction between covariation and co-occurrence of characters, and we discuss these results in relation to the extensive biogeographic variation and phenotypic plasticity (seasonal polyphenism) in Pieris wing-melanization patterns.     

Why the short face? Developmental disintegration of the neurocranium drives convergent evolution in neotropical electric fishes

Convergent evolution is widely viewed as strong evidence for the influence of natural selection on the origin of phenotypic design. However, the emerging evo‐devo synthesis has highlighted other processes that may bias and direct phenotypic evolution in the presence of environmental and genetic variation. Developmental biases on the production of phenotypic variation may channel the evolution of convergent forms by limiting the range of phenotypes produced during ontogeny. Here, we study the evolution and convergence of brachycephalic and dolichocephalic skull shapes among 133 species of Neotropical electric fishes (Gymnotiformes: Teleostei) and identify potential developmental biases on phenotypic evolution. We plot the ontogenetic trajectories of neurocranial phenotypes in 17 species and document developmental modularity between the face and braincase regions of the skull. We recover a significant relationship between developmental covariation and relative skull length and a significant relationship between developmental covariation and ontogenetic disparity. We demonstrate that modularity and integration bias the production of phenotypes along the brachycephalic and dolichocephalic skull axis and contribute to multiple, independent evolutionary transformations to highly brachycephalic and dolichocephalic skull morphologies.     

The "eyespot module" and eyespots as modules: development, evolution, and integration of a complex phenotype

Organisms are inherently modular, yet modules also evolve in response to selection for functional integration or functional specialization of traits. For serially repeated homologous traits, there is a clear expectation that selection on the function of individual traits will reduce the integration between traits and subdivide a single ancestral module. The eyespots on butterfly wings are one example of serially repeated morphological traits that share a common developmental mechanism but are subject to natural and sexual selection for divergent functions. Here, I test two hypotheses about the organization of the eyespot pattern into independent dorsal-ventral and anterior-posterior modules, using a graphical modeling technique to examine patterns of eyespot covariation among and within wing surfaces in the butterfly Bicyclus anynana. Although there is a hierarchical and complex pattern of integration among eyespots, the results show a surprising mismatch between patterns of eyespot integration and the developmental and evolutionary eyespot units identified in previous empirical studies. These results are discussed in light of the relationships between developmental, functional, and evolutionary modules, and they suggest that developmental sources of independent trait variation are often masked by developmental sources of trait integration.     

A DEVELOPMENTAL CONSTRAINT IN CERION,WITH COMMENTS ON THE DEFINITION AND INTERPRETATION OF CONSTRAINT IN EVOLUTION

Since orthodox evolutionary theory is functionalist, constraints attain their most important positive meaning as channels of change imposed by historical and formal determinants, rather than by immediate natural selection. Since ontogeny is the usual locus of expression for these determinants, developmental constraint is an appropriate, general term. A particular developmental constraint in Cerion, most variable of West Indian land snails, stands out for two reasons: 1) it is simply and inexorably defined as a consequence both of formal principles (coiling of tube about axis) and of historical contingencies in Cerion's invariant allometry of growth; 2) it is pervasive in its influence, underlying major patterns of variation in every Cerion study I have ever undertaken. I refer to this pattern as the “jigsaw constraint.” When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this obvious fact is promoted from trivial to important because complex allometries impose a substantial set of further consequences for form upon this basic trade-off of whorl size and whorl number. I show that this complex of consequences dominates patterns of natural variation in Cerion at all levels (among shells within samples, between samples in the geographic variation of single species, and between species in multitaxon faunas). It also sets patterns of hybridization between taxa. This paper is primarily a compendium of such examples. It is designed to illustrate the importance of this constraint by the fundamental criterion of relative frequency.     

Evolution of a complex phenotype with biphasic ontogeny: Contribution of development versus function and climatic variation to skull modularity in toads

The theory of morphological integration and modularity predicts that if functional correlations among traits are relevant to mean population fitness, the genetic basis of development will be molded by stabilizing selection to match functional patterns. Yet, how much functional interactions actually shape the fitness landscape is still an open question. We used the anuran skull as a model of a complex phenotype for which we can separate developmental and functional modularity. We hypothesized that functional modularity associated to functional demands of the adult skull would overcome developmental modularity associated to bone origin at the larval phase because metamorphosis would erase the developmental signal. We tested this hypothesis in toad species of the Rhinella granulosa complex using species phenotypic correlation pattern (P‐matrices). Given that the toad species are distributed in very distinct habitats and the skull has important functions related to climatic conditions, we also hypothesized that differences in skull trait covariance pattern are associated to differences in climatic variables among species. Functional and hormonal‐regulated modules are more conspicuous than developmental modules only when size variation is retained on species P‐matrices. Without size variation, there is a clear modularity signal of developmental units, but most species have the functional model as the best supported by empirical data without allometric size variation. Closely related toad species have more similar climatic niches and P‐matrices than distantly related species, suggesting phylogenetic niche conservatism. We infer that the modularity signal due to embryonic origin of bones, which happens early in ontogeny, is blurred by the process of growth that occurs later in ontogeny. We suggest that the species differing in the preferred modularity model have different demands on the orbital functional unit and that species contrasting in climate are subjected to divergent patterns of natural selection associated to neurocranial allometry and T3 hormone regulation.     

MULTILEVEL ANALYSIS OF MORPHOMETRIC DATA FROM NATURAL PLANT POPULATIONS: INSIGHTS INTO ONTOGENETIC,GENETIC, AND SELECTIVE CORRELATIONS IN DALECHAMPIA SCANDENS

I examined patterns of covariation of three morphometric blossom characters [gland area (GA), gland–stigma distance (GSD), and bract length (BL)] within genets, among genets, and among populations of the tropical vine, Dalechampia scandens (Euphorbiaceae). Covariance between BL and GA was evenly distributed among the three levels. This observation, coupled with developmental information, indicates that the two characters change size similarly during development, that there is probably genetic covariance between them (apparently caused by pleiotropy), and that the genetic covariance may have constrained (at least proximally) the course of population differentiation with respect to these characters. Most covariance between GSD and GA occurred at the among-population level. This observation, coupled with developmental information, indicates that there is negligible ontogenetic covariance and that within populations there is probably little or no genetic covariance between the two characters. Among-population covariance has probably been caused by natural selection operating in a correlated fashion on characters that functionally interact in pollination.     

EVOLUTION OF VARIATION AND VARIABILITY UNDER FLUCTUATING,STABILIZING, AND DISRUPTIVE SELECTION

How variation and variability (the capacity to vary) may respond to selection remain open questions. Indeed, effects of different selection regimes on variational properties, such as canalization and developmental stability are under debate. We analyzed the patterns of among‐ and within‐individual variation in two wing‐shape characters in populations of Drosophila melanogaster maintained under fluctuating, disruptive, and stabilizing selection for more than 20 generations. Patterns of variation in wing size, which was not a direct target of selection, were also analyzed. Disruptive selection dramatically increased phenotypic variation in the two shape characters, but left phenotypic variation in wing size unaltered. Fluctuating and stabilizing selection consistently decreased phenotypic variation in all traits. In contrast, within‐individual variation, measured by the level of fluctuating asymmetry, increased for all traits under all selection regimes. These results suggest that canalization and developmental stability are evolvable and presumably controlled by different underlying genetic mechanisms, but the evolutionary responses are not consistent with an adaptive response to selection on variation. Selection also affected patterns of directional asymmetry, although inconsistently across traits and treatments.     

Morphological intergration between development compartments in the Drosophila wing

Abstract. Developmental integration is the covariation among morphological structures due to connections between the developmental processes that built them. Here we use the methods of geometric morphometrics to study integration in the wing of Drosophila melanogaster . In particular, we focus on the hypothesis that the anterior and posterior wing compartments are separate developmental units that vary independently. We measured both variation among genetically diverse individuals and random differences between body sides of single individuals (fluctuating asymmetry, FA). For both of these sources of variation, the patterns of variation identified by principal component analyses all involved landmarks in both the anterior and posterior compartments simultaneously. Analyses focusing exclusively on the covariation between the anterior and posterior compartments, by the partial least-squares method, revealed pervasive integration of the two compartments, for both individual variation and FA. These analyses clearly indicate that the anterior and posterior compartments are not separate units of variation, but that the covariation between compartments is sufficient to account for nearly all the variation throughout the entire wing. We conclude that variation among individuals as well as the developmental perturbations responsible for FA generate shape variation primarily through developmental processes that are integrated across both compartments. In contrast, much less of the shape variation in our sample can be attributed to the localized processes that establish the identity of particular wing veins.     

Effects of developmental and functional interactions on mouse cranial variability through late ontogeny

The mammalian skull performs a variety of functions and its growth and development mirrors this complexity. Cranial growth and development have been actively studied for many years. Despite this interest, the variation in the patterns and processes of skull growth has attracted little attention. An important and unanswered question is the extent to which patterns of cranial covariation and variation are dynamically reworked throughout postnatal growth. To address this question, we examine patterns of variability in random-bred mouse skulls aged 35, 90, and 150 days. Using a battery of both Procrustes coordinate and Euclidean distance-based methods, we measure mean shape, canalization, developmental stability, and morphological integration in these skulls. We predict that the patterns of variability are dynamic, particularly between the youngest and the two oldest age groups due to the influence of functional effects such as postweaning mastication. We also hypothesize that patterns of variability are structured by the same functional and developmental factors that have been shown to influence cranial growth in primates. Our results indicate that contrary to our predictions, patterns of canalization, developmental stability, and morphological integration are stabilized before 35 days. The mean shape, however, changed significantly with growth. We found that only the facial region showed significant integration as predicted by the functional matrix model used in other studies of integration. These results indicate that phenotypic integration in these mice does not closely match those found for primate species, suggesting that comparisons between species should be made with care.     

Morphological integration and developmental progress during fish ontogeny in two contrasting habitats

SUMMARY Morphological integration can respond to environmental conditions, a response that may be dynamic through ontogeny. Among fishes, brook charrs ( Salvelinus fontinalis ) display a trophic polymorphism that makes it a good species for analyzing the ontogeny of morphological integration. To better understand the processes regulating variation and integration, we assess the ontogenetic dynamics of covariances and developmental progress for populations of S. fontinalis from two habitats that differ in water velocity; lake and stream. Geometric morphometrics and developmental progress were evaluated on 751 and 198 specimens, respectively. In both habitats, most ossification events occur before the transition from alevin to juvenile. This threshold defines two distinct periods. During the first period representing free-embryos and alevins, there are important shape changes and rapid ossification, integration tends to be relatively low and decreasing and the variance of shape drastically decreases. During the juvenile period, the rate of shape change decreases and the onset of ossification is nearly complete, plus integration increases and shape variance stabilizes. While we find two distinct developmental periods, we nonetheless find a notable stability underlying the ontogenetic dynamics of variability as well as gradual change in the structure of covariation within each habitat. Our results imply that the variability of juvenile body shape does not seem to retain signals of variability determined early in ontogeny and warrants caution in using juvenile as guides to the earlier causes of variability. Overall, this study highlights the difficulty of inferring causes of integration from studies of static covariance.     

Drosophila wing modularity revisited through a quantitative genetic approach

To predict the response of complex morphological structures to selection it is necessary to know how the covariation among its different parts is organized. Two key features of covariation are modularity and integration. The Drosophila wing is currently considered a fully integrated structure. Here, we study the patterns of integration of the Drosophila wing and test the hypothesis of the wing being divided into two modules along the proximo‐distal axis, as suggested by developmental, biomechanical, and evolutionary evidence. To achieve these goals we perform a multilevel analysis of covariation combining the techniques of geometric morphometrics and quantitative genetics. Our results indicate that the Drosophila wing is indeed organized into two main modules, the wing base and the wing blade. The patterns of integration and modularity were highly concordant at the phenotypic, genetic, environmental, and developmental levels. Besides, we found that modularity at the developmental level was considerably higher than modularity at other levels, suggesting that in the Drosophila wing direct developmental interactions are major contributors to total phenotypic shape variation. We propose that the precise time at which covariance‐generating developmental processes occur and/or the magnitude of variation that they produce favor proximo‐distal, rather than anterior‐posterior, modularity in the Drosophila wing.