Roles for modularity and constraint in the evolution of cranial diversity among Anolis lizards

Complex organismal structures are organized into modules, suites of traits that develop, function, and vary in a coordinated fashion. By limiting or directing covariation among component traits, modules are expected to represent evolutionary building blocks and to play an important role in morphological diversification. But how stable are patterns of modularity over macroevolutionary timescales? Comparative analyses are needed to address the macroevolutionary effect of modularity, but to date few have been conducted. We describe patterns of skull diversity and modularity in Caribbean Anolis lizards. We first diagnose the primary axes of variation in skull shape and then examine whether diversification of skull shape is concentrated to changes within modules or whether changes arose across the structure as a whole. We find no support for the hypothesis that cranial modules are conserved as species diversify in overall skull shape. Instead we find that anole skull shape and modularity patterns independently converge. In anoles, skull modularity is evolutionarily labile and may reflect the functional demands of unique skull shapes. Our results suggest that constraints have played little role in limiting or directing the diversification of head shape in Anolis lizards.     

The Influence of Modularity on Cranial Morphological Disparity in Carnivora and Primates (Mammalia)

Background

Although variation provides the raw material for natural selection and evolution, few empirical data exist about the factors controlling morphological variation. Because developmental constraints on variation are expected to act by influencing trait correlations, studies of modularity offer promising approaches that quantify and summarize patterns of trait relationships. Modules, highly-correlated and semi-autonomous sets of traits, are observed at many levels of biological organization, from genes to colonies. The evolutionary significance of modularity is considerable, with potential effects including constraining the variation of individual traits, circumventing pleiotropy and canalization, and facilitating the transformation of functional structures. Despite these important consequences, there has been little empirical study of how modularity influences morphological evolution on a macroevolutionary scale. Here, we conduct the first morphometric analysis of modularity and disparity in two clades of placental mammals, Primates and Carnivora, and test if trait integration within modules constrains or facilitates morphological evolution.

Principal Findings

We used both randomization methods and direct comparisons of landmark variance to compare disparity in the six cranial modules identified in previous studies. The cranial base, a highly-integrated module, showed significantly low disparity in Primates and low landmark variance in both Primates and Carnivora. The vault, zygomatic-pterygoid and orbit modules, characterized by low trait integration, displayed significantly high disparity within Carnivora. 14 of 24 results from analyses of disparity show no significant relationship between module integration and morphological disparity. Of the ten significant or marginally significant results, eight support the hypothesis that integration within modules constrains morphological evolution in the placental skull. Only the molar module, a highly-integrated and functionally important module, showed significantly high disparity in Carnivora, in support of the facilitation hypothesis.

Conclusions

This analysis of within-module disparity suggested that strong integration of traits had little influence on morphological evolution over large time scales. However, where significant results were found, the primary effect of strong integration of traits was to constrain morphological variation. Thus, within Primates and Carnivora, there was some support for the hypothesis that integration of traits within cranial modules limits morphological evolution, presumably by limiting the variation of individual traits.     

Does nasal echolocation influence the modularity of the mammal skull?

In vertebrates, changes in cranial modularity can evolve rapidly in response to selection. However, mammals have apparently maintained their pattern of cranial integration throughout their evolutionary history and across tremendous morphological and ecological diversity. Here, we use phylogenetic, geometric morphometric and comparative analyses to test the hypothesis that the modularity of the mammalian skull has been remodelled in rhinolophid bats due to the novel and critical function of the nasal cavity in echolocation. We predicted that nasal echolocation has resulted in the evolution of a third cranial module, the ‘nasal dome’, in addition to the braincase and rostrum modules, which are conserved across mammals. We also test for similarities in the evolution of skull shape in relation to habitat across rhinolophids. We find that, despite broad variation in the shape of the nasal dome, the integration of the rhinolophid skull is highly consistent with conserved patterns of modularity found in other mammals. Across their broad geographical distribution, cranial shape in rhinolophids follows two major divisions that could reflect adaptations to dietary and environmental differences in African versus South Asian distributions. Our results highlight the potential of a relatively simple modular template to generate broad morphological and functional variation in mammals.     

Challenges in identifying and interpreting organizational modules in morphology

Form is a rich concept that agglutinates information about the proportions and topological arrangement of body parts. Modularity is readily measurable in both features, the variation of proportions (variational modules) and the organization of topology (organizational modules). The study of variational modularity and of organizational modularity faces similar challenges regarding the identification of meaningful modules and the validation of generative processes; however, most studies in morphology focus solely on variational modularity, while organizational modularity is much less understood. A possible cause for this bias is the successful development in the last twenty years of morphometrics, and specially geometric morphometrics, to study patters of variation. This contrasts with the lack of a similar mathematical framework to deal with patterns of organization. Recently, a new mathematical framework has been proposed to study the organization of gross anatomy using tools from Network Theory, so‐called Anatomical Network Analysis (AnNA). In this essay, I explore the potential use of this new framework—and the challenges it faces in identifying and validating biologically meaningful modules in morphological systems—by providing working examples of a complete analysis of modularity of the human skull and upper limb. Finally, I suggest further directions of research that may bridge the gap between variational and organizational modularity studies, and discuss how alternative modeling strategies of morphological systems using networks can benefit from each other.     

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.     

SIZE VARIATION,GROWTH STRATEGIES,AND THE EVOLUTION OF MODULARITY IN THE MAMMALIAN SKULL

Allometry is a major determinant of within‐population patterns of association among traits and, therefore, a major component of morphological integration studies. Even so, the influence of size variation over evolutionary change has been largely unappreciated. Here, we explore the interplay between allometric size variation, modularity, and life‐history strategies in the skull from representatives of 35 mammalian families. We start by removing size variation from within‐species data and analyzing its influence on integration magnitudes, modularity patterns, and responses to selection. We also carry out a simulation in which we artificially alter the influence of size variation in within‐taxa matrices. Finally, we explore the relationship between size variation and different growth strategies. We demonstrate that a large portion of the evolution of modularity in the mammalian skull is associated to the evolution of growth strategies. Lineages with highly altricial neonates have adult variation patterns dominated by size variation, leading to high correlations among traits regardless of any underlying modular process and impacting directly their potential to respond to selection. Greater influence of size variation is associated to larger intermodule correlations, less individualized modules, and less flexible responses to natural selection.     

Evolution of the morphological innovations of feathers

Feathers are complex assemblages of multiple morphological innovations. Recent research on the development and evolution of feathers has produced new insights into the origin and diversification of the morphological innovations in feathers. In this article, I review and discuss the contribution of three different factors to the evolution of morphological innovations in feathers: feather tubularity, hierarchical morphological modularity, and the co-option molecular signaling modules. The developing feather germ is a tube of epidermis with a central dermal pulp. The tubular organization of the feather germ and follicle produces multiple axes over which morphological differentiation can be organized. Feather complexity is organized into a hierarchy of morphological modules. These morphological modules evolved through the innovative differentiation along multiple different morphological axes created by the tubular feather germ. Concurrently, many of the morphological innovations of feathers evolved through the evolutionary co-option of plesiomorphic molecular signaling modules. Gene co-option also reveals a role for contingency in the evolution of hierarchical morphological innovations.     

Modularity in the skull and cranial vasculature of laboratory mice: implications for the evolution of complex phenotypes

The generation of coordinated morphological change over time results from the interconnectedness of evolution and development. The modular architecture of development results in varying degrees of integration and independence among parts of the phenotype, and facilitates the production of phenotypic variation in complex anatomical units composed of multiple tissue types. Here we use geometric morphometrics to investigate modularity in the arterial Circle of Willis (CW) and skull of the CD-1 laboratory mouse. We contrast a hypothesis of tight integration between these tissues with a hypothesis of more modular organization, to determine the level at which natural selection works to generate coordinated change. We report a complex pattern of covariation that indicates that the skull and CW are highly integrated and developmentally linked. Further, we report higher levels of fluctuating asymmetry in the CW than in the skull, suggesting a greater potential for lability in this tissue. These results suggest that epigenetic interactions or genetic influences on regional development are more important determinants of covariation structure than the factors that produce covariation within individual tissues or organ systems.     

Evolution of networks for body plan patterning; interplay of modularity, robustness and evolvability

A major goal of evolutionary developmental biology (evo-devo) is to understand how multicellular body plans of increasing complexity have evolved, and how the corresponding developmental programs are genetically encoded. It has been repeatedly argued that key to the evolution of increased body plan complexity is the modularity of the underlying developmental gene regulatory networks (GRNs). This modularity is considered essential for network robustness and evolvability. In our opinion, these ideas, appealing as they may sound, have not been sufficiently tested. Here we use computer simulations to study the evolution of GRNs' underlying body plan patterning. We select for body plan segmentation and differentiation, as these are considered to be major innovations in metazoan evolution. To allow modular networks to evolve, we independently select for segmentation and differentiation. We study both the occurrence and relation of robustness, evolvability and modularity of evolved networks. Interestingly, we observed two distinct evolutionary strategies to evolve a segmented, differentiated body plan. In the first strategy, first segments and then differentiation domains evolve (SF strategy). In the second scenario segments and domains evolve simultaneously (SS strategy). We demonstrate that under indirect selection for robustness the SF strategy becomes dominant. In addition, as a byproduct of this larger robustness, the SF strategy is also more evolvable. Finally, using a combined functional and architectural approach, we determine network modularity. We find that while SS networks generate segments and domains in an integrated manner, SF networks use largely independent modules to produce segments and domains. Surprisingly, we find that widely used, purely architectural methods for determining network modularity completely fail to establish this higher modularity of SF networks. Finally, we observe that, as a free side effect of evolving segmentation and differentiation in combination, we obtained in-silico developmental mechanisms resembling mechanisms used in vertebrate development.     

The Evolution of Modularity in the Mammalian Skull II: Evolutionary Consequences

Changes in patterns and magnitudes of integration may influence the ability of a species to respond to selection. Consequently, modularity has often been linked to the concept of evolvability, but their relationship has rarely been tested empirically. One possible explanation is the lack of analytical tools to compare patterns and magnitudes of integration among diverse groups that explicitly relate these aspects to the quantitative genetics framework. We apply such framework here using the multivariate response to selection equation to simulate the evolutionary behavior of several mammalian orders in terms of their flexibility, evolvability and constraints in the skull. We interpreted these simulation results in light of the integration patterns and magnitudes of the same mammalian groups, described in a companion paper. We found that larger magnitudes of integration were associated with a blur of the modules in the skull and to larger portions of the total variation explained by size variation, which in turn can exert a strong evolutionary constraint, thus decreasing the evolutionary flexibility. Conversely, lower overall magnitudes of integration were associated with distinct modules in the skull, to smaller fraction of the total variation associated with size and, consequently, to weaker constraints and more evolutionary flexibility. Flexibility and constraints are, therefore, two sides of the same coin and we found them to be quite variable among mammals. Neither the overall magnitude of morphological integration, the modularity itself, nor its consequences in terms of constraints and flexibility, were associated with absolute size of the organisms, but were strongly associated with the proportion of the total variation in skull morphology captured by size. Therefore, the history of the mammalian skull is marked by a trade-off between modularity and evolvability. Our data provide evidence that, despite the stasis in integration patterns, the plasticity in the magnitude of integration in the skull had important consequences in terms of evolutionary flexibility of the mammalian lineages.     

Association between integration structure and functional evolution in the opercular four‐bar apparatus of the threespine stickleback,Gasterosteus aculeatus (Pisces: Gasterosteidae)

Phenotypes may evolve to become integrated in response to functional demands. Once evolved, integrated phenotypes, often modular, can also influence the trajectory of subsequent responses to selection. Clearly, connecting modularity and functionally adaptive evolution has been challenging. The teleost skull and jaw structures are useful for understanding this connection because of the key roles that these structures play in feeding in novel environments with different prey resources. In the present study, we examined such a structure in the threespine stickleback: the opercular four‐bar lever that functions in jaw opening. Comparing oceanic and two fresh‐water populations, we find marked phenotypic divergence in the skull opercular region, and the major axes of morphological and functional variation of the lever are found to be highly correlated. All three populations share the same global skull integration structure, and a conserved, strongly‐supported modular organization is evident in the region encompassing the lever. Importantly, a boundary between two modules that subdivides the lever apparatus corresponds to the region of most prominent morphological evolution. The matched modular phenotypic and functional architecture of head and jaw structures of stickleback therefore may be important for facilitating their rapid adaptive transitions between highly divergent habitats. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 375–390.     

Cranial modularity shifts during mammalian evolution

The mammalian skull has been studied as several separate functional components for decades, but the study of modularity is a more recent, integrative approach toward quantitative examination of independent subsets of highly correlated traits, or modules. Although most studies of modularity focus on developmental and genetic systems, phenotypic modules have been noted in many diverse morphological structures. However, few studies have provided empirical data for comparing modules across higher taxonomic levels, limiting the ability to assess the broader evolutionary significance of modularity. This study uses 18-32 three-dimensional cranial landmarks to analyze phenotypic modularity in 106 mammalian species and demonstrates that cranial modularity is generally conserved in the evolution of therian mammals (marsupials and placentals) but differs between therians and monotremes, the two extant subclasses of Mammalia. Within therians, cluster analyses identify six distinct modules, but only three modules display significant integration in all species. Monotremes display only two highly integrated modules. Specific hypotheses of functional and developmental influences on cranial bones were tested. Theoretical correlation matrices for bones were constructed on the basis of shared function, tissue origin, or mode of ossification, and all three of these models are significantly correlated with observed correlation matrices for the mammalian cranium.     

In search of morphological modules: a systematic review

Morphological modularity arises in complex living beings due to a semi‐independent inheritance, development, and function of body parts. Modularity helps us to understand the evolvability and plasticity of organismal form, and how morphological variation is structured during evolution and development. For this reason, delimiting morphological modules and establishing the factors involved in their origins is a lively field of inquiry in biology today. Although it is thought that modularity is pervasive in all living beings, actually we do not know how often modularity is present in different morphological systems. We also do not know whether some methodological approaches tend to reveal modular patterns more easily than others, or whether some factors are more related to the formation of modules or the integration of the whole phenotype. This systematic review seeks to answer these type of questions through an examination of research investigating morphological modularity from 1958 to present. More than 200 original research articles were gathered in order to reach a quantitative appraisal on what is studied, how it is studied, and how the results are explained. The results reveal an heterogeneous picture, where some taxa, systems, and approaches are over‐studied, while others receive minor attention. Thus, this review points out various trends and gaps in the study of morphological modularity, offering a broad picture of current knowledge and where we can direct future research efforts.     

Evolvability of the Primate Pelvic Girdle

The ilium and ischiopubic bones of the pelvis arise from different regulatory pathways, and as a result, they may be modular in their organization such that features on one bone may be morphologically integrated with each other, but not with features on the other pelvic bone. Modularity at this gross level of organization can act to increase the ability of these structures to respond to selection pressures (i.e., their evolvability). Furthermore, recent work has suggested that the evolution of the human pelvis was facilitated by low levels of integration and high levels of evolvability relative to other African apes. However, the extent of morphological integration and modularity of the bones of the pelvic girdle is not well understood, especially across the entire order of primates. Therefore, the hypothesis that the ilium and ischiopubis constitute separate modules was tested using three-dimensional landmark data that were collected from 752 pelves from 35 primate species. In addition, the hypothesis that the human pelvis demonstrates greatest evolvability was tested by comparing it to all other primates. The results demonstrate that regardless of phylogeny and locomotor function, the primate pelvis as a whole is characterized by low levels of overall integration and high levels of evolvability. In addition, the results support the developmental hypothesis of separate ilium and ischiopubis modular units. Finally, all primates, including humans, apparently share a common pattern of integration, modularity, and evolvability in the pelvis.     

Mosaic heterochrony and evolutionary modularity: the trilobite genus Zacanthopsis as a case study

Logical connections exist between evolutionary modularity and heterochrony, two unifying and structuring themes in the expanding field of evolutionary developmental biology. The former sees complex phenotypes as being made up of semi-independent units of evolutionary transformation; the latter requires such a modular organization of phenotypes to occur in a localized or mosaic fashion. This conceptual relationship is illustrated here by analyzing the evolutionary changes in the cranidial ontogeny of two related species of Cambrian trilobites. With arguments from comparative developmental genetics and functional morphology, we delineate putative evolutionary modules within the cranidium and examine patterns of evolutionary changes in ontogeny at both global and local scales. Results support a case of mosaic heterochrony, that is, a combination of local heterochronies affecting the different parts individuated in the cranidium, leading to the complex pattern of allometric repatterning observed at the global scale. Through this example, we show that recasting morphological analyses of complex phenotypes with a priori knowledge or hypotheses about their organizational and variational properties can significantly improve our interpretation and understanding of evolutionary changes among related taxa, fossil and extant. Such considerations open avenues to investigate the large-scale dynamics of modularity and its role in phenotypic evolution.     

Modular genetic architecture of floral morphology in Nicotiana: quantitative genetic and comparative phenotypic approaches to floral integration

Animal‐pollinated flowers are complex structures that may require a precise configuration of floral organs for proper function. As such, they represent an excellent system with which we can examine the role of phenotypic integration and modularity in morphological evolution. We use complementary quantitative genetic and comparative phenotypic approaches to examine correlations among floral characters in Nicotiana alata, N. forgetiana and their artificial fourth‐generation hybrids. Flowers of both species share basic patterns of genetic and phenotypic correlations characterized by at least two integrated character suites that are relatively independent of each other and are not disrupted by four generations of recombination in hybrids. We conclude that these integrated character suites represent phenotypic modules that are the product of a modular genetic architecture. Intrafloral modularity may have been critical for rapid specialization of these species to different pollinators.     

Evolutionary integration of the frog cranium

Evolutionary integration (covariation) of traits has long fascinated biologists because of its potential to elucidate factors that have shaped morphological evolution. Studies of tetrapod crania have identified patterns of evolutionary integration that reflect functional or developmental interactions among traits, but no studies to date have sampled widely across the species-rich lissamphibian order Anura (frogs). Frogs exhibit a vast range of cranial morphologies, life history strategies, and ecologies. Here, using high-density morphometrics we capture cranial morphology for 172 anuran species, sampling every extant family. We quantify the pattern of evolutionary modularity in the frog skull and compare patterns in taxa with different life history modes. Evolutionary changes across the anuran cranium are highly modular, with a well-integrated “suspensorium” involved in feeding. This pattern is strikingly similar to that identified for caecilian and salamander crania, suggesting replication of patterns of evolutionary integration across Lissamphibia. Surprisingly, possession of a feeding larval stage has no notable influence on cranial integration across frogs. However, late-ossifying bones exhibit higher integration than early-ossifying bones. Finally, anuran cranial modules show diverse morphological disparities, supporting the hypothesis that modular variation allows mosaic evolution of the cranium, but we find no consistent relationship between degree of within-module integration and disparity.