The developmental renaissance in adaptationism

Divergent structural leaf trait spectra in succulent versus non-succulent plant taxa

BACKGROUND AND SCOPE

Plant functional traits are the result of natural selection to optimize carbon gain, leading to a broad spectrum of traits across environmental gradients. Among plant traits, leaf water storage capacity is paramount for plant drought resistance. We explored whether leaf-succulent taxa follow trait correlations similar to those of non-leaf-succulent taxa to evaluate whether both are similarly constrained by relationships between leaf water storage and climate.

METHODS

We tested the relationships among three leaf traits related to water storage capacity and resource use strategies in 132 species comprising three primary leaf types: succulent, sclerophyllous, and leaves with rapid returns on water investment, referred to as fast return. Correlation coefficients among specific leaf area (SLA), water mass per unit of area (WMA), and saturated water content (SWC) were tested, along with relationships between leaf trait spectra and aridity determined from species occurrence records.

RESULTS

Both SWC and WMA at a given SLA were ~10-fold higher in succulent leaves than in non-succulent leaves. While SWC actually increased with SLA in non-succulent leaves, no relationship was detected between SWC and SLA in succulent leaves, although WMA decreased with SLA in all leaf types. A principal component analysis (PCA) revealed that succulent taxa occupied a widely different mean trait space than either fast-return (P < 0.0001) or sclerophyllous (P < 0.0001) taxa along the first PCA axis, which explained 63 % of mean trait expression among species. However, aridity only explained 12 % of the variation in PCA1 values. This study is among the first to establish a structural leaf trait spectrum in succulent leaf taxa and quantify contrasts in leaf water storage among leaf types relative to specific leaf area.

CONCLUSIONS

Trait coordination in succulent leaf taxa may not follow patterns similar to those of widely studied non-succulent taxa.

Temperature and Turgor "Limitation" and Environmental "Control" in Xylem Biology and Dendrochronology

Trees and other woody plants are immensely ecologically important, making it essential to understand the causes of relationships between tree structure and function. To help these efforts, we highlight persistent traditions in plant biology of appealing to environmental factors "limiting" or "controlling" woody plant features. Examples include the idea that inevitable drops in cell turgor with plant height limit cell expansion and thus leaf size and tree height; that low temperatures prohibit lignification of cells and thus the growth of woody plants at high elevation; and notions from dendrochronology and related fields that climate factors such as rainfall and temperature "control" growth ring features. We show that notions of "control," "limitation," and the like imply that selection would favor a given trait value, but that these would-be favored values are developmentally impossible to produce. Such "limitation" scenarios predict trait frequency distributions that are very narrow and are abruptly curtailed at the upper limit of developmental possibility (the right-hand side of the distribution). Such distributions have, to our knowledge, never been observed, so we see little empirical support for "limitation" hypotheses. We suggest that, as a more productive starting point, plant biologists should examine adaptation hypotheses, in which developmental possibility is wide (congruent with the wide ranges of trait variation that really are observed), but only some of the possible variants are favored. We suggest that (1) the traditional the proximate/ultimate causation distinction, (2) purging scenarios of teleology/anthropomorphism, and (3) stating hypotheses in terms of developmental potential and natural selection are three simple ways of making "limitation" hypotheses clearer with regard to biological process and thus empirically testable.

The ontogeny of disparity in Cupressaceae seed cones

Ontogenetic shape change has long been recognized to be important in generating patterns of morphological diversity and may be especially important in plant reproductive structures. We explore how seed cone disparity in Cupressaceae changes over ontogeny by comparing pollination-stage and mature cones. We sampled cones at pollen and seed release and measured cone scales using basic morphometric shape variables. We used multivariate statistical methods, particularly hypervolume overlap calculations, to measure morphospace occupation and disparity. Cone scales at both pollination and maturity exhibit substantial variability, although the disparity is greater at maturity. Mature cone scales are also more clustered in trait space, showing less overlap with other taxa than at pollination. These patterns reflect two growth strategies that generate closed cones over maturation, either through thin laminar scales or relatively thick, peltate scales, resulting in two distinct regions of morphospace occupation. Disparity patterns in Cupressaceae seed cones change over ontogeny, reflecting shifting functional demands that require specific patterns of cone scale growth. The evolution of Cupressaceae reproductive disparity therefore represents selection for trajectories of ontogenetic shape change, a phenomenon that should be widespread across seed plants.

Plant morpho evo-devo

Evo-devo is often thought of as being the study of which genes underlie which phenotypes. However, evo-devo is much more than this, especially in plant science. In leaf scars along stems, cell changes across wood growth rings, or flowers along inflorescences, plants trace a record of their own development. Plant morpho evo-devo provides data that genes could never furnish on themes such as heterochrony, the evolution of temporal phenotypes, modularity, and phenotype-first evolution. As plant science surges into increasingly -omic realms, it is essential to keep plant morpho evo-devo in full view as an honored member of the evo-devo canon, ensuring that plant scientists can, wherever they are, generate fundamental insights at the appropriate level of biological organization.

Where do we stand with the covariation framework in primate societies?

Comparative study of the social systems of macaques has revealed correlated variations between species in multiple social traits such as the asymmetry of dominance relationships, preferential treatment of kin, patterns of aggression and reconciliation, modes of socialization, and access to food resources. Macaques can be classified on a scale of four categories of social styles, ranging from the least to the most tolerant species. This led to the development of the covariation framework, which addresses the constraints responsible for the linkages between social traits, and their consequences on the evolution of primate social systems. Decades of research have provided a wealth of information that supports, complements, expands, or challenges the covariation framework. In this article, I review this body of knowledge and consider covariation in its two aspects, that is, as a pattern and as a hypothesis. I first consider the extent to which social styles can be invariant, the strength of correlations between traits, and the possible extension of the framework to nonhuman primates other than macaques. I then discuss how to formulate hypotheses, identify sources of linkage between traits, make predictions about the effects of social constraints, assess the tolerance dimension of social styles, and consider the breaking of linkages between traits. Whereas socioecological studies aim to understand how adaptation to the ecological environment determines the shape of social systems, the covariation framework is a complementary research program that seeks to unravel the internal processes that restrict or channel change in social behavior.

The vessel wall thickness-vessel diameter relationship across woody angiosperms

PREMISE

Comparative anatomy is necessary to identify the extremes of combinations of functionally relevant structural traits, to ensure that physiological data cover xylem anatomical diversity adequately, and thus achieve a global understanding of xylem structure-function relations. A key trait relationship is that between xylem vessel diameter and wall thickness of both the single vessel and the double vessel+adjacent imperforate tracheary element (ITE).

METHODS

We compiled a comparative data set with 1093 samples, 858 species, 350 genera, 86 families, and 33 orders. We used broken linear regression and an algorithm to explore changes in parameter values from linear regressions using subsets of the data set to identify a threshold, at 90-µm vessel diameter, in the wall thickness-diameter relationship.

RESULTS

Below 90 µm diameter for vessels, virtually any wall thickness could be associated with virtually any diameter. Below this threshold, selection is free to favor a very wide array of combinations, such as very thick walls and narrow vessels in ITE-free herbs, or very thin-walled, wide vessels in evergreen dryland pioneers. Above 90 µm, there was a moderate positive relationship.

CONCLUSIONS

Our analysis shows that the space of vessel wall thickness-diameter combinations is very wide, with selection apparently eliminating individuals with vessel walls "too thin" for their diameter. Most importantly, our survey revealed poorly studied plant hydraulic syndromes (functionally significant trait combinations). These data suggest that the full span of trait combinations, and thus the minimal set of hydraulic syndromes requiring study to span woody plant functional diversity adequately, remains to be documented.

Body-axis organization in tetrapods: a model-system to disentangle the developmental origins of convergent evolution in deep time

Convergent evolution is a central concept in evolutionary theory but the underlying mechanism has been largely debated since On the Origin of Species. Previous hypotheses predict that developmental constraints make some morphologies more likely to arise than others and natural selection discards those of the lowest fitness. However, the quantification of the role and strength of natural selection and developmental constraint in shaping convergent phenotypes on macroevolutionary timescales is challenging because the information regarding performance and development is not directly available. Accordingly, current knowledge of how embryonic development and natural selection drive phenotypic evolution in vertebrates has been extended from studies performed at short temporal scales. We propose here the organization of the tetrapod body-axis as a model system to investigate the developmental origins of convergent evolution over hundreds of millions of years. The quantification of the primary developmental mechanisms driving body-axis organization (i.e. somitogenesis, homeotic effects and differential growth) can be inferred from vertebral counts, and recent techniques of three-dimensional computational biomechanics have the necessary potential to reveal organismal performance even in fossil forms. The combination of both approaches offers a novel and robust methodological framework to test competing hypotheses on the functional and developmental drivers of phenotypic evolution and evolutionary convergence.

Fossils and plant evolution: structural fingerprints and modularity in the evo-devo paradigm

Fossils constitute the principal repository of data that allow for independent tests of hypotheses of biological evolution derived from observations of the extant biota. Traditionally, transformational series of structure, consisting of sequences of fossils of the same lineage through time, have been employed to reconstruct and interpret morphological evolution. More recently, a move toward an updated paradigm was fueled by the deliberate integration of developmental thinking in the inclusion of fossils in reconstruction of morphological evolution. The vehicle for this is provided by structural fingerprints-recognizable morphological and anatomical structures generated by (and reflective of) the deployment of specific genes and regulatory pathways during development. Furthermore, because the regulation of plant development is both modular and hierarchical in nature, combining structural fingerprints recognized in the fossil record with our understanding of the developmental regulation of those structures produces a powerful tool for understanding plant evolution. This is particularly true when the systematic distribution of specific developmental regulatory mechanisms and modules is viewed within an evolutionary (paleo-evo-devo) framework. Here, we discuss several advances in understanding the processes and patterns of evolution, achieved by tracking structural fingerprints with their underlying regulatory modules across lineages, living and fossil: the role of polar auxin regulation in the cellular patterning of secondary xylem and the parallel evolution of arborescence in lycophytes and seed plants; the morphology and life history of early polysporangiophytes and tracheophytes; the role of modularity in the parallel evolution of leaves in euphyllophytes; leaf meristematic activity and the parallel evolution of venation patterns among euphyllophytes; mosaic deployment of regulatory modules and the diverse modes of secondary growth of euphyllophytes; modularity and hierarchy in developmental regulation and the evolution of equisetalean reproductive morphology. More generally, inclusion of plant fossils in the evo-devo paradigm has informed discussions on the evolution of growth patterns and growth responses, sporophyte body plans and their homology, sequences of character evolution, and the evolution of reproductive systems.

Convergent developmental patterns underlie the repeated evolution of adhesive toe pads among lizards

How developmental modifications produce key innovations, which subsequently allow for rapid diversification of a clade into new adaptive zones, has received much attention. However, few studies have used a robust comparative framework to investigate the influence of evolutionary and developmental constraints on the origin of key innovations, such as the adhesive toe pad of lizards. Adhesive toe pads evolved independently at least 16 times in lizards, allowing us to examine whether the patterns observed are general evolutionary phenomena or unique, lineage-specific events. We performed a high-resolution comparison of plantar scale development in 14 lizard species in Anolis and geckos, encompassing five independent origins of toe pads (one in Anolis, four in geckos). Despite substantial evolutionary divergence between Anolis and geckos, we find that these clades have undergone similar developmental modifications to generate their adhesive toe pads. Relative to the ancestral plantar scale development, in which scale ridges form synchronously along the digit, both padded geckos and Anolis exhibit scansor formation in a distal-to-proximal direction. Both clades have undergone developmental repatterning and, following their origin, modifications in toe pad morphology occurred through relatively minor developmental modifications, suggesting that developmental constraints governed the diversification of the adhesive toe pad in lizards.

A poplar option: the 'within-individual approach' for elucidating xylem structure-function relationships

This article comments on:

Baer AB, Fickle JC, Medina J, Robles C, Pratt RB, Jacobsen AL. 2021. Xylem biomechanics, water storage, and density within roots and shoots of an angiosperm tree species. Journal of Experimental Botany 72, 7984–7997.

Xylem biomechanics, water storage, and density within roots and shoots of an angiosperm tree species

Xylem is a complex tissue that forms the bulk of tree bodies and has several functions, including to conduct water, store water and nutrients, and biomechanically support the plant body. We examined how xylem functional traits varied at different positions within 9-year-old Populus balsamifera subsp. trichocarpa. Whole trees were excavated, and xylem samples were collected at 1-m increments along the main root-to-shoot axis of six trees, from root tip to shoot tip. We examined biomechanical and water-storage traits of the xylem, including using a non-invasive imaging technique to examine water content within long, intact branches (high-resolution computed tomography; microCT). Xylem density, strength, and stiffness were greater in shoots than roots. Along the main root-to-shoot axis, xylem strength and stiffness were greatest at shoot tips, and the tissue became linearly weaker and less stiff down the plant and through the root. Roots had greater water storage with lower biomechanical support, and shoots had biomechanically stronger and stiffer xylem with lower water storage. These findings support trade-offs among xylem functions between roots and shoots. Understanding how xylem functions differ throughout tree bodies is important in understanding whole-tree functioning and how terrestrial plants endure numerous environmental challenges over decades of growth.

Tip-to-base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities

In the stems of terrestrial vascular plants studied to date, the diameter of xylem water-conducting conduits D widens predictably with distance from the stem tip L approximating D ∝ L^b , with b ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip-to-base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip-to-base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip-to-base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.

Why call it developmental bias when it is just development?

The concept of developmental constraints has been central to understand the role of development in morphological evolution. Developmental constraints are classically defined as biases imposed by development on the distribution of morphological variation.

This opinion article argues that the concepts of developmental constraints and developmental biases do not accurately represent the role of development in evolution. The concept of developmental constraints was coined to oppose the view that natural selection is all-capable and to highlight the importance of development for understanding evolution. In the modern synthesis, natural selection was seen as the main factor determining the direction of morphological evolution. For that to be the case, morphological variation needs to be isotropic (i.e. equally possible in all directions). The proponents of the developmental constraint concept argued that development makes that some morphological variation is more likely than other (i.e. variation is not isotropic), and that, thus, development constraints evolution by precluding natural selection from being all-capable.

This article adds to the idea that development is not compatible with the isotropic expectation by arguing that, in fact, it could not be otherwise: there is no actual reason to expect that development could lead to isotropic morphological variation. It is then argued that, since the isotropic expectation is untenable, the role of development in evolution should not be understood as a departure from such an expectation. The role of development in evolution should be described in an exclusively positive way, as the process determining which directions of morphological variation are possible, instead of negatively, as a process precluding the existence of morphological variation we have no actual reason to expect.

This article discusses that this change of perspective is not a mere question of semantics: it leads to a different interpretation of the studies on developmental constraints and to a different research program in evolution and development. This program does not ask whether development constrains evolution. Instead it asks questions such as, for example, how different types of development lead to different types of morphological variation and, together with natural selection, determine the directions in which different lineages evolve.

The comparative method is not macroevolution: across-species evidence for within-species process

It is common for studies that employ the comparative method for the study of adaptation, i.e. documentation of potentially adaptive across-species patterns of trait-environment or trait-trait correlation, to be designated as "macroevolutionary." Authors are justified in using "macroevolution" in this way by appeal to definitions such as "evolution above the species level." I argue that regarding the comparative method as "macroevolutionary" is harmful because it hides in serious ways the true causal content of hypotheses tested with the comparative method. The comparative method is a means of testing hypotheses of adaptation and their alternatives. Adaptation is a population level phenomenon, involving heritable interindividual variation that is associated with fitness differences. For example, given heritable intrapopulational variation, more streamlined individuals in populations of fast-moving aquatic animals have higher locomotory efficiency and thus better survivorship and more resources directed to reproduction than less streamlined ones. Direct evidence consistent with this population-level scenario includes the observation that many unrelated species of fast-moving aquatic animals have similar streamlined shapes, an example of the comparative method. Crucial to note in this example is that although the data are observed across species, the comparative method for studying adaptation tests hypotheses regarding standard population-level natural selection with no content that can be construed as "macro." Even less "macro," individual-level developmental dynamics can limit or bias the range of variants available for selection. Calling any of these studies "macroevolutionary" implies that some additional process is at work, shrouding the need to test adaptation hypotheses and study the range of variants that can be produced in development.

Corner's rules pass the test of time: little effect of phenology on leaf-shoot and other scaling relationships

BACKGROUND AND AIMS

Twig cross-sectional area and the surface area of leaves borne on it are expected to be isometrically correlated across species (Corner's rules). However, how stable this relationship remains in time is not known. We studied inter- and intraspecific twig leaf area-cross-sectional area (la-cs) and other scaling relationships, including the leaf-shoot mass (lm-sm) scaling relationship, across a complete growing season. We also examined the influence of plant height, deciduousness and the inclusion of reproductive buds on the stability of the scaling relationships, and we discuss results from a functional perspective.

METHODS

We collected weekly current-year twigs of six Patagonian woody species that differed in growth form and foliar habit. We also used prominent inflorescences from Embothrium coccineum (Proteaceae) to assess whether reproductive buds alter the la-cs isometric relationship. Mixed effects models were fitted to obtain parameter estimates and to test whether interaction terms were non-significant (invariant) for the scaling relationships.

KEY RESULTS

The slope of the la-cs scaling relationship remained invariant across the growing season. Two species showed contrasting and disproportional (allometric) la-cs scaling relationships (slope ≠ 1). Scaling relationships varied significantly across growth form and foliar habit. The lm-sm scaling relationship differed between reproductive- and vegetative-origin twigs in E. coccineum, which was explained by a significantly lower leaf mass per area in the former.

CONCLUSIONS

Although phenology during the growing season appeared not to change leaf-shoot scaling relationships across species, we show that scaling relationships departed from the general trend of isometry as a result of within-species variation, growth form, foliar habit and the type of twig. The identification of these functional factors helps to understand variation in the general trend of Corner's rules.

Interplay of mesoscale physics and agent-like behaviors in the parallel evolution of aggregative multicellularity

Myxobacteria and dictyostelids are prokaryotic and eukaryotic multicellular lineages, respectively, that after nutrient depletion aggregate and develop into structures called fruiting bodies. The developmental processes and resulting morphological outcomes resemble one another to a remarkable extent despite their independent origins, the evolutionary distance between them and the lack of traceable homology in molecular mechanisms. We hypothesize that the morphological parallelism between the two lineages arises as the consequence of the interplay within multicellular aggregates between generic processes, physical and physicochemical processes operating similarly in living and non-living matter at the mesoscale (~10^-3-10^-1 m) and agent-like behaviors, unique to living systems and characteristic of the constituent cells, considered as autonomous entities acting according to internal rules in a shared environment. Here, we analyze the contributions of generic and agent-like determinants in myxobacteria and dictyostelid development and their roles in the generation of their common traits. Consequent to aggregation, collective cell-cell contacts mediate the emergence of liquid-like properties, making nascent multicellular masses subject to novel patterning and morphogenetic processes. In both lineages, this leads to behaviors such as streaming, rippling, and rounding-up, as seen in non-living fluids. Later the aggregates solidify, leading them to exhibit additional generic properties and motifs. Computational models suggest that the morphological phenotypes of the multicellular masses deviate from the predictions of generic physics due to the contribution of agent-like behaviors of cells such as directed migration, quiescence, and oscillatory signal transduction mediated by responses to external cues. These employ signaling mechanisms that reflect the evolutionary histories of the respective organisms. We propose that the similar developmental trajectories of myxobacteria and dictyostelids are more due to shared generic physical processes in coordination with analogous agent-type behaviors than to convergent evolution under parallel selection regimes. Insights from the biology of these aggregative forms may enable a unified understanding of developmental evolution, including that of animals and plants.

Plant Evolutionary Ecology in the Age of the Extended Evolutionary Synthesis

Plant ecology is increasingly turning to evolutionary questions, just as evolutionary biology pushes out of the strictures of the Modern Synthesis into what some regard as an "Extended Evolutionary Synthesis." As plant ecology becomes increasingly evolutionary, it is essential to ask how aspects of the Extended Synthesis might impinge on plant ecological theory and practice. I examine the contribution of plant evolutionary ecology to niche construction theory, as well as the potential for developmental systems theory and genes-as-followers adaptive evolution, all important post-Modern Synthesis themes, in providing novel perspectives for plant evolutionary ecology. I also examine ways that overcoming dichotomies such as "genetic vs. plastic" and "constraint vs. adaptation" provide fertile opportunities for plant evolutionary ecologists. Along the same lines, outgrowing vague concepts such as "stress" and replacing them with more precise terminology in all cases provides vastly increased causal clarity. As a result, the synthetic path that plant ecologists are blazing, becoming more evolutionary every year, bodes extremely well for the field, with vast potential for expansion into important scientific territory.

Bark in Woody Plants: Understanding the Diversity of a Multifunctional Structure

Most biological structures carry out multiple functions. Focusing on only one function to make adaptive inferences overlooks that manifold selection pressures and tradeoffs shape the characteristics of a multifunctional structure. Focusing on single functions can only lead to a partial picture of the causes underlying diversity and the evolutionary origin of the structure in question. I illustrate this discussion using bark as a study case. Bark comprises all the tissues surrounding the xylem in woody plants. Broadly, bark includes an inner and mostly living region and an outer, dead one. Of all plant structures, bark has the most complex anatomical structure and ontogenetic origin involving two (and often three) different meristems. Traditionally, the wide diversity in bark traits, mainly bark thickness, has been interpreted as the result of the selective pressures imposed by fire regime. However, recent research has shown that explanations based on fire regime cannot account for salient patterns of bark variation globally including the very strong inner bark thickness-stem diameter scaling, which is likely due to metabolic needs, and the very high intracommunity variation in total, inner, and outer bark thickness, and in inner:outer proportions. Moreover, explanations based on fire disregard that in addition to fire protection, bark carries out several other crucial functions for plants including translocation of photosynthates; storage of starch, soluble sugars, water, and other compounds; protection from herbivores, pathogens, and high temperatures; wound closure, as well as mechanical support, photosynthesis, and likely being involved in xylem embolism repair. All these functions are crucial for plant performance and are involved in synergistic (e.g., storage of water and insulation) and trade-off relationships (e.g., protection from fire vs photosynthetic activity). Focusing on only one of these functions, protection from fire has provided an incomplete picture of the selective forces shaping bark diversity and has severely hindered our incipient understanding of the functional ecology of this crucial region of woody stems. Applying a multifunctional perspective to the study of bark will allow us to address why we observe such high intracommunity variation in bark traits, why some bark trait combinations are ontogenetically impossible or penalized by selection, how bark is coordinated functionally with other plant parts, and as a result, to understand how bark contributes to the vast diversity of plant ecological strategies across the globe.

Mosaic evolution, preadaptation, and the evolution of evolvability in apes

A major goal in postsynthesis evolutionary biology has been to better understand how complex interactions between traits drive movement along and facilitate the formation of distinct evolutionary pathways. I present analyses of a character matrix sampled across the haplorrhine skeleton that revealed several modules of characters displaying distinct patterns in macroevolutionary disparity. Comparison of these patterns to those in neurological development showed that early ape evolution was characterized by an intense regime of evolutionary and developmental flexibility. Shifting and reduced constraint in apes was met with episodic bursts in phenotypic innovation that built a wide array of functional diversity over a foundation of shared developmental and anatomical structure. Shifts in modularity drove dramatic evolutionary changes across the ape body plan in two distinct ways: (1) an episode of relaxed integration early in hominoid evolution coincided with bursts in evolutionary rate across multiple character suites; (2) the formation of two new trait modules along the branch leading to chimps and humans preceded rapid and dramatic evolutionary shifts in the carpus and pelvis. Changes to the structure of evolutionary mosaicism may correspond to enhanced evolvability that has a "preadaptive" effect by catalyzing later episodes of dramatic morphological remodeling.

Reproductive Ontogeny and the Evolution of Morphological Diversity in Conifers and Other Plants

Biologists often study morphological evolution through form and function relationships. But biological structures can perform multiple functional roles, complicating efforts to understand the evolutionary significance of any one relationship. Plant reproductive organs perform multiple roles in a sequence, however, which provides a unique opportunity to understand how structures evolve to meet multiple functional demands. Using conifers as a study group, we discuss how a shared developmental trajectory links the performance of sequential functional roles. Variation in development among lineages can underlie morphological diversity; pollination-stage seed cones in Pinaceae conifers function similarly but show diverse forms reflecting differences in developmental rate. As cones develop further, the morphologies that they use to perform later functional roles are influenced by the specific developmental patterns used to meet earlier demands, which may ultimately limit morphological diversity. However, we also show how selective pressures relating to the final functional stage (seed dispersal) may influence cone anatomy and morphology over all previous stages, highlighting the complex linkages among form, function, and development. We end by discussing the potential relationships between functional ontogeny and morphological disparity in plant reproductive structures more broadly, suggesting that the complex functional roles associated with seed plant reproduction probably underlie the high disparity in this group.

Convergent xylem widening among organs across diverse woody seedlings

Xylem conduit diameter (D_max ) of woody angiosperm adults scales with plant size and widens from the stem apex downwards. We hypothesized that, notwithstanding relative growth rate (RGR), growth form or leaf habit, woody seedling conduit D_max scales linearly with plant size across species; this scaling should be applicable to all vegetative organs, with consistent conduit widening from leaf via stem to main root and coupling with whole-leaf area and whole-stem xylem area. To test these hypotheses, organ-specific xylem anatomy traits and size-related traits in laboratory-grown seedlings were analyzed across 55 woody European species from cool-temperate and Mediterranean climates. As hypothesized, conduit D_max of each organ showed similar scaling with plant size and consistent basipetal widening from the leaf midvein via the stem to the main root across species, independently of growth form, RGR and leaf habit. We also found a strong correlation between D_max and average leaf area, and between stem xylem area and whole-plant leaf area. We conclude that seedlings of ecologically wide-ranging woody species converge in their allometric scaling of conduit diameters within and across plant organs. These relationships will contribute to modeling of water transport in woody vegetation that accounts for the whole life history from the trees' regeneration phase to adulthood.

How a growing organismal perspective is adding new depth to integrative studies of morphological evolution

Over the past half century, the field of Evolutionary Developmental Biology, or Evo-devo, has integrated diverse fields of biology into a more synthetic understanding of morphological diversity. This has resulted in numerous insights into how development can evolve and reciprocally influence morphological evolution, as well as generated several novel theoretical areas. Although comparative by default, there remains a great gap in our understanding of adaptive morphological diversification and how developmental mechanisms influence the shape and pattern of phenotypic variation. Herein we highlight areas of research that are in the process of filling this void, and areas, if investigated more fully, that will add new insights into the diversification of morphology. At the centre of our discussion is an explicit awareness of organismal biology. Here we discuss an organismal framework that is supported by three distinct pillars. First, there is a need for Evo-devo to adopt a high-resolution phylogenetic approach in the study of morphological variation and its developmental underpinnings. Secondly, we propose that to understand the dynamic nature of morphological evolution, investigators need to give more explicit attention to the processes that generate evolutionarily relevant variation at the population level. Finally, we emphasize the need to address more thoroughly the processes that structure variation at micro- and macroevolutionary scales including modularity, morphological integration, constraint, and plasticity. We illustrate the power of these three pillars using numerous examples from both invertebrates and vertebrates to emphasize that many of these approaches are already present within the field, but have yet to be formally integrated into many research programs. We feel that the most exciting new insights will come where the traditional experimental approaches to Evo-devo are integrated more thoroughly with the principles of this organismal framework.

Spandrels and trait delimitation: No such thing as "architectural constraint"

Forty years ago, Gould and Lewontin used the metaphor of a building's "spandrels" to highlight that organismal traits could be the inevitable consequence of organismal construction, with no alternative configurations possible. Because adaptation by natural selection requires variation, regarding a trait incapable of variation as an adaptation could be a serious error. Gould and Lewontin's exhortation spurred biologists' efforts to investigate biases and limitations in development in their studies of adaptation, a major methodological advance. But in terms of the metaphor itself, over the past 40 years there are virtually no examples of "spandrels" in the primary literature. Moreover, multiple serious confusions in the metaphor have been identified and clarified, for example, that the "spandrels" of San Marco are pendentives, and pendentives are perfect examples of adaptation. I look back over the sparse empirical fruits of the "spandrels" metaphor, and ask what the clarifications of the past 40 years mean for biological theory and practice. I conclude that if there is anything to be rescued from the clarified spandrels metaphor, it is not "constraint" at all. Instead, it is the still-unresolved issue of trait delimitation, which is how to parse organisms into subsets that are tractable and biologically appropriate for study.

Evolution of development of pollen performance

With the origin of pollination in ancient seed plants, the male gametophyte ("pollen") began to evolve a new and unique life history stage, the progamic phase, a post-pollination period in which pollen sexual maturation occurs in interaction with sporophyte-derived tissues. Pollen performance traits mediate the timing of the fertilization process, often in competition with other pollen, via the speed of pollen germination, sperm development, and pollen tube growth. Studies of pollen development rarely address the issue of performance or its evolution, which involves linking variation in developmental rates to relative fitness within populations or to adaptations on a macroevolutionary scale. Modifications to the pollen tube pathway and changes in the intensity of pollen competition affect the direction and strength of selection on pollen performance. Hence, pollen developmental evolution is always contextual-it involves both the population biology of pollen reaching stigmas and the co-evolution of sporophytic traits, such as the pollen tube pathway and mating system. For most species, performance evolution generally reflects a wandering history of periods of directional selection and relaxed selection, channeled by developmental limitations, a pattern that favors the accumulation of diversity and redundancy in developmental mechanisms and the genetic machinery. Developmental biologists are focused on finding universal mechanisms that underlie pollen function, and these are largely mechanisms that have evolved through their effects on performance. Here, we suggest ways in which studies of pollen performance or function could progress by cross-fertilization between the "evo" and "devo" fields.

Carbon limitation, stem growth rate and the biomechanical cause of Corner's rules

Background and aims

Corner's rules describe a global spectrum from large-leaved plants with thick, sparingly branched twigs with low-density stem tissues and thick piths to plants with thin, highly branched stems with high-density stem tissues and thin piths. The hypothesis was tested that, if similar crown areas fix similar amounts of carbon regardless of leaf size, then large-leaved species, with their distantly spaced leaves, require higher stem growth rates, lower stem tissue densities and stiffnesses, and therefore thicker twigs.

Methods

Structural equation models were used to test the compatibility of this hypothesis with a dataset on leaf size, shoot tip spacing, stem growth rate and dimensions, and tissue density and mechanics, sampling 55 species drawn from across the angiosperm phylogeny from a morphologically diverse dry tropical community.

Key results

Very good fit of structural equation models showed that the causal model is highly congruent with the data.

Conclusions

Given similar amounts of carbon to allocate to stem growth, larger-leaved species require greater leaf spacing and therefore greater stem extension rates and longer stems, in turn requiring lower-density, more flexible, stem tissues than small-leaved species. A given stem can have high resistance to bending because it is thick (has high second moment of area I) or because its tissues are stiff (high Young's modulus E), the so-called E-I trade-off. Because of the E-I trade-off, large-leaved species have fast stem growth rates, low stem tissue density and tissue stiffness, and thick twigs with wide piths and thick bark. The agreement between hypothesis and data in structural equation analyses strongly suggests that Corner's rules emerge as the result of selection favouring the avoidance of self-shading in the context of broadly similar rates of carbon fixation per unit crown area across species.

Nearly 200 years of sustained selection have not overcome the leaf area-stem size relationship in the poinsettia

Organismal parts often covary in their proportions, a phenomenon known as allometry. One way of exploring the causes of widespread allometric patterns is with artificial selection, to test whether or not it is possible to move populations into "empty" allometric space not occupied by the wild type. Domesticated organisms have been subject to many generations of selection, making them ideal model systems. We used the domesticated Christmas poinsettia Euphorbia pulcherrima in combination with wild populations to examine the origin of the proportionality between leaf area and stem size, which scales predictably across nearly all plants. In accordance with the stated aims of breeders to produce more compact plants, we predicted that domesticated poinsettias would have greater leaf area for a given stem volume than the tall, lanky wild ancestors. Our data rejected this prediction, showing instead that domesticates have leaf area-stem volume relationships identical to the wild ancestors. Presumably the metabolic dependence between stems and leaves makes the leaf area-stem volume relationship difficult to overcome. The relative fixity of this relationship leads to predictable covariation in other traits: The fuller outlines of domestic poinsettias involve significantly shorter internodes, and given a constant leaf area-stem volume relationship, smaller individual leaf areas. At the same time, domestic poinsettias are subject to selection favoring breakage resistance, which is achieved via thicker stems for a given length rather than stiffer stem tissue resistance to bending. Our results show that domesticated poinsettias differ from wild plants in a suite of traits including leaf size, internode distances, and stem length-diameter relations, but despite over 200 years of selection favoring rounded outlines, there has been no change in the total leaf area-stem volume relationship, helping to predict which changes are likely achievable and which will not be under continued artificial selection and in the wild.

A rich diversity of opercle bone shape among teleost fishes

The opercle is a prominent craniofacial bone supporting the gill cover in all bony fish and has been the subject of morphological, developmental, and genetic investigation. We surveyed the shapes of this bone among 110 families spanning the teleost tree and examined its pattern of occupancy in a principal component-based morphospace. Contrasting with expectations from the literature that suggest the local morphospace would be only sparsely occupied, we find primarily dense, broad filling of the morphological landscape, indicating rich diversity. Phylomorphospace plots suggest that dynamic evolution underlies the observed spatial patterning. Evolutionary transits through the morphospaces are sometimes long, and occur in a variety of directions. The trajectories seem to represent both evolutionary divergences and convergences, the latter supported by convevol analysis. We suggest that that this pattern of occupancy reflects the various adaptations of different groups of fishes, seemingly paralleling their diverse marine and freshwater ecologies and life histories. Opercle shape evolution within the acanthomorphs, spiny ray-finned fishes, appears to have been especially dynamic.

Dinosaurs, Chameleons, Humans, and Evo-Devo Path: Linking Étienne Geoffroy's Teratology, Waddington's Homeorhesis, Alberch's Logic of "Monsters," and Goldschmidt Hopeful "Monsters"

Since the rise of evo-devo (evolutionary developmental biology) in the 1980s, few authors have attempted to combine the increasing knowledge obtained from the study of model organisms and human medicine with data from comparative anatomy and evolutionary biology in order to investigate the links between development, pathology, and macroevolution. Fortunately, this situation is slowly changing, with a renewed interest in evolutionary developmental pathology (evo-devo-path) in the past decades, as evidenced by the idea to publish this special, and very timely, issue on "Developmental Evolution in Biomedical Research." As all of us have recently been involved, independently, in works related in some way or another with evolution and developmental anomalies, we decided to join our different perspectives and backgrounds in the present contribution for this special issue. Specifically, we provide a brief historical account on the study of the links between evolution, development, and pathologies, followed by a review of the recent work done by each of us, and then by a general discussion on the broader developmental and macroevolutionary implications of our studies and works recently done by other authors. Our primary aims are to highlight the strength of studying developmental anomalies within an evolutionary framework to understand morphological diversity and disease by connecting the recent work done by us and others with the research done and broader ideas proposed by authors such as Étienne Geoffroy Saint-Hilaire, Waddington, Goldschmidt, Gould, and Per Alberch, among many others to pave the way for further and much needed work regarding abnormal development and macroevolution.

The significance and scope of evolutionary developmental biology: a vision for the 21st century

Evolutionary developmental biology (evo-devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo-devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines-from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself-and discuss why evo-devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo-devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

Evolution of the vertebrate skeleton: morphology, embryology, and development

Two major skeletal systems-the endoskeleton and exoskeleton-are recognized in vertebrate evolution. Here, we propose that these two systems are distinguished primarily by their relative positions, not by differences in embryonic histogenesis or cell lineage of origin. Comparative embryologic analyses have shown that both types of skeleton have changed their mode of histogenesis during evolution. Although exoskeletons were thought to arise exclusively from the neural crest, recent experiments in teleosts have shown that exoskeletons in the trunk are mesodermal in origin. The enameloid and dentine-coated postcranial exoskeleton seen in many vertebrates does not appear to represent an ancestral condition, as previously hypothesized, but rather a derived condition, in which the enameloid and dentine tissues became accreted to bones. Recent data from placoderm fossils are compatible with this scenario. In contrast, the skull contains neural crest-derived bones in its rostral part. Recent developmental studies suggest that the boundary between neural crest- and mesoderm-derived bones may not be consistent throughout evolution. Rather, the relative positions of bony elements may be conserved, and homologies of bony elements have been retained, with opportunistic changes in the mechanisms and cell lineages of development.

Evolution of Drosophila sex comb length illustrates the inextricable interplay between selection and variation

In spite of the diversity of possible biological forms observed in nature, a limited range of morphospace is frequently occupied for a given trait. Several mechanisms have been proposed to explain this bias in the distribution of phenotypes including selection, drift, and developmental constraints. Despite extensive work on phenotypic bias, the underlying developmental mechanisms explaining why particular regions of morphological space remain unoccupied are poorly understood. To address this issue, we studied the sex comb, a group of modified bristles used in courtship that shows marked morphological diversity among Drosophila species. In many Drosophila species including Drosophila melanogaster, the sex comb rotates 90° to a vertical position during development. Here we analyze the effect of changing D. melanogaster sex comb length on the process of rotation. We find that artificial selection changes the number of bristles per comb without a proportional change in the space available for rotation. As a result, when increasing sex comb length, rather than displaying a similar straight vertical shape observed in other Drosophila species, long sex combs bend because rotation is blocked by a neighboring row of bristles. Our results show ways in which morphologies that would be favored by natural selection are apparently impossible to achieve developmentally. These findings highlight the potential role of development in modifying selectable variation in the evolution of Drosophila sex comb length.

Cone size is related to branching architecture in conifers

The relationship between branch diameter and leaf size has been widely used to understand how vegetative resources are allocated in plants. Branching architecture influences reproductive allocation as well, but fewer studies have explored this relationship at broad phylogenetic or ecological scales. In this study, we tested whether pollen-producing and seed-producing cone size scales with branch diameter in conifers, a diverse and globally distributed lineage of nonflowering seed plants. Branch diameter and cone size were analyzed using multiple regression models and evolutionary models of trait evolution for a data set of 293 extant conifer species within an explicit phylogenetic framework. Branch diameter is a strong predictor of cone size across conifer species, particularly for pollen cones and dry seed cones. However, these relationships are complex in detail because leaf morphology and seed dispersal biology influence the specific ways in which they are expressed. The ubiquity and strength of these scaling relationships across conifers suggest that reproductive and vegetative morphologies are coupled in the group, and it is therefore difficult to disentangle the evolution of cone size from the evolution of branching architecture.

Long-term morphological stasis maintained by a plant-pollinator mutualism

Many major branches in the Tree of Life are marked by stereotyped body plans that have been maintained over long periods of time. One possible explanation for this stasis is that there are genetic or developmental constraints that restrict the origin of novel body plans. An alternative is that basic body plans are potentially quite labile, but are actively maintained by natural selection. We present evidence that the conserved floral morphology of a species-rich flowering plant clade, Malpighiaceae, has been actively maintained for tens of millions of years via stabilizing selection imposed by their specialist New World oil-bee pollinators. Nine clades that have lost their primary oil-bee pollinators show major evolutionary shifts in specific floral traits associated with oil-bee pollination, demonstrating that developmental constraint is not the primary cause of morphological stasis in Malpighiaceae. Interestingly, Malpighiaceae show a burst in species diversification coinciding with the origin of this plant-pollinator mutualism. One hypothesis to account for radiation despite morphological stasis is that although selection on pollinator efficiency explains the origin of this unique and conserved floral morphology, tight pollinator specificity subsequently permitted greatly enhanced diversification in this system.

Bark functional ecology: evidence for tradeoffs, functional coordination, and environment producing bark diversity

The causes underlying bark diversity are unclear. Variation has been frequently attributed to environmental differences across sites. However, variation may also result from tradeoffs and coordination between bark's multiple functions. Bark traits may also covary with wood and leaf traits as part of major dimensions of plant variation. To assess hypotheses regarding tradeoffs and functional coordination, we measured bark traits reflecting protection, storage, mechanics, and photosynthesis in branches of 90 species spanning a wide phylogenetic and environmental range. We also tested associations between bark, wood, and leaf traits. We partitioned trait variation within species, and within and across communities to quantify variation associated with across-site differences. We observed associations between bark mechanics and storage, density and thickness, and thickness and photosynthetic activity. Increasing bark thickness contributed significantly to stiffer stems and greater water storage. Bark density, water content, and mechanics covaried strongly with the equivalent wood traits, and to a lesser degree with leaf size, xylem conductivity, and vessel diameter. Most variation was observed within sites and had low phylogenetic signal. Compared with relatively minor across-site differences, tradeoffs and coordination among functions of bark, leaves, and wood are likely to be major and overlooked factors shaping bark ecology and evolution.

Diversity in time and space: wanted dead and alive

Current patterns of biological diversity are influenced by both historical and present-day factors, yet research in ecology and evolution is largely split between paleontological and neontological studies. Responding to recent calls for integration, we provide a conceptual framework that capitalizes on data and methods from both disciplines to investigate fundamental processes. We highlight the opportunities arising from a combined approach with four examples: (i) which mechanisms generate spatial and temporal variation in diversity; (ii) how traits evolve; (iii) what determines the temporal dynamics of geographical ranges and ecological niches; and (iv) how species-environment and biotic interactions shape community structure. Our framework provides conceptual guidelines for combining paleontological and neontological perspectives to unravel the fundamental processes shaping life on Earth.

More on how and why: a response to commentaries

We are grateful to the commentators for taking the time to respond to our article. Too many interesting and important points have been raised for us to tackle them all in this response, and so in the below we have sought to draw out the major themes. These include problems with both the term 'ultimate causation' and the proximate-ultimate causation dichotomy more generally, clarification of the meaning of reciprocal causation, discussion of issues related to the nature of development and phenotypic plasticity and their roles in evolution, and consideration of the need for an extended evolutionary synthesis.

Identifying constraints in the evolution of primate societies

The evolutionary study of social systems in non-human primates has long been focused on ecological determinants. The predictive value of socio-ecological models remains quite low, however, in particular because such equilibrium models cannot integrate the course of history. The use of phylogenetic methods indicates that many patterns of primate societies have been conserved throughout evolutionary history. For example, the study of social relations in macaques revealed that their social systems are made of sets of correlated behavioural traits. Some macaque species are portrayed by marked social intolerance, a steep dominance gradient and strong nepotism, whereas others display a higher level of social tolerance, relaxed dominance and a weaker influence of kinship. Linkages between behavioural traits occur at different levels of organization, and act as constraints that limit evolutionary responses to external pressures. Whereas these constraints can exert strong stabilizing selection that opposes the potential changes required by the ecological environment, selective mechanisms may have the potential to switch the whole social system from one state to another by acting primarily on some key behavioural traits that could work as pacemakers.

Developmental constraints on behavioural flexibility

We suggest that variation in mammalian behavioural flexibility not accounted for by current socioecological models may be explained in part by developmental constraints. From our own work, we provide examples of constraints affecting variation in behavioural flexibility, not only among individuals, but also among species and higher taxonomic units. We first implicate organizational maternal effects of androgens in shaping individual differences in aggressive behaviour emitted by female spotted hyaenas throughout the lifespan. We then compare carnivores and primates with respect to their locomotor and craniofacial adaptations. We inquire whether antagonistic selection pressures on the skull might impose differential functional constraints on evolvability of skulls and brains in these two orders, thus ultimately affecting behavioural flexibility in each group. We suggest that, even when carnivores and primates would theoretically benefit from the same adaptations with respect to behavioural flexibility, carnivores may nevertheless exhibit less behavioural flexibility than primates because of constraints imposed by past adaptations in the morphology of the limbs and skull. Phylogenetic analysis consistent with this idea suggests greater evolutionary lability in relative brain size within families of primates than carnivores. Thus, consideration of developmental constraints may help elucidate variation in mammalian behavioural flexibility.