The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries... more The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries. However, the quantitative distribution of floral morphological diversity (disparity) among lineages and the relative contribution of functional modules (perianth, androecium and gynoecium) to total floral disparity have rarely been addressed. Focusing on a major angiosperm order (Ericales), we compiled a dataset of 37 floral traits scored for 381 extant species and nine fossils. We conducted morphospace analyses to explore phylogenetic, temporal and functional patterns of disparity. We found that the floral morphospace is organized as a continuous cloud in which most clades occupy distinct regions in a mosaic pattern, that disparity increases with clade size rather than age, and that fossils fall in a narrow portion of the space. Surprisingly, our study also revealed that among functional modules, it is the androecium that contributes most to total floral disparity in Ericales. We discuss our findings in the light of clade history, selective regimes as well as developmental and functional constraints acting on the evolution of the flower and thereby demonstrate that quantitative analyses such as the ones used here are a powerful tool to gain novel insights into the evolution and diversity of flowers.
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material, and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian palaeontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between grey and ringed seals. We analysed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human–Neanderthal distance, but still within that of morphologically similar species pairs known to hybridize. By contrast, morphological and developmental analyses show grey and ringed seals to be highly disparate, and that the hybrid is a predictable intermediate. Genetic analyses of the parent populations reveal introgression in the wild, suggesting that grey–ringed seal hybridization is not limited to captivity. Taken together, we postulate that there is considerable potential for mammalian hybridization between phenotypically disparate taxa.
Petals, the inner organs in a differentiated perianth, generally play an important role in pollin... more Petals, the inner organs in a differentiated perianth, generally play an important role in pollinator attraction. As such they exhibit an extraordinary diversity of shapes, sizes, and colors. Being involved in pollinator attraction and reward, they are privileged targets of evolution. The corolla of the Ranunculaceae species Nigella damascena consists of elaborate nectariferous petals, made of a stalk, upper, and lower lips forming a nectar pouch, shiny pseudonectaries, and pilose ears. While the main events of petal development are properly described, a few is known about the pattern of organ size and shape covariation and the cellular dynamics during development. In this study, we investigated the relationships between morphogenesis and growth of N. damascena petals using geometric morphometrics coupled with the study of cell characteristics. First, we found that petal shape and size dynamics are allometric during development and that their covariation suggests that petal shape ch...
A large part of extant and past mammalian morphological diversity is related to variation in size... more A large part of extant and past mammalian morphological diversity is related to variation in size through allometric effects. Previous studies suggested that craniofacial allometry is the dominant pattern underlying mammalian skull shape variation, but cranial allometries were rarely characterized within cranial units such as individual bones. Here, we used 3D geometric morphometric methods to study allometric patterns of the whole skull (global) and of cranial units (local) in a postnatal developmental series of nine-banded armadillos (Dasypus novemcinctus ssp.). Analyses were conducted at the ontogenetic and static levels, and for successive developmental stages. Our results support craniofacial allometry as the global pattern along with more local allometric trends, such as the relative posterior elongation of the infraorbital canal, the tooth row reduction on the maxillary, and the marked development of nuchal crests on the supraoccipital with increasing skull size. Our study al...
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian paleontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between gray and ringed seals. We analyzed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology, and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human-Neanderthal distance, but still within that of morphologically similar species-pairs known to h...
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material, and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian palaeontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between grey and ringed seals. We analysed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human–Neanderthal distance, but still within that of morphologically similar species pairs known to ...
Proceedings of the Royal Society B: Biological Sciences
The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries... more The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries. However, the quantitative distribution of floral morphological diversity (disparity) among lineages and the relative contribution of functional modules (perianth, androecium and gynoecium) to total floral disparity have rarely been addressed. Focusing on a major angiosperm order (Ericales), we compiled a dataset of 37 floral traits scored for 381 extant species and nine fossils. We conducted morphospace analyses to explore phylogenetic, temporal and functional patterns of disparity. We found that the floral morphospace is organized as a continuous cloud in which most clades occupy distinct regions in a mosaic pattern, that disparity increases with clade size rather than age, and that fossils fall in a narrow portion of the space. Surprisingly, our study also revealed that among functional modules, it is the androecium that contributes most to total floral disparity in Ericales. We disc...
The Cambrian explosion was the equivalent of the ignition of an "anatomic bomb", and it... more The Cambrian explosion was the equivalent of the ignition of an "anatomic bomb", and its fallout is the bewildering disparity of metazoan bodyplans. Despite many advances, this "Mother of all Evolutionary Radiations" still remains poorly understood, in large part due to the absence of an objective morphological framework for the representation of the ensemble of phyla in an explicit, all-encompassing morphological space. We address this enduring challenge, by showing that such quintessential representation problem can be solved. A morphospace of phyla is here rendered descriptively possible, representable and analyzable in quantitative terms. Coopting data from Brusca and Brusca, Schram, Nielsen, and others, morphological characters originally coded for phylogenetic purposes have been used in multivariate ordination analyses, allowing the graphical projection of about 30 extinct and extant phyla onto single spaces of lower dimensionality. From various dataset par...
Morphospaces are mathematical representations used for studying the evolution of morphological di... more Morphospaces are mathematical representations used for studying the evolution of morphological diversity and for the evaluation of evolved shapes among theoretically possible ones. Although widely used in zoology, they--with few exceptions--have been disregarded in plant science and in particular in the study of broad-scale patterns of floral structure and evolution. Here we provide basic information on the morphospace approach; we review earlier morphospace applications in plant science; and as a practical example, we construct and analyze a floral morphospace. Morphospaces are usually visualized with the help of ordination methods such as principal component analysis (PCA) or nonmetric multidimensional scaling (NMDS). The results of these analyses are then coupled with disparity indices that describe the spread of taxa in the space. We discuss these methods and apply modern statistical tools to the first and only angiosperm-wide floral morphospace published by Stebbins in 1951. Despite the incompleteness of Stebbins’ original dataset, our analyses highlight major, angiosperm-wide trends in the diversity of flower morphology and thereby demonstrate the power of this previously neglected approach in plant science.
Analyses of morphological disparity have been used to characterize and investigate the evolution ... more Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation. We propose best practice guidelines for disparity analyses, while noting that there can be no ‘one-size-fits-all’ approach. The available tools should always be used in the context of a specific biological question that will determine data and method selection at every stage of the analysis.
Symmetry is a pervasive feature of organismal shape and the focus of a large body of research in ... more Symmetry is a pervasive feature of organismal shape and the focus of a large body of research in Biology. Here, we consider complex patterns of symmetry where a phenotype exhibits a hierarchically structured combination of symmetries. We extend the Procrustes ANOVA for the analysis of nested symmetries and the decomposition of the overall morphological variation into components of symmetry (among-individual variation) and asymmetry (directional and fluctuating asymmetry). We illustrate its use with the Aristotle's lantern, the masticatory apparatus of 'regular' sea urchins, a complex organ displaying bilateral symmetry nested within five-fold rotational symmetry. Our results highlight the importance of characterising the full symmetry of a structure with nested symmetries. Higher order rotational symmetry appears strongly constrained and developmentally stable compared to lower level bilateral symmetry. This contrast between higher and lower levels of asymmetry is discussed in relation to the spatial pattern of the lantern morphogenesis. This extended framework is applicable to any biological object exhibiting nested symmetries, regardless of their type (e.g., bilateral, rotational, translational). Such cases are extremely widespread in animals and plants, from arthropod segmentation to angiosperm inflorescence and corolla shape. The method therefore widens the research scope on developmental instability, canalization, developmental modularity and morphological integration. The anatomical organisation of almost all organisms appears to follow precise patterns of symmetry in which body parts are repeated and geometrically arranged according to specific positions and orientations. As an important feature of morphological phenotypes, symmetry is at the core of numerous research programs in evolutionary and developmental biology, such as the origin of symmetry in the corolla of flowers 1 , the mechanisms of phyllotaxis and branching in plants 2 , and the patterns of segmentation and serial homology 3. Deviation from symmetry, or asymmetry, is also critical to our understanding of the origin of phenotypic variation. The measurement of directional asymmetry, the population average systematic deviation from perfect symmetry, and of fluctuating asymmetry, the small random deviations from perfect symmetry or around the average directional asymmetry, underlies research on developmental instability, canalization, morphological integration, and developmental modularity 4-7. The study of biological forms has greatly benefited from the advent of geometric morphometric methods (GMMs), a powerful array of mathematical and statistical tools for quantifying and analysing shape and shape variation. Geometric morphometric frameworks have also been devised for the study of symmetry and asymmetry in bilaterally symmetric structures, and these frameworks have been recently generalized to all types of symmetry 8-11. Morphometric analyses of symmetric structures allow the partitioning of the total variation
The measurement of morphological variation in macroevolutionary studies is increasingly based on ... more The measurement of morphological variation in macroevolutionary studies is increasingly based on mor-phospaces constructed from discrete character data. This trend mostly results from the appropriation of phylogenetic data matrices as character spaces for carrying out disparity analyses. Phylogenetic matrices provide morphological descriptions of taxa as combinations of character states and thus appear, if not conceptually, at least mathematically, comparable to discrete character datasets found in numerical taxonomy or built for disparity purposes. Hence, phylo-genetic matrices seem to constitute an abundant source of data readily available for morphospace analyses. Discrete character spaces have generally been described as more flexible than morphospaces capturing continuous shape variation. The discrete coding of morphology allows morphospaces to accommodate more disparate morphologies and the ability of discrete character schemes to handle missing data is also often emphasized. This flexibility comes at a cost, however. Multivariate ordinations of such spaces often provide deceptive visualizations and may invite the use of inappropriate methodologies for their exploration. The large amount of missing data that typifies many phylogenetic datasets is also problematic for the measurement of dissimilarity among taxa and can therefore be detrimental to the assessment of morphological disparity. Here, the properties of discrete character spaces are described and common pitfalls discussed. Graphical and methodological approaches are suggested to circumvent or limit their impact, and greater caution is recommended when using discrete character data for mor-phospace and disparity inferences.
Proceedings of the National Academy of Sciences of America, 2013
There are few putative macroevolutionary trends or rules that
withstand scrutiny. Here, we test ... more There are few putative macroevolutionary trends or rules that
withstand scrutiny. Here, we test and verify the purported tendency
for animal clades to reach their maximum morphological variety
relatively early in their evolutionary histories (early high disparity). We present a meta-analysis of 98 metazoan clades radiating throughout the Phanerozoic. The disparity profiles of
groups through time are summarized in terms of their center
of gravity (CG), with values above and below 0.50 indicating topand bottom-heaviness, respectively. Clades that terminate at one of
the“bigfive”mass extinction events tend to have truncated trajectories, with a significantly top-heavy CG distribution overall. The
remaining 63 clades show the opposite tendency, with a significantly bottom-heavy mean CG (relatively early high disparity).
Resampling tests are used to identify groups with a CG significantly above or below 0.50; clades not terminating at a mass extinction are three times more likely to be significantly bottomheavy than top-heavy. Overall, there is no clear temporal trend in
disparity profile shapes from the Cambrian to the Recent, and early
high disparity is the predominant pattern throughout the Phanerozoic. Our results do not allow us to distinguish between ecological
and developmental explanations for this phenomenon. To the extent that ecology has a role, however, the paucity of bottom-heavy
clades radiating in the immediate wake of mass extinctions suggests
that early high disparity more probably results from the evolution
of key apomorphies at the base of clades rather than from physical
drivers or catastrophic ecospace clearing.
Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform,... more Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform, carnivorous ecdysozoans with an armoured, extensible proboscis. These worms were an important component of marine communities during the Palaeozoic, but were especially abundant and diverse in the Cambrian. Today, they comprise just seven genera in four families. Priapulids were among the first groups used to test hypotheses concerning the morphological disparity of Cambrian fossils relative to the extant fauna. A previous study sampled at the generic level, concluding that Cambrian genera embodied marginally less morphological diversity than their extant counterparts. Here, we sample predominantly at the species level and include numerous fossils and some extant forms described in the last fifteen years. Empirical morphospaces for priapulids, archaeopriapulids and palaeoscolecids are relatively insensitive to changes in the taxon or character sample: their overall form has altered little, despite the markedly improved sampling. Cambrian and post-Cambrian genera occupy adjacent rather than broadly overlapping regions of these spaces, and Cambrian species still show lower morphological disparity than their post-Cambrian counterparts. Crucially, the significance of this difference has increased with improved taxon sampling over research time. In contrast with empirical morphospaces, the phylogeny of priapulids, archaeopriapulids and palaeoscolecids derived from morphological characters is extremely sensitive to details of taxon sampling and the manner in which characters are weighted. However, the extant Priapulidae and Halicryptidae invariably resolve as sister families, with this entire clade subsequently being sister group to the Maccabeidae. In our most inclusive trees, the extant Tubiluchidae are separated from these other living taxa by a number of small, intervening fossil clades.
The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries... more The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries. However, the quantitative distribution of floral morphological diversity (disparity) among lineages and the relative contribution of functional modules (perianth, androecium and gynoecium) to total floral disparity have rarely been addressed. Focusing on a major angiosperm order (Ericales), we compiled a dataset of 37 floral traits scored for 381 extant species and nine fossils. We conducted morphospace analyses to explore phylogenetic, temporal and functional patterns of disparity. We found that the floral morphospace is organized as a continuous cloud in which most clades occupy distinct regions in a mosaic pattern, that disparity increases with clade size rather than age, and that fossils fall in a narrow portion of the space. Surprisingly, our study also revealed that among functional modules, it is the androecium that contributes most to total floral disparity in Ericales. We discuss our findings in the light of clade history, selective regimes as well as developmental and functional constraints acting on the evolution of the flower and thereby demonstrate that quantitative analyses such as the ones used here are a powerful tool to gain novel insights into the evolution and diversity of flowers.
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material, and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian palaeontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between grey and ringed seals. We analysed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human–Neanderthal distance, but still within that of morphologically similar species pairs known to hybridize. By contrast, morphological and developmental analyses show grey and ringed seals to be highly disparate, and that the hybrid is a predictable intermediate. Genetic analyses of the parent populations reveal introgression in the wild, suggesting that grey–ringed seal hybridization is not limited to captivity. Taken together, we postulate that there is considerable potential for mammalian hybridization between phenotypically disparate taxa.
Petals, the inner organs in a differentiated perianth, generally play an important role in pollin... more Petals, the inner organs in a differentiated perianth, generally play an important role in pollinator attraction. As such they exhibit an extraordinary diversity of shapes, sizes, and colors. Being involved in pollinator attraction and reward, they are privileged targets of evolution. The corolla of the Ranunculaceae species Nigella damascena consists of elaborate nectariferous petals, made of a stalk, upper, and lower lips forming a nectar pouch, shiny pseudonectaries, and pilose ears. While the main events of petal development are properly described, a few is known about the pattern of organ size and shape covariation and the cellular dynamics during development. In this study, we investigated the relationships between morphogenesis and growth of N. damascena petals using geometric morphometrics coupled with the study of cell characteristics. First, we found that petal shape and size dynamics are allometric during development and that their covariation suggests that petal shape ch...
A large part of extant and past mammalian morphological diversity is related to variation in size... more A large part of extant and past mammalian morphological diversity is related to variation in size through allometric effects. Previous studies suggested that craniofacial allometry is the dominant pattern underlying mammalian skull shape variation, but cranial allometries were rarely characterized within cranial units such as individual bones. Here, we used 3D geometric morphometric methods to study allometric patterns of the whole skull (global) and of cranial units (local) in a postnatal developmental series of nine-banded armadillos (Dasypus novemcinctus ssp.). Analyses were conducted at the ontogenetic and static levels, and for successive developmental stages. Our results support craniofacial allometry as the global pattern along with more local allometric trends, such as the relative posterior elongation of the infraorbital canal, the tooth row reduction on the maxillary, and the marked development of nuchal crests on the supraoccipital with increasing skull size. Our study al...
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian paleontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between gray and ringed seals. We analyzed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology, and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human-Neanderthal distance, but still within that of morphologically similar species-pairs known to h...
An increasing number of mammalian species have been shown to have a history of hybridization and ... more An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material, and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian palaeontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between grey and ringed seals. We analysed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human–Neanderthal distance, but still within that of morphologically similar species pairs known to ...
Proceedings of the Royal Society B: Biological Sciences
The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries... more The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries. However, the quantitative distribution of floral morphological diversity (disparity) among lineages and the relative contribution of functional modules (perianth, androecium and gynoecium) to total floral disparity have rarely been addressed. Focusing on a major angiosperm order (Ericales), we compiled a dataset of 37 floral traits scored for 381 extant species and nine fossils. We conducted morphospace analyses to explore phylogenetic, temporal and functional patterns of disparity. We found that the floral morphospace is organized as a continuous cloud in which most clades occupy distinct regions in a mosaic pattern, that disparity increases with clade size rather than age, and that fossils fall in a narrow portion of the space. Surprisingly, our study also revealed that among functional modules, it is the androecium that contributes most to total floral disparity in Ericales. We disc...
The Cambrian explosion was the equivalent of the ignition of an "anatomic bomb", and it... more The Cambrian explosion was the equivalent of the ignition of an "anatomic bomb", and its fallout is the bewildering disparity of metazoan bodyplans. Despite many advances, this "Mother of all Evolutionary Radiations" still remains poorly understood, in large part due to the absence of an objective morphological framework for the representation of the ensemble of phyla in an explicit, all-encompassing morphological space. We address this enduring challenge, by showing that such quintessential representation problem can be solved. A morphospace of phyla is here rendered descriptively possible, representable and analyzable in quantitative terms. Coopting data from Brusca and Brusca, Schram, Nielsen, and others, morphological characters originally coded for phylogenetic purposes have been used in multivariate ordination analyses, allowing the graphical projection of about 30 extinct and extant phyla onto single spaces of lower dimensionality. From various dataset par...
Morphospaces are mathematical representations used for studying the evolution of morphological di... more Morphospaces are mathematical representations used for studying the evolution of morphological diversity and for the evaluation of evolved shapes among theoretically possible ones. Although widely used in zoology, they--with few exceptions--have been disregarded in plant science and in particular in the study of broad-scale patterns of floral structure and evolution. Here we provide basic information on the morphospace approach; we review earlier morphospace applications in plant science; and as a practical example, we construct and analyze a floral morphospace. Morphospaces are usually visualized with the help of ordination methods such as principal component analysis (PCA) or nonmetric multidimensional scaling (NMDS). The results of these analyses are then coupled with disparity indices that describe the spread of taxa in the space. We discuss these methods and apply modern statistical tools to the first and only angiosperm-wide floral morphospace published by Stebbins in 1951. Despite the incompleteness of Stebbins’ original dataset, our analyses highlight major, angiosperm-wide trends in the diversity of flower morphology and thereby demonstrate the power of this previously neglected approach in plant science.
Analyses of morphological disparity have been used to characterize and investigate the evolution ... more Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation. We propose best practice guidelines for disparity analyses, while noting that there can be no ‘one-size-fits-all’ approach. The available tools should always be used in the context of a specific biological question that will determine data and method selection at every stage of the analysis.
Symmetry is a pervasive feature of organismal shape and the focus of a large body of research in ... more Symmetry is a pervasive feature of organismal shape and the focus of a large body of research in Biology. Here, we consider complex patterns of symmetry where a phenotype exhibits a hierarchically structured combination of symmetries. We extend the Procrustes ANOVA for the analysis of nested symmetries and the decomposition of the overall morphological variation into components of symmetry (among-individual variation) and asymmetry (directional and fluctuating asymmetry). We illustrate its use with the Aristotle's lantern, the masticatory apparatus of 'regular' sea urchins, a complex organ displaying bilateral symmetry nested within five-fold rotational symmetry. Our results highlight the importance of characterising the full symmetry of a structure with nested symmetries. Higher order rotational symmetry appears strongly constrained and developmentally stable compared to lower level bilateral symmetry. This contrast between higher and lower levels of asymmetry is discussed in relation to the spatial pattern of the lantern morphogenesis. This extended framework is applicable to any biological object exhibiting nested symmetries, regardless of their type (e.g., bilateral, rotational, translational). Such cases are extremely widespread in animals and plants, from arthropod segmentation to angiosperm inflorescence and corolla shape. The method therefore widens the research scope on developmental instability, canalization, developmental modularity and morphological integration. The anatomical organisation of almost all organisms appears to follow precise patterns of symmetry in which body parts are repeated and geometrically arranged according to specific positions and orientations. As an important feature of morphological phenotypes, symmetry is at the core of numerous research programs in evolutionary and developmental biology, such as the origin of symmetry in the corolla of flowers 1 , the mechanisms of phyllotaxis and branching in plants 2 , and the patterns of segmentation and serial homology 3. Deviation from symmetry, or asymmetry, is also critical to our understanding of the origin of phenotypic variation. The measurement of directional asymmetry, the population average systematic deviation from perfect symmetry, and of fluctuating asymmetry, the small random deviations from perfect symmetry or around the average directional asymmetry, underlies research on developmental instability, canalization, morphological integration, and developmental modularity 4-7. The study of biological forms has greatly benefited from the advent of geometric morphometric methods (GMMs), a powerful array of mathematical and statistical tools for quantifying and analysing shape and shape variation. Geometric morphometric frameworks have also been devised for the study of symmetry and asymmetry in bilaterally symmetric structures, and these frameworks have been recently generalized to all types of symmetry 8-11. Morphometric analyses of symmetric structures allow the partitioning of the total variation
The measurement of morphological variation in macroevolutionary studies is increasingly based on ... more The measurement of morphological variation in macroevolutionary studies is increasingly based on mor-phospaces constructed from discrete character data. This trend mostly results from the appropriation of phylogenetic data matrices as character spaces for carrying out disparity analyses. Phylogenetic matrices provide morphological descriptions of taxa as combinations of character states and thus appear, if not conceptually, at least mathematically, comparable to discrete character datasets found in numerical taxonomy or built for disparity purposes. Hence, phylo-genetic matrices seem to constitute an abundant source of data readily available for morphospace analyses. Discrete character spaces have generally been described as more flexible than morphospaces capturing continuous shape variation. The discrete coding of morphology allows morphospaces to accommodate more disparate morphologies and the ability of discrete character schemes to handle missing data is also often emphasized. This flexibility comes at a cost, however. Multivariate ordinations of such spaces often provide deceptive visualizations and may invite the use of inappropriate methodologies for their exploration. The large amount of missing data that typifies many phylogenetic datasets is also problematic for the measurement of dissimilarity among taxa and can therefore be detrimental to the assessment of morphological disparity. Here, the properties of discrete character spaces are described and common pitfalls discussed. Graphical and methodological approaches are suggested to circumvent or limit their impact, and greater caution is recommended when using discrete character data for mor-phospace and disparity inferences.
Proceedings of the National Academy of Sciences of America, 2013
There are few putative macroevolutionary trends or rules that
withstand scrutiny. Here, we test ... more There are few putative macroevolutionary trends or rules that
withstand scrutiny. Here, we test and verify the purported tendency
for animal clades to reach their maximum morphological variety
relatively early in their evolutionary histories (early high disparity). We present a meta-analysis of 98 metazoan clades radiating throughout the Phanerozoic. The disparity profiles of
groups through time are summarized in terms of their center
of gravity (CG), with values above and below 0.50 indicating topand bottom-heaviness, respectively. Clades that terminate at one of
the“bigfive”mass extinction events tend to have truncated trajectories, with a significantly top-heavy CG distribution overall. The
remaining 63 clades show the opposite tendency, with a significantly bottom-heavy mean CG (relatively early high disparity).
Resampling tests are used to identify groups with a CG significantly above or below 0.50; clades not terminating at a mass extinction are three times more likely to be significantly bottomheavy than top-heavy. Overall, there is no clear temporal trend in
disparity profile shapes from the Cambrian to the Recent, and early
high disparity is the predominant pattern throughout the Phanerozoic. Our results do not allow us to distinguish between ecological
and developmental explanations for this phenomenon. To the extent that ecology has a role, however, the paucity of bottom-heavy
clades radiating in the immediate wake of mass extinctions suggests
that early high disparity more probably results from the evolution
of key apomorphies at the base of clades rather than from physical
drivers or catastrophic ecospace clearing.
Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform,... more Priapulids and their extinct relatives, the archaeopriapulids and palaeoscolecids, are vermiform, carnivorous ecdysozoans with an armoured, extensible proboscis. These worms were an important component of marine communities during the Palaeozoic, but were especially abundant and diverse in the Cambrian. Today, they comprise just seven genera in four families. Priapulids were among the first groups used to test hypotheses concerning the morphological disparity of Cambrian fossils relative to the extant fauna. A previous study sampled at the generic level, concluding that Cambrian genera embodied marginally less morphological diversity than their extant counterparts. Here, we sample predominantly at the species level and include numerous fossils and some extant forms described in the last fifteen years. Empirical morphospaces for priapulids, archaeopriapulids and palaeoscolecids are relatively insensitive to changes in the taxon or character sample: their overall form has altered little, despite the markedly improved sampling. Cambrian and post-Cambrian genera occupy adjacent rather than broadly overlapping regions of these spaces, and Cambrian species still show lower morphological disparity than their post-Cambrian counterparts. Crucially, the significance of this difference has increased with improved taxon sampling over research time. In contrast with empirical morphospaces, the phylogeny of priapulids, archaeopriapulids and palaeoscolecids derived from morphological characters is extremely sensitive to details of taxon sampling and the manner in which characters are weighted. However, the extant Priapulidae and Halicryptidae invariably resolve as sister families, with this entire clade subsequently being sister group to the Maccabeidae. In our most inclusive trees, the extant Tubiluchidae are separated from these other living taxa by a number of small, intervening fossil clades.
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Papers by Sylvain Gerber
withstand scrutiny. Here, we test and verify the purported tendency
for animal clades to reach their maximum morphological variety
relatively early in their evolutionary histories (early high disparity). We present a meta-analysis of 98 metazoan clades radiating throughout the Phanerozoic. The disparity profiles of
groups through time are summarized in terms of their center
of gravity (CG), with values above and below 0.50 indicating topand bottom-heaviness, respectively. Clades that terminate at one of
the“bigfive”mass extinction events tend to have truncated trajectories, with a significantly top-heavy CG distribution overall. The
remaining 63 clades show the opposite tendency, with a significantly bottom-heavy mean CG (relatively early high disparity).
Resampling tests are used to identify groups with a CG significantly above or below 0.50; clades not terminating at a mass extinction are three times more likely to be significantly bottomheavy than top-heavy. Overall, there is no clear temporal trend in
disparity profile shapes from the Cambrian to the Recent, and early
high disparity is the predominant pattern throughout the Phanerozoic. Our results do not allow us to distinguish between ecological
and developmental explanations for this phenomenon. To the extent that ecology has a role, however, the paucity of bottom-heavy
clades radiating in the immediate wake of mass extinctions suggests
that early high disparity more probably results from the evolution
of key apomorphies at the base of clades rather than from physical
drivers or catastrophic ecospace clearing.
withstand scrutiny. Here, we test and verify the purported tendency
for animal clades to reach their maximum morphological variety
relatively early in their evolutionary histories (early high disparity). We present a meta-analysis of 98 metazoan clades radiating throughout the Phanerozoic. The disparity profiles of
groups through time are summarized in terms of their center
of gravity (CG), with values above and below 0.50 indicating topand bottom-heaviness, respectively. Clades that terminate at one of
the“bigfive”mass extinction events tend to have truncated trajectories, with a significantly top-heavy CG distribution overall. The
remaining 63 clades show the opposite tendency, with a significantly bottom-heavy mean CG (relatively early high disparity).
Resampling tests are used to identify groups with a CG significantly above or below 0.50; clades not terminating at a mass extinction are three times more likely to be significantly bottomheavy than top-heavy. Overall, there is no clear temporal trend in
disparity profile shapes from the Cambrian to the Recent, and early
high disparity is the predominant pattern throughout the Phanerozoic. Our results do not allow us to distinguish between ecological
and developmental explanations for this phenomenon. To the extent that ecology has a role, however, the paucity of bottom-heavy
clades radiating in the immediate wake of mass extinctions suggests
that early high disparity more probably results from the evolution
of key apomorphies at the base of clades rather than from physical
drivers or catastrophic ecospace clearing.