Phylogeny and jaw ontogeny of beloniform fishes

To investigate jaw evolution in beloniform fishes, we reconstructedthe phylogeny of 54 species using fragments of two nuclear (RAG2and Tmo-4C4) and two mitochondrial (cytochrome b and 16S rRNA)genes. Our total molecular evidence topology refutes the monophylyof needlefishes (Belonidae) and halfbeaks (Hemiramphidae), butsupports the monophyly of flyingfishes (Exocoetidae) and sauries(Scomberesocidae). Flyingfishes are nested within halfbeaks,and sauries are nested within needlefishes. Optimization ofjaw characters on the tree reveals a diverse array of evolutionarychanges in ontogeny. During their development, needlefishespass through a "halfbeak" stage that closely resembles the adultcondition in the hemiramphid halfbeaks. The reconstruction ofjaw transitions falsifies the hypothesis that halfbeaks arepaedomorphic derivatives of needlefishes. Instead, halfbeaksmake up a basal paraphyletic grade within beloniforms, and theneedlefish jaw morphology is relatively derived. The parallelbetween needlefish ontogeny and beloniform phylogeny is discussed,and clades amenable to future morphological analysis are proposed.     

Modifications of the falciform process in the eye of beloniformes (Teleostei: Atherinomorpha): evolution of a curtain-like septum in the eye

In order to comparatively analyze curtain-like septa in the eyes of visually orientated "close-to-surface-predators" among atherinomorph teleosts, we examined the eyes of 24 atherinomorph species under a binocular microscope with regard to the falciform process and related structures in the vitreous cavity. Additionally, falciform process samples were analyzed by transmission electron microscopy. All the studied representatives of the Cyprinodontiformes and Atheriniformes, and of one of the beloniform suborder, Adrianichthyioidei, possess a "typical" processus falciformis. In the eyes of the representatives of the other beloniform suborder, Belonoidei, however, pigmented structures that originate in the region of the optic disc and protrude into the vitreous cavity were noted. In the Hemiramphidae (halfbeaks) and Exocoetidae (flying fishes) these pigmented structures have a more cone-like shape, whereas in the Belonidae (needlefishes) and Scomberesocidae (sauries) horizontally oriented heavily pigmented curtain-like septa occur that divide the vitreous cavity dorsoventrally. It is suggested that the "typical" processus falciformis represents a plesiomorphic feature within the Atherinomorpha, whereas the pigmented modifications of the falciform process must be seen as a synapomorphic character state of the Belonoidei. The curtain-like septum of the Belonidae and Scomberesocidae might have evolved from the cone-like structures that are found in the Exocoetoidea. The functional significance of the pigmented structures in the eye is as yet not clear, except for the curtain-like septum found in Belonidae. It might play a role in visual orientation near the water surface at Snell's window.     

Ontogeny discombobulates phylogeny: paedomorphosis and higher-level salamander relationships

Evolutionary developmental biology ("evo-devo") has revolutionized evolutionary biology but has had relatively little impact on systematics. We show that similar large-scale developmental changes in distantly related lineages can dramatically mislead phylogenetic analyses based on morphological data. Salamanders are important model systems in many fields of biology and are of special interest in that many species are paedomorphic and thus never complete metamorphosis. A recent study of higher-level salamander phylogeny placed most paedomorphic families in a single clade based on morphological data. Here, we use new molecular and morphological data to show that this result most likely was caused by the misleading effects of paedomorphosis. We also provide a well-supported estimate of higher-level salamander relationships based on combined molecular and morphological data. Many authors have suggested that paedomorphosis may be problematic in studies of salamander phylogeny, but this hypothesis has never been tested with a rigorous phylogenetic analysis. We find that the misleading effects of paedomorphosis on phylogenetic analysis go beyond the sharing of homoplastic larval traits by paedomorphic adults, and the problem therefore is not solved by simply excluding suspected paedomorphic characters. Instead, two additional factors are critically important in causing paedomorphic species to be phylogenetically "misplaced": (1) the absence of clade-specific synapomorphies that develop during metamorphosis in nonpaedomorphic taxa and allow their "correct" placement and (2) parallel adaptive changes associated with the aquatic habitat of the larval stage. Our results suggest that the effects of paedomorphosis on phylogenetic analyses may be complex, difficult to detect, and can lead to results that are both wrong and statistically well supported by parsimony and Bayesian analyses.     

Ontogeny–a way forward for systematics, a way backward for phylogeny

In the history of biology, the term 'evolution' has carried a dual meaning, viz. ontogeny (the unfolding of the germ) versus phylogeny (descent with modification). A problem in modern biology is the question of whether it is ontogeny which creates phylogeny, or whether it is phylogeny which moulds ontogeny. The paper explores the relationship of ontogeny to phylogeny in the context of 'pattern cladism'. The conclusion is that the analysis of ontogeny provides a direct method for classification ('a way forward for systematics'), which is a logical prerequisite for a phylogenetic interpretation of ontogenetic sequences ('a way backward for phylogeny'). The ontogenetic process of growth, subdivision and differentiation is related to the 'morphogenetic tree theory' on the basis of Von Baer's "laws of individual development". This conceptual relation shows that ontogeny creates phylogeny in an upward direction within the morphogenetic tree, whereas phylogeny (by means of natural selection) moulds ontogeny in a downward direction. A conflict originates with the conventions of Linnaean classification if ontogenetic divergence is proposed as a causal agent in the origin of higher taxa. It is proposed to solve this conflict by viewing individual organisms (or reproductive communities) not as constituents, but as representatives of higher taxa.     

Enzymatic digestion in stomachless fishes: how a simple gut accommodates both herbivory and carnivory

The lack of a stomach is not uncommon amongst teleost fishes, yet our understanding of this reductive specialisation is lacking. The absence of a stomach does not restrict trophic preference, resulting in fishes with very similar alimentary morphology capable of digesting differing diets. We examined the digestive biochemistry of four beloniform fishes: two herbivorous halfbeaks (Hemiramphidae) and two carnivorous needlefish (Belonidae) to determine how these fishes digest their respective diets with their simple, short gut. We found that although the halfbeaks showed significantly greater α-amylase activity than that of the needlefish (P < 0.01), trypsin, lipase, aminopeptidase and maltase activity were not substantially different between the two families. We also found that habitat (freshwater vs. marine) appears to play a significant role in digestive capability, as the two freshwater taxa and the two marine taxa were significantly different (ANOSIM; dietary Gobal R = 0.544, P = 0.001, habitat Global R = 0.437, P = 0.001), despite their phyletic and dietary similarities. Our findings offer partial support for the adaptive modulation hypothesis, support the Plug-Flow Reactor model of digestion in herbivorous halfbeaks and also support the compartmental model of digestion but suggest that another model is required to describe stomachless carnivorous needlefish.     

ONTOGENY AND THE HIERARCHY OF TYPES

Abstract— The long history of belief in a parallelism between ontogeny and a hierarchical order of natural things is reviewed. The meaning of von Baerian recapitulation is analyzed and its implications for cladistic methodology are discussed at two levels: ontogeny and homology. The basic problem inherent in the purported parallelism is that the order of natural things (i.e., the taxic approach to homology) is part of the "world of being" of Platonic ideas, whereas ontogeny and phylogeny (i.e., the transformational approach to homology) belong to Plato's "world of becoming." These two "genera of existence," as Plato put it, being and becoming, are incompatible but complementary views of nature.     

The utility of cranial ontogeny for phylogenetic inference: a case study in crocodylians using geometric morphometrics

The degree to which the ontogeny of organisms could facilitate our understanding of phylogenetic relationships has long been a subject of contention in evolutionary biology. The famed notion that ‘ontogeny recapitulates phylogeny’ has been largely discredited, but there remains an expectation that closely related organisms undergo similar morphological transformations throughout ontogeny. To test this assumption, we used three‐dimensional geometric morphometric methods to characterize the cranial morphology of 10 extant crocodylian species and construct allometric trajectories that model the post‐natal ontogenetic shape changes. Using time‐calibrated molecular and morphological trees, we employed a suite of comparative phylogenetic methods to assess the extent of phylogenetic signal in these trajectories. All analyses largely demonstrated a lack of significant phylogenetic signal, indicating that ontogenetic shape changes contain little phylogenetic information. Notably, some Mantel tests yielded marginally significant results when analysed with the morphological tree, which suggest that the underlying signal in these trajectories is correlated with similarities in the adult cranial morphology. However, despite these instances, all other analyses, including more powerful tests for phylogenetic signal, recovered statistical and visual evidence against the assumption that similarities in ontogenetic shape changes are commensurate with phylogenetic relatedness and thus bring into question the efficacy of using allometric trajectories for phylogenetic inference.     

Ontogeny tends to recapitulate phylogeny in digital organisms

Abstract Biologists have long debated whether ontogeny recapitulates phylogeny and, if so, why. Two plausible explanations are that (i) changes to early developmental stages are selected against because they tend to disrupt later development and (ii) simpler structures often precede more complex ones in both ontogeny and phylogeny if the former serve as building blocks for the latter. It is difficult to test these hypotheses experimentally in natural systems, so we used a computational system that exhibits evolutionary dynamics. We observed that ontogeny does indeed recapitulate phylogeny; traits that arose earlier in a lineage's history also tended to be expressed earlier in the development of individuals. The relative complexity of traits contributed substantially to this correlation, but a significant tendency toward recapitulation remained even after accounting for trait complexity. This additional effect provides evidence that selection against developmental disruption also contributed to the conservation of early stages in development.     

Paedomorphosis as an Evolutionary Driving Force: Insights from Deep-Sea Brittle Stars

Heterochronic development has been proposed to have played an important role in the evolution of echinoderms. In the class Ophiuroidea, paedomorphosis (retention of juvenile characters into adulthood) has been documented in the families Ophiuridae and Ophiolepididae but not been investigated on a broader taxonomic scale. Historical errors, confusing juvenile stages with paedomorphic species, show the difficulties in correctly identifying the effects of heterochrony on development and evolution. This study presents a detailed analysis of 40 species with morphologies showing various degrees of juvenile appearance in late ontogeny. They are compared to a range of early ontogenetic stages from paedomorphic and non-paedomorphic species. Both quantitative and qualitative measurements are taken and analysed. The results suggest that strongly paedomorphic species are usually larger than other species at comparable developmental stage. The findings support recent notions of polyphyletic origin of the families Ophiuridae and Ophiolepididae. The importance of paedomorphosis and its correct recognition for the practice of taxonomy and phylogeny are emphasized.     

The multiple fuzzy origins of woodiness within Balsaminaceae using an integrated approach. Where do we draw the line?

Background and Aims

The family Balsaminaceae is essentially herbaceous, except for some woodier species that can be described as ‘woody’ herbs or small shrubs. The family is nested within the so-called balsaminoid clade of Ericales, including the exclusively woody families Tetrameristaceae and Marcgraviaceae, which is sister to the remaining families of the predominantly woody order. A molecular phylogeny of Balsaminaceae is compared with wood anatomical observations to find out whether the woodier species are derived from herbaceous taxa (i.e. secondary woodiness), or whether woodiness in the family represents the ancestral state for the order (i.e. primary woodiness).

Methods

Wood anatomical observations of 68 Impatiens species and Hydrocera triflora, of which 47 are included in a multigene phylogeny, are carried out using light and scanning electron microscopy and compared with the molecular phylogenetic insights.

Key Results

There is much continuous variation in wood development between the Impatiens species studied, making the distinction between herbaceousness and woodiness difficult. However, the most woody species, unambiguously considered as truly woody shrubs, all display paedomorphic wood features pointing to secondary woodiness. This hypothesis is further supported by the molecular phylogeny, demonstrating that these most woody species are derived from herbaceous (or less woody) species in at least five independent clades. Wood formation in H. triflora is mostly confined to the ribs of the stems and shows paedomorphic wood features as well, suggesting that the common ancestor of Balsaminaceae was probably herbaceous.

Conclusions

The terms ‘herbaceousness’ and ‘woodiness’ are notoriously difficult to use in Balsaminaceae. However, anatomical observations and molecular sequence data show that the woodier species are derived from less woody or clearly herbaceous species, demonstrating that secondary woodiness has evolved in parallel.     

Molecular systematics, biogeography and population structure of neotropical freshwater needlefishes of the genus Potamorrhaphis

Phylogenetic relationships of populations and species within Potamorrhaphis, a genus of freshwater South American needlefishes, were assessed using mitochondrial cytochrome b sequences. Samples were obtained from eight widely distributed localities in the Amazon and Orinoco rivers, and represented all three currently recognized species of Potamorrhaphis. The phylogeny of haplotypes corresponded imperfectly to current morphological species identities: haplotypes from P. guianensis, the most widespread species, did not make up a monophyletic clade. Geography played a strong role in structuring genetic variation: no haplotypes were shared between any localities, indicating restricted gene flow. Possible causes of this pattern include limited dispersal and the effects of current and past geographical barriers. The haplotype phylogeny also showed a complex relationship between fishes from different river basins. Based on the geographical distribution of clades, we hypothesize a connection between the middle Orinoco and Amazon via rivers of the Guianas. More ancient divergence events may have resulted from Miocene alterations of river drainage patterns. We also present limited data for two other Neotropical freshwater needlefish genera: Belonion and Pseudotylosurus. Pseudotylosurus showed evidence of substantial gene flow between distant localities, indicating ecological differences from Potamorrhaphis.     

The evolutionary continuum from lung development to homeostasis and repair

A functional, developmental, and comparative biological approach is probably the most effective way for arranging gene regulatory networks (GRNs) in their biological contexts. Evolutionary developmental biology allows comparison of GRNs during development across phyla. For lung evolution, the parathyroid hormone-related protein (PTHrP) GRN exemplifies a continuum from ontogeny to phylogeny, homeostasis, and repair. PTHrP signaling between the lung endoderm and mesoderm stimulates lipofibroblast differentiation by downregulating the myofibroblast Wnt signaling pathway and upregulating the protein kinase A-dependent cAMP signaling pathway, inducing the lipofibroblast phenotype. Leptin secreted by the lipofibroblast, in turn, binds to its receptor on the alveolar type II cell, stimulating surfactant synthesis to ensure alveolar homeostasis. Failure of the PTHrP/PTHrP receptor signaling mechanism causes transdifferentiation of lipofibroblasts to myofibroblasts, which are the hallmark for lung fibrosis. We have shown that by targeting peroxisome proliferator-activated receptor gamma, the downstream target for lipofibroblast PTHrP signaling, we can prevent lung fibrosis. We speculate that the recapitulation of the myofibroblast phenotype during transdifferentiation is consistent with lung injury as lung evolution in reverse. Repair recapitulates ontogeny because it is programmed to express the cross talk between epithelium and mesoderm through evolution. This model demonstrates how epithelial-mesenchymal cross talk, when seen as a recapitulation of ontogeny and phylogeny (in both a forward and reverse direction), predicts novel, effective diagnostic and therapeutic targets.     

Free from mitochondrial DNA: Nuclear genes and the inference of species trees among closely related darter lineages (Teleostei: Percidae: Etheostomatinae)

Investigations into the phylogenetics of closely related animal species are dominated by the use of mitochondrial DNA (mtDNA) sequence data. However, the near-ubiquitous use of mtDNA to infer phylogeny among closely related animal lineages is tempered by an increasing number of studies that document high rates of transfer of mtDNA genomes among closely related species through hybridization, leading to substantial discordance between phylogenies inferred from mtDNA and nuclear gene sequences. In addition, the recent development of methods that simultaneously infer a species phylogeny and estimate divergence times, while accounting for incongruence among individual gene trees, has ushered in a new era in the investigation of phylogeny among closely related species. In this study we assess if DNA sequence data sampled from a modest number of nuclear genes can resolve relationships of a species-rich clade of North American freshwater teleost fishes, the darters. We articulate and expand on a recently introduced method to infer a time-calibrated multi-species coalescent phylogeny using the computer program ^*BEAST. Our analyses result in well-resolved and strongly supported time-calibrated darter species tree. Contrary to the expectation that mtDNA will provide greater phylogenetic resolution than nuclear gene data; the darter species tree inferred exclusively from nuclear genes exhibits a higher frequency of strongly supported nodes than the mtDNA time-calibrated gene tree.     

Relationships among four genera of mojarras (Teleostei: Perciformes: Gerreidae) from the western Atlantic and their tentative placement among percomorph fishes

A phylogenetic study of the percoid family Gerreidae at both lower and higher taxonomic levels is presented based on DNA sequence data of four genes: mitochondrial 12S and 16S, and nuclear genes rhodopsin and recombination activating gene 1 (RAG1). The taxonomic sampling includes four genera of Gerreidae from the western Atlantic, 39 additional percomorph representatives and two outgroups. Phylogenetic results confirm the monophyly of the Gerreidae and suggest that the family is divided into two sub-groups ( Diapterus auratus plus Eugerres plumieri and Eucinostomus gula plus Gerres cinereus ), which correspond to two previously defined taxonomic assemblages characterized by the shape of the preoperculum. Gerreids are placed at an intermediate position in the percomorph tree between two basal clades (L and Q) and a terminal clade N (grouping tetraodontiforms, acanthuroids, lophiiforms, caproids and several percoids). In addition, topology tests indicate that two traditional assemblages, Labroidei (seven representatives sampled) and Percoidei (22 representatives sampled) are not natural groups. Labrids and scarids appear to be more closely related to gerreids and to the members of clade N than to any other basal percomorphs, including their labroid 'allies' sampled in this study, Embiotocidae, Pomacentridae and Cichlidae, which are all nested within clade Q that also includes atherinomorphs, mugiliforms and Chandidae. The percoid taxa included in this study are widely distributed among various percomorph lineages. The percomorph phylogeny obtained is highly congruent with results from recent molecular studies.     

Trait-dependency of trophic interactions in zooplankton food webs

Anthropogenic change in the abundance or identity of dominant top predators may induce reorganizations in whole food webs. Predicting these reorganizations requires identifying the biological rules that govern trophic niches. However, we still lack a detailed understanding of the respective contributions of body size, behaviour (e.g. match between predator hunting mode and prey antipredator strategy), phylogeny and/or ontogeny in determining both the presence and strength of trophic interactions. Here, we address this question by measuring zooplankton numerical response to fish predators in lake enclosures. We compared the fit to zooplankton count data of models grouping zooplankters based either on 1) body sizes, 2) antipredator behaviour, 3) body size combined with antipredator behaviour or on 4) phylogeny combined with ontogeny (i.e. different life stages of copepods). Body size was a better predictor of zooplankton numerical response to fish than antipredator behaviour, but combining body size and behaviour provided even better predictions. Models based on phylogeny combined with ontogeny clearly outperformed those based on other zooplankton grouping rules, except when phylogeny was poorly resolved. Removing ontogenetic information plagued the predictive power of the highly-resolved (genus-level) phylogenetic grouping but not of medium-resolved or poorly-resolved phylogenetic grouping. Our results support the recent use of phylogeny as a superior surrogate for traits controlling trophic niches, and further highlight the added value of combining phylogeny with ontogenetic traits. Further improvements in our mechanistic understanding of how trophic networks are shaped are bound to uncovering the trophic traits captured by phylogeny and ontogeny, but that currently remain hidden to us.     

HGT-Gen: a tool for generating a phylogenetic tree with horizontal gene transfer

Horizontal gene transfer (HGT) is a common event in prokaryotic evolution. Therefore, it is very important to consider HGT in the study of molecular evolution of prokaryotes. This is true also for conducting computer simulations of their molecular phylogeny because HGT is known to be a serious disturbing factor for estimating their correct phylogeny. To the best of our knowledge, no existing computer program has generated a phylogenetic tree with HGT from an original phylogenetic tree. We developed a program called HGT-Gen that generates a phylogenetic tree with HGT on the basis of an original phylogenetic tree of a protein or gene. HGT-Gen converts an operational taxonomic unit or a clade from one place to another in a given phylogenetic tree. We have also devised an algorithm to compute the average length between any pair of branches in the tree. It defines and computes the relative evolutionary time to normalize evolutionary time for each lineage. The algorithm can generate an HGT between a pair of donor and acceptor lineages at the same evolutionary time. HGT-Gen is used with a sequence-generating program to evaluate the influence of HGT on the molecular phylogeny of prokaryotes in a computer simulation study.

Availability

The database is available for free at http://www.grl.shizuoka.ac.jp/˜thoriike/HGT-Gen.html     

Measures of clade confidence do not correlate with accuracy of phylogenetic trees

Metrics of phylogenetic tree reliability, such as parametric bootstrap percentages or Bayesian posterior probabilities, represent internal measures of the topological reproducibility of a phylogenetic tree, while the recently introduced aLRT (approximate likelihood ratio test) assesses the likelihood that a branch exists on a maximum-likelihood tree. Although those values are often equated with phylogenetic tree accuracy, they do not necessarily estimate how well a reconstructed phylogeny represents cladistic relationships that actually exist in nature. The authors have therefore attempted to quantify how well bootstrap percentages, posterior probabilities, and aLRT measures reflect the probability that a deduced phylogenetic clade is present in a known phylogeny. The authors simulated the evolution of bacterial genes of varying lengths under biologically realistic conditions, and reconstructed those known phylogenies using both maximum likelihood and Bayesian methods. Then, they measured how frequently clades in the reconstructed trees exhibiting particular bootstrap percentages, aLRT values, or posterior probabilities were found in the true trees. The authors have observed that none of these values correlate with the probability that a given clade is present in the known phylogeny. The major conclusion is that none of the measures provide any information about the likelihood that an individual clade actually exists. It is also found that the mean of all clade support values on a tree closely reflects the average proportion of all clades that have been assigned correctly, and is thus a good representation of the overall accuracy of a phylogenetic tree.     

Balfour, Garstang and de Beer: The First Century of Evolutionary Embryology

Evolution has been integrated with embryology during two greatperiods: the latter half of the 19th C as evolutionary morphology/embryology,and the latter third of the 20th C as evolutionary developmentalbiology. My mandate was to use the contributions of three embryologists/morphologists:Francis (Frank) Balfour (1851–1882), Walter Garstang (1868–1949)and Gavin de Beer (1899–1972) to discuss the foundationsof evolutionary embryology in the UK from 1870 (when "everyaspiring zoologist was an embryologist, and the one topic ofprofessional conversation was evolution," Bateson, 1922, p.56), through the 1920s ("ontogeny does not recapitulate phylogeny,it creates it," Garstang, 1922, p. 81) to the 1970s ("homologyof phenotypes does not imply similarity in genotypes," de Beer,1971, p. 15). Evolutionary embryology was driven by a comparativeembryological approach that sought homology of adult structuresin germ layers and ancestry in embryos, and sought to differentiatelarval adaptations from retained ancestral characters. An initialemphasis on a phylogenetic mechanism (recapitulation) slowlygave way to more mechanistic approaches that included heterochronyand the integration of embryology with physiological genetics.Germ layers, homology, larval evolution, larval origins of thevertebrates, paedomorphosis and heterochrony underpinned theorigins of evolutionary embryology, and so I discuss each ofthese topics.     

Data congruence, paedomorphosis and salamanders

Background

The retention of ancestral juvenile characters by adult stages of descendants is called paedomorphosis. However, this process can mislead phylogenetic analyses based on morphological data, even in combination with molecular data, because the assessment if a character is primary absent or secondary lost is difficult. Thus, the detection of incongruence between morphological and molecular data is necessary to investigate the reliability of simultaneous analyses. Different methods have been proposed to detect data congruence or incongruence. Five of them (PABA, PBS, NDI, LILD, DRI) are used herein to assess incongruence between morphological and molecular data in a case study addressing salamander phylogeny, which comprises several supposedly paedomorphic taxa. Therefore, previously published data sets were compiled herein. Furthermore, two strategies ameliorating effects of paedomorphosis on phylogenetic studies were tested herein using a statistical rigor. Additionally, efficiency of the different methods to assess incongruence was analyzed using this empirical data set. Finally, a test statistic is presented for all these methods except DRI.

Results

The addition of morphological data to molecular data results in both different positions of three of the four paedomorphic taxa and strong incongruence, but treating the morphological data using different strategies ameliorating the negative impact of paedomorphosis revokes these changes and minimizes the conflict. Of these strategies the strategy to just exclude paedomorphic character traits seem to be most beneficial. Of the three molecular partitions analyzed herein the RAG1 partition seems to be the most suitable to resolve deep salamander phylogeny. The rRNA and mtDNA partition are either too conserved or too variable, respectively. Of the different methods to detect incongruence, the NDI and PABA approaches are more conservative in the indication of incongruence than LILD and PBS.

Conclusion

Paedomorphosis induces strong conflicts and can mislead the phylogenetic analyses even in combined analyses. However, different strategies are efficiently minimizing these problems. Though the exploration of different methods to detect incongruence is preferable NDI and PABA are more conservative than the others and NDI is computational less extensive than PABA.     

MORPHOMETRICS AND CLADISTICS: MEASURING PHYLOGENY IN THE SEA URCHIN ECHINOCARDIUM

A phylogenetic approach to the study of evolutionary patterns is based on taxic homologies (synapomorphies). In contrast, the recognition of evolutionary processes (namely heterochronies) involves analysis of the entire morphology. Recent developments in geometric morphometry permit analysis of morphological similarities grounded in operational homologies. Such morphometric techniques are explored (1) at the level of evolutionary processes, and (2) as a complement in exploration of phylogenetic relationships. To examplify this, we perform a two-part study of the ontogeny and phylogeny of the spatangoid sea urchin Echinocardium. First, a phylogenetic analysis of ten Recent species in the genus is performed on 18 informative characters of the test. Second, morphological divergences among the species are analyzed using procrustean (superimposition) methods based on 49 homologous points. An additive distance tree is built from a matrix of morphometric distances among adult specimens. This tree is fully congruent with the phyletic results. Ontogenetic processes are explored by inserting ontogenetic series into the analysis. A distance tree including the juvenile stages shows that the general evolutionary trend of the genus is peramorphic, but species-to-species comparisons attest that no general clinal trend exists. Our analysis emphasizes the importance of morphometric approaches in evolutionary studies (1) for the understanding of heterochronies; (2) to trace the morphological implications of phylogenetic patterns; and (3) to estimate the impact of homoplasies.