Did incumbency play a role in maintaining boundaries between Late Ordovician brachiopod realms?

Studies of ecosystem level changes in the geological record have found that the major extinction events eliminated many incumbent clades that had been ecologically dominant for long intervals. Surviving clades that had not been able to compete with the extinct incumbents were then able to evolve adaptations that allowed them to move into the niches vacated by the incumbents. Underlying this pattern is the inability of clades that do not occupy a particular niche to evolve adaptations that would permit them to compete with incumbent clades that are already successfully occupying that niche. The zoogeographic distributions of brachiopods in the Late Ordovician of Laurentia may also have been maintained by incumbency, which was disrupted by the end-Ordovician extinction event. Following the extinction event, an Early Silurian zoogeographic reorganization occurred, during which surviving clades evolved into the vacated epeiric sea niches in the Early Silurian. Just as incumbency plays a role in long-term evolutionary patterns, zoogeographic realms and provinces are also partially maintained by incumbency.     

Molecular evidence for the compilospecies model of reticulate evolution in Armeria (Plumbaginaceae)

Cladistic analyses of the nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) sequences from 55 samples corresponding to 34 taxa in the genus Armeria reveal that ITS sequence diversity among and within species utterly conflicts with patterns of morphological similarity. Three facts are apparent from the results here reported: (1) different samples of a single subspecies, A. villosa subsp. longiaristata, appear in three of the five major clades; (2) samples of at least one of the six subspecies of A. villosa appear in four of the five major clades; and (3) the composition of major clades shows greater congruence with the geographic origin of plants than with the traditional systematic arrangement based primarily on morphology. Specifically, the clades here termed Ia, II, III, and IV each encompass terminals restricted to geographically delimited areas. There are alternative explanations for the ITS pattern, but the most likely one is that nucleotide positions supporting the major clades are due, in some of the samples, to concerted evolution following horizontal transfer (gene flow) rather than to recency of common ancestry. This interpretation is consistent with previous systematic and experimental evidence and implies that reticulation in Armeria may be extensive. Harlan and de Wet (1963, Evolution 17:497-501) proposed the compilospecies concept to account for situations in which a genetically "aggressive" species captures portions of the genome of other sympatric species by means of extensive introgression. Evidence of extensive reticulation, ecological diversification, and geographic pattern indicates that A. villosa may fit the compilospecies concept, which is here supported on molecular grounds for the first time.     

The threefold parallelism of Agassiz and Haeckel,and polarity determination in phylogenetic systematics

A parallel exists between the threefold parallelism of Agassiz and Haeckel and the three valid methods of polarity determination in phylogenetic systematics. The structural gradation among taxa within a linear hierarchy, ontogenetic recapitulation, and geological succession of the threefold parallelism resemble outgroup comparison, the ontogenetic method, and the paleontological method, respectively, which are methods of polarity determination in phylogenetic systematics. The parallel involves expected congruence among similar components of the distribution of character states among organisms. The threefold parallelism is a manifestation of a world view based on linear hierarchies, whereas polarity determination is part of the methodology of phylogenetic systematics which assumes that organisms are grouped into a nested hierarchy. The threefold parallelism facilitated the ranking of previously established taxa into linear hierarchies consisting mostly of paraphyletic groups. In contrast, methods of polarity determination identify apomorphies that determine and diagnose monophyletic taxa (clades) in the nested genealogical hierarchy. Taxa in linear hierarchies are defined by sets of character states, whereas clades are defined by common ancestry. Although the threefold parallelism was ostensibly abandoned with the rejection of Haeckel's biogenetic law, some of its components continue to facilitate the progressive scenarios that are common in evolutionary thought. Although a general view of progression in organismal history may be invalid, the progressive or directional sequence of character state changes that results in the characterization of a particular clade has considerable heuristic value. Agassiz's ostensibly nested hierarchy and other pre-Darwinian classifications do not provide support for the view that the natural system can be discovered without recourse to the principle of common descent.     

Heterochrony in Tingidae (Insecta: Heteroptera): paedomorphosis and/or peramorphosis?

Tingidae (Heteroptera: Insecta) exhibit cephalic tubercles that present a very diverse shape in larvae but that are much simpler in adults. A phylogeny based on adult and last instar characters showed that these tubercles evolved independently from simple to complex states in two clades, and reversed from complex to simple in some taxa. These homoplasies are analysed in the light of ontogenetic sequences and interpreted as heterochronic events. The general trend of evolution of the cephalic tubercles in Tingidae is in mosaic, and could be generally peramorphic, with some isolated cases of paedomorphosis.  Journal compilation © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 71–80. No claim to original French government works.     

Lowering sample size in comparative analyses can indicate a correlation where there is none: example from Rensch's rule in primates

The fact that characters may co-vary in organism groups because of shared ancestry and not always because of functional correlations was the initial rationale for developing phylogenetic comparative methods. Here we point out a case where similarity due to shared ancestry can produce an undesired effect when conducting an independent contrasts analysis. Under special circumstances, using a low sample size will produce results indicating an evolutionary correlation between characters where an analysis of the same pattern utilizing a larger sample size will show that this correlation does not exist. This is the opposite effect of increased sample size to that expected; normally an increased sample size increases the chance of finding a correlation. The situation where the problem occurs is when co-variation between the two continuous characters analysed is clumped in clades; e.g. when some phylogenetically conservative factors affect both characters simultaneously. In such a case, the correlation between the two characters becomes contingent on the number of clades sharing this conservative factor that are included in the analysis, in relation to the number of species contained within these clades. Removing species scattered evenly over the phylogeny will in this case remove the exact variation that diffuses the evolutionary correlation between the two characters - the variation contained within the clades sharing the conservative factor. We exemplify this problem by discussing a parallel in nature where the described problem may be of importance. This concerns the question of the presence or absence of Rensch's rule in primates.     

Pronounced Peramorphosis in Lissamphibians—Aviturus exsecratus (Urodela,Cryptobranchidae) from the Paleocene–Eocene Thermal Maximum of Mongolia

Background

The oldest and largest member of giant salamanders (Cryptobranchidae) Aviturus exsecratus appears in the latest Paleocene (near the Paleocene–Eocene Thermal Maximum) of Mongolia. Based on femoral and vertebral morphology and metrics, a terrestrial adaptation has been supposed for this species.

Methodology/Principal Findings

A detailed morphological reinvestigation of published as well as unpublished material reveals that this salamander shows a vomerine dentition that is posteriorly shifted and arranged in a zigzag pattern, a strongly developed olfactory region within the cranial cavity, and the highest bone ossification and relatively longest femur among all fossil and recent cryptobranchids.

Conclusions/Significance

The presence of these characteristics indicates a peramorphic developmental pattern for Aviturus exsecratus. Our results from Av. exsecratus indicate for the first time pronounced peramorphosis within a crown-group lissamphibian. Av. exsecratus represents a new developmental trajectory within both fossil and recent lissamphibian clades characterized by extended ontogeny and large body size, resembling the pattern known from late Paleozoic eryopines. Moreover, Av. exsecratus is not only a cryptobranchid with distinctive peramorphic characters, but also the first giant salamander with partially terrestrial (amphibious) lifestyle. The morphology of the vomers and dentaries suggests the ability of both underwater and terrestrial feeding.     

Homology and errors

A recent review of the homology concept in cladistics is critiqued in light of the historical literature. Homology as a notion relevant to the recognition of clades remains equivalent to synapomorphy. Some symplesiomorphies are “homologies” inasmuch as they represent synapomorphies of more inclusive taxa; others are complementary character states that do not imply any shared evolutionary history among the taxa that exhibit the state. Undirected character‐state change (as characters optimized on an unrooted tree) is a necessary but not sufficient test of homology, because the addition of a root may alter parsimonious reconstructions. Primary and secondary homology are defended as realistic representations of discovery procedures in comparative biology, recognizable even in Direct Optimization. The epistemological relationship between homology as evidence and common ancestry as explanation is again emphasized. An alternative definition of homology is proposed. © The Willi Hennig Society 2012.     

The phylogeography of palm cockatoos, Probosciger aterrimus, in the dynamic Australo-Papuan region

Aims We aimed to investigate the effects of historical land–sea boundary and vegetation dynamics in the Australo‐Papuan region on the genetic structure of palm cockatoo populations. In doing so, we also sought to clarify the intraspecific taxonomic status of palm cockatoos, and to examine the potential conservation implications of our results. Location New Guinea and northern Australia. Methods We examined mtDNA (domain III, control region) genetic structure in 71 palm cockatoos from 17 locations across their Australo‐Papuan range. Results Twenty polymorphic sites over 242‐base pairs defined 12 haplotypes that were arranged in a 95% confidence parsimony network of six one‐step clades. Half of these were linked in one clade that included birds from eastern New Guinea–Australia, and the other half included birds from western New Guinea. Nested clade analyses revealed strong and significant genetic structure between these two clades. The average nucleotide divergence between eastern and western birds is c. 3.3%. Within the western clade there was a non‐random distribution of haplotypes according to sampling location alone, but the locations did not cluster significantly, probably due to low sample sizes. A non‐random distribution of haplotypes emerged within one of the one‐step clades from the east of the range (once rare haplotypes were removed), although the historic mechanism that may have created this pattern is unclear. The underlying low nucleotide divergence (0.39%) among haplotypes within the eastern clade suggests relatively recent common ancestry. Main conclusions Our results suggest genetic isolation of the eastern and western clades sometime during the Pleistocene. The continual reappearance of land bridges associated with Pleistocene glacio‐eustatic cycles within the eastern part of the range provides an explanation for our results. We suggest that the occurrence of two deep marine troughs maintained a narrow mountainous barrier between eastern and western birds throughout much of the Pleistocene at a time when extensive land bridges formed elsewhere in the species’ range, and that this has maintained their genetic distinctiveness. Our results provide little support for the current accepted subspecies; the western clade is roughly congruent with Probosciger aterrimus goliath (with caveats), but the otherwise unstructured small genetic distances cast considerable doubt on the remaining subspecies. The eastern and western lineages are endemic to each area and should therefore be considered for independent conservation status and management.     

Phylogenetic relationships among families of the Scaphopoda (Mollusca)

Phylogenetic relationships among families in the molluscan class Scaphopoda were analysed using morphological characters and cladistic parsimony methods. A maximum parsimony analysis of 34 discrete characters, treated as unordered and equally weighted, from nine ingroup terminal taxa produced a single most parsimonious tree; supplementary analyses of tree length frequency distribution and Bremer support indices indicate a strong phylogenetic signal from the data and moderate to minimally supported clades. The traditional major division of the class, the orders Dentaliida and Gadilida, is supported as both taxa are confirmed as monophyletic clades. Within the Dentaliida, two clades are recognized, the first comprised of the families Dentaliidae and Fustiariidae, the second of the Rhabdidae and Calliodentaliidae; together, these groups comprise a third clade, which has the Gadilinidae as sister. Within the Gadilida, a nested series of relationships is found among [Entalinidae, [Pulsellidae, [Wemersoniellidae, Gadilidae]]]. These results lend cladistic support to earlier hypotheses of shared common ancestry for some families, but are at variance with other previous hypotheses of evolution in the Scaphopoda. Furthermore, analysis of constituent Gadilinidae representatives provide evidence for paraphyly of this family. The relationships supported here provide a working hypothesis that the development of new characters and greater breadth of taxonomic sampling can test, with a suggested primary goal of establishing monophyly at the family level.     

Evidence for heterochrony in the cranial evolution of fossil crocodyliforms

The southern supercontinent of Gondwana was home to an extraordinary diversity of stem‐crocodylians (Crocodyliformes) during the Late Cretaceous. The remarkable morphological disparity of notosuchian crocodyliforms indicates that this group filled a wide range of ecological roles more frequently occupied by other vertebrates. Among notosuchians, the distinctive cranial morphology and large body sizes of Baurusuchidae suggest a role as apex predators in ecosystems in which the otherwise dominant predatory theropod dinosaurs were scarce. Large‐bodied crocodyliforms, modern and extinct, are known to have reached large sizes by extending their growth period. In a similar way, peramorphic heterochronic processes may have driven the evolution of the similarly large baurusuchids. To assess the presence of peramorphic processes in the cranial evolution of baurusuchids, we applied a geometric morphometric approach to investigate ontogenetic cranial shape variation in a comprehensive sample of notosuchians. Our results provide quantitative morphological evidence that peramorphic processes influenced the cranial evolution of baurusuchids. After applying size and ancestral ontogenetic allometry corrections to our data, we found no support for the action of either hypermorphosis or acceleration, indicating that these two processes alone cannot explain the shape variation observed in Notosuchia. Nevertheless, the strong link between cranial shape variation and size increase in baurusuchids suggests that peramorphic processes were involved in the emergence of hypercarnivory in these animals. Our findings illustrate the role of heterochrony as a macroevolutionary driver, and stress, once more, the usefulness of geometric morphometric techniques for identifying heterochronic processes behind evolutionary trends.     

New crinoids from the Baltic region (Estonia): fossil tip‐dating phylogenetics constrains the origin and Ordovician–Silurian diversification of the Flexibilia (Echinodermata)

This study documents previously unknown taxonomic and morphological diversity among early Palaeozoic crinoids. Based on highly complete, well preserved crown material, we describe two new genera from the Ordovician and Silurian of the Baltic region (Estonia) that provide insight into two major features of the geological history of crinoids: the early evolution of the flexible clade during the Great Ordovician Biodiversification Event (GOBE), and their diversification history surrounding the end‐Ordovician mass extinction. The unexpected occurrence of a highly derived sagenocrinid, Tintinnabulicrinus estoniensis gen. et. sp. nov., from Upper Ordovician (lower Katian) rocks of the Baltic palaeocontinent provides high‐resolution temporal, taxonomic and palaeobiogeographical constraints on the origin and early evolution of the Flexibilia. The Silurian (lower Rhuddanian, Llandovery) Paerticrinus arvosus gen. et sp. nov. is the oldest known Silurian crinoid from Baltica and thus provides the earliest Baltic record of crinoids following the aftermath of the end‐Ordovician mass extinction. A Bayesian ‘fossil tip‐dating’ analysis implementing the fossilized birth–death process and a relaxed morphological clock model suggests that flexibles evolved c. 3 million years prior to their oldest fossil record, potentially involving an ancestor–descendant relationship (via ‘budding’ cladogenesis or anagenesis) with the paraphyletic cladid Cupulocrinus. The sagenocrinid subclade rapidly diverged from ‘taxocrinid’ grade crinoids during the final stages of the GOBE, culminating in maximal diversity among Ordovician crinoid faunas on a global scale. Remarkably, diversification patterns indicate little taxonomic turnover among flexibles across the Late Ordovician mass extinction. However, the elimination of closely related clades may have helped pave the way for their subsequent Silurian diversification and increased ecological role in post‐Ordovician Palaeozoic marine communities. This study highlights the significance of studies reporting faunas from undersampled palaeogeographical regions for clade‐based phylogenetic studies and improving estimates of global biodiversity through geological time.     

A genome evolution-based framework for measures of originality for clades

To evaluate the originality of a species for determining its conservation priority, most indices use the branching pattern and the branch length of a phylogenetic tree to represent the diversification pattern and the number of characters. One limitation of these indices is their lack of consideration of the dynamic process, such as character changes and distribution along lineages during evolution. In this study, we propose a robust framework incorporating the underlying dynamic processes under a framework of genome evolution to model character changes and distribution along different lineages in a given phylogenetic tree. Our framework provides a more transparent modeling, instead of the simple surrogates of branching pattern and branch length previously employed. Nonrandom extinction has been found to be clustered within old and species-poor clades, thus it is desirable to combine the evaluation of originality of clades, which will provide a more complete picture and a useful tool for setting global conservation priorities. Using a phylogenetic tree consisting of 70 species of New World terrestrial Carnivora, we demonstrate that the index derived from our framework can discern the difference in originality of clades. Moreover, we demonstrate that the originality of clades and species in a tree changes with different scenarios of dynamic processes, which were neglected by previous indices. We find that the originality of clades should be one of the criteria for setting global conservation priorities.     

Molecular phylogeography of common garter snakes (Thamnophis sirtalis) in western North America: implications for regional historical forces

Complete ND2 and partial ND4 and cytochrome b mitochondrial DNA (mtDNA) sequences were analysed to evaluate the phylogeographic patterns of common garter snakes (Thamnophis sirtalis) in western North America. This species is widely distributed throughout North America, and exhibits extensive phenotypic variation in the westernmost part of its range. The overall phylogeographic pattern based on mtDNA sequences is concordant with results from studies of other species in this region, implicating historical vicariant processes during the Pleistocene and indicating bottleneck effects of recent dispersal into postglacial habitat. Indeed, the topology is statistically consistent with the hypothesis of both southern (Great Basin and California) and northern (Haida Gwaii) refugia. Specifically, we identified genetic breaks among three major clades: Northwest Coastal populations, Intermountain populations, and all California populations. The California clade contained the only other well-supported branching patterns detected; relationships among populations within the two northern clades were indistinguishable. These molecular splits contrast sharply with all prior geographical analyses of phenotypic variation in T. sirtalis in this region. Our results suggest that the extensive phenotypic variation in western T. sirtalis has been shaped more by local evolutionary forces than by shared common ancestry. Consequently, we consider all morphologically based subspecies designations of T. sirtalis in this region invalid because they do not reflect reciprocal monophyly of the mtDNA sequences.     

An Energetic Representation of Protein Architecture that Is Independent of Primary and Secondary Structure

Protein fold classification often assumes that similarity in primary, secondary, or tertiary structure signifies a common evolutionary origin. However, when similarity is not obvious, it is sometimes difficult to conclude that particular proteins are completely unrelated. Clearly, a set of organizing principles that is independent of traditional classification could be valuable in linking different structural motifs and identifying common ancestry from seemingly disparate folds. Here, a four-dimensional ensemble-based energetic space spanned by a diverse set of proteins was defined and its characteristics were contrasted with those of Cartesian coordinate space. Eigenvector decomposition of this energetic space revealed the dominant physical processes contributing to the more or less stable regions of a protein. Unexpectedly, those processes were identical for proteins with different secondary structure content and were also identical among different amino-acid types. The implications of these results are twofold. First, it indicates that excited conformational states comprising the protein native state ensemble, largely invisible upon inspection of the high-resolution structure, are the major determinant of the energetic space. Second, it suggests that folds dissimilar in sequence or structure could nonetheless be energetically similar if their respective excited conformational states are considered, one example of which was observed in the N-terminal region of the Arc repressor switch mutant. Taken together, these results provide a surface area-based framework for understanding folds in energetic terms, a framework that may eventually yield a means of identifying common ancestry among structurally dissimilar proteins.     

Freshman Undergraduate Biology Students�� Difficulties with the Concept of Common Ancestry

All life on earth descended from a single common ancestor that existed several billion years ago; thus, any pair of organisms will have had a common ancestor at some point in their history. This concept is fundamental to an understanding of evolution and phylogeny. Developing an understanding of this concept is an important goal of evolution education and a part of most high school and college biology curricula. This study examines freshman undergraduate biology majors’ understanding and application of the concept of common ancestry. We used a survey that asked students to provide a brief definition of common ancestry and to rate their confidence that different pairs of organisms shared a common ancestor. Our results show that, although many students in our sample could give a satisfactory definition of common ancestry, the overwhelming majority failed to apply their definitions correctly when assessing the likelihood that the pairs of organisms shared common ancestors. Instead, we found that these students do not treat common ancestry as a binary (yes/no) trait, but instead see it as a continuum from less probable to more probable. These students are more likely to think that closely related organisms have a common ancestor than those that are more distantly related and that humans are less likely to be connected to common ancestors than nonhuman organisms. This pattern is highly consistent from student to student and has important implications for teaching evolution.     

Large‐brained frogs mature later and live longer

Brain sizes vary substantially across vertebrate taxa, yet, the evolution of brain size appears tightly linked to the evolution of life histories. For example, larger brained species generally live longer than smaller brained species. A larger brain requires more time to grow and develop at a cost of exceeded gestation period and delayed weaning age. The cost of slower development may be compensated by better homeostasis control and increased cognitive abilities, both of which should increase survival probabilities and hence life span. To date, this relationship between life span and brain size seems well established in homoeothermic animals, especially in mammals. Whether this pattern occurs also in other clades of vertebrates remains enigmatic. Here, we undertake the first comparative test of the relationship between life span and brain size in an ectothermic vertebrate group, the anuran amphibians. After controlling for the effects of shared ancestry and body size, we find a positive correlation between brain size, age at sexual maturation, and life span across 40 species of frogs. Moreover, we also find that the ventral brain regions, including the olfactory bulbs, are larger in long‐lived species. Our results indicate that the relationship between life history and brain evolution follows a general pattern across vertebrate clades.     

Universal common ancestry,LUCA, and the Tree of Life: three distinct hypotheses about the evolution of life

Common ancestry is a central feature of the theory of evolution, yet it is not clear what “common ancestry” actually means; nor is it clear how it is related to other terms such as “the Tree of Life” and “the last universal common ancestor”. I argue these terms describe three distinct hypotheses ordered in a logical way: that there is a Tree of Life is a claim about the pattern of evolutionary history, that there is a last universal common ancestor is an ontological claim about the existence of an entity of a specific kind, and that there is universal common ancestry is a claim about a causal pattern in the history of life. With these generalizations in mind, I argue that the existence of a Tree of Life entails a last universal common ancestor, which would entail universal common ancestry, but neither of the converse entailments hold. This allows us to make sense of the debates surrounding the Tree, as well as our lack of knowledge about the last universal common ancestor, while still maintaining the uncontroversial truth of universal common ancestry.     

Historical introgression and the role of selective vs. neutral processes in structuring nuclear genetic variation (AFLP) in a circumpolar marine fish, the capelin (Mallotus villosus)

The capelin (Mallotus villosus) is a widespread marine fish species for which previous work has identified geographically distinct mtDNA clades, the frontiers of which are well within adult and larval dispersal capabilities. Here, we use AFLPs to test for the presence of nuclear gene flow among clades. In addition, we evaluate genetic structuring within one clade, the Northwest Atlantic (NWA). We found that each of the mtDNA clades corresponds with a unique nuclear DNA genetic cluster. Within the NWA clade, we detected individuals with small but significant amounts of genetic ancestry from other clades, likely due to historical introgression. Further support for historical introgression comes from analyses of variance in locus-specific differentiation, which support introgression between some clades and divergence without gene flow between others. Within the NWA, we identified two genetic clusters that correspond to sites in geographically adjacent areas. However, these clusters differ primarily at 'outlier' loci, and a genetic subdivision (K=2) was not supported by genetic clustering programs using neutral loci. Significant neutral F(ST) differentiation was found only between sites that otherwise differed at outlier loci. Thus, these populations may be in the initial stages of 'isolation by adaptation'. These results suggest strong between-clade reproductive isolation despite opportunities for gene flow and support the hypothesis that selection can contribute to divergence in otherwise 'open' systems.     

Molecular phylogenetics of squirrelfishes and soldierfishes (Teleostei: Beryciformes: Holocentridae): Reconciling more than 100 years of taxonomic confusion

Squirrelfishes and soldierfishes (Holocentridae) are among the most conspicuous species in the nocturnal reef fish community. However, there is no clear consensus regarding their evolutionary relationships, which is reflected in a complicated taxonomic history. We collected DNA sequence data from multiple single copy nuclear genes and one mitochondrial gene sampled from over fifty percent of the recognized holocentrid species and infer the first species-level phylogeny of the Holocentridae. Our results strongly support the monophyly of the clades Myripristinae (soldierfishes) and Holocentrinae (squirrelfishes). The molecular phylogenies differ with regard to previous hypotheses of relationships within the Myriprisitinae, resolving a clade of cryptic reef associated and deep water non-reef dwelling lineages (Corniger+Plectrypops+Ostichthys) that is the sister lineage to a monophyletic Myripristis. Within Holocentrinae, Neoniphon and Sargocentron are strongly supported as paraphyletic, while Holocentrus is nested within Sargocentron. Using Bayesian ancestral state reconstruction methods, we demonstrate the taxonomically diagnostic characters for Neoniphon and Sargocentron likely represent character states with a complex evolutionary history that is not reflective of shared common ancestry. We propose a new classification for Holocentrinae, recognizing four lineages that are treated as genera: SargocentronFowler, 1904, HolocentrusScopoli, 1777, FlameoJordan and Evermann, 1898, and NeoniphonCastelnau, 1875.     

Keeping it simple: flowering plants tend to retain, and revert to, simple leaves

? A wide range of factors (developmental, physiological, ecological) with unpredictable interactions control variation in leaf form. Here, we examined the distribution of leaf morphologies (simple and complex forms) across angiosperms in a phylogenetic context to detect patterns in the directions of changes in leaf shape. ? Seven datasets (diverse angiosperms and six nested clades, Sapindales, Apiales, Papaveraceae, Fabaceae, Lepidium, Solanum) were analysed using maximum likelihood and parsimony methods to estimate asymmetries in rates of change among character states. ? Simple leaves are most frequent among angiosperm lineages today, were inferred to be ancestral in angiosperms and tended to be retained in evolution (stasis). Complex leaves slowly originated ('gains') and quickly reverted to simple leaves ('losses') multiple times, with a significantly greater rate of losses than gains. Lobed leaves may be a labile intermediate step between different forms. The nested clades showed mixed trends; Solanum, like the angiosperms in general, had higher rates of losses than gains, but the other clades had higher rates of gains than losses. ? The angiosperm-wide pattern could be taken as a null model to test leaf evolution patterns in particular clades, in which patterns of variation suggest clade-specific processes that have yet to be investigated fully.