Punctuated equilibrium in a neontological context

The theory of punctuated equilibrium, which proposes that biological species evolve rapidly when they originate rather than gradually over time, has sparked intense debate between palaeontologists and evolutionary biologists about the mode of character evolution and the importance of natural selection. Difficulty in interpreting the fossil record prevented consensus, and it remains disputed as to what extent gradual change in established species is responsible for phenotypic differences between species. Against the historical background of the concept of evolution concentrated in speciation events, we review attempts to investigate tempo and mode of evolution using present-day species since the introduction of the theory of punctuated equilibrium in 1972. We discuss advantages, disadvantages, and prospects of using neontological data, methodological advances, and the findings of some recent studies.     

Detection of punctuated equilibrium from molecular phylogenies

Abstract The theory of ‘punctuated equilibrium’ hypothesises that most morphological change in species takes place in rapid bursts triggered by speciation. Eldregde and Gould postulated the theory in 1972, as an alternative to the idea that morphological change slowly accumulates in the course of time, a then common belief they dubbed ‘phyletic gradualism’. Ever since its introduction the theory of punctuated equilibrium has been the subject of speculation rather than empirical validation. Here I present a method to detect punctuated evolution without reference to fossil data, based on the phenotypes of extant species and on their relatedness as revealed by molecular phylogeny. The method involves a general mathematical model describing morphological differentiation of two species over time. The two parameters in the model, the rates of punctual (cladogenetic) and gradual (anagenetic) change, are estimated from plots of morphological diversification against time since divergence of extant species.     

Positive correlation between diversification rates and phenotypic evolvability can mimic punctuated equilibrium on molecular phylogenies

The hypothesis of punctuated equilibrium proposes that most phenotypic evolution occurs in rapid bursts associated with speciation events. Several methods have been developed that can infer punctuated equilibrium from molecular phylogenies in the absence of paleontological data. These methods essentially test whether the variance in phenotypes among extant species is better explained by evolutionary time since common ancestry or by the number of estimated speciation events separating taxa. However, apparent "punctuational" trait change can be recovered on molecular phylogenies if the rate of phenotypic evolution is correlated with the rate of speciation. Strong support for punctuational models can arise even if the underlying mode of trait evolution is strictly gradual, so long as rates of speciation and trait evolution covary across the branches of phylogenetic trees, and provided that lineages vary in their rate of speciation. Species selection for accelerated rates of ecological or phenotypic divergence can potentially lead to the perception that most trait divergence occurs in association with speciation events.     

Species,speciation and human evolution

Human evolution is considered from the perspective of the recognition concept of species, which views species as an epiphenomenon of shared fertilisation systems in sexually reproducing organisms. It is argued that this concept predicts the controversial pattern of punctuated equilibrium, and offers an understanding of the hominid fossil evidence in line with that pattern. Changes in the nature of the fertilization system in the human lineage over time are discussed in relation to the pattern of morphological continuity between proposed species.     

The tempo and mode of three-dimensional morphological evolution in male reproductive structures

Various evolutionary forces may shape the evolution of traits that influence the mating decisions of males and females. Phenotypic traits that males and females use to judge the species identify of potential mates should evolve in a punctuated fashion, changing significantly at the time of speciation but changing little between speciation events. In contrast, traits experiencing sexual selection or sexually antagonistic interactions are generally expected to change continuously over time because of the directional selection pressures imposed on one sex by the actions of the other. To test these hypotheses, we used spherical harmonic representations of the shapes of male mating structures in reconstructions of the evolutionary tempo of these structures across the history of the Enallagma damselfly clade. Our analyses show that the evolution of these structures is completely consistent with a punctuated model of evolutionary change and a constant evolutionary rate throughout the clade's history. In addition, no interpopulation variation in shape was detected across the range of one species. These results indicate that male mating structures in this genus are used primarily for identifying the species of potential mates and experience little or no selection from intraspecific sexual selection or sexual antagonism. The implications of these results for speciation are discussed.     

Statistical limitations on molecular evolution

Complexity of functions evolving in an evolution process are expected to be limited by the time length of an evolution process among other factors. This paper outlines a general method of deriving function-complexity limitations based on mathematical statistics and independent from details of a biological or genetic mechanism of the evolution of the function. Limitations on the emergence of life are derived, these limitations indicate a possibility of a very fast evolution and are consistent with "RNA world" hypothesis. The discussed method is general and can be used to characterize evolution of more specific biological organism functions and relate functions to genetic structures. The derived general limitations indicate that a co-evolution of multiple functions and species could be a slow process, whereas an evolution of a specific function might proceed very fast, so that no trace of intermediate forms (species) is preserved in fossil records of phenotype or DNA structure; this is consistent with a picture of "punctuated equilibrium".     

The correlated evolution of three-dimensional reproductive structures between male and female damselflies

For many taxa, species are defined by the morphologies of reproductive structures. In many odonates, these structures are the cerci of males (used to hold females during mating) and the thoracic plates of females where the male cerci contact the females' bodies. A previous study showed that the shapes of cerci of Enallagma males (Zygoptera: Coenagrionidae) are best explained by an evolutionary model of punctuated change at the time of speciation, with a homogeneous rate of change across the entire phylogeny of the genus. In the present study, we examine the evolution of shape change in the corresponding female plates. We found that, like male cerci, the shapes of Enallagma female thoracic plates could best be explained by an evolutionary model of punctuated change at the time of speciation, with a homogeneous rate of change across the clade. Moreover, the evolutionary contrasts quantifying the rates of change in male cerci and female thoracic plates were positively related across the history of the clade, demonstrating that these male and female structures evolve in a correlated fashion. This pattern of evolution suggests that these structures are primary signals of species identity during mating.     

Extant mammal body masses suggest punctuated equilibrium

Is gradual microevolutionary change within species simultaneously the source of macroevolutionary differentiation between species? Since its first publication, Darwin''s original idea that phenotypic differences between species develop gradually over time, as the accumulation of small selection-induced changes in successive generations has been challenged by palaeontologists claiming that, instead, new species quickly acquire their phenotypes to remain virtually unchanged until going extinct again. This controversy, widely known as the ‘punctuated equilibrium’ debate, remained unresolved, largely owing to the difficulty of distinguishing biological species from fossil remains. We analysed body masses of 2143 existing mammal species on a phylogeny comprising 4510 (i.e. nearly all) extant species to estimate rates of gradual (anagenetic) and speciational (cladogenetic) evolution. Our Bayesian estimates from mammals as well as separate sub-clades such as primates and carnivores suggest that gradual evolution is responsible for only a small part of body size variation between mammal species.     

Punctuated genome size evolution in Liliaceae

Most angiosperms possess small genomes (mode 1C = 0.6 pg, median 1C = 2.9 pg). Those with truly enormous genomes (i.e. > or = 35 pg) are phylogenetically restricted to a few families and include Liliaceae - with species possessing some of the largest genomes so far reported for any plant as well as including species with much smaller genomes. To gain insights into when and where genome size expansion took place during the evolution of Liliaceae and the mode and tempo of this change, data for 78 species were superimposed onto a phylogenetic tree and analysed. Results suggest that genome size in Liliaceae followed a punctuated rather than gradual mode of evolution and that most of the diversification evolved recently rather than early in the evolution of the family. We consider that the large genome sizes of Liliaceae may have emerged passively rather than being driven primarily by selection.     

Statistical Limitations on Molecular Evolution

Abstract

Complexity of functions evolving in an evolution process are expected to be limited by the time length of an evolution process among other factors. This paper outlines a general method of deriving function-complexity limitations based on mathematical statistics and independent from details of a biological or genetic mechanism of the evolution of the function. Limitations on the emergence of life are derived, these limitations indicate a possibility of a very fast evolution and are consistent with “RNA world” hypothesis. The discussed method is general and can be used to characterize evolution of more specific biological organism functions and relate functions to genetic structures. The derived general limitations indicate that a co-evolution of multiple functions and species could be a slow process, whereas an evolution of a specific function might proceed very fast, so that no trace of intermediate forms (species) is preserved in fossil records of phenotype or DNA structure; this is consistent with a picture of “punctuated equilibrium”.     

Patterns of evolution among lower Jurassic crinoids

The crinoid families Isocrinidae and Pentacrinitidae are well represented in the British Lower Jurassic, allowing a detailed investigation of their evolution at this time. The pseudopelagic pentacrinitids diverged from the benthic isocrinids during the Triassic. Seirocrinus arose from Pentacrinites through hyper‐morphosis prior to the start of the Jurassic but both genera subsequently show extreme evolutionary conservatism, perhaps attributable to constraints imposed by their unusual life‐style. From a single species in the Hettangian, the benthic isocrinids diversified in the Sinemurian and Carixian. A “central lineage”;, comprising three species of Chladocrinus, evolved along a peramorphocline by “intermittent anagenesis”;, prolonged periods of evolutionary stasis punctuated by short periods of gradualistic change. Two main offshoots, Balanocrinus and Hispidocrinus gen. nov., represent shifts into new habitats. Balanocrinus arose through progenesis, with subsequent evolution along a paedomorphocline, while Hispidocrinus may have arisen through extreme peramorphosis, although its spinose brachials are an evolutionary innovation. Both genera show an apparently punctuated mode of evolution. There is some correlation between morphology and faciès, and also between the emergence of new species and times of high sea level. Benthic isocrinids appear to have been greatly affected by the early Toarcian anoxic event but the pseudopelagic pentacrinitids were not.     

Do arms races punctuate evolutionary stasis? Unified insights from phylogeny,phylogeography and microevolutionary processes

One of the major controversies in evolutionary biology concerns the processes underlying macroevolutionary patterns in which prolonged stasis is disrupted by rapid, short-term evolution that leads species to new adaptive zones. Recent advances in the understanding of contemporary evolution have suggested that such rapid evolution can occur in the wild as a result of environmental changes. Here, we examined a novel hypothesis that evolutionary stasis is punctuated by co-evolutionary arms races, which continuously alter adaptive peaks and landscapes. Based on the phylogeny of long-mouthed weevils in the genus Curculio , likelihood ratio tests showed that the macroevolutionary pattern of the weevils coincides with the punctuational evolution model. A coalescent analysis of a species, Curculio camelliae , the mouthpart of which has diverged considerably among populations because of an arms race with its host plant, further suggested that major evolutionary shifts had occurred within 7000 generations. Through a microevolutionary analysis of the species, we also found that natural selection acting through co-evolutionary interactions is potentially strong enough to drive rapid evolutionary shifts between adaptive zones. Overall, we posit that co-evolution is an important factor driving the history of organismal evolution.     

Speciation and Bursts of Evolution

A longstanding debate in evolutionary biology concerns whether species diverge gradually through time or by rapid punctuational bursts at the time of speciation. The theory of punctuated equilibrium states that evolutionary change is characterised by short periods of rapid evolution followed by longer periods of stasis in which no change occurs. Despite years of work seeking evidence for punctuational change in the fossil record, the theory remains contentious. Further there is little consensus as to the size of the contribution of punctuational changes to overall evolutionary divergence. Here we review recent developments which show that punctuational evolution is common and widespread in gene sequence data.     

Stabilité de l'espèce et évolution: La théorie de l'équilibre intermittent («Punctuated equilibrium)

Paleontologists' traditional conceptions on evolution do not seem to be in perfect accord with these of biologists. The gradual phyletism is compatible neither with E. Mayr's model of the Founder, nor with the actual stability of most of the paleontological species. The punctuated equilibrium model proposed by N. Eldredge & S.J. Gould offers a solution which integrates both paleontological observations and the data of modern biology.     

Adaptation,punctuation and information: a rate-distortion approach to non-cognitive 'learning plateaus' in evolutionary process

We extend recent information-theoretic phase transition approaches to evolutionary and cognitive process via the Rate Distortion and Joint Asymptotic Equipartition Theorems, in the circumstance of interaction with a highly structured environment. This suggests that learning plateaus in cognitive systems and punctuated equilibria in evolutionary process are formally analogous, even though evolution is not cognitive. Extending arguments by Adami et al. (2000), we argue that 'adaptation' is the process by which a distorted genetic image of a coherently structured environment is imposed upon a species.     

A Phylogenetic Perspective on Sequence Evolution in Microsatellite Loci

We examined the evolution of the repeat regions of three noncoding microsatellite loci in 58 species of the Polistinae, a subfamily of wasps that diverged over 140 million years ago. A phylogenetic approach allows two new kinds of approaches to studying microsatellite evolution: character mapping and comparative analysis. The basic repeat structure of the loci was highly conserved, but was often punctuated with imperfections that appear to be phylogenetically informative. Repeat numbers evolved more rapidly than other changes in the repeat region. Changes in number of repeats among species seem consistent with the stepwise mutation model, which is based on slippage during replication as the main source of mutations. Changes in repeat numbers can occur even when there are very few tandem repeats but longer repeats, especially perfect repeats led to greater rates of evolutionary change. Species phylogenetically closer to the one from which we identified the loci had longer stretches of uninterrupted repeats and more different motifs, but not longer total repeat regions. The number of perfect repeats increased more often than it decreased. However, there was no evidence that some species have consistently greater numbers of repeats across loci than other species have, once ascertainment bias is eliminated. We also found no evidence for a population size effect posited by one form of the directionality hypothesis. Overall, phylogenetic variation in repeat regions can be explained by adding neutral evolution to what is already known about the mutation process. The life cycle of microsatellites appears to reflect a balance between growth by slippage and degradation by an essentially irreversible accumulation of imperfections. Received: 13 April 1999 / Accepted: 8 September 1999     

Pruning the tree of life: k-core percolation as selection mechanism

We propose a model for evolution aiming to reproduce statistical features of fossil data, in particular the distributions of extinction events, the distribution of species per genus and the distribution of lifetimes, all of which are known to be of power-law type. The model incorporates both species-species interactions and ancestral relationships. The main novelty of this work is to show the feasibility of k-core percolation as a selection mechanism. We give theoretical predictions for the observable distributions, confirm their validity by computer simulation and compare with fossil data. A key feature of the proposed model is a co-evolving fitness landscape determined by the topology of the underlying species interactions, ecological niches emerge naturally. The predicted distributions are independent of the rate of speciation, i.e. whether one adopts an gradualist or punctuated view of evolution.     

TRANSLATING BETWEEN MICROEVOLUTIONARY PROCESS AND MACROEVOLUTIONARY PATTERNS: THE CORRELATION STRUCTURE OF INTERSPECIFIC DATA

As species evolve along a phylogenetic tree, we expect closely related species to retain some phenotypic similarities due to their shared evolutionary histories. The amount of expected similarity depends both on the hierarchical phylogenetic structure, and on the specific magnitude and types of evolutionary changes that accumulate during each generation. In this study, we show how models of microevolutionary change can be translated into the resulting macroevolutionary patterns. We illustrate how the structure of phenotypic covariances expected in interspecific measurements can be derived, and how this structure depends on the microevolutionary forces guiding phenotypic change at each generation. We then explore the covariance structure expected from several simple microevolutionary models of phenotypic evolution, including various combinations of random genetic drift, directional selection, stabilizing selection, and environmental change, as well as models of punctuated or burst-like evolution. We find that stabilizing selection leads to patterns of exponential decrease of between species covariance with phylogenetic distance. This is different from the usual linear patterns of decrease assumed in most comparative and systematic methods. Nevertheless, linear patterns of decrease can result from many processes in addition to random genetic drift, such as directional and fluctuating selection as well as modes of punctuated change. Our framework can be used to develop methods for (1) phylogenetic reconstruction; (2) inference of the evolutionary process from comparative data; and (3) conducting or evaluating statistical analyses of comparative data while taking phylogenetic history into account.     

Convergently evolved muscle architecture enables high-performance ballistic movement in salamanders

Elastically powered ballistic movements, such as tongue projection, are common in nature, likely due to benefits such as increased acceleration and distance of movement, and decreased thermal sensitivity imparted by elastic mechanisms. Within Plethodontidae, both muscle-powered and elastically powered ballistic tongue projection occur. Thus, we examine how elastically powered ballistic tongue projection morphology has evolved from muscle powered projection at the level of the projector muscles (m. subarcualis rectus [SAR]). We find that two main SAR morphologies have evolved within Plethodontidae. The first SAR morphology is conducive to elastically powered ballistic projection. This ballistic SAR morphology has evolved multiple, independent times within Plethodontidae, and results from the correlated evolution of several traits including increased collagen aponeuroses, larger SAR muscles, and the loss of inner myofibers attaching directly to the tongue skeleton. While the independent evolution of ballistic SAR morphology has arrived at a similar anatomical design, other tongue structures such as tongue attachment and skeleton folding type varies among species with a ballistic SAR morphology. The second morphology is conducive to muscle-powered projection and is similar to morphology found in an outgroup, Salamandridae. The SAR of these species have inner myofibers that attach to the tongue skeleton, limiting projection distance, coupled with reduced collagen aponeuroses present in relatively small projector muscles. This SAR morphology has likely been retained from ancestors or may be related to feeding ecology. Overall, a ballistic SAR morphology has evolved repeatedly and independently due to the correlated evolution of several SAR traits, including the loss of inner myofibers, which is likely a defining feature of ballistic projection.     

Punctuated stasis and collateral evolution in the Devonian lineage of Monograptus hercynicus

The changes in the sicula of Early Devonian monograptid graptolite populations show a pattern that is described as punctuated stasis: two longer periods of stasis separated by a transitional population of shorter duration between them. The pattern differs from punctuated equilibrium in that anagenesis is involved rather than cladogenesis. The pattern has been observed in several widely separated areas (Nevada, Thuringia, Poland, Ural Mountains) and is an example of collateral evolution in graptolites. In this case, the collateral morphologic change occurs relatively rapidly in a large, globally distributed taxon. The study permits accurate correlation of the horizon of appearance of Monograptus hercynicus Pemer in the upper delta Zone (conodont zonation) of late Lochkovian age (Devonian) on a global scale. The partial integration of these two biostratigraphies constrains correlations between the shelly, shallow-water biofacies and the deeper graptolitic biofacies. The pattern of punctuated stasis has been discovered in Lower Devonian strata of central Nevada, where 22 horizons in sequence were sampled. Statistical analyses were performed on 13 characters encompassing measurements of the sicula, thecae, and rhabdosome. Most of these characters do not exhibit sustained trends or punctuated changes. However, sicular width exhibits an abrupt increase over a short stratigraphic interval and is used to classify members of the group into four quantitatively defined morphotypes. This natural perturbation in the pattern of change is used to divide the lineage into paleospecies. This makes the segments of the lineage easy to identify and imparts stability to the classification and precision in biostratigraphy. The variation studies enable the ranges of the morphotypes to be used to further subdivide the stratigraphic record of the late Lochkovian. □ Graptolites, Devonian, Lochkovian, evolution, punctuated stasis, biostratigraphy, MONOGRAPTUS, phylogeny.