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Species selection in the molecular age
Abstract
Since the 1980s, a renewed understanding of molecular development has afforded an unprecedented level of knowledge of the mechanisms by which phenotype in animals and plants has evolved. In this volume, top scientists in these fields provide perspectives on how molecular data in biology help to elucidate key questions in estimating paleontological divergence and in understanding the mechanisms behind phenotypic evolution. Paleobiological questions such as genome size, digit homologies, genetic control cascades behind phenotype, estimates of vertebrate divergence dates, and rates of morphological evolution are addressed, with a special emphasis on how molecular biology can inform paleontology, directly and indirectly, to better understand life's past. Highlighting a significant shift towards interdisciplinary collaboration, this is a valuable resource for students and researchers interested in the integration of organismal and molecular biology.
... Simultaneously, traits associated with species that have relatively lower speciation rates will decrease in frequency. The pattern of covariation between rate variation 170 and trait variation determines how the relative frequency of traits will change over time (Arnold and Fristrup 1982, Simpson 2010b, Simpson and Mller 2012, just as the covariance 172 between fitness and trait variation among organisms determines how trait values will change by natural selection (Lande and Arnold 1983, Price 1970, Price 1972, Rice 2004). ...
The mere existence of speciation and extinction make macro-evolutionary processes possible. Speciation and extinction introduce discontinuities in the microevolutionary change within lineages by initiating, disrupting, and terminating the continuity of species lineages. Within a clade, speciation and extinction become potent means of macroevolution in and of themselves. This process, termed species selection, is a macroevolutionary analogue of natural selection, with species playing an analogous part akin to that played by organisms in microevolution. That said, it has proven difficult to think about levels of selection. The concept of species sorting was introduced to help our thinking on this issue by identifying two aspects inherent in hierarchical systems can confuse our attempts to understand them: uncertainty in the level that selection acts and uncertainty about if the pattern of selection is in fact caused at all. Thanks to insights from evolutionary transitions in individuality, we now know more about how to identify the level of selection and how to parse the causal structure in hierarchical evolutionary circumstances. We know that if the fitness of organisms causes the fitness of more inclusive species then they must covary. However, there is no evidence of such a covariance between fitnesses at these two levels. This covariance is just not observed; neither between cells and organisms nor between organisms and species. Rather, speciation and extinction rates appear to be completely divorced from organismal fitness. With this insight, the concept of species sorting shrinks so that it only covers the two processes of species selection and drift. I argue that we are better off focusing on understanding the processes of species selection and drift and that there is therefore no further need for the concept of species sorting.
The fitness of groups is often considered to be the average fitness among constituent members. This assumption has been useful for developing models of multilevel selection, but its uncritical adoption has held back our understanding of how multilevel selection actually works in nature. If group fitness is only equal to mean member fitness, then it is a simple task to erode the importance of group-level selection in all multilevel scenarios—species selection could then be reduced to organismal selection as easily as group selection can. Because selection from different levels can act on a single trait, body size, for example, there must be a way to translate one level of fitness to another. This allows the calculation of the contributions of selection at each level. If high-level fitness is not a simple function of member fitness, then how do they interlace? Here we reintroduce Leigh Van Valen’s argument for the inclusion of expansion as a component of fitness. We show that expansion is an integral part of fitness even if one does not subscribe to the energetic view of fitness from which Van Valen originally derived it. From a hierarchical perspective, expansion is the projection of demographic fitness from one level to the next level up; differential births and deaths at one level produce differential expansion one level above. Including expansion in our conceptual tool kit helps allay concerns about our ability to identify the level of selection using a number of methods as well as allowing for the various forms of multilevel selection to be seen as manifestations of the same basic process.
Evolvability is best addressed from a multi-level, macroevolutionary perspective through a comparative approach that tests for among-clade differences in phenotypic diversification in response to an opportunity, such as encountered after a mass extinction, entering a new adaptive zone, or entering a new geographic area. Analyzing the dynamics of clades under similar environmental conditions can (partially) factor out shared external drivers to recognize intrinsic differences in evolvability, aiming for a macroevolutionary analog of a common-garden experiment. Analyses will be most powerful when integrating neontological and paleontological data: determining differences among extant populations that can be hypothesized to generate large-scale, long-term contrasts in evolvability among clades; or observing large-scale differences among clade histories that can by hypothesized to reflect contrasts in genetics and development observed directly in extant populations. However, many comparative analyses can be informative on their own, as explored in this overview. Differences in clade-level evolvability can be visualized in diversity-disparity plots, which can quantify positive and negative departures of phenotypic productivity from stochastic expectations scaled to taxonomic diversification. Factors that evidently can promote evolvability include modularity—when selection aligns with modular structure or with morphological integration patterns; pronounced ontogenetic changes in morphology, as in allometry or multiphase life cycles; genome size; and a variety of evolutionary novelties, which can also be evaluated using macroevolutionary lags between the acquisition of a trait and phenotypic diversification, and dead-clade-walking patterns that may signal a loss of evolvability when extrinsic factors can be excluded. High speciation rates may indirectly foster phenotypic evolvability, and vice versa. Mechanisms are controversial, but clade evolvability may be higher in the Cambrian, and possibly early in the history of clades at other times; in the tropics; and, for marine organisms, in shallow-water disturbed habitats.
A fuller understanding of the role of developmental bias in shaping large‐scale evolutionary patterns requires integrating bias (the probability distribution of variation accessible to an ancestral phenotype) with clade dynamics (the differential survival and production of species and evolutionary lineages). This synthesis could proceed as a two‐way exchange between the developmental data available to neontologists and the strictly phenotypic but richly historical and dynamic data available to paleontologists. Analyses starting in extant populations could aim to predict macroevolution in the fossil record from observed developmental bias, while analyses starting in the fossil record, particularly the record of extant species and lineages, could aim to predict developmental bias from macroevolutionary patterns, including the broad range of extinct phenotypes. Analyses in multivariate morphospaces are especially effective when coupled with phylogeny, theoretical and developmental models, and diversity–disparity plots. This research program will also require assessing the “heritability” of an ancestral bias across phylogeny, and the tendency for bias change in strength and orientation over evolutionary time. Such analyses will help find a set of general rules for the macroevolutionary effects of developmental bias, including its impact on and interactions with the other intrinsic and extrinsic factors governing the movement, expansion, and contraction of clades in morphospace.
HIGHLIGHTS
• A fuller, more synthetic understanding of the macroevolutionary role of developmental bias requires the integration of bias with clade dynamics—that is, the differential survival and production of species and evolutionary lineages
Approaches to macroevolution require integration of its two fundamental components, within a hierarchical framework. Following a companion paper on the origin of variation, I here discuss sorting within an evolutionary hierarchy. Species sorting—sometimes termed species selection in the broad sense, meaning differential origination and extinction owing to intrinsic biological properties—can be split into strict-sense species selection, in which rate differentials are governed by emergent, species-level traits such as geographic range size, and effect macroevolution, in which rates are governed by organism-level traits such as body size; both processes can create hitchhiking effects, indirectly causing the proliferation or decline of other traits. Several methods can operationalize the concept of emergence, so that rigorous separation of these processes is increasingly feasible. A macroevolutionary tradeoff, underlain by the intrinsic traits that influence evolutionary dynamics, causes speciation and extinction rates to covary in many clades, resulting in evolutionary volatility of some clades and more subdued behavior of others; the few clades that break the tradeoff can achieve especially prolific diversification. In addition to intrinsic biological traits at multiple levels, extrinsic events can drive the waxing and waning of clades, and the interaction of traits and events are difficult but important to disentangle. Evolutionary trends can arise in many ways, and at any hierarchical level; descriptive models can be fitted to clade trajectories in phenotypic or functional spaces, but they may not be diagnostic regarding processes, and close attention must be paid to both leading and trailing edges of apparent trends. Biotic interactions can have negative or positive effects on taxonomic diversity within a clade, but cannot be readily extrapolated from the nature of such interactions at the organismic level. The relationships among macroevolutionary currencies through time (taxonomic richness, morphologic disparity, functional variety) are crucial for understanding the nature of evolutionary diversification. A novel approach to diversity-disparity analysis shows that taxonomic diversifications can lag behind, occur in concert with, or precede, increases in disparity. Some overarching issues relating to both the origin and sorting of clades and phenotypes include the macroevolutionary role of mass extinctions, the potential differences between plant and animal macroevolution, whether macroevolutionary processes have changed through geologic time, and the growing human impact on present-day macroevolution. Many challenges remain, but progress is being made on two of the key ones: (a) the integration of variation-generating mechanisms and the multilevel sorting processes that act on that variation, and (b) the integration of paleontological and neontological approaches to historical biology.
Differences in the relative diversification rates of species with variant traits are known as species selection. Species selection can produce a macroevolutionary change in the frequencies of traits by changing the relative number of species possessing each trait over time. But species selection is not the only process that can change the frequencies of traits, phyletic microevolution of traits within species and phylogenetic trait evolution among species, the tempo and mode of microevolution can also change trait frequencies. Species selection, phylogenetic, and phyletic processes can all contribute to large-scale trends, reinforcing or canceling each other out. Even more complex interactions among macroevolutionary processes are possible when multiple covarying traits are involved. Here I present a multilevel macroevolutionary framework that is useful for understanding how macroevolutionary processes interact. It is useful for empirical studies using fossils, molecular phylogenies, or both. I illustrate the framework with the macroevolution of coloniality and photosymbiosis in scleractinian corals using a time-calibrated molecular phylogeny. I find that standing phylogenetic variation in coloniality and photosymbiosis deflects the direction of macroevolution from the vector of species selection. Variation in these traits constrains species selection and results in a 200 million year macroevolutionary equilibrium.
The shallow, tropical reef environment differs from other marine environ-ments in its more intense competition for space, more limited nutrient con-centrations, proliferation of clonal animals, and a greater habitat complexity. The evolutionary consequences of these ecologic peculiarities are still poorly understood, but they seem to cause greater turnover rates of reef taxa than nonreef taxa and an especially volatile record of reefs on geologic timescales. The boom and bust pattern of Phanerozoic reef construction is impossi-ble to explain by linear responses to physicochemical changes. Threshold effects appear to be involved not only in reef crises but also in reef expan-sions. Long-term climate change seems to influence the biotic composition of reefs, but neither climate nor sea-level nor chemical changes in the oceans can elucidate the waxing and waning of reefs. Biological factors affecting spa-tial competition are thus probably more important than geologic controls on reef evolution.
Aim To evaluate the influence of geographical distribution on the extinction risk of benthic marine invertebrates using data from the fossil record, both during times of background extinction and across a mass‐extinction episode. Total geographical range is contrasted with proxies of global abundance to assess the relationships between the two essential components of geographical distribution and extinction risk.
Location A global occurrence data base of fossil benthic macro‐organisms from the Triassic and Jurassic periods was used for this study.
Methods Geographical distributions and biodiversity dynamics were assessed for each genus (all taxa) or species (bivalves) based on a sample‐standardized data set and palaeogeographical reconstructions. Geographical ranges were measured by the maximum great circle distance of a taxon within a stratigraphic interval. Global abundance was assessed by the number of localities at which a taxon was recorded. Widespread and rare taxa were separated using median and percentile values of the frequency distributions of occurrences.
Results The frequency distribution of geographical ranges is very similar to that for modern taxa. Although no significant correlation could be established between local abundance and geographical range, proxies of global abundance are strongly correlated with geographical range. Taxon longevities are correlated with both mean geographical range and mean global abundance, but range size appears to be more critical than abundance in determining extinction risk. These results are valid when geographical distribution is treated as a trait of taxa and when assessed for individual geological stages.
Main conclusions Geographical distribution is a key predictor of extinction risk of Triassic and Jurassic benthic marine invertebrates. An important exception is in the end‐Triassic mass extinction, which equally affected geographically restricted and widespread genera, as well as common and rare genera. This suggests that global diversity crises may curtail the role of geographical distribution in determining extinction risk.
Species traits may influence rates of speciation and extinction, affecting both the patterns of diversification among lineages and the distribution of traits among species. Existing likelihood approaches for detecting differential diversification require complete phylogenies; that is, every extant species must be present in a well-resolved phylogeny. We developed 2 likelihood methods that can be used to infer the effect of a trait on speciation and extinction without complete phylogenetic information, generalizing the recent binary-state speciation and extinction method. Our approaches can be used where a phylogeny can be reasonably assumed to be a random sample of extant species or where all extant species are included but some are assigned only to terminal unresolved clades. We explored the effects of decreasing phylogenetic resolution on the ability of our approach to detect differential diversification within a Bayesian framework using simulated phylogenies. Differential diversification caused by an asymmetry in speciation rates was nearly as well detected with only 50% of extant species phylogenetically resolved as with complete phylogenetic knowledge. We demonstrate our unresolved clade method with an analysis of sexual dimorphism and diversification in shorebirds (Charadriiformes). Our methods allow for the direct estimation of the effect of a trait on speciation and extinction rates using incompletely resolved phylogenies.
More-diverse communities are thought to be ecologically stable because a greater number of ecological interactions among members allows for the increases in robustness and resilience. Diversity-stability relationships have mostly been studied on short ecological time scales but one study has identified such patterns over million-year time scales in reef communities. Here we propose and test a hypothesis for the mechanism of large-scale diversity-stability relationships in reefs. The extinction of community members destabilizes the community as a whole, unless there is sufficient diversity to buffer the community from the stochastic loss of members, thereby preventing collapse. If genera have high extinction rates, any variation in diversity among communities will result in a diversity-stability relationship. Conversely, in the absence of other mechanisms, the stability of low extinction communities is expected to be independent of diversity. We compare the extinction rates of six reef-building metazoan taxa to patterns of reef community stability and reef volume. We find that extinction of reef-builders occurs independent of reef volume, and that the strength of the diversity-stability relationship varies positively with extinction rate.
Major modifications to the pharyngeal jaw apparatus are widely regarded as a recurring evolutionary key innovation that has enabled adaptive radiation in many species-rich clades of percomorph fishes. However one of the central predictions of this hypothesis, that the acquisition of a modified pharyngeal jaw apparatus will be positively correlated with explosive lineage diversification, has never been tested. We applied comparative methods to a new time-calibrated phylogeny of labrid fishes to test whether diversification rates shifted at two scales where major pharyngeal jaw innovations have evolved: across all of Labridae and within the subclade of parrotfishes.
Diversification patterns within early labrids did not reflect rapid initial radiation. Much of modern labrid diversity stems from two recent rapid diversification events; one within julidine fishes and the other with the origin of the most species-rich clade of reef-associated parrotfishes. A secondary pharyngeal jaw innovation was correlated with rapid diversification within the parrotfishes. However diversification rate shifts within parrotfishes are more strongly correlated with the evolution of extreme dichromatism than with pharyngeal jaw modifications.
The temporal lag between pharyngeal jaw modifications and changes in diversification rates casts doubt on the key innovation hypothesis as a simple explanation for much of the richness seen in labrids and scarines. Although the possession of a secondarily modified PJA was correlated with increased diversification rates, this pattern is better explained by the evolution of extreme dichromatism (and other social and behavioral characters relating to sexual selection) within Scarus and Chlorurus. The PJA-innovation hypothesis also fails to explain the most dominant aspect of labrid lineage diversification, the radiation of the julidines. We suggest that pharyngeal jaws might have played a more important role in enabling morphological evolution of the feeding apparatus in labrids and scarines rather than in accelerating lineage diversification.
The uneven distribution of species richness is a fundamental and unexplained pattern of vertebrate biodiversity. Although species richness in groups like mammals, birds, or teleost fishes is often attributed to accelerated cladogenesis, we lack a quantitative conceptual framework for identifying and comparing the exceptional changes of tempo in vertebrate evolutionary history. We develop MEDUSA, a stepwise approach based upon the Akaike information criterion for detecting multiple shifts in birth and death rates on an incompletely resolved phylogeny. We apply MEDUSA incompletely to a diversity tree summarizing both evolutionary relationships and species richness of 44 major clades of jawed vertebrates. We identify 9 major changes in the tempo of gnathostome diversification; the most significant of these lies at the base of a clade that includes most of the coral-reef associated fishes as well as cichlids and perches. Rate increases also underlie several well recognized tetrapod radiations, including most modern birds, lizards and snakes, ostariophysan fishes, and most eutherian mammals. In addition, we find that large sections of the vertebrate tree exhibit nearly equal rates of origination and extinction, providing some of the first evidence from molecular data for the importance of faunal turnover in shaping biodiversity. Together, these results reveal living vertebrate biodiversity to be the product of volatile turnover punctuated by 6 accelerations responsible for >85% of all species as well as 3 slowdowns that have produced "living fossils." In addition, by revealing the timing of the exceptional pulses of vertebrate diversification as well as the clades that experience them, our diversity tree provides a framework for evaluating particular causal hypotheses of vertebrate radiations.
Evolution involves both deterministic and random processes, both of which are known to contribute to directional evolutionary change. A number of studies have shown that when fitness is treated as a random variable, meaning that each individual has a distribution of possible fitness values, then both the mean and variance of individual fitness distributions contribute to directional evolution. Unfortunately the most general mathematical description of evolution that we have, the Price equation, is derived under the assumption that both fitness and offspring phenotype are fixed values that are known exactly. The Price equation is thus poorly equipped to study an important class of evolutionary processes.
I present a general equation for directional evolutionary change that incorporates both deterministic and stochastic processes and applies to any evolving system. This is essentially a stochastic version of the Price equation, but it is derived independently and contains terms with no analog in Price's formulation. This equation shows that the effects of selection are actually amplified by random variation in fitness. It also generalizes the known tendency of populations to be pulled towards phenotypes with minimum variance in fitness, and shows that this is matched by a tendency to be pulled towards phenotypes with maximum positive asymmetry in fitness. This equation also contains a term, having no analog in the Price equation, that captures cases in which the fitness of parents has a direct effect on the phenotype of their offspring.
Directional evolution is influenced by the entire distribution of individual fitness, not just the mean and variance. Though all moments of individuals' fitness distributions contribute to evolutionary change, the ways that they do so follow some general rules. These rules are invisible to the Price equation because it describes evolution retrospectively. An equally general prospective evolution equation compliments the Price equation and shows that the influence of stochastic processes on directional evolution is more diverse than has generally been recognized.
The analysis of the tempo and mode of evolution has a strong tradition in paleontology. Recent advances in molecular phylogenetic reconstruction make it possible to complement this work by using data from extant species.
Data on numbers of marine families within 91 metazoan classes known from the Phanerozoic fossil record are analyzed. The distribution of the 2800 fossil families among the classes is very uneven, with most belonging to a small minority of classes. Similarly, the stratigraphic distribution of the classes is very uneven, with most first appearing early in the Paleozoic and with many of the smaller classes becoming extinct before the end of that era. However, despite this unevenness, a Q -mode factor analysis indicates that the structure of these data is rather simple. Only three factors are needed to account for more than 90% of the data. These factors are interpreted as reflecting the three great “evolutionary faunas” of the Phanerozoic marine record: a trilobite-dominated Cambrian fauna, a brachiopod-dominated later Paleozoic fauna, and a mollusc-dominated Mesozoic-Cenozoic, or “modern,” fauna. Lesser factors relate to slow taxonomic turnover within the major faunas through time and to unique aspects of particular taxa and times. Each of the three major faunas seems to have its own characteristic diversity so that its expansion or contraction appears as being intimately associated with a particular phase in the history of total marine diversity. The Cambrian fauna expands rapidly during the Early Cambrian radiations and maintains dominance during the Middle to Late Cambrian “equilibrium.” The Paleozoic fauna then ascends to dominance during the Ordovician radiations, which increase diversity dramatically; this new fauna then maintains dominance throughout the long interval of apparent equilibrium that lasts until the end of the Paleozoic Era. The modern fauna, which slowly increases in importance during the Paleozoic Era, quickly rises to dominance with the Late Permian extinctions and maintains that status during the general rise in diversity to the apparent maximum in the Neogene. The increase in diversity associated with the expansion of each new fauna appears to coincide with an approximately exponential decline of the previously dominant fauna, suggesting possible displacement of each evolutionary fauna by its successor.
Cohort analysis provides an effective method of analysing taxonomic survivorship in the fossil record where large data sets are available. An analysis of the Stratigraphic ranges of about 8,500 fossil genera and subgenera shows that survivorship patterns are substantially the same throughout the Phanerozoic. These patterns are used to calculate an average value for mean species duration among fossil invertebrates (11.1 Myr.). Also, the extra extinctions near the Permo-Triassic boundary are shown to be equivalent to about 85 Myr of normal, background extinction.
Stratigraphic range data are used to derive time scales on which taxonomic survivorship curves for genera and families are as nearly as possible independent of their times of origin. These time scales correct for temporal variations in overall extinction rates caused by major extinctions and the pull of the Recent. Survivorship curves for genera and families on their respective time scales are well fit by Pareto distributions differing only in their scale parameter.
Phylogenies reconstructed from gene sequences can be used to investigate the tempo and mode of species diversification. Here we develop and use new statistical methods to infer past patterns of speciation and extinction from molecular phylogenies. Specifically, we test the null hypothesis that per-lineage speciation and extinction rates have remained constant through time. Rejection of this hypothesis may provide evidence for evolutionary events such as adaptive radiations or key adaptations. In contrast to previous approaches, our methods are robust to incomplete taxon sampling and are conservative with respect to extinction. Using simulation we investigate, first, the adverse effects of failing to take incomplete sampling into account and, second, the power and reliability of our tests. When applied to published phylogenies our tests suggest that, in some cases, speciation rates have decreased through time.
This analysis examines the evolution of the greater diversity of species with non-planktonic larval types relative to species with planktonic larval types in the turritellid gastropods. Two mechanisms for generating diversity gradients in larval types have been proposed in the literature: species selection and factors in development that are mediated by organismal adaptation. In order to examine the relevance of these two proposed mechanisms, a phylogenetic analysis of the turritellids using molecular phylogeny suggests that species selection is not the only process driving the trend toward increasing numbers of non-planktonic species through time. Developmental processes, apart from those involving organismal adaptation are implicated as playing a role in this trend. -from Authors
Bringing together the viewpoints of leading ecologists concerned with the processes that generate patterns of diversity, and evolutionary biologists who focus on mechanisms of speciation, this book opens up discussion in order to broaden understanding of how speciation affects patterns of biological diversity, especially the uneven distribution of diversity across time, space and taxa studied by macroecologists. The contributors discuss questions such as: Are species equivalent units, providing meaningful measures of diversity? To what extent do mechanisms of speciation affect the functional nature and distribution of species diversity? How can speciation rates be measured using molecular phylogenies or data from the fossil record? What are the factors that explain variation in rates? Written for graduate students and academic researchers, the book promotes a more complete understanding of the interaction between mechanisms and rates of speciation and these patterns in biological diversity.
A diffusion model of the distribution of a phenotypic character in a group of species is developed and analyzed. The model incorporates the combined effects of phyletic evolution, speciation and extinction. Directed speciation is modeled by assuming there is some bias to phenotypic changes during speciation. Species selection is modeled by assuming there is some dependence of either speciation or extinction rates on the phenotypic character. Three examples are analyzed to illustrate the use of the model. A model of completely random changes due to both phyletic evolution and speciation shows how between-species differences are established. A model of directed speciation due to multiplicative changes during speciation shows how a simple assumption about the speciation process can produce macroevolutionary trends. A model of species selection due to differences in extinction rates shows how the efficacy of species selection depends on the between-species variance produced both by speciation and by phyletic evolution.
The loss of sex and other recurrent unidirectional evolutionary trends can be treated as a simple equilibrium balanced by group selection. Genera of large mammals and large foraminiferans have a selective disadvantage of 0.02 to 0.05 per million years. The components of this selection act in opposite directions and in each case are much larger than the total selection, suggesting that most interspecific competition is with organisms of rather similar size. A related conjecture is disproved. Group selection is an important cause of evolution, including that of man.
Cracraft, J. (Department of Anatomy, University of Illinois at the Medical Center, P.O. Box 6998, Chicago, Illinois 60680,
and Division of Birds, Field Museum of Natural History, Chicago, Illinois 60605) 1982. A nonequilibrium theory for the rate-control
of speciation and extinction and the origin of macroevolutionary patterns. Syst. Zool., 31:348–365.—Macroevolutionary analysis
focuses on the explanation of general patterns of diversity among monophyletic groups. Three general patterns can be recognized:
radiation diversity in which species richness increases through time, reduction diversity in which richness decreases, and
steadystate diversity in which it remains relatively constant. Most clades show some combination of these three patterns.
Clades also exhibit a pattern of phenotypic change that can be superimposed on the pattern of diversification. With speciation,
descendant species can become either markedly apomorphic or remain plesiomorphic relative to the ancestral condition. There
is no necessary correlation between a particular phenotypic pattern and a specific pattern of diversity.
Explanations of macroevolutionary patterns have been of two principal types: adaptationist and species selectionist. The adaptationist
approach has explained patterns of diversity in terms of the occupation of “adaptive zones,” driven by natural selection.
“Adaptive zones” are tautological constructs and lack ontological status. The other macroevolutionary explanation, species
selection, is a pattern of species survival through time and as such is neither a process nor a theory of macroevolutionary
change. A deterministic theory of macroevolution lies in explaining inter-cladal diversity patterns by variation in the rate-controls
of speciation and extinction.
Speciation rate is controlled by factors intrinsic to species such as morphogenetic complexity (Wiley and Brooks, 1982) and
by external factors. It is hypothesized here that the primary external rate control of speciation is lithospheric complexity,
which relates directly to the number of geological and climatic barriers promoting geographic isolation. It is also proposed
that plate tectonic activity controls extinction rates through mechanical influences on habitats and its effects on environmental
climatic change. Extinction rates are postulated to be lower in environments of higher favorableness, which is defined in
terms of relatively high mean annual temperature, low range of mean annual temperature, and high annual rainfall dispersed
evenly throughout the year.
The interaction of relative environmental complexity and favorableness can be used to make predictions about diversity patterns
among clades and to investigate the relative importance of internal morphogenetic complexity. [Macroevolution; diversity;
speciation rates; extinction rates; lithospheric complexity; climatic change.]
Alleles, individuals, and species are all examples of entities possessing variation in the properties that underlie natural selection: branching (reproduction), persistence (survivorship), and heritability of characters. This suggests that the logic embodied in the theory of natural selection can be abstracted from its usual application to the level of individuals to encompass selection operating among any biological entities for which these essential properties can be meaningfully defined. This approach leads to a unified perspective of adaptation, selection, and fitness at all levels. Expanded versions of the Price covariance selection equations provide a convenient and useful conceptual vehicle for this discussion. The advantages of a hierarchical approach are twofold: it permits exploration of concepts and ideas across levels by analogy, and it focuses attention upon the mechanisms that account for different evolutionary dynamics at each level rather than obscuring these biologically unique properties with argument by extension from a single “special” level.
We point out that the choice of a single measure of evolutionary change restricts the context in which “other level” processes will be perceived. We illustrate the limited forms in which higher and lower level selection can be recognized from the unique perspective provided by any given level through extensions of Price's formula.
An exploration of the implications of such an approach leads us to the assertion that the development of a unified theory of evolution demands the recognition and incorporation of hierarchical structure as a conceptual foundation.
Paleontologists have a long tradition of the use of mathematical models to assist in describing and understanding patterns of diversification through time (e.g., Raup et al. 1973; Stanley 1975; Sepkoski 1978; Raup 1985; Foote 1988; Gilinsky and Good 1989). This is natural, as the information, phylogenetic and otherwise, that paleontologists work with comes equipped with a temporal dimension, albeit approximate, which endows these phylogenies with information about the tempo of evolution as well as the genealogical relationships among the lineages. Mathematical and statistical modeling are the tools for unlocking the quantitative information in the phylogenies.
Recently, molecular phylogenetics (e.g., Hillis et al. 1996) has created a new source of phylogenies with a temporal dimension, now frequently provided by molecular clocks. Many people have applied mathematical models to these phylogenies as well. Here, I highlight the areas of overlap as well as differences in what the simple models in the two fields have to tell us. To save ink, I will hereafter refer to paleontology as P and molecular phylogenetics as MP.
First I review the use of simple mathematical models to extract information about the tempo of evolution from phylogenies in the fields of P and MP. The same models, or variants thereof—these being the birth, birth-death, and Moran models—are used in the two areas, but there are differences in what they tell us, arising from differences in the nature of the phylogenies themselves. Finally, I address a high-profile assault on this common framework of understanding that has recently been launched.
This is one of the two simplest mathematical models used in P and MP (the other is the Moran process—discussed below) and, in its stochastic form, was one of the first stochastic processes to have been studied (Yule 1924; Kendall 1948, 1949 …
SYNOPSIS. Functional morphological analysis has revealed the existence of three functionally and morphologically different mechanisms underlying the tongue-parasphenoid and pharyngeal-parasphenoid bites in advanced teleost fishes. The bite is specialized differently in Pristolepis and the Anabantoidei, and in a primitive condition in both the Nandidae andChanniformes. These taxa belong to at least three unrelated lineages and do not share a commonancestry as was previously postulated. It has been possible to show how an originally primitive character can acquire a new biological and phylogenetic meaning by being integrated into a specialized functional complex. Based on functional data on the pharyngeal jaw apparatus, a new hypothesis is proposed stating that the Cichlidae, Embiotocidae, Labridae, Odacidae and Scaridae represent a monophyletic assemblage. This case study has demonstrated that reciprocal illumination of functional morphological and phylogenetic findings can lead to: (1) better tested and more precise phylogenetic hypotheses; (2) the construction of new hypotheses on the basis of specialized character complexes which were unrecognized by the use of a purely descriptive morphological approach
Temporal patterns of origination and extinction are essential components of many paleontological studies, but it has been difficult to obtain accurate rate estimates because the observed record of first and last appearances is distorted by the incompleteness of the fossil record. Here I analyze observed first and last appearances of marine animal and microfossil genera in a way that explicitly takes incompleteness and its variation into consideration. This approach allows estimates of true rates of origination and extinction throughout the Phanerozoic. Substantial support is provided for the proposition that most rate peaks in the raw data are real in the sense that they do not arise as a consequence of temporal variability in the overall quality of the fossil record. Even though the existence of rate anomalies is supported, their timing is nevertheless open to question in many cases. If one assumes that rates of origination and extinction are constant through a given stratigraphic interval, then peaks in revised origination rates tend to be displaced backward and extinction peaks forward relative to the peaks in the raw data. If, however, one assumes a model of pulsed turnover, with true originations concentrated at lower interval boundaries and true extinctions concentrated at upper interval boundaries, the apparent timing of extinction peaks is largely reliable at face value. Thus, whereas rate anomalies may well be real, precisely when they occurred is a question that cannot be answered definitively without independent support for a model of smooth versus pulsed rate variation. The pattern of extinction, particularly the major events, is more faithfully represented in the fossil record than that of origination. There is a tendency for the major extinction events to occur during stages in which the quality of the record is relatively high and for recoveries from extinctions to occur when the record is less complete. These results imply that interpretations of origination and extinction history that depend only on the existence of rate anomalies are fairly robust, whereas interpretations of the timing of events and the temporal covariation between origination and extinction may require substantial revision.
Mathematical modeling of cladogenesis and fossil preservation is used to explore the expected behavior of commonly used measures of taxonomic diversity and taxonomic rates with respect to interval length, quality of preservation, position of interval in a stratigraphic succession, and taxonomic rates themselves. Particular attention is focused on the independent estimation of origination and extinction rates. Modeling supports intuitive and empirical arguments that single-interval taxa, being especially sensitive to variation in preservation and interval length, produce many undesirable distortions of the fossil record. It may generally be preferable to base diversity and rate measures on estimated numbers of taxa extant at single points in time rather than to adjust conventional interval-based measures by discarding single-interval taxa. A combination of modeling and empirical analysis of fossil genera supports two major trends in marine animal evolution. (1) The Phanerozoic decline in taxonomic rates is unlikely to be an artifact of secular improvement in the quality of the fossil record, a point that has been argued before on different grounds. (2) The post-Paleozoic rise in diversity may be exaggerated by the essentially complete knowledge of the living fauna, but this bias is not the principal cause of the pattern. The pattern may partly reflect a secular increase in preservation nevertheless. Apparent temporal variation in taxonomic rates can be produced artificially by variation in preservation rate. Some empirical arguments suggest, however, that much of the short-term variation in taxonomic rates observed in the fossil record is real. (1) For marine animals as a whole, the quality of the fossil record of a higher taxon is not a good predictor of its apparent variability in taxonomic rates. (2) For a sample data set covering a cross-section of higher taxa in the Ordovician, most of the apparent variation in origination and extinction rates is not statistically attributable to independently measured variation in preservation rates. (3) Previous work has shown that standardized sampling to remove effects of variable preservation and sampling yields abundant temporal variation in estimated taxonomic rates. While modeling suggests which rate measures are likely to be most accurate in principle, the question of how best to capture true variation in taxonomic rates remains open.
For more than two decades, Jack Sepkoski's hypothesis of three great 'evolutionary faunas' has dominated thinking about the Phanerozoic evolution of marine animals. This theory combines pattern description with process modelling: diversity trajectories of major taxonomic groups are sorted into three categories, and the trajectories are predicted by coupled logistic equations. Here I use a re-creation of Sepkoski's classic three-phase coupled logistic model and an empirical analysis of his genus-level compendium to re-examine his claims about diversity dynamics. I employ a 'focal-group' variant of the proportional volatility G-statistic to deter-mine whether variation in turnover rates of focal taxonomic groups can be explained by the average rates for each group through time combined with average rates across all groups within each temporal bin. If growth is exponential and ecological interactions between pairs of groups are always similarly strong, then groups will wax and wane very predictably, and these statistics will always be insignificant. If instead growth is density-dependent and there are no inter-actions, significant volatility should be confined to periods of rapid radiations, such as those following major mass extinctions. Finally, if unusually strong pairwise interactions directly cause certain groups to succeed or fail, then significant volatility in each competing group should be present during replacement episodes that are not tied to overall radiations or extinctions. Additionally, if clustering groups into faunas is informative, then summed faunal diversity histories will replicate the observed volatility of all groups treated separately. To illustrate the focal-group method, I apply it to diversity data for the major groups of Cenozoic North American mammals. Surprisingly, the tests show that although some orders experience significant radiations and extinctions, orders with visually similar trajectories such as archaic, mostly Paleocene mammals fail to share dynamic properties. Volatility is far greater in Sepkoski's marine data, with almost every class showing significant and strong deviations from background turnover rates. However, Sepkoski's three-phase model predicts these patterns inconsistently. As expected, the Cambrian and Paleozoic faunas show high volatility during the hand-off between them. However, the hypothesized twin late Paleozoic and Jurassic/early Cretaceous hand-offs between the Paleozoic and Modern faunas are not marked by excessively rapid declines and increases. Instead, the Modern evolutionary fauna shows highly unusual dynamic behaviour starting in the mid-Cretaceous, coincident with the Mesozoic marine revolution and well after the Paleozoic fauna's decline. Sepkoski's categorization also generally fails to summarize overall volatility during long stretches of the Paleozoic and Mesozoic. Consult the copyright statement on the inside front cover for non-commercial copying policies.
Birth-death models, and their subsets—the pure birth and pure death models—have a long history of use for informing thinking about macroevolutionary patterns. Here we illustrate with examples the wide range of questions they have been used to address, including estimating and comparing rates of diversification of clades, investi-gating the "shapes" of clades, and some rather surprising uses such as estimating speciation rates from data that are not resolved below the level of the genus. The raw data for inference can be the fossil record or the molecular phylogeny of a clade, and we explore the similarites and differences in the behavior of the birth-death models when applied to these different forms of data.
A new method for estimating divergence times when evolutionary rates are variable across lineages is proposed. The method, called nonparametric rate smoothing (NPRS), relies on minimization of ancestor-descendant local rate changes and is motivated by the likelihood that evolutionary rates are autocorrelated in time. Fossil information pertaining to minimum and/or maximum ages of nodes in a phylogeny is incorporated into the algorithms by constrained optimization techniques. The accuracy of NPRS was examined by comparison to a clock-based maxi-mum-likelihood method in computer simulations. NPRS provides more accurate estimates of divergence times when (1) sequence lengths are sufficiently long, (2) rates are truly nonclocklike, and (3) rates are moderately to highly autocorrelated in time. The algorithms were applied to estimate divergence times in seed plants based on data from the chloroplast rbcL gene. Both constrained and unconstrained NPRS methods tended to produce divergence time estimates more consistent with paleobotanical evidence than did clock-based estimates.
This paper discusses why hierarchy demands that sorting and selection be disentangled. It then presents and illustrates an expanded taxonomy of sorting for a hierarchical world. For each of three levels (genes, organisms, and species), we show how sorting can arise from selection at the focal level itself, and as a consequence either of downward causation from processes acting on individuals at higher levels or upward causation from lower levels. We then discuss how hierarchy might illuminate a range of evolutionary questions based on both the logical structure of hierarchy and the historical pathways of its construction - for hierarchy is a property of nature, not only a conceptual scheme for organization.-from Authors
All groups for which data exist go extinct at a rate that is constant for a given group. When this is recast in ecological form (the effective environment of any homogeneous, group of organisms deteriorates at a stochasti- cally constant rate), no definite exceptions exist although a few are possible. Extinction rates are similar within some very broad categories and vary regularly with size of area inhabited. A new unit of rates for discrete phenomena, the macarthur, is introduced. Laws are appropriate in evolutionary biology. Truth needs more than correct predictions. The Law of Extinction is evidence for ecological significance and comparability of taxa. A ncn- Markovian hypothesis to explain the law invokes mutually incompatible optima within an adaptive zone. A self-perpetuating fluctuation results which can be stated in terms of an unstudied aspect of zero-sum game theory. The hypothesis can be derived from a view that momentary fitness is the amount of control of resources, which remain constant in total amount. The hypothesis implies that long-term fitness has only two components and that events of mutualism are rare. The hypothesis largely explains the observed pattern of
molecular evolution.
Abundance is one of the primary factors believed to influence extinction yet little is known about its relationship to extinction rates over geologic time. Using data from the Paleobiology Database we show that abundance was an important factor in the extinction dynamics of marine bivalve genera over the post-Paleozoic. Contrary to expectations, Our analyses reveal a nonlinear relationship between abundance and extinction rates, with rare and abundant genera exhibiting rates elevated over those of genera of moderate abundance. This U-shaped pattern is a persistent feature of the post-Paleozoic history of marine bivalves and provides one possible explanation for why we find strong support for heterogeneous extinction rates among genera grouped by similarity in abundance yet effectively no not relationship among these rates when using models of directional selection on abundance.
Species selection in the broad sense-also termed species sorting-shapes evolutionary patterns through differences in speciation and extinction rates (and their net outcome, Often termed the emergent fitness of clades) that arise by interaction of intrinsic biological traits with the environment. Effect-macroevolution occurs when those biotic traits, such as body size or fecundity, reside at the organismic level. Strict-sense species selection occurs when those traits are emergent at the species level, such is geographic range or population size. The fields of paleontology, comparative phylogenetic analysis, macroecology, and conservation biology are rich in examples of species sorting, but relatively few instances have been well documented, so the extent and efficacy of the specific processes remain poorly, known. A general formalization of these processes remains challenging, but approaches drawing oil hierarchical covariance models appear promising. Analyses integrating paleontological and neontological data for a single set of clades would be especially powerful.
After briefly introducing the hierarchical perspective, I discuss several theoretical issues concerning the nature of species selection and Vrba's effect hypothesis, and I review recent empirical evidence supporting hierarchical approaches to macroevolution. I argue that Vrba's influential definition of species selection is flawed in two ways. First, species selection does not require emergent traits because higher-level selection acting on aggregate traits can oppose lower-level selection. Second, clades do not play the same role in species selection that populations play in organismic selection. If explanatory and ontological reductionism are distinguished, then even though effect macroevolution does not involve a distinct macroevolutionary process, effect hypothesis explanations can be irreducible. A few well-documented cases of species selection and effect macroevolution suggest the need for a hierarchical expansion of neodarwinism.
The presumed geometry of clam and brachiopod clades (brachiopod declines matched closely by clam increases) has long served as primary data for the classic case of gradual replacement by competition in geological time. Agassiz invoked the geometric argument to assert the general superiority of clams, and it remains the standard textbook illustration today. Yet, like so many classic stories, it is not true. The supposed replacement of brachiopods by clams is not gradual and sequential. It is a product of one event: the Permian extinction (which affected brachiopods profoundly and clams relatively little). When Paleozoic and post-Paleozoic times are plotted separately, numbers of clam and brachiopod genera are positively correlated in each phase. Each group pursues its characteristic and different history in each phase—clams increasing, brachiopods holding their own. The Permian extinction simply reset the initial diversities. The two groups seem to track each other in each phase and a plot of brachiopod vs. clam residuals (each from their own within-phase regressions against time) yields significantly positive association. Some of this tracking may be an artifact of available rock volumes; we could, however, detect no effect of stage lengths. Passive extrapolation of microevolutionary theory into the vastness of geological time has often led paleontologists astray. Competitive interaction may rule in local populations, but differential response to mass extinctions (surely not a matter of conventional competition) may set the relative histories of large groups through geological time. Similarly, adaptive superiority in design cannot, in the usual sense of optimal engineering, have much to do with the macroevolutionary success of clams. The interesting question lies one step further back: what in the inherited Bauplan of a clam permits flexibility in design and why are other groups, however successful in their own domain, unable to alter their basic design.
Animal taxa show remarkable variability in species richness across phylogenetic groups. Most explanations for this disparity postulate that taxa with more species have phenotypes or ecologies that cause higher diversification rates (i.e., higher speciation rates or lower extinction rates). Here we show that clade longevity, and not diversification rate, has primarily shaped patterns of species richness across major animal clades: more diverse taxa are older and thus have had more time to accumulate species. Diversification rates calculated from 163 species-level molecular phylogenies were highly consistent within and among three major animal phyla (Arthropoda, Chordata, Mollusca) and did not correlate with species richness. Clades with higher estimated diversification rates were younger, but species numbers increased with increasing clade age. A fossil-based data set also revealed a strong, positive relationship between total extant species richness and crown group age across the orders of insects and vertebrates. These findings do not negate the importance of ecology or phenotype in influencing diversification rates, but they do show that clade longevity is the dominant signal in major animal biodiversity patterns. Thus, some key innovations may have acted through fostering clade longevity and not by heightening diversification rate.
The Red Queen describes a view of nature in which species continually evolve but do not become better adapted. It is one of the more distinctive metaphors of evolutionary biology, but no test of its claim that speciation occurs at a constant rate has ever been made against competing models that can predict virtually identical outcomes, nor has any mechanism been proposed that could cause the constant-rate phenomenon. Here we use 101 phylogenies of animal, plant and fungal taxa to test the constant-rate claim against four competing models. Phylogenetic branch lengths record the amount of time or evolutionary change between successive events of speciation. The models predict the distribution of these lengths by specifying how factors combine to bring about speciation, or by describing how rates of speciation vary throughout a tree. We find that the hypotheses that speciation follows the accumulation of many small events that act either multiplicatively or additively found support in 8% and none of the trees, respectively. A further 8% of trees hinted that the probability of speciation changes according to the amount of divergence from the ancestral species, and 6% suggested speciation rates vary among taxa. By comparison, 78% of the trees fit the simplest model in which new species emerge from single events, each rare but individually sufficient to cause speciation. This model predicts a constant rate of speciation, and provides a new interpretation of the Red Queen: the metaphor of species losing a race against a deteriorating environment is replaced by a view linking speciation to rare stochastic events that cause reproductive isolation. Attempts to understand species-radiations or why some groups have more or fewer species should look to the size of the catalogue of potential causes of speciation shared by a group of closely related organisms rather than to how those causes combine.
Species selection as a potential driver of macroevolutionary trends has been relegated to a largely philosophical position in modern evolutionary biology. Fundamentally, species selection is the outcome of heritable differences in speciation and extinction rates among lineages when the causal basis of those rate differences can be decoupled from genotypic (within-population) fitnesses. Here, we discuss the rapidly growing literature on variation in species diversification rates as inferred from molecular phylogenies. We argue that modern studies of diversification rates demonstrate that species selection is an important process influencing both the evolution of biological diversity and distributions of phenotypic traits within higher taxa. Explicit recognition of multi-level selection refocuses our attention on the mechanisms by which traits influence speciation and extinction rates.
Recent application of time-varying birth-death models to molecular phylogenies suggests that a decreasing diversification rate can only be observed if there was a decreasing speciation rate coupled with extremely low or no extinction. However, from a paleontological perspective, zero extinction rates during evolutionary radiations seem unlikely. Here, with a more comprehensive set of computer simulations, we show that substantial extinction can occur without erasing the signal of decreasing diversification rate in a molecular phylogeny. We also find, in agreement with the previous work, that a decrease in diversification rate cannot be observed in a molecular phylogeny with an increasing extinction rate alone. Further, we find that the ability to observe decreasing diversification rates in molecular phylogenies is controlled (in part) by the ratio of the initial speciation rate (Lambda) to the extinction rate (Mu) at equilibrium (the LiMe ratio), and not by their absolute values. Here we show in principle, how estimates of initial speciation rates may be calculated using both the fossil record and the shape of lineage through time plots derived from molecular phylogenies. This is important because the fossil record provides more reliable estimates of equilibrium extinction rates than initial speciation rates.
Diversification rate is one of the most important metrics in macroecological and macroevolutionary studies. Here I demonstrate that diversification analyses can be misleading when researchers assume that diversity increases unbounded through time, as is typical in molecular phylogenetic studies. If clade diversity is regulated by ecological factors, then species richness may be independent of clade age and it may not be possible to infer the rate at which diversity arose. This has substantial consequences for the interpretation of many studies that have contrasted rates of diversification among clades and regions. Often, it is possible to estimate the total diversification experienced by a clade but not diversification rate itself. I show that the evidence for ecological limits on diversity in higher taxa is widespread. Finally, I explore the implications of ecological limits for a variety of ecological and evolutionary questions that involve inferences about speciation and extinction rates from phylogenetic data.
A new method to estimate the diversification rate of a lineage from a phylogeny of recent species is presented. This uses survival models to analyse the ages of the species as derived from the phylogeny. Survival models can analyse missing data where the exact date of death is unknown (censoring). This approach allows us to include missing data (species not included in a detailed phylogenetic study) in the analysis, provided a minimum age is known for these species. Three models are presented, with emphasis on temporal variation in diversification rates. The maximum likelihood method and Akaike information criteria are used to derive estimators and tests of hypotheses. A simulation study demonstrates that the method is able to detect a temporal variation in diversification rate only when it is present, avoiding type I and type II errors. A lineage with ten species may be sufficient to detect a temporal variation in diversification rate even with 50 per cent of missing data. An application is presented with data from a phylogeny of birds of the genus Ramphocelus.
Species richness varies dramatically among groups of organisms, yet the causes of this variation remain poorly understood. Variation in species-level diversification rates may partially explain differential species richness among clades, but older clades should also be more diverse, because they will have had more time to accumulate species. Surprisingly, studies that have investigated this question have reached dramatically different conclusions: several claim to find no such age-diversity relationship, whereas a recent and more inclusive study reported that clade age and not diversification rate explains the variation in species richness among animal taxa. Here I address the relationship between clade age and species richness using a model-based approach that controls for variation in diversification rates among clades. I find that species richness is effectively independent of clade age in four of five data sets. Even extreme among-clade variation in diversification rates cannot account for the absence of a positive age-diversity relationship in angiosperms, birds, and teleost fishes. I consider two alternative explanations for these results and find that a clade volatility model positing correlated speciation-extinction dynamics does not underlie these patterns. Rather, ecological limits on clade growth, such as geographic area, appear to mediate temporal declines in diversification within higher taxa.
Clades diversify in an ecological context, but most macroevolutionary models do not directly encapsulate ecological mechanisms that influence speciation and extinction. A data set of 245 chordate, arthropod, mollusk, and magnoliophyte phylogenies had a majority of clades that showed rapid lineage accumulation early with a slowing more recently, whereas a small but significant minority showed accelerated lineage accumulation in their recent histories. Previous analyses have demonstrated that macroevolutionary birth-death models can replicate the pattern of slowing lineage accumulation only by a strong decrease in speciation rate with increasing species richness and extinction rate held extremely low or absent. In contrast, the metacommunity model presented here could generate the full range of patterns seen in the real phylogenies by simply manipulating the degree of ecological differentiation of new species at the time of speciation. Specifically, the metacommunity model predicts that clades showing decelerating lineage accumulation rates are those that have diversified by ecological modes of speciation, whereas clades showing accelerating lineage accumulation rates are those that have diversified primarily by modes of speciation that generate little or no ecological diversification. A number of testable predictions that integrate data from molecular systematics, community ecology, and biogeography are also discussed.
The discipline-wide effort to database the fossil record at the occurrence level has made it possible to estimate marine invertebrate extinction and origination rates with much greater accuracy. The new data show that two biotic mechanisms have hastened recoveries from mass extinctions and confined diversity to a relatively narrow range over the past 500 million years (Myr). First, a drop in diversity of any size correlates with low extinction rates immediately afterward, so much so that extinction would almost come to a halt if diversity dropped by 90%. Second, very high extinction rates are followed by equally high origination rates. The two relationships predict that the rebound from the current mass extinction will take at least 10 Myr, and perhaps 40 Myr if it rivals the Permo-Triassic catastrophe. Regardless, any large event will result in a dramatic ecological and taxonomic restructuring of the biosphere. The data also confirm that extinction and origination rates both declined through the Phanerozoic and that several extinctions in addition to the Permo-Triassic event were particularly severe. However, the trend may be driven by taxonomic biases and the rates vary in accord with a simple log normal distribution, so there is no sharp distinction between background and mass extinctions. Furthermore, the lack of any significant autocorrelation in the data is inconsistent with macroevolutionary theories of periodicity or self-organized criticality.
Gradual evolutionary change by natural selection operates so slowly within established species that it cannot account for the major features of evolution. Evolutionary change tends to be concentrated within speciation events. The direction of transpecific evolution is determined by the process of species selection, which is analogous to natural selection but acts upon species within higher taxa rather than upon individuals within populations. Species selection operates on variation provided by the largely random process of speciation and favors species that speciate at high rates or survive for long periods and therefore tend to leave many daughter species. Rates of speciation can be estimated for living taxa by means of the equation for exponential increase, and are clearly higher for mammals than for bivalve mollusks.