Niles Eldredge

Punctuated equilibria; the tempo and mode of evolution reconsidered

We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade—speciation may be random with respect to the direction of a trend (Wrights rule). As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation—not phyletic evolution—must be the dominant mode of evolution. We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerichs data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation. Phyletic gradualism was an a priori assertion from the start—it was never “seen” in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an “unnecessary difficulty.” We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.

Individuals, hierarchies and processes: towards a more complete evolutionary theory

Hierarchy is a central phenomenon of life. Yet it does not feature as such in traditional biological theory. The genealogical hierarchy is a nested organization of entities at ascending levels. There are phenomena common to all levels: (1) Entities such as genomic constituents, organisms, demes, and species are individuals. (2) They have aggregate characters (statistics of characters of subparts), but also emergent characters (arising from organization among subparts). Character variation changes by (3) introduction of novelty and (4) sorting by differential birth and death. Causation of introduction and sorting of variation at each level may be (5) upward from lower levels, (6) downward from higher levels, or (7) lodged at the focal level. The term “selection” applies to only one of the possible processes which cause sorting at a focal level. Neo-Darwinian explanations are too narrow, both in the levels (of genotypes and phenotypes) and in the directive process (selection) which are stressed. The acknowledgment of additional, hierarchical phenomena does not usually extend beyond lip service. We urge that interlevel causation should feature centrally in explanatory hypotheses of evolution. For instance, a ready explanation for divergence in populations is “selection of random mutants.” But upward causation from genome dynamics (or downward causation from the hierarchical organism) to the directed introduction of mutants may be more important in a given case. Similarly, a long-term trend is traditionally explained as additive evolution in populations. But sorting among species may be the cardinal factor, and the cause may not be species selection but upward causation from lower levels. A general theory of biology is a theory of hierarchical levels—how they arise and interact. This is a preliminary contribution mainly to the latter question.

The dynamics of evolutionary stasis

Abstract The fossil record displays remarkable stasis in many species over long time periods, yet studies of extant populations often reveal rapid phenotypic evolution and genetic differentiation among populations. Recent advances in our understanding of the fossil record and in population genetics and evolutionary ecology point to the complex geographic structure of species being fundamental to resolution of how taxa can commonly exhibit both short-term evolutionary dynamics and long-term stasis.

Phylogeny and Paleontology

Paleontologists have traditionally regarded the temporal sequence of fossils as central to the concept of phylogeny. In recent years the significance of the time aspect has been questioned by a number of systematists who argue that the fossil record can offer only a very incomplete picture of phylogeny and that temporal criteria are generally less reliable than morphologic ones in working out relationships. We intend to explore these different opinions by considering both the nature of paleontological data and some methodological generalizations concerning their use.

Phylogenetics and Material Cultural Evolution

Cultural artefacts, like genes and languages, reflect their history. The methodology of inference of that history, however, has been a contentious question. Recent applications of biological phylogenetic methodology to infer historical patterns of material culture are often explicitly justified on the grounds that essentially similar processes underlie evolution in both biological and material cultural realms. Conventional phylogenetic techniques, while helpful in some cases, do not provide a general theoretical and operational framewok for reconstructing material cultural history. Critical analyses of the diversity patterns of two musical instruments, the stringed psaltery and the brasswind cornet, reveal paths of information transfer and the origins of innovation unique to the cultural context that are unlike those in biological systems.

Trilobite biogeography in the Middle Devonian: geological processes and analytical methods

Phylogenetic patterns of trilobite clades were used to deduce biogeographic patterns during the Middle Devonian, a time of active plate collision between North America (Laurentia) and other plates, coincident with several major episodes of sea-level rise and fall. The mapping of biogeographic states onto phylogenies for asteropyginid and proetid trilobites indicated that dur- ing their history these trilobite clades often shifted the areas they occupied, and also underwent vicariant differentiation, followed by range expansion, followed by subsequent vicariance. Biogeo- graphic patterns in these individual phylogenies were evaluated and synthesized using a modified version of Brooks Parsimony Analysis, which is discussed. This method makes it possible using cladistic methods to distinguish between episodes of vicariance and episodes of dispersal. Two types of dispersal are recognized herein: (1) the individualistic responses of certain taxa in a single clade that cannot be generalized, i.e., traditional ad hoc dispersal, and (2) those patterns of con- gruent range expansion that are replicated across several clades. The latter are not treated as true dispersal, expansion of a taxons range over a barrier accompanied by diversification, but rather as a result of the temporary removal of barriers to marine taxa, due either to relative sea-level rise or to the collision of formerly disjunct plates. These are interpreted as changes in the structure of areas, and this type of dispersal is referred to as geo-dispersal. Geo-dispersal was found to have occurred in the Middle Devonian trilobite fauna of Eastern North America. Biogeographic analysis indicated that Eastern North America is a strongly supported area, with the Appalachian and Michigan Basins as sister areas. Armorica and the Canadian Arctic are also sister areas. Congruence was found between area cladograms produced by vicariance and dispersal analyses for Middle Devonian trilobites, suggesting that in some cases the geological processes governing vicariance, such as sea-level changes, were the same as those that caused dispersal.

Reconstruction of hominid phylogeny: A testable framework based on cladistic analysis*

The cladistic method of systematic analysis is applied to the taxa of Hominidae (s.l.), from Dryopithecus to Homo, Pan and Pongo. This method is based on determination of shared derived character states among all taxa studied, and only this kind of similarity links taxa into subgroups. Each branching point is characterized by its reconstructed morphotype or list of deduced characters common to it and (at least some of) its descendants. Ancestor-descendant relationships are not sought but concentration is on hypotheses as to relative recency of common ancestry. Morphologically, Dryopithecus is found to be conservative, Gigantopithecus widely divergent and Ramapithecus linked with later men. Australopithecus africanus is also conservative but potentially linked to Homo, while A. robustus is divergent.

A study of stasis and change in two species lineages from the Middle Devonian of New York state

More than 5000 measurements were taken on over 1000 specimens of two species of brachiopods, Mediospirifer audaculus and Athyris spiriferoides, from the Middle Devonian Hamilton Group of New York state. Statistical analyses were performed on these data, with specimens partitioned by their occurrence in one of many paleoenvironments and stratigraphic horizons. Neither species showed substantial morphological departures between first appearance and extinction (the range of the Hamilton Group, roughly 5 m.y.). However, oscillations in morphology were discovered in both taxa. For the two species we studied, groups of organisms occurring in a single paleoenvironment undergo moderate morphological change through time; however, the net sum of changes through time in all paleoenvironments in which these species occur is essentially zero. Therefore, stasis may be partly a property of the organization of species into different environmental populations. Different “environmental populations” may evolve, but they will typically do so in several different “directions,” generally producing no net change. The difference between the morphology of species in different environments over the whole interval of the Hamilton Group is also nil, thereby ruling out any major role that ecophenotypic effects could play in the patterns recognized herein.

Levels of selection and macroevolutionary patterns in the turritellid gastropods

This analysis examines the evolution of the greater diversity of species with non-plank- tonic larval types relative to species with planktonic larval types in the turritellid gastropods. This sort of trend has been documented in both the fossil and recent biota of several gastropod families. Two mechanisms for generating diversity gradients in larval types have been proposed in the literature. The first, species selection, focuses on the population biology of larval types. The second proposes that factors in development that are mediated by organismal adaptation are responsible. Turritellids have been cited as a classic example of species selection. In order to examine the relevance of these two proposed mechanisms, a phylogenetic analysis of the turritellids using molecular sequence data was performed to determine the evolution of larval types in this clade. The resultant 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 in- volving organismal adaptation (except in the trivial sense), are implicated as playing a role in this trend. In particular, these processes may involve changes in the timing of germ-line sequestration in organisms. Germ-line sequestration governs how accessible organisms are to heritable variation during ontogeny. Embryological evidence from gastropods suggests that non-planktonic species have early germ-line sequestration relative to planktonic species, making them more resistant to developmental change. Thus, non-planktonic lineages will only rarely revert to a planktonic larval mode.

The evolutionary ecology of technological innovations

Technological evolution has been compared to biological evolution by many authors over the last two centuries. As a parallel experiment of innovation involving economic, historical, and social components, artifacts define a universe of evolving properties that displays episodes of diversification and extinction. Here, we critically review previous work comparing the two types of evolution. Like biological evolution, technological evolution is driven by descent with variation and selection, and includes tinkering, convergence, and contingency. At the same time, there are essential differences that make the two types of evolution quite distinct. Major distinctions are illustrated by current specific examples, including the evolution of cornets and the historical dynamics of information technologies. Due to their fast and rich development, the later provide a unique opportunity to study technological evolution at all scales with unprecedented resolution. Despite the presence of patterns suggesting convergent trends between man-made systems end biological ones, they provide examples of planned design that have no equivalent with natural evolution.