David Jablonski, Michael J. Benton, Robert A. Gastaldo,
Charles R. Marshall, and J. John Sepkoski, Jr.
Macroevolution can be defined simply as evolution above the species level, and its subject matter includes the origins and fates of major novelties such as tetrapod limbs and insect wings, the waxing and waning of multi-species lineages over long time-scales, and the impact of continental drift and other physical processes on the evolutionary process. With its unique time perspective, paleontology has a central role to play in this area: the fossil record provides a direct, empirical window onto large-scale evolutionary patterns, and thus is invaluable both as a document of macroevolutionary phenomena, and as a natural laboratory for the framing and testing of macroevolutionary hypotheses. This is a vibrant field (if underpopulated relative to the wealth of material and questions within its domain), with a steady stream of papers, books and symposia and an increasing interaction with a broad range of disciplines from astrophysics to developmental biology. The result has been a number of insights into the processes that have shaped the major evolutionary patterns of present-day and ancient organisms.
One striking macroevolutionary pattern that has emerged from the fossil record is that major groups and evolutionary novelties have not originated randomly in time and space. The Cambrian Explosion at the beginning of the Paleozoic Era established virtually all of the major body plans seen in present-day oceans, along with a number of extinct, enigmatic groups, a remarkable evolutionary burst reflected in the sudden appearance in the fossil record of all but one of the living marine phyla and many of the marine classes. Equally intriguing is the realization that both morphological divergence and the production of higher taxa are less prolific after this early Paleozoic pulse, although similar evolutionary pulses have also been documented in vascular plants with the invasion of land, and in terrestrial insects soon thereafter. One major question has been the relative role of development (e.g. changes in canalization) and ecology (e.g. ecospace availability) in promoting and then damping these evolutionary explosions.
In marine and terrestrial systems alike, secondary bursts of evolutionary novelty appear to be concentrated in the wake of mass extinctions, when taxonomic and morphologic divergence is again accentuated (although not so dramatically as in the early Paleozoic). The timing of these originations lends support to a macroevolutionary hypothesis of incumbency -- which early occupants of a resource or habitat can dominate until an extinction or other perturbation clears the way for other groups. Still unclear is whether such mechanisms can explain spatial patterns in the origin of novelties, such as the preferential onshore origination of higher taxa in Paleozoic and post-Paleozoic marine invertebrates (and some possible analogs in Paleozoic and post-Paleozoic terrestrial plants), and the preferential tropical origins of many marine invertebrate orders and some terrestrial plant groups.
The mechanisms underlying the origins of novelties remain poorly understood. Interactions with two biological fields have been especially valuable in exploring this macroevolutionary problem, and are sure to accelerate in the future. Phylogenetic analysis, including molecular techniques, can give a genealogical context for the production of novel features, providing essential data on the morphology, ecology and other aspects of probable ancestors or less-derived outgroups; in exchange, paleontology provides data on the age, distribution, and characters of extinct taxa that can significantly change the interpretation of character state polarities and the sequence of evolutionary transitions. Developmental biology can provide insights at several levels. Developmental patterns at the organismic level, for example, can help to explain how relatively small changes in the timing of development, or heterochrony, can have dramatic morphological results. At the molecular level, knowledge of the operation and phylogenetic distribution of regulatory gene systems can elucidate how those genetic control systems, some of them extremely ancient, were altered or commandeered to generate novel morphologies. Selected references: Bengtson 1994, Benton 1987, 1996, DiMichele & Aronson 1992, Donoghue et al. 1989, Erwin et al. 1987, 1997, Foote 1996, Jablonski 1993, Jablonski & Bottjer 1990a,b,c, Knoll et al. 1984, Lipps & Signor 1992, McKinney & McNamara 1991, McNamara 1995, Padian et al. 1994, Smith 1994, Wagner 1995a.
Much macroevolutionary research was triggered by the realization that many species appeared to be almost static morphologically after their first appearance in the fossil record rather than evolving continuously. This led to the hypothesis of punctuated equilibrium, which holds that most evolutionary change accrues at the branchpoints of species' histories rather than over the duration of established species. The fossil record does indeed exhibit many examples of stasis, but contains not only instances of punctuated equilbria, but also punctuated anagenesis (evolutionary histories consisting of stasis and rapid change without branching), gradual anagenesis (constant, directional evolution without branching) and even the gradualistic splitting of lineages that characterized evolution in classical textbooks. Still unclear is whether these different evolutionary tempos and modes are distributed unevenly among taxa, habitats, regions, or ecological categories.
These observations have prompted an approach to certain macroevolutionary patterns, such as trends in the morphology or species-richness within lineages that allowed for the operation of evolutionary processes simultaneously at several hierarchical levels. Thus, directional changes or shifts in the number of species contained within subclades of larger groups might be attributed to sorting among units at several different levels, from the familiar one of bodies within populations to species or higher taxa within clades. Factors that accelerate speciation rates within one segment of a clade, for example, might shape the long-term development of that clade as a whole. Differences in speciation or extinction rates have long been recognized among and within higher taxa (e.g. ammonites vs. bivalves, lytoceratid vs. acanthoceratid ammonites), but much more work is needed on the biotic factors that might confer these rate differentials. Further, separating species selection in the strict sense from the waxing or waning of groups as a byproduct of processes at the individual level has proven difficult in most instances. More generally, empirical research and quantitative theory have lagged far behind the qualitative theoretical arguments in this area.
As in the origin of novelties, phylogenetic information can be extremely valuable in analyzing macroevolutionary dynamics: many macroevolutionary hypotheses can be stated in terms of predictions about phylogeny, for example on the repeated association of particular characters with changes in evolutionary rate or the topology of an evolutionary tree, or on the variation in branch lengths in different regions of an evolutionary tree. Phylogenetic methods that take into account both the strengths and weaknesses of paleontological data are being developed and debated; this interaction with systematic biology will continue to be important, not only for paleobiological research, but as a basis for testing hypotheses about molecular evolution, including constancy or variation in rates among groups and through time (which can be assessed directly for groups with accurate phylogenies and a rich fossil record).
The relative roles of physical and biotic factors in shaping macroevolutionary patterns also remain hotly debated, and surely vary among taxa, times, and places. On the biotic side, rate differentials among higher taxa imply a role for such inherent biological factors as genetic population structure or intrinsic growth rates; the Mesozoic Marine Revolution and other apparent escalations in predation intensity and in the defenses of potential prey suggest that diffuse interactions among many disparate taxa can have long-term consequences; and the effects of incumbency discussed above argue for at least a pre-emptive mode of competitive interaction over macroevolutionary timescales. On the physical side, changes in climate, geography, and perhaps atmospheric or ocean chemistry have proven to have overriding effects in some cases, and many of the biotic replacements once thought to reflect competitive interactions now appear to have been mediated by mass extinction (e.g. the replacement of dinosaurs by mammals at the end of the Mesozoic Era). Still problematic is the issue of spatial and temporal scale: the theoretical expectations from ecological theories of competition and predator-prey interactions are difficult to extrapolate meaningfully to macroevolutionary scales, so that new expectations tuned to the dynamics of clades over millions of years need to be developed.
The role of extinction, particularly mass extinction events, has been a major research area. Whether extinction peaks merely accelerate processes set in motion during times of less intense turnover or play a more special role in the evolutionary process is an area of active research, and the role of extinction events may vary with their magnitude and timing. Some have argued that mass extinctions appear to break the dominance of incumbent groups and open opportunities for diversification and evolutionary novelties, which may help to explain how such events can have such dramatic evolutionary effects despite accounting for only a small proportion of the total extinction that has occurred over the past half-billion years. More work is needed on selectivity during mass extinctions, particularly comparative approaches among extinction events of different intensities. Such studies can also be important for understanding, and quite possibly predicting, extinction and recovery patterns in the present-day biota as human-caused environmental purturbations increase in extent and severity.
The history of multicellular life has been characterized by stable, if weakly bounded, ecological/evolutionary associations at several scales, from communities and community types to the three evolutionary faunas of Phanerozoic oceans. The marine evolutionary faunas have intriguing parallels on land, in the four evolutionary floras among vascular plants and the three evolutionary faunas among tetrapod vertebrates. In each instance, the major faunas/floras appear to have their own characteristic dominant taxa, levels of diversity (increasing in stepwise fashion), and, at least in the marine faunas, characteristic rates of origination and extinction for those dominant taxa (decreasing in turn among the evolutionary faunas --which may help to explain the coincident long-term evolutionary behaviors of their constituent taxa). Still uncertain is exactly what maintains the apparent environmental separation among marine faunas for much of the Phanerozoic, whether similar environmental (or latitudinal) separation occurs in the terrestrial equivalents, and the extent of mixing and dilution among the faunas or floras over their histories. The problem of biotic interaction and ecologic scale emerges again when considering the characteristic diversity levels: why should marine diversity at the family level, for example, hold steady for most of the Paleozoic despite extensive turnover (one possible implication is the attainment of a global equilibrium, reset or perhaps never even reached in post-Paleozoic times), and to what extent are the successive higher characteristic diversity levels attributable to physical versus biological controls? Selected references: Clyde & Fisher 1997, Erwin & Anstey 1995, Gould & Eldredge 1993, Grantham 1995, Hitchin & Benton,1997, Jablonski 1986, 1995, Jablonski & Sepkoski 1996, McShea 1994, Norrell & Novacek 1992, Patzkowsky 1995, Sepkoski 1992, 1996, 1997, Valentine 1990, Vermeij 1987, 1994, Wagner 1996.
Macroevolutionary study has benefited from a steady infusion of quantitative methods from many other fields, from bootstrapping and time-series statistics to the scaling up of survivorship and rarefaction analysis, as well as the generation of novel approaches to morphometrics, phylogenetics and sampling theory. These quantitative tools will greatly expand the scope and depth of macroevolutionary questions addressed in the fossil record, as will the growing array of computerized databases -- which as noted above should begin to incorporate a broader spectrum of information, such as localities and environments (see databases report).
Mathematical models have also been used to good effect. The most extensively explored methods to date have been models of taxonomic diversification, in which the dynamics of clades or entire faunas are modeled with simple equations to explore the effects of variations in extinction, origination, and the strengths of interaction among and within groups. New work on Morphological diversification and its relative concordance or discordance with taxonomic patterns shows much promise. Models of individual body forms, such as Raup's classic work on molluscan shells, have enjoyed less attention lately, but the ability to generate a theoretical morphospace based on a few generative parameters, and then to explore the occupation of that morphospace by living and fossil organisms, remains a macroevolutionary avenue with great potential. In all of these modeling approaches, the aim has not been simply to reduce complex macroevolutionary patterns to a few simple explanatory equations, but to improve understanding of how specific changes in each variable in turn would shape macroevolutionary phenomena, and to develop a baseline of expectations for random behaviors against which the patterns of the fossil record can be compared (such as long-term trends in morphology or taxonomic diversity).
The ultimate source of virtually all these data are the rocks and sediments of the fossil record, and the quantitative assessment of potential biases represents an important, expanding area for macroevolution. Taphonomic studies, from actualistic assessment of the fidelity of local assemblages to the quantification of the megabiases of available outcrop area and stratigraphic gap analysis, have begun to provide taxon-, environment- and time-specific assessments of the nature of paleontological data--and for the most part, the news is good on the reliability of paleontological data. Methods for placing statistical confidence limits on such variables as stratigraphic ranges, taxon durations, diversity levels, and extinction intensities are all being developed or refined, although the multitude of sampling situations presented by the stratigraphic record requires much further work before general methods are fully available. Even now, these approaches allow the investigator to steer a more rigorous course between the defeatist view, that the fossil record is too incomplete and biased to yield robust conclusions, and the literalist one, that the fossil record should be taken strictly at face value whatever its theoretical shortcomings or biases. Selected references: Benton & Hitchin 1996, Donovan & Paul in press, Foote 1996a,b, Gilinsky & Signor 1991, Kidwell & Flessa 1995, Marshall 1997, Wagner 1995b.
Prospects And Needs
Macroevolution is an interdisciplinary field, and so one important goal is to strengthen interchange with other branches of paleontology -- systematics, biostratigraphy and paleoecology, for example can be crucial to developing rigorous datasets for maroevolutionary study. Interactions with other areas of geology, such as stratigraphy and geochemistry are also important, providing a temporal framework and invaluable information on the environmental context -- and perhaps even the forcing factors - of macroevolutionary patterns. Electronic databases will play an important role, but targets should expand beyond the synoptic taxonomic databases that have dominated the field, to include morphological, ecological, phylogenetic and biogeographic information; collecting and standardizing such data will be a major challenge in many instances.
Interchange with the biological sciences is probably at an all-time high; this has been beneficial in both directions, and should be encouraged. Phylogenetics and developmental biology have already been mentioned as rewarding avenues of interaction. The potential for interaction with ecology is great, but much care is needed to consider the strengths and limitations of the fossil record in terms of temporal and spatial scale. Ecologists and microevolutionists are beginning to appreciate the importance of events over larger time scales than the decades to centuries that are their usual bounds, and a new effort is needed from both neontological and paleontological sides to get beyond a simple extrapolation of ecological phenomena into macroevolutionary time scales (see paleoecology report, presumably).
Besides an ongoing, healthy exchange with cognate fields in geology and biology, more integration is needed within macroevolution itself. Taxonomic/phylogenetic approaches and morphological approaches need to be more fully linked, although this is beginning. Each approach has its strengths and may be better-suited to a particular set of macroevolutionary problems, but neither is "closer" to the evolutionary process, because understanding evolutionary process at any hierarchical level requires knowledge of genealogy and the sorting dynamics on the one hand, and of the phenotypes that interact with the physical and biotic environment on the other. Cross-comparisons among different approaches to reconstructing evolutionary history, for example morphology- or molecule-based phylogenies versus stratigraphic distributions, will be also continue to grow in importance, and should help to produce a more rigorous approach to historical analysis in which methods are fitted to the strengths and weaknesses of the system under study. Finally, as might be expected in a young science (at least in its present incarnation), empirical analyses have tended to lag behind theory. With a few exceptions, relatively few detailed studies are available for any given problem; this is at least partly a function of the labor-intensive nature of macroevolutionary research, which necessarily must encompass significant periods of time, numerous taxa and/or many characters. As hypotheses are framed in more specific and rigorous terms, and as methods for assessing and even factoring out sampling and other biases become increasingly available, this imbalance should be remedied, and a rich balance of data and theory will continue to inform research on large-scale patterns of evolution.
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Prof. David Jablonski--Topic Coordinator
University of Chicago
Dept. of Geophysical Science
5734 S. Ellis Avenue
Chicago, IL 60637 USA
Prof. Michael J. Benton
Dept. of Geology, University of Bristol
Wills Memorial Building, Queen's Road
Bristol BS8 1RJ, UNITED KINGDOM
Prof. Robert A. Gastaldo
Department of Geology
210 Petrie Hall
Auburn, AL 36849-3501 USA
Dr. Charles Marshall
Dept. of Earth & Space Sciences
University of California
Los Angeles, CA 90024-1567
Dr. J. John Sepkoski
Dept. of Geophyiscal Sciences
University of Chicago
5734 South Ellis Ave
Chicago, IL 60637 USA
This page is maintained for the Paleo21 Organizing Committee by Norman MacLeod and H. Richard Lane. Corrections, inquiries about, and updates to any of the information shown above should be directed to Norm and/or Rich.