chapter sixteen The Shape of Evolution: The MBL Model and Clade Shape John Huss introduction In 1971, University of Chicago paleontologist Tom Schopf organized a sym- posium on models in paleobiology at the Geological Society of America meet- ing in Atlantic City, New Jersey. In a fi eld populated by specialists on every- thing from inarticulate brachiopods to pterodactyls, Schopf was skeptical that paleobiology would ever move beyond the description and classifi cation of fossils to the construction and testing of general models and theories unless the fi eld were radically reoriented. The session resulted in the seminal book Models in Paleobiology (1972), perhaps best known as the initial vehicle for Niles Eldredge’s and Stephen Jay Gould’s theory of punctuated equilibrium. Other contributors to the volume included Dave Raup, whose chapter con- cerned models of morphology, and ecologist Dan Simberloff, who provided a primer on models in biogeography. Encouraged by the success of that meeting, Schopf cornered three of his fellow symposiasts—Raup, Gould, and Simberloff—for two informal brain- storming sessions in the winter of 1972 at the Marine Biological Labora- tory (MBL) in Woods Hole, Massachusetts (Jack Sepkoski, then a geology graduate student at Harvard, also participated). Above all, the group sought to shake up what they perceived to be a moribund fi eld. They began looking for means by which paleontology might move toward becoming a more pre- dictive, general, and theoretical—in a word, nomothetic—science, and turned to recent developments in ecology and biogeography for inspiration (Raup et al. 1973). For an idiographic science like paleontology, mired in taxonomic The Shape of Evolution:The MBL Model and Clade Shape 327 nomenclature and description, the prospect of a predictive model that did not depend on the identity of particular taxa was promising.1 MacArthur and Wilson’s equilibrium model of island biogeography was especially appealing as an exemplar because it predicted the equilibrium number of species on an island as a function of immigration rate of new species and extinction rate of species already present, regardless of taxonomic affinity (MacArthur and Wilson 1963, 1967). It helped that Simberloff had collaborated with E. O. Wilson on experimental tests of island biogeography, which was one of the main reasons Schopf brought him on board. The question was whether an island biogeographic model, with speciation rate substituting for immigration rate, could be ramped up to geologic timescales. Schopf was hopeful. Schopf arrived at the fi rst MBL meeting with the multivolume, data- rich Treatise of Invertebrate Paleontology in tow, and Simberloff was accessorized with one of the fi rst handheld calculators. The group spent long days exchang- ing ideas and poring over raw data for patterns and trends, but Simberloff, skilled in mathematical modeling and ecological data analysis, found most of the data too fragmentary to analyze. It was against this backdrop of despera- tion and frustration, on the last day of the fi rst MBL meeting, that the question arose: “what if we were to remove Darwin from evolution?” In other words, what would phylogenetic patterns look like without natural selection as a guiding force? In effect, this question suggested a null hypothesis for evaluat- ing the effects of natural selection on phylogenetic patterns (A couple of years earlier Simberloff had used an analogous approach, generating null communi- ties by random sampling to assess the effects of interspecifi c competition on the composition of island communities [Simberloff 1970]). The meeting ad- journed with the agreement that Raup would write a simulation program that refl ected this picture of evolution. Thus was born the MBL model. the mbl model The MBL model began as a computer model in which evolutionary trees grew by a stochastic branching process in discrete time subject to an equilibrium constraint. It “removed Darwin from evolution” in two ways. First, the model was neutral at the lineage level. By assigning each lineage the same probabili- ties of persistence, branching, and extinction, no lineage had an advantage over any other. Second, natural selection simply went unrepresented in the model. Instead, phylogenetic evolution was modeled as a Monte Carlo pro- cess. Each simulation run would begin with a single lineage and a random number “seed.” At each time unit, the computer would generate a random (or 328 Chapter Sixteen pseudorandom—the distinction need not concern us here) number which would determine, according to three preset probabilities, the lineage’s fate: (1) extinction, (2) persistence to the next time unit and branching to form a sis- ter lineage, or (3) persistence to the next time unit without branching. Each new lineage that evolved followed these same rules with the same probabili- ties. Initially (in the pre- equilibrial phase) the probability of branching would exceed that of extinction until the total number of species reached a preset equilibrium, at which point the probabilities of branching and extinction would be made equal to one another, and the total number of species would oscillate around the equilibrium value. The oscillations were damped by an algorithm that adjusted extinction and branching probabilities in proportion to the square of the deviation from equilibrium (Simberloff 1974). A simula- tion run would terminate when the end of the time scale was reached, when all lineages became extinct, or when a total of 500 species had evolved throughout the course of the run, whichever came fi rst (a limitation of computer memory). A taxonomic routine subdivided the resulting phylogeny into clades (sets of lineages sharing a common ancestor) once a certain minimum size (defi ned as the sum of the durations of all lineages descended from the ancestor) was reached. If a group within an existing clade had grown large enough to meet the minimum size requirement, a new clade would be established (Raup et al. 1973). The model operated at a single taxonomic rank—no subtaxa were rec- ognized. Thus, once a new clade was cropped out of an existing clade, the re- sidual lineages formed a paraphyletic group, which, strictly speaking, is not a clade. The MBL authors did not distinguish between monophyletic and para- phyletic topologies, calling both “clades,” and I will follow their usage herein. The simulation output was a phylogeny, or evolutionary tree. Although in principle the information could simply have been represented numerically, the outputs were instead represented in the familiar iconography of paleontol- ogy: phylogenies of clades, and plots of originations, extinctions, and diversity (number of lineages) versus time. In the phylogenies of clades, each clade is represented by a “spindle,” varying in width in proportion to the number of lineages in it during any given time interval. The decision to represent simula- tion results in this way was not arbitrary. By capitalizing on the working pale- ontologist’s familiarity with such diagrams, the paper was able to attract much attention. It is doubtful that presenting the same results as tables of numbers would have had as strong an impact on the paleontological community. The MBL simulation results were surprising. This relatively simple proba- bilistic branching model, primed by a random number seed, could simulate a The Shape of Evolution:The MBL Model and Clade Shape 329 variety of familiar- looking evolutionary patterns: adaptive radiation, competi- tive replacement, and simultaneous extinction events (Raup et al. 1973). An expanded version of the model incorporating morphologic change at each time interval produced morphologically coherent taxa, morphologic trends, variation in evolutionary rates, and correlated character evolution, despite the absence of natural selection in the model (Raup and Gould 1974). Using this same morphologically explicit version of the MBL model, Schopf, who was increasingly committed to the neutralist view that natural selection intrinsi- cally favored no species over any other, spearheaded an effort to demonstrate that differences in evolutionary rate among taxa as disparate as clams and horses were more apparent than real, our judgments of rate being biased by differences in the morphologic complexity of the taxa involved (Schopf et al. 1975).2 Once the group devised measures of clade size and shape, the computer- generated clades appeared to be startling simulacra of those com- piled from the fossil record, both real and simulated clades exhibiting random variation around a mean shape (Gould et al. 1977). The question, both then and today, was what to make of these results. Did they suggest we change our views on the fossil record, or rather the methods by which we draw inferences from it? Did they warrant a fundamental rethink- ing of our picture of evolution or simply offer a novel perspective on it? Did they call into question what paleontologists thought they knew about adap- tive radiations, mass extinctions, adaptive trends, and evolutionary rates, or did they simply suggest that more stringent evidentiary standards were called for? Was the capacity of the MBL model to simulate evolutionary patterns in the absence of selection evidence for non- Darwinian evolution? Or were the results simply blown out of proportion? There is evidence of mixed agendas within the group itself, and reactions of the paleontological community were also varied. Modifi ed versions of the MBL model have far outlived the MBL collaboration—the model has truly taken on a life of its own (Sepkoski 1978; Wassersug et al. 1979; Simberloff et al. 1981; Simberloff 1987; Sepkoski and Kendrick 1993; Uhen 1996; Foote et al. 1999a, b; Hubbell 2001). In my dis- cussion here, I shall mainly focus on the arguments concerning clade shape in the 1973 and 1977 papers. I am particularly concerned with the visual argumentation on which these two papers rely in connection with scaling problems pointed out by Steven Stanley (1979) and coworkers (Stanley et al.