Histo-icalBiology, 1996, Vol 12,pp I 1I-157 © 1996 OPA (Overseas Publishers Association) Reprints available directly from the publisher Amsterdam B V Published in The Netherlands Photocopying available by license only By Harwood Academic Publishers GmbH Printed in Malaysia TESTING THE QUALITY OF THE FOSSIL RECORD BY GROUPS AND BY MAJOR HABITATS MICHAEL J BENTON and REBECCA HITCHIN Department of Geology, University of Bristol, Bristol, B 58 IRJ, United Kingdom (Received February 9 1996; in final form March 25, 1996) The evolution of life is a form of history and, as Karl Popper pointed out, that makes much of palaeontology and evolutionary biology metaphysical and not scientific, since direct testing is not possible: history cannot be re-run However, it is possible to cross-compare three sources of data on phylogeny stratigraphic, cladistic, and molecular Three metrics for comparing cladograms with stratigraphic information allow cross-testing of () the order of branching with the stratigraphic order of fossils, and of (2) the relative amount of cladistically-implied gap in proportion to known fossil record. Results of the metrics, based upon a data set of 376 cladograms, show that there are statistically significant differences in the results for echinoderms, fishes, and tetrapods Matching of rank- order data on stratigraphic age of first appearances and branching points in cladograms, using Spearman Rank Correlation (SRC), is poorer than reported before, with only 148 of the 376 cladograms tested (39 %) showing statistically significant matching Tests of the relative amount of cladistically-implied gap, using the Relative Completeness Index (RCI), indicated excellent results, with 288 of the cladograms tested (77 %) having records more than 50% complete. Assessment of the Stratigraphic Consistency Index (SCI), the relative number of nodes i a cladogram having younger taxa located above them than immediately below them showed that 79 % of cladograms tested scored over 0 500 This indicates that node order was generally consistent with stratigraphic order of first finds, contrary to the findings of the rather cruder SRC metric. Group-by-group results were variable SRC results, based on significances of age-clade rank order equivalence, were best for tetrapods, moderate for echinoderms, and worst for fishes Best RCI values were for fishes, and poorer for tetrapods and echinoderms SCI values were best for echinoderms, poorer for tetrapods, and poorest for fishes Arbitrary scoring of the three metrics suggests provisionally that the echinoderms and tetrapods have a better fossil record than fishes. Comparison of marine groups (echinoderms + fishes) versus continental groups (tetrapods) showed that many more continental cladograms (50 %) showed significant SRC matching of age and clade order than marine cladograms (25 %) Continental cladograms also yielded higher SCI values, with 87 % achieving values equal to, or higher than, 0 5, while for marine cladograms, the value was 72 % Marine cladograms, on the other hand, scored better for relative completeness, with 79% showing RCI values greater than 50 %, compared to 74 % for continental cladograms The aggregate results suggest that continental cladograms are better than marine cladograms by two metrics (SRC, SCI) to one (RCI). KEY WORDS: Cladistics, phylogeny, stratigraphy, evolution, Popper, fossil record. INTRODUCTION Fossil data offer unique information on evolution, giving the only evidence for the existence of groups that are now extinct, and providing precise data on the order of events in phylogeny Two serious problems with the fossil record have been identified, however, and these must be tackled before palaeontological data can be used III 112 M J BENTON AND R HITCHIN seamlessly in combination with information on living organisms These problems are that ( 1) studies of the history of life may be viewed as non-scientific, as metaphysical (Popper, 1960, 1972, 1976), and that (2) the fossil record is clearly incomplete, but that incompleteness cannot be quantified against an absolute knowledge of the true pattern of the history of life. A potential resolution of these two issues is presented here It is proposed that large parts of Darwinian evolution can be cast in a way that is falsifiable, and phylogeny reconstruction, the methods used to discover the true shape of the history of life, offer means of mutual cross-falsification, a solution to the dilemma that Popper highlighted. The mutual cross-testing approach also offers the opportunity of finding measures of the completeness of the fossil record, in part and in whole. POPPER, EVOLUTION, AND METAPHYSICS Popper (1960, 1972, 1976) charged that the historical sciences, such as the study of the history of life, are metaphysics and not science This is a serious challenge, and potentially damaging to the acceptance of conclusions from large parts of evolutionary biology and palaeontology (Platnick and Gaffney, 1977, 1978 a, 1978 b; Halstead, 1980). Popper based his conclusion on his discovery (Popper, 1959, 1965) of a clear operational demarcation between science and metaphysics: in his terms, scientific theories are capable of falsification, while metaphysical propositions are not. Popper did not use the word 'metaphysics' in a pejorative sense, although many before him and after him have done so, sometimes equating metaphysics with mumbo-jumbo or pseudo-science Indeed, Popper argued very clearly that a theory in metaphysics is not testable, but that does not mean that it is wrong Indeed, in the history of many subjects, what was once metaphysics may become science when the tools or insights become available to design adequate tests that offer the possibility of falsification. Popper considered the nature of Darwinian evolution in various of his writings, but came to the firm conclusion, through a series of his works in which he considered the correct method for studying and analysing history, that evolution is more metaphysics than science Popper ( 1976, p 172) compared the natural theological explanation that nature is perfect because God created it that way, with Darwinian evolution: "Now to the degree that Darwinism creates the same impression, it is not much better than the theistic view of adaptation; it is therefore important to show that Darwinism is not a scientific theory, but metaphysical But its value for science as a metaphysical research programme is very great, especially if it is admitted that it may be criticized, and improved upon " Popper argued that Darwinism was metaphysics, and not science, for two reasons, ( 1) the theory is not testable, and indeed is almost tautological, and (2) the evolution of life happened once, and it is impossible to generalise law-like statements from a single set of events Darwinian evolution is untestable, Popper ( 1976) argued, because it cannot make predictions about what might happen on another planet where life originates The key tenets of Darwinism, natural selection, diversification, and adaptation in a sense simply describe what has happened, and is happening, but these phenomena cannot be framed in a precise way that permits of their falsification. Popper repeats the favourite argument of Creationists that adaptation is virtually a tautologous proposition: adaptation and fitness are defined as survival value, and they can be measured by actual success in survival, but such a definition is virtually untestable The second criticism of evolution was that life has evolved once only, so far as we know, and cannot be tested Hence, the scientist, used to applying the TESTING THE QUALITY OF THE FOSSIL RECORD 113 experimental hypothesis-testing approach is powerless How can a scientific study of phylogeny be instituted if it is impossible to re-run evolutionary history many times, altering variables on each run so that the fundamental principles may be determined? Put another way, and still applying the Popperian method of science, how can evolution be falsified? What single test could be imagined? lThis critique, in simple form, is beloved of creationists, who use it to deny that evolution is scientific l The usual riposte, convincing, but not overwhelming, is that evolution is a complex amalgam of numerous hypotheses, any one of which may be tested, and many of which have been tested repeatedly by evolutionary ecologists, geneticists, palaeobiologists, and systematists. It is important first to divide evolution into two main propositions: ( 1) the assertion that life has evolved (literally 'unrolled') during the course of geological time; and, (2) that evolution has occurred by natural selection As Platnick and Gaffney (1978 a), Patterson (1978), and others have argued, the first proposition probably offers most difficulty Natural selection can be subjected to test in a variety of ways, some of them in the form of short-term laboratory experiments, others by longer-term natural experiments, but all of these are, and have been, repeatable time and time again, with different organisms, and in different settings There is no need to get sucked into the Creationists' tautology'argument, as Popper did (see Ridley, 1993, chapter 3 for an excellent overview). The first proposition, not original to Darwin, simply that life has evolved, from simple to complex organisms, from low to high diversity, and over a long period of time, is at first sight harder to tackle, for the reasons given by Popper Patterson (1978, pp 145-147) accepted Popper's description of evolution as a pattern