428 Nature Vol. 293 8 October 1981 it is possible greatly to alter the degree of concentrations of copper high enough to but its contents do not force us to change sexual isolation of D. pseudoobscura from kill populations not exposed to this selec our views on the genetic mechanisms of the its close relative D. persimilis by artificial tive agent. The grasses on the mine flower origin of species. (] selection in less than 20 generationsl2 • In about a week earlier than do the surround the same way, selection of different parts ing non-tolerant populations and have an of an originally freely interbreeding increased ability for self-fertilization. Drosophila or house fly population for These genetic differences - which have I . Maynard Smith, J. The Evolution 0/ Sex (Cambridge University Pres:Ii, 1978). bristle number, climbing ability or the evolved in the one hundred years since the 2. Cronin, J.E., Boaz, N.T .. Stringer, C,(j , and Rak, Y. ability to tolerate insecticides can lead to mines were opened - are enough to lead to Nature292. 113 (1981). 3. Goldschmidt, R.B. The Material Basis Of Evolution (Yale the incidental evolution of considerable considerable reproductive isolation Uni .... ersity Press. 1940). reproductive isolation within a few tens of between mine populations and their 4. Eldredge, N. and Gould, S.J. Models in Paleobiology (ed. I3 15 Schopf, T.J.M.) (Freeman, San FranCisco, 1972). generations - • Even Drosophila 17 • ancestors in the nearby pastures 5. Berry, A.J. and Kadri, A.B.H. J. Zool., Lond 172, 369 populations kept at different temperatures Once evolutionary change in the fossil (1974). for five years in the laboratory evolve record - even in a record as well charac 6. Mackie, G.1.. and fluggins, D.G. J. Fish. Res. lJd. Can. 33, 1652 (1976). considerable reproductive isolation among terized as that unearthed by Williamson - 7. Russell-Hunter, W.O. In The Pulmonates (ed. FreHcr. V.& themselves l6 • These are periods trivial in is placed in the context of the known ability Peake, J.F.) (Academic, New York , 1978). 8. Claylon, G.A., Morris, J.A. and Robertson, A. 1. Genet. relation to those interpreted by Williamson of living organisms to respond to the forces 55 , 131 (1957). and others as 'punctuations' in the fossil of classical darwinian natural selection, it 9. Falconer, D.S. Cenet. Res. Comb. 22,29 (1973). 10. Woodworlh, C.M., I.eng, E.K . and Jugenheimer, K.W. record which can only be explained by new becomes clear that it is not necessary to Agron. J. 44, 60 (1952). evolutionary mechanisms. invoke evolutionary forces 'qualitatively II. Knight, G. R. , Robertson, A. and Waddinglon, e.H. The efficacy of gradual evolutionary different' from those emphasized by Evolution 10, 14 (1956). 12. Koopman, R.F. Evolution 4, 135 (1950). change in producing genetic subdivision Darwin. Depending on the time scale to 13 . Thoday, J.I\1. Proc. Roy. Soc. B182, 109(1972). and the reduction of gene flow in natural which the investigator is accustomed, one 14. Hurd, I..E. and Eisenberg, 1{'~1. Am. Nat. 109, 353 (1975) . populations subject to the forces of man's punctuated equilibrium may be 15. King, J.e. Cold Spring Harb. Symp. quont. Bioi. 20,311 gradual selection is particularly well seen in another's evolutionary gradualism. (1955). 16. Kilias. G .• Alahiotis. S .N . and PeJcranos, ~1. Evolut;on34. plants. Some annual grasses have evolved Williamson describes an extraordinarily 730 (1980). an ability to grow on mines polluted by complete page in the history of evolution 17. McNeilly, T. and Anlonovics, J. Heredity2J, 205 (1968).
Diversity changes of marine organisms through the Phanerozoic from A. Hallam ONE of the most important palaeon from the Cambrian to the present. To periods by tabulating new species reported tological debates of the past decade obtain reliable estimates of the extent of in the Zoological Record, and established a appears to have been resolved in an article change through time it is clearly desirable strong correlation between apparent published in this week's Nature (see page to be able to eliminate possible species numbers and the present areal 435). The issue at stake is whether the complications due to preservational bias distribution of rocks per system. This diversity of life has increased substantially and the problems inherent in making suggested that perhaps geological systems in the five hundred million years since the taxonomic distinctions among fossils. with more available rock have more species Cambrian period or whether it is only Originally, Valentine made the claim and hence more species are described, imperfections in the fossil record that give that marine invertebrate generic and family [n the analysis published this week, five an appearance of increasing diversity while diversity increased substantially from the independent measures of diversity, for the true pattern is of equilibrium or even early Palaeozoic to the Cenozoic. This was different groups of organisms and decrease in diversity. challenged by Raup soon afterwards different taxonomic levels, are shown to be The two chief protagonists in the debate, (Science 1977; 1065, 1972). He argued that highly inter-correlated, even when the David Raup and James Valentine, are now the large Mesozoic-Cenozoic increase may ubiquitous correlation with time is in agreement and have joined forces with be an artefact. The Cenozoic record removed. A simple underlying pattern is two other American palaeontologists, includes extant taxa, which biases it revealed of a rise from a low valUe in the John Sepkoski and Richard Bambach, who towards an overestimate, and Cambrian through the Palaeozoic, have made notable contributions to the preservational factors may have produced followed by a low Triassic value, after subject. Together they demonstrate that a major bias in the reverse direction further which there is a more or less accelerating there is indeed, in their choice of phrase, a back in time_ He generated a computer increase through the Mesozoic and genuine "evolutionary signal" discernable simulation model based on an increased Cenozoic, the last era containing the in the fossil record of marine organisms probability of destruction of fossils with highest diversity, The sharp drop at the end despite possible preservational and time, and showed that Valentine's data of the Palaeozoic is clearly the result of the taxonomic noise. The agreement is a firm could be accounted for by postulating Permian mass extinction. In sharp contrast vote of confidence in the reliability of the diversity rise from the Cambrian to a mid the end Cretaceous mass extinction has no fossil record even though the conclusion Palaeozoic peak followed by a drop to an effect on the general trend. Above the drawn from it is a sort of compromise - appreciably more modest level which family level the pattern of diversity change diversity has increased since the Cambrian remained stable through the Mesozoic and is different, due probably to the tendency but the pattern is not one of a simple rise. Cenozoic. for morphologically distinct groups to It is the existence of such massive data In a later paper (Paleobiology 2; 289, appear rapidly during the early phases of compilations as the American Treatise on 1976), Raup estimated the number of major radiations_ Invertebrate Paleontology and the species described for each of the geological Palaeontologists will no doubt continue Geological Society of London's The to argue about the meaning of diversity Fossil Record that have allowed change data, but at least they are now palaeontologists to analyse the pattern of A. Hallam is Lapworth Professor ofGeology in apparently assured that the data reflect the diversity change of preservable organisms the University of Birmingham, real world. 1'_.
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