- Email: [email protected]
Extinction: Bad genes or bad luck?
rapid increase in temperature, for instance. Bacteria respond to such changes by altering metabolic pathways, whereas larger animals make behavioral adjustments. More specifically, a certain bacterial species may respond by producing heat-shock proteins (at the expense of other metabolites), while a certain elephant species may respond by spending more time resting in the shade (at the expense of foraging). But at such a level of detail, a given response does not necessarily apply even to closely related taxa, much less from microorganisms to macroorganisms. Another bacterial species may enter a temporary ‘shut down’ phase; another elephant species may spend more time wallowing in ponds. The most intriguing patterns discussed in the book relate not so much to size as to unitary versus modular growth forms. Insects and birds are typical examples of the former: mobile creatures with determinate growth, having a germ line sequestered from the soma so that the organism from zygote to zygote represents the unit of genetic variation. Such organisms exhibit senescence and therefore a limited life span. Modular organisms, on the other hand, include bacteria, fungi, plants and colonial animals such as corals and bryozoans. They are constructed by repeated production of a basic unit: the bacterial cell, the fungal hypha, the plant leaf or the coral polyp. They need not exhibit senescence; they potentially grow and live indefinitely. In such organisms a single genetic individual may comprise a large number of physiological individuals. However, somatic mutations and other analogous genetic changes mean that such a genetic individual may in fact be a mosaic. While unitary organisms tend to move around in search of resources, modular ones depend on their tremendously plastic growth form to get at resources. While there are now a number of important treatments of implications of modular growth form for plants and animals2,3, microorganisms have been essentially left out of these considerations. An important contribution of this book is to point out that bacteria with their microcolonies and fungi with their hyphae should also be thought of as modular organisms. It also points out that most ecological investigations are based on the comparative or correlation the only apapproach - often proach that is feasible with macroorganisms. Andrews suggests that many interesting ideas generated through this approach could be 244
profitably employed to devise experiments with small organisms, either modular organisms like bacteria, or perhaps unitary ones like nematodes. While attractive, this suggestion is not backed by any real-life examples of how the potential of microorganisms can actually be tapped in this fashion. Andrews has obviously been inspired by the masterly syntheses of Bonneti, looking at life cycles and complexity of organisms across a wide size range. He attempts in this book an equally ambitious synthesis in terms of a whole range of tactics of resource acquisition, allocation and reproduction. Unlike Bonner, he fails to develop any coherent argument; instead he produces a patchwork that fails to grip the imagination. Nevertheless, the book represents a
TREE vol. 7, no. 7, July
1992
useful compendium that should considerable pedagogic value.
be of
Madhav Gadgil Centrefor Ecological Sciences, Indian Institute of Science, Bangalore 560012, India References 1 Southwood, T.R.E. (1988) Oikos 52, 3-18 2 Jackson, J.B.C., Buss, L.W. and Cook, R.E., eds (1985) Population Biology and Evolution of Clonal Organisms, Yale University Press 3 Harper, J.L., Rosen, B.R. and White, J., eds (1986) The Growth and Form of Modular Organisms, The Royal Society 4 Bonner, J.T. (1965) Size and Cycle: An Essay on the Structure of Biology,
Princeton University Press 5 Bonner, J.T. (1988) The Evolution of Complexity by Means of Natural University Press Selection, Princeton
Extinction Extinction: Bad Genesor Bad Luck? by David M. Raup, W. W. Norton&Co., 1991. f13.95 hbk (xvii + 210 pages) ISBN 0 393 03008 3 Extinction is the inevitable flip side to evolution, and yet the study of extinction has lagged far behind its counterpart. However, over the last decade or so the study of past extinctions has finally received some impetus, mainly from the geological and palaeontological fraternity who have recognized a series of mass extinction events stretching back through the fossil record. Mass extinctions are relatively brief intervals (meaning, to a geologist, less than a few million years) when extinction rates are greatly elevated above background values. The greatest of these occurred at the end of the Permian when as many as 96% of the Earth’s species disappeared. Slightly less devastating, but more famous, is the end-Cretaceous event that removed the dinosaurs along with 75% of all species. In fact, palaeontologists cannot really resolve species extinctions due to the poor fossil record of most species, although the data are better for genera and better again for families. It was one of Raup’s early contributions to extinction study that allows us to extrapolate species extinctions from genus and family data using rarefaction curves’. A good deal of the study of mass extinctions has focused upon the end-Cretaceous event due to the compelling evidence of an associated meteorite impact at this time. Raup
enters this field with a truly radical unification of all extinction studies on both the small and large scale. Extinction: Bad Genes or Bad Luck? begins with a brief exposition of extinction as seen in the fossil record and then goes on to document what is apparently one of the few studied (or understood?) extinctions in historical time, that of the heath hen of Tympanuchus (a subspecies cupido). From this example it appears that species must initially suffer a ‘first strike’ to reduce their populations down to below the minimum viable population, before extinction inevitably follows due to some random event such as disease. Up to this point Raup’s book is a clearly constructed and valuable contribution to the sparse literature on extinction, particularly in the elegant use of statistical data for which Raup is justly renowned. However, the main contention of the book is that the first strike for nearly all species is caused by meteorite impact (the answer to the question in the title therefore becomes ‘bad luck’). Raup has carefully constructed a houseofcardsinordertocometothis astonishing conclusion. (I) Mass extinction events are not distinct from background extinctions - they just represent the extreme ‘tail’ in a continuum of extinction rates. (2) As one of the mass extinctions in the continuum is known to be associated with a meteorite impact - the endCretaceous event -then why not all of them? (3) There is a positive correlation between extinction rates and the diameters of the few known terrestrial impact craters.
TREE
vol.
7, no.
7, July
1992
However, Raup himself contradicts the first claim, as the end-Permian event is simply too great to be considered as part of this continuum. Also, several studies have shown that the selectivity of extinctions during mass extinction intervals appears to be different from that operating during background times. Regarding claim (21, the evidence for meteorite impact at the end of the Cretaceous is good, although it may have little to do with the actual extinctions. There is simply no evidence at all
.. .
by Niles Eldredge,
(246 paged
Prentice Hall, 7997. ISBN 0 73 583659 X
On Methuselah’s Trail: Living Fossilsand the Great Extinctions by
rent extinction theory, for its elegant use of statistics, and for its carefully constructed arguments for an idea that all mass extinctions are caused by meteorite impact - that is surely wrong.
something general about its effect1z2. But paleontology has been much more successful in the latter than the former. Many workers now would agree that mass extinction is ‘more frequent, more rapid, more extensive in impact and more qualitatively different in effect’ than previously believed3, and that, at least in some cases, it alters the long-term patterns of evolutionary history. Even if it does not, as Gould has suggested, operate as an autonomous ‘third tier’ of process3, by the very magnitude and rapidity that define it, it acts as an incredibly strong dose of historical contingency, which alters the shape and subsequent biosphere. The world after mass extinction isn’t just the world minus what disappeared; it is often a qualitatively different place4. So to the barricades we go, armed withtheinformationthatthesituation is even worse than we thought. If we causethe equivalent of a mass extinction in the next few centuries, we will not only bequeath to our descendants an impoverished world, we will havealteredfundamentallytheshape of things to come. What a satisfying response to the anthropocentric rantings of those who say that both humans and extinction are parts of nature and the outcome can therefore be nothing but natura15. But is this all we can do? With more than a score of ‘mass extinctions’ to choose from, can we not say anything socially - or at least neontologically - useful about the causes of extinction? Niles Eldredge has written a popular book of the sort that many paleontologists would probably like to have written. It is a book that other paleontologists should read to become inspired, and that neontologists should read to get some feeling for what kinds of questions they can reasonably expect paleontologists to answer for them. The book is actually something of
a hybrid. It is half a personal response to the impending crisis, and half an outgrowth of Eldredge’s technical work over the last decade. Readers familiarwith his previous books6,7wiII see in this one a logical extension of Eldredge’s ideas about what factors govern the origin of species. And he is quite unambiguous about the result: ‘I set out to write this book with a simple, the goal .. . of finding overarching theory of extinction’ he writes in the Prologue. And he concludes, ‘I am convinced I have found it. ... It sees ecosystem collapse and the extinction of species as the outcome, generally, of earthbound causes. These causes are simply the flip side of the same factors that build and maintain ecosystems and lead to the evolution of species in the first place.’ Eldredge’s basic conclusion is that ‘change of size and location of habitat underlies most extinction events, ranging from isolated species disappearances through the most massive of extinctions’. And he concludes that ‘Global climate change, especially global cooling, seems to have been the prime cause of habitat disruption.’ One can quibble about the details, for example Eldredge’s statement of an emerging ‘consensus’ about the effects of global cooling at mass extinction, without disagreeing with the importance of ecosystem collapse. Yet despite the power of Eldredge’s conclusion, a reader might still be left asking: so that’s all there is? All that paleontology has to contribute to solving this crisis is to note that ecosystem collapse is bad, pass this information along toconservationists and go back to studying fossils? The argument forthe importance of preserving whole ecosystems (and, by inference, the role of catastrophic ecosystem collapse and random killing during mass extinction) is surely strengthened if what Eldredge says is
Paul B. Wignall Dept of Earth Sciences, University
of Leeds, Leeds, UK LS2 9JT
Reference 1 Raup, D.M. (1979) Science 206, 217-218
and more Extinction
The Miner’s Canary: Unraveling the Mysteries of Extinction
$79.50
for meteorite impacts at times of other major mass extinction events. Finally, the biggest impact craters did not form at moments of mass extinction. For example the 70 km diameter Manicouagan Crater near Quebec used to be implicated in the Triassic/Jurassic boundary mass extinction but the crater has now been more accurately dated as Late Triassic - over 20 million years older. Despite its astounding main conelusion, this is a book worth reading for its succinct summary of cur-
Peter
Freeman & pages) ISBN
Douglas
Ward,
Co., 7992. $78.95 0 7767 2203 8
W.H. (212
Some paleontologists are in a quandary of late. They have been successful (apparently) in documenting episodes of substantial extinction, and they have suggested and tested hypotheses of cause for at least some of these episodes. At the same time, neontologists have been growing increasingly concerned over extinction today, to the point where it is now commonplace to hear comparisons between what happened to the dinosaurs and what is happening now in tropical rain forests. The quandary lies here: in the midst of what all informed sources see as nothing short of a crisis in the history of life, what should be the response of a profession that claims to know more than any other about that history? Should we hope for some of the scraps of ‘biodiversity’ funding to trickle down to research on ancient biodiversity, whether or not it is relevant to the Recent? Should we abandon fossils and devote ourselves to making inventories of the Recent representatives of our respective groups? (Paleontologists, it turns out, are often the only systematists working on many living groups.) In the mid-1980s a few paleontologists began to see that perhaps they did have something to offer to neontologists. If extinction is so common in Earth history, maybe paleontology can indicate either (I) something general about its cause, or (2)
245
profitably employed to devise experiments with small organisms, either modular organisms like bacteria, or perhaps unitary ones like nematodes. While attractive, this suggestion is not backed by any real-life examples of how the potential of microorganisms can actually be tapped in this fashion. Andrews has obviously been inspired by the masterly syntheses of Bonneti, looking at life cycles and complexity of organisms across a wide size range. He attempts in this book an equally ambitious synthesis in terms of a whole range of tactics of resource acquisition, allocation and reproduction. Unlike Bonner, he fails to develop any coherent argument; instead he produces a patchwork that fails to grip the imagination. Nevertheless, the book represents a
TREE vol. 7, no. 7, July
1992
useful compendium that should considerable pedagogic value.
be of
Madhav Gadgil Centrefor Ecological Sciences, Indian Institute of Science, Bangalore 560012, India References 1 Southwood, T.R.E. (1988) Oikos 52, 3-18 2 Jackson, J.B.C., Buss, L.W. and Cook, R.E., eds (1985) Population Biology and Evolution of Clonal Organisms, Yale University Press 3 Harper, J.L., Rosen, B.R. and White, J., eds (1986) The Growth and Form of Modular Organisms, The Royal Society 4 Bonner, J.T. (1965) Size and Cycle: An Essay on the Structure of Biology,
Princeton University Press 5 Bonner, J.T. (1988) The Evolution of Complexity by Means of Natural University Press Selection, Princeton
Extinction Extinction: Bad Genesor Bad Luck? by David M. Raup, W. W. Norton&Co., 1991. f13.95 hbk (xvii + 210 pages) ISBN 0 393 03008 3 Extinction is the inevitable flip side to evolution, and yet the study of extinction has lagged far behind its counterpart. However, over the last decade or so the study of past extinctions has finally received some impetus, mainly from the geological and palaeontological fraternity who have recognized a series of mass extinction events stretching back through the fossil record. Mass extinctions are relatively brief intervals (meaning, to a geologist, less than a few million years) when extinction rates are greatly elevated above background values. The greatest of these occurred at the end of the Permian when as many as 96% of the Earth’s species disappeared. Slightly less devastating, but more famous, is the end-Cretaceous event that removed the dinosaurs along with 75% of all species. In fact, palaeontologists cannot really resolve species extinctions due to the poor fossil record of most species, although the data are better for genera and better again for families. It was one of Raup’s early contributions to extinction study that allows us to extrapolate species extinctions from genus and family data using rarefaction curves’. A good deal of the study of mass extinctions has focused upon the end-Cretaceous event due to the compelling evidence of an associated meteorite impact at this time. Raup
enters this field with a truly radical unification of all extinction studies on both the small and large scale. Extinction: Bad Genes or Bad Luck? begins with a brief exposition of extinction as seen in the fossil record and then goes on to document what is apparently one of the few studied (or understood?) extinctions in historical time, that of the heath hen of Tympanuchus (a subspecies cupido). From this example it appears that species must initially suffer a ‘first strike’ to reduce their populations down to below the minimum viable population, before extinction inevitably follows due to some random event such as disease. Up to this point Raup’s book is a clearly constructed and valuable contribution to the sparse literature on extinction, particularly in the elegant use of statistical data for which Raup is justly renowned. However, the main contention of the book is that the first strike for nearly all species is caused by meteorite impact (the answer to the question in the title therefore becomes ‘bad luck’). Raup has carefully constructed a houseofcardsinordertocometothis astonishing conclusion. (I) Mass extinction events are not distinct from background extinctions - they just represent the extreme ‘tail’ in a continuum of extinction rates. (2) As one of the mass extinctions in the continuum is known to be associated with a meteorite impact - the endCretaceous event -then why not all of them? (3) There is a positive correlation between extinction rates and the diameters of the few known terrestrial impact craters.
TREE
vol.
7, no.
7, July
1992
However, Raup himself contradicts the first claim, as the end-Permian event is simply too great to be considered as part of this continuum. Also, several studies have shown that the selectivity of extinctions during mass extinction intervals appears to be different from that operating during background times. Regarding claim (21, the evidence for meteorite impact at the end of the Cretaceous is good, although it may have little to do with the actual extinctions. There is simply no evidence at all
.. .
by Niles Eldredge,
(246 paged
Prentice Hall, 7997. ISBN 0 73 583659 X
On Methuselah’s Trail: Living Fossilsand the Great Extinctions by
rent extinction theory, for its elegant use of statistics, and for its carefully constructed arguments for an idea that all mass extinctions are caused by meteorite impact - that is surely wrong.
something general about its effect1z2. But paleontology has been much more successful in the latter than the former. Many workers now would agree that mass extinction is ‘more frequent, more rapid, more extensive in impact and more qualitatively different in effect’ than previously believed3, and that, at least in some cases, it alters the long-term patterns of evolutionary history. Even if it does not, as Gould has suggested, operate as an autonomous ‘third tier’ of process3, by the very magnitude and rapidity that define it, it acts as an incredibly strong dose of historical contingency, which alters the shape and subsequent biosphere. The world after mass extinction isn’t just the world minus what disappeared; it is often a qualitatively different place4. So to the barricades we go, armed withtheinformationthatthesituation is even worse than we thought. If we causethe equivalent of a mass extinction in the next few centuries, we will not only bequeath to our descendants an impoverished world, we will havealteredfundamentallytheshape of things to come. What a satisfying response to the anthropocentric rantings of those who say that both humans and extinction are parts of nature and the outcome can therefore be nothing but natura15. But is this all we can do? With more than a score of ‘mass extinctions’ to choose from, can we not say anything socially - or at least neontologically - useful about the causes of extinction? Niles Eldredge has written a popular book of the sort that many paleontologists would probably like to have written. It is a book that other paleontologists should read to become inspired, and that neontologists should read to get some feeling for what kinds of questions they can reasonably expect paleontologists to answer for them. The book is actually something of
a hybrid. It is half a personal response to the impending crisis, and half an outgrowth of Eldredge’s technical work over the last decade. Readers familiarwith his previous books6,7wiII see in this one a logical extension of Eldredge’s ideas about what factors govern the origin of species. And he is quite unambiguous about the result: ‘I set out to write this book with a simple, the goal .. . of finding overarching theory of extinction’ he writes in the Prologue. And he concludes, ‘I am convinced I have found it. ... It sees ecosystem collapse and the extinction of species as the outcome, generally, of earthbound causes. These causes are simply the flip side of the same factors that build and maintain ecosystems and lead to the evolution of species in the first place.’ Eldredge’s basic conclusion is that ‘change of size and location of habitat underlies most extinction events, ranging from isolated species disappearances through the most massive of extinctions’. And he concludes that ‘Global climate change, especially global cooling, seems to have been the prime cause of habitat disruption.’ One can quibble about the details, for example Eldredge’s statement of an emerging ‘consensus’ about the effects of global cooling at mass extinction, without disagreeing with the importance of ecosystem collapse. Yet despite the power of Eldredge’s conclusion, a reader might still be left asking: so that’s all there is? All that paleontology has to contribute to solving this crisis is to note that ecosystem collapse is bad, pass this information along toconservationists and go back to studying fossils? The argument forthe importance of preserving whole ecosystems (and, by inference, the role of catastrophic ecosystem collapse and random killing during mass extinction) is surely strengthened if what Eldredge says is
Paul B. Wignall Dept of Earth Sciences, University
of Leeds, Leeds, UK LS2 9JT
Reference 1 Raup, D.M. (1979) Science 206, 217-218
and more Extinction
The Miner’s Canary: Unraveling the Mysteries of Extinction
$79.50
for meteorite impacts at times of other major mass extinction events. Finally, the biggest impact craters did not form at moments of mass extinction. For example the 70 km diameter Manicouagan Crater near Quebec used to be implicated in the Triassic/Jurassic boundary mass extinction but the crater has now been more accurately dated as Late Triassic - over 20 million years older. Despite its astounding main conelusion, this is a book worth reading for its succinct summary of cur-
Peter
Freeman & pages) ISBN
Douglas
Ward,
Co., 7992. $78.95 0 7767 2203 8
W.H. (212
Some paleontologists are in a quandary of late. They have been successful (apparently) in documenting episodes of substantial extinction, and they have suggested and tested hypotheses of cause for at least some of these episodes. At the same time, neontologists have been growing increasingly concerned over extinction today, to the point where it is now commonplace to hear comparisons between what happened to the dinosaurs and what is happening now in tropical rain forests. The quandary lies here: in the midst of what all informed sources see as nothing short of a crisis in the history of life, what should be the response of a profession that claims to know more than any other about that history? Should we hope for some of the scraps of ‘biodiversity’ funding to trickle down to research on ancient biodiversity, whether or not it is relevant to the Recent? Should we abandon fossils and devote ourselves to making inventories of the Recent representatives of our respective groups? (Paleontologists, it turns out, are often the only systematists working on many living groups.) In the mid-1980s a few paleontologists began to see that perhaps they did have something to offer to neontologists. If extinction is so common in Earth history, maybe paleontology can indicate either (I) something general about its cause, or (2)
245