TREE vol. 3, no. 6, June 1988

Ciilpin) and even catastrophes such as disease epidemics or rare but devastating environmental events (Ewens, Brockwell, Gani and Pesnick) have been considered.

Viable population theory has pro- gressed rapidly in the last decade. As more factors have been taken into consideration, the models have be- c3me more realistic (and more com- plex) and the estimated MVPs have become larger. Several types of analysis now suggest that MVPs are lilrger than previously believed, of the order of several thousand indi- viduals for higher vertebrates. Can this be true? Unfortunately, there is slrne evidence, such as Newmark’s recent analysis2 of mammal extinc- tions in the National Parks of the Lnited States, and Belovsky’s chap- tc:r in this volume, that these esti- mates are of the right order of mag- t-itude. Fortunately, most current models assume that only a single population exists and that it is not af:tively managed. It is likely that substantial reductions in estimated MVPs can be achieved if multiple populations of a species exist and hAmans actively manage these populations, for example, by moving ir-dividuals between the populations when this appears desirable.

Extinction is a matter of probabili- tis3.s. The probability of extinction in- ct eases as population size decreases and as the period under consider-

- Problems of Sex

The Evolution of Sex: An Examination of Current Ideas

edited by R.E. Michod and Bruce R. Lcvin, Sinauer Associates Inc., 1988. $;?9.95 (pbk) $55 lhbk) (ix + 342 pages) ISBN 0 87893 459 6 (pbk) 0 87893 458 8 (hbk)

The Evolution of Sex and its Consequences: Experientia Supplementum Vol. 55

edited by S.C. Stearns, BirkhBuser Vdag AG, 1987. SFr 80. (403 pages) ISBN 3 7643 1807 4

A~1 unparalleled decade of activity since the publication of George V\Jilliams’l and John Maynard Smith’s2 books on the evolution of SEX has marked an open season on the problems of the evolutionary origins and maintenance of sex. For those not directly involved in the research it has been all but impos- sible to keep up with events. Fortu- nately, two sets of editors believe that it is time to take stock of the

ation lengthens. Given enough time, most species eventually become ex- tinct. Because of the probabilistic nature of extinction, we must, as Schaffer points out in his chapter, define more precisely what we mean by preservation. Should we, for ex- ample, consider that a population with a 95% probability of surviving for the next 100 years is in danger or adequately protected? Captive- population managers in the United States have faced the time-frame problem by adopting the ‘Front Royal Rule’, that captive populations should be managed so as to pre- serve 90% of their existing genetic variation for 200 years3. No consen- sus has been reached on comparable guidelines for wild populations.

This volume does not provide step-by-step instructions for those wanting to perform a PVA for a par- ticular species or population. It does summarize the current body of theory available for such work. This theory needs further testing. In his chapter, Belovsky, using data on the distribution of boreal mammals of various body sizes on mountaintops in western North America, makes a first attempt to test the demographic extinction model presented by Goodman earlier in the volume. He concludes that this model ‘appears to capture the important parameters of the extinction process’ but rightly urges others to carry out additional

testing with other data sets. In contrast to the progress on the

theoretical side, there have been only a few applications of the theory to real life+s. As it is now clear that there is no ‘magic number’, that is, a single MVP size applicable to most species and populations, there is an urgent need for such work. In his concluding overview, Soul6 predicts that, as case studies accumulate, generalizable protocols for the estimation of MVPs will be de- veloped within a decade. We hope so. Time is short.

Katherine Rails and Jonathan Ballou

National Zoological Park, Smithsonian Institution, Washington, DC 20008, USA.

References 1 The Global 2000 Report to the President: Entering the Twenty-first Century. (Vol. 2). Technical Report (1980), pp. 328-331, Washington Government Printing Office 2 Newmark, W. (1987) Nature 325, 430-432 3 Soul&M., Gilpin, M., Conway, W. and Foose, T. (1986) Zoo Biol. 5, 101-l 14 4 Shaffer, M. (1983) Int. Conf. BearRes. Manage. 5,133-139 5 Harris, R. in Reproductive Biology of Black-footed Ferrets and Small Population Biology as they Relate to Conservation (Seal, U.S. and Bogan, M., eds), Yale University Press (in press)

situation and, given the quality and coherence of the resulting volumes, we should be grateful. Michod and Levin’s book is narrower in scope, dealing simply with the different theories which have been put for- ward, while Stearns’ seeks also to review the associated topics of sex allocation, sex determining mechan- isms and sexual selection. The audi- ences for the two works will, there- fore, be slightly different.

Michod and Levins’ strategy has been to ask the synthesizers (Wil- liams, Maynard Smith, Crow, Felsen- stein) to assess the current position, and the advocates of particular theories to summarize their views. Among notable contributions are those by Bernstein and co-authors (including Michod) on the DNA re- pair theory, Seger and Hamilton on the role of parasite-host coevolu- tion, and Hickey and Rose on plas- mid manipulation. A particular strength of Michod and Levin’s book is that the various authors criticize and respond to criticisms of each

others chapters. Many edited volumes attempt this, but none that we have seen do it so successfully. Among the most interesting ex- changes is that between Maynard Smith, who has reasons to doubt that the DNA repair theory is the whole story, and Bernstein et al. who believe that it is. Many of the authors seem to have been working on their chapters until the summer of 1987, which makes the book even more valuable.

We had two minor criticisms of Michod and Levin’s book. The first is that it is more about ideas than data. Much of the excitement associated with the more recent work on the maintenance of sex is that theories have become testable, and tests are being made. The best examples are provided by those attempting to correlate parasite load and life his- tory with the occurrence of sex and the amount of recombination. Perhaps it will take another decade before the data can be assessed so precisely as the theories. Our second

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criticism is that we do not agree with Trivers’ argument for why recom- bination rates might be lower in males than in females. He suggests that because sexual selection results in very few males contributing dis- proportionately to the next genera- tion, there is selection to maintain their co-adapted genotypes, hence less crossing over. But the gametes are one-half related to their father, whatever the crossover rate. And who is to say whether of a particular- ly successful combination of alleles from linked loci in a male are both maternally inherited, both paternally inherited, or are from different parents? In the latter case crossing over would be favoured. Trivers has a knack of getting things right, so perhaps we are wrong (though if we are, we made the same error inde- pendently), but we should like to see a formal population genetic model telling us why.

Stearns’ volume emanates from a series of papers published in Ex- perientia as long ago as 1985, so it is

not quite so up to date as Michod and Levin. Nevertheless, we found it interesting. The fifteen chapters are allocated to eight sections: an intro- duction to the problems, necessary background, the major hypotheses, experimental tests, the comparative evidence, sex allocation, sexual selection, and the consequences. New and interesting to us were Hoekstra’s theoretical enquiries into the evolution of bipolar sexuality - which is still, it seems, a mystery - and Herbert’s review of enzyme polymorphism studies in cyclical and obligate parthenogens; given the statements of several theoreticians that the problem of sex is likely to be allied to that of the maintenance of genetic variability (Bell, Bremmer- man, Seger and Hamilton), this is a most timely review.

Tellingly, the shortest chapter in the book is Bierzychudek’s fine sum- mary of the experiments designed to test the various theories of sex -the ‘queen of problems in evolutionary biology’ has yet to capture the im-

agination of field ecologists. This is unfortunate for all concerned. As Bell (in Michod and Levin) makes clear, the problem of sex is as much of interest for highlighting our ignor- ance about the environment as it is a problem of adaptation. If, as many evolutionists believe, sex is selected for its genetic consequences, then there is something going on in nat- ure, be it genotype by environment interactions, frequency dependent selection, fitness decays through time, host-parasite cycles, or what- ever, which is strong enough to pay the twofold cost of sex and which we do not know much about.

References 1 Williams, G.C. (1975) Sexand Evolution, Princeton University Press 2 Maynard Smith, J. (1978) TheEvolution ofSex, Cambridge University Press

Paul Harvey and Austin Burt

Zoology Department, Unlverslty of Oxford, South Parks Road, Oxford OX1 3PS. UK.

Angiosperm Paleobotany

The Origins of Angiospermsand their Biological Consequences

edited by E.M.Friis, W.G. Chaloner and P.R. Crane, Cambridge Univer- sity Press, 1987. f27.50/$#4.50 Ix + 358 pages) ISBN 0 52132357 6

What can paleontology contribute to evolutionary biology? Certainly, fos- sils document patterns of change in morphology and diversity through time. But the geological record con- strains evolutionary thinking in another way; it allows us to evaluate paleontological pattern in the con- text of a changing Earth.

During the last decade, a period when many biologists equated paleobiological debate about evol- ution with arguments over punctu- ated equilibria, a revolution was beginning in historical geology. The revolution is founded on innov- ative research in disciplines such as isotopic geochemistry, paleocean- ography, paleomagnetics, sediment- ology, and climatic and biogeo- chemical modelling; its result is an increasingly sophisticated under- standing of biogeochemical, clima- tic, and tectonic change in Earth his- tory.

One well-known revelation is that at least one episode of mass extinc- tion (at the end of the Cretaceous Period) coincided with a major ecol-

ogical disruption attributed to bolide impact or cataclysmic volcanic activ- ity. However, there is another observation that is fundamentally important to biologists. Surface en- vironments on our planet are in a continual state of change. Climate fluctuates on various time scales; fluxes and reservoirs of biogeo- chemical cycles vary; sea levels rise and fall; oceanic water masses form and decay; continents grow, split apart, and recombine. At the same time, organisms that prey upon, compete against, or live in symbiotic association with any given taxon of interest come and go. This is the context of evolution; the contingen- cies of Earth history must surely con- tribute strongly to the biological structure of the evolutionary instant in which we live.

The paleobotanical documentation of early angiosperm evolution has advanced remarkably in recent years, and this alone would justify a major review. But what I found most illuminating about The Origins of Angiosperms and their Biological Consequences is its authors’ attempts to interpret emerging mor- phological and systematic patterns in light of increasingly refined data on Cretaceous and Cenozoic en- vironmental change.

The world into which early angio- sperms radiated could hardly have

been more different from today’s. AI present, the continents are almost maximally dispersed; during the Early Cretaceous the break up of Pangaea had just begun. Today, sea level is near it: Phanerozoic low; then it was high enough to flood continental interiors with substantial seas. The present climate is among the coldest in the past 600 million years; Cretaceous climates were among the warmest. The Cretaceous atmosphere was certainly much en- riched in CO2 relative to today’s, and more controversial data suggest that p02 may also have been higher - factors of importance in any consid- eration of evolving photosynthetic and photorespiratory physiologies. The biological environment of early angiosperms included dinosaurs as major herbivores, a limited diversity of insects to act as pollination vec- tors, and a host of now extinct or relict gymnosperms and ferns as competitors. Some researchers be- lieve that because of seasonal arid- ity, tropical rain forests - those exuberant expressions of flowering plant diversity - did not exist in the equatorial lowlands of the Cre- taceous Period.

Most contributors to Origins con- sider the effects of environmental change on evolving angiosperms, and most argue a pure case. Thus, Upchurch and Wolfe see angiosperm

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