Sex car?
Sex serves to clean the genome - at least according to one of the numerous theories Thomas P. Weber, Spectrum of Science, October 2006 Birds - do it; bees - do it; even educated fleas - do it' - and Cole Porter could have continued his list indefinitely: 'Falling in love' or, to put it less romantically, sex is an almost universal phenomenon in multicellular organisms. It is precisely this universality that causes enormous headaches for evolutionary biologists. Because sex is an expensive business. As far as reproduction is concerned, males are only needed to fertilize the female's eggs, and a few would suffice for this task. In most cases, however, a sex ratio of 1:1 is evolutionarily stable and almost half of the reproductive effort is wasted. On the other hand, females who reproduce asexually and therefore only give birth to females would have twice as high a reproduction rate and could thus easily crowd out their competitors who indulge in sex and produce males. Despite this huge cost disadvantage, sexual reproduction has evolved into most groups in the plant and animal kingdoms. Hardly any group of organisms can get along completely without sex; even many normally asexual creatures reproduce sexually under certain environmental conditions. And then there is also something like the rotifers of the class Bdelloidea, which have happily reproduced exclusively virginally for tens of millions of years and have even developed 370 species. Over the past few decades, the great and small lights of evolutionary biology have floated more than twenty hypotheses to explain what benefits can outweigh the costs of sexual reproduction. Does sexuality cleanse the genome of harmful mutations? Or does sex allow rapid genetic adaptation to a constantly changing environment? So far, not a single assumption has been able to assert itself as a clear favourite. Many explanations sound thoroughly plausible, but they only work if important parameters such as the mutation rate assume certain values; and plausibility is not a substitute for pending experimental confirmation. George Williams, one of the most important evolutionary biologists of the 20th century, also failed with his attempt at an explanation - and remarked in 1975 that his failure at least put him in good company. Christian Göldenboog's latest foray into evolutionary biology summarizes the scientific debate about the evolution of sexuality in an entertaining and readable way. The reader gets to know some members of the good society identified by Williams and their ways of thinking in more detail. John Maynard Smith, who died in 2004 and whose book »The Evolution of Sex«, published in 1978, is a milestone in the debate, presents the most important theses and theories on the emergence and maintenance of sex in conversation with the author at the beginning of the book. We then learn more about the theory of female choice from Maynard Smith, from Wolf Reik from Cambridge (England) about the so-called imprinting of the genome during reproduction, from Kim Nasmyth from Oxford about the molecular basis of eggs and sperm, and the population geneticist Luigi Cavalli-Sforza teaches us about the benefits of sex-enhanced genetic diversity and the scientifically untenable illusions of racism (Spektrum der Wissenschaft 4/2000, p. 105). Göldenboog writes consistently competently and excitingly and loosens up the chapters with interesting historical excursions. Unfortunately, the book ends somewhat unsatisfied, because a common thread and a clear conclusion are missing. For the sake of his readers, Göldenboog should have taken a more frequent stand against luminaries like Maynard Smith or Cavalli-Sforza instead of hiding behind the mostly page-long transcripts of the interviews. There is still no unequivocal answer to the title question "Why sex?" - empirical evidence is still far behind theory - but many comparative and experimental studies promise new insights. For example, genomics make it possible to measure the accumulation of harmful mutations in many organisms more reliably than ever before; only if the values fall within a narrow interval can sex serve to "clean up" the genome. And many scientists are increasingly trying to assess the benefits of sexual reproduction in the field. Göldenboog's book provides insufficient insight into these developments and thematically remains too much in the orbit of his illustrious interlocutors. Fortunately, the author refrains from flippant analogies between animal and human sexual behavior, but fails to portray the sensitive subject without resorting to clichés. Göldenboog relies exclusively on male authority, and in too many places the tone of an academic men's get-together prevails. The study of sexuality in the natural sciences is characterized by deep-seated cultural clichés and prejudices like no other topic. It is therefore unfortunate that scientists like Lynn Margulis or Joan Roughgarden do not have their say with their controversial but debatable theories on the evolution of sexuality or on sexual selection. Scientific and cultural power of interpretation are still firmly in male hands. Thomas P. Weber The reviewer is a research assistant at the Institute of Animal Ecology at Lund University (Sweden) and an author.
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