In This Article Expand or collapse the "in this article" section Paradox of Sex

  • Introduction
  • Books
  • The Origin of Sex
  • Definitions of Sex and Asex
  • Asexuality and Apomixis in Plants
  • Asexual Reproduction in Animals
  • The Distribution of Asexuality in Animals and Plants
  • The History of the Paradox of Sex
  • Modeling Epistasis, Recombination, and the Benefits of Sex
  • Changing Environments and Sexual Advantages
  • The Red Queen
  • Muller’s Ratchet
  • Kondrashov’s Hatchet
  • Pluralistic Models of the Advantage of Sex
  • Geographic Parthenogenesis
  • Ancient Asexual Scandals
  • Applications for Society

Evolutionary Biology Paradox of Sex
Isa Schön, Koen Martens
  • LAST REVIEWED: 01 September 2022
  • LAST MODIFIED: 29 November 2018
  • DOI: 10.1093/obo/9780199941728-0035


The evolution of sex can be divided into two major but rather different topics, the origin and the maintenance of sex. The origin of sex is speculative because it is difficult to reconstruct. Sex originated early in the evolution of life and has general features being shared by all higher organisms, such as the generation of haploid gametes (eggs and sperm) and their fusion which is accompanied by the exchange of genetic material (see the Origin of Sex and Definitions of Sex and Asex). The maintenance of sex is a much more debated topic in evolutionary biology. Because sex is costly in evolutionary terms, but at the same time widespread among eukaryotes, it presents an evolutionary paradox. More than twenty-five different hypotheses have been put forward by theoretical evolutionary research in the 1970s and 1980s to explain why sex is evolutionary advantageous (see the History of the Paradox of Sex). The majority of these hypotheses can be divided into two major groups. (1) Sex provides novel and mostly advantageous genetic variation in offspring by reshuffling genes during recombination and meiosis. Natural selection can act on this variation and sex thus provides the opportunity for novel fast adaptations, for example to changing environments (see Changing Environments and Sexual Advantages and the Red Queen). (2) Sex removes negative, deleterious mutations through meiosis and recombination (see Modeling Epistasis, Recombination, and the Benefits of Sex, Kondrashov’s Hatchet, and Muller’s Ratchet). Certain animals and fungi might have persisted without any sex for millions of years, the so-called Ancient Asexual Scandals. The majority of asexuals are of more recent origin (see the Distribution of Asexuality in Animals and Plants), and many different forms of nonsexual reproduction have originated (see Definitions of Sex and Asex, Asexuality and Apomixis in Plants, and Asexual Reproduction in Animals). Asexual reproduction can furthermore be combined with occasional sexual reproduction. This amazing diversity in reproductive modes is translated in a plethora of hypotheses on the paradox of sex. Testing these hypotheses, or combinations of several of these, is the challenge we face. Indeed, there is still no general explanation for the maintenance of sex that is applicable to all eukaryotes, and the paradox of sex remains largely unresolved to date. The paradox of sex is not only an academically relevant question, but has also many applied aspects, for example in agriculture, medicine, and human reproductive technologies (see Applications for Society).


Several books provide a good introduction to the topic. Williams 1975 and Maynard Smith 1978 were among the first to recognize the paradox of sex. Their theoretical approach was complemented in Bell 1982, which also described the variety and taxonomic distribution of asexuality in eukaryotes, while Suomalainen, et al. 1987 provided an overview of the karyological and cytogenetic mechanisms of asexuality. Margulis and Sagan 1986 is a highly controversial book on the origin of sex. Avise 2008 and Martens 1998 provide overviews on asexuality in certain animal groups, while Schön, et al. 2009 approaches the topic from an asexual point of view.

  • Avise, John. 2008. Clonality: The genetics, ecology, and evolution of sexual abstinence in vertebrates. Oxford: Oxford Univ. Press.

    DOI: 10.1093/acprof:oso/9780195369670.001.0001

    Although this book focuses mainly on vertebrates, it provides a very accessible introduction into the topic, distinguishing between parthenogenesis and gynogenesis and clonality at different levels (within and between individuals) and environments (nature versus lab). Very suitable for all readers, especially undergraduates.

  • Bell, Graham. 1982. The masterpiece of nature: The evolution and genetics of sexuality. Berkeley: Univ. of California Press.

    A real classic in the field and often cited because it was Bell who stated that “the paradox of sex is the queen of problems in evolutionary biology” (p. 19). Discusses all existing evolutionary theories of the 1980s, plus provides the most complete overview of the occurrence of asexuality in animal taxa. Well written and accessible for students of all levels.

  • Margulis, Lynn, and Dorion Sagan. 1986. Origins of sex: Three billion years of genetic recombination. Bio-Origin Series. New Haven, CT, and London: Yale Univ. Press.

    This book is highly controversial because it uses cannibalism to explain the origin of sex. It is pleasant to read because it is well written and suitable for all levels.

  • Martens, Koen, ed. 1998. Sex and parthenogenesis: Evolutionary ecology of reproductive modes in non-marine ostracods. Leiden, The Netherlands: Backhuys.

    This edited multi-author book provides a general introduction to the paradox of sex and then describes and discusses various aspects of sex and parthenogenesis in the model group, nonmarine ostracods (Ostracoda, Crustacea), including paleontological, ecological, and genetic studies. Suitable for all students.

  • Maynard Smith, John. 1978. The evolution of sex. Cambridge, UK: Cambridge Univ. Press.

    Provides the theoretical background for thirty years of research on the paradox of sex and is a real groundbreaker. Because of the modeling and theoretical approach, it might be more suitable for advanced students. Interested undergraduate students might be better off with Maynard Smith’s Evolutionary genetics (2nd ed. Oxford: Oxford Univ. Press, 1998).

  • Schön, Isa, Koen Martens, and Peter van Dijk, eds. 2009. Lost sex: The evolutionary biology of parthenogenesis. Dordrecht, The Netherlands: Springer.

    DOI: 10.1007/978-90-481-2770-2

    This multi-author edited book covers various topics of the research field, including three major hypotheses and twelve animal and two plant case studies, illustrating the amazing variety of reproductive modes. Also, related topics such as clonality, asexual species, geographic parthenogenesis, and applied aspects are provided. It is the most recent overview of the topic, suitable for readers of all levels.

  • Suomalainen, Esko, Anssi Saura, and Juhani Lokki. 1987. Cytology and evolution in parthenogenesis. Boca Raton, FL: CRC Press.

    This book focuses on karyological variation in parthenogenesis, describing the different mechanisms and their consequences for generating genetic variability. Suitable for readers of all levels.

  • Williams, George C. 1975. Sex and evolution. Princeton, NJ: Princeton Univ. Press.

    George Williams provides an extensive argument for why the prevalence of sex is in conflict with evolutionary theory. A real classic using different model systems, diverse forms of sexuality; also describes the effect of sex on organic and biotic evolution. Very accessible.

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