The Queen of Problems in Evolutionary Biology


Sexual reproduction is widespread amongst higher eukaryotes. But why are there not many more organisms reproducing asexually? After all, an asexual organism avoids making males, can thus in theory reproduce more efficiently, and one asexual mutant should quickly lead to the extinction of a sexual source population. Owing to such arguments, understanding the evolution and maintenance of sexual reproduction has in 1982 been called ‘the queen of problems in evolutionary biology’. In the last four decades, we have gained tremendous insights into this issue: perhaps most importantly, we understand more clearly that the powerful arguments against sex are not fully realised in all species. There are also recent insights into understanding the maintenance of sex in species where the problem is (partially) realised. Experimental evolution studies show that outcrossing can speed up the response to selection, while studies on natural populations emphasise the importance of niche differentiation and parasites.

Key Concepts

  • Sexual organisms dominate amongst higher eukaryotes.
  • Sexuals should in theory suffer from severe costs of sex, such as the cost of males.
  • All else equal, sexual populations should quickly go extinct due to these costs of sex.
  • On the genetic level, sexuality leads to fixed combinations of alleles across loci – also known as linkage disequilibrium (LD) – whereas sexual reproduction tends to break down these combinations to make sure that all combinations are possible, causing the population to be in a state of linkage equilibrium.
  • Since the breakdown of linkage disequilibrium is the most striking genetic effect of sexual reproduction, this has been the focus of most of the proposed benefits for sex.
  • In many species, the costs of sex might not be realised; the exact costs are affected by species‐specific constraints on the evolution of asexuality as well as ecological differentiation between sexuals and asexuals and life‐history traits.

Keywords: maintenance of sex; cost of sex; asexuality; geographic parthenogenesis; response to selection; niche differentiation; mutation accumulation

Figure 1. The cost of males in a dioecious species (Maynard Smith, ). If every female produces on average two offspring, in sexuals this will be approximately equally divided between male and females; asexually reproducing females will, however, be able to produce two female offspring. As a result, the growth rate of the asexuals will be twice as high as that of the sexuals, which translates into a two‐fold cost of sex.
Figure 2. Asexual reproduction sweeping to fixation in an initially mostly sexual population, under a two‐fold cost of sex. The model assumes a very large population and an initial frequency of asexual reproduction of 10−3.


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Further Reading

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Meirmans, Stephanie, and Meirmans, Patrick G(Aug 2019) The Queen of Problems in Evolutionary Biology. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0028485]