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Evolution of sex

The costs and benefits of sex: new insights from old asexual lineages


Discussions that are aimed at understanding the maintenance of sexual reproduction are in a bit of a quagmire owing to the many competing theories that have been proposed. Also, one of the central observations — that asexual lineages are typically short lived — still needs to be properly quantified. Exciting new results on ancient asexual organisms show that lineages can persist for many millions of generations without recombination. Understanding how they do so might well provide crucial new insights into the problem of sex.

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Figure 1: Inferring the age of asexual lineages.
Figure 2: Candidate ancient asexuals.
Figure 3: Maximum-likelihood gene trees for hsp82 in rotifers.


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I am very grateful to D. Mark Welch, K. Martens, I. Schön, S. West and an anonymous referee for their insightful comments on earlier drafts of this article.

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A polyploid of hybrid origin in which there are four copies of each homologous chromosome per cell, two derived from each parent species.


A form of reproduction in which the male gametes are a different size to the female gametes.


Reproduction that is exclusively through mitotic cell division, with no form of meiosis.


A type of statistical analysis to test the reliability of certain branches in the evolutionary tree. The bootstrap proceeds by re-sampling the original data, with replacement, to create a series of bootstrap samples of the same size as the original data. The bootstrap value of a node is the percentage of times that a node is present in the set of trees that is constructed from the new data sets.


A clade is a lineage of organisms/alleles comprising an ancestor and its descendants. Clade selection is a form of group selection arising when a trait alters the probability of extinction of lineages that carry it, or the rate of production of new lineages, relative to those lineages that do not have the trait.


The position in a coding region of DNA sequence at which the presence of any of the four bases is translated into the same amino acid.


The non-reciprocal transfer of genetic information between homologous genes, as a consequence of mismatch repair after heteroduplex formation.


The principle that any gene or protein has a near-constant rate of evolution in all branches of a clade, which means that the amount of sequence divergence between two sequences will be proportional to the amount of time elapsed since their shared ancestor existed.


A member of the rotifer class Monogononta, the sister clade to the Bdelloidea. They are cyclical parthenogens.


The irreversible accumulation of deleterious mutations in asexual populations of finite size. The average load of mutations increases over generations because the class of individuals that carry the smallest number of mutant alleles is occasionally lost by genetic drift. In the absence of recombination, this class can never be re-created.


The increase in the harmful effect of a mutation when other deleterious mutations are simultaneously present in a genome.


The principle that, as trait changes during evolution are rare, the hypothesis that minimizes the number of events that are required to explain the current distribution of a trait should be preferred.


The production of offspring by a female with no genetic contribution from a male.


A method for testing for constancy of base substitution rate in part of an evolutionary tree. If species A and B are sister taxa and C is an outgroup, then the number of substitutions that separate the pairs AC and BC are expected to be equal.


A measure of the variation around the central class of a distribution (the average squared deviation of the observations from their mean value).

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Butlin, R. The costs and benefits of sex: new insights from old asexual lineages. Nat Rev Genet 3, 311–317 (2002).

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