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Experimental Evolution and Evolvability Heredity (2008) 100, 464–470 & 2008 Nature Publishing Group All rights reserved 0018-067X/08 $30.00 www.nature.com/hdy SHORT REVIEW Experimental evolution: experimental evolution and evolvability N Colegrave1 and S Collins1,2 1Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, UK and 2Max Planck Institute for Plant Breeding Research, Cologne, Germany The suggestion that there are characteristics of living organ- architecture. In each case, we summarize what studies of isms that have evolved because they increase the rate of experimental evolution have told us so far and speculate on evolution is controversial and difficult to study. In this review, we where progress might be made in the future. We show that, examine the role that experimental evolution might play in while experimental evolution has helped us to begin to resolving this issue. We focus on three areas in which understand the evolutionary dynamics of traits that affect experimental evolution has been used previously to examine evolvability, many interesting questions remain to be answered. questions of evolvability; the evolution of mutational supply, the Heredity (2008) 100, 464–470; doi:10.1038/sj.hdy.6801095; evolution of genetic exchange and the evolution of genetic published online 23 January 2008 Keywords: experimental evolution; evolvability; mutator; sex; robustness Introduction Many features of organisms have been proposed as evolvability traits, and most can be loosely classified into Natural selection produces organisms that are well one of the three categories. First, there are characters that adapted to their environment and the explanation of how directly increase the input of genetic variation. The complex adaptation may arise has been one of the great elevated mutation rate observed in many species of successes of the neo-Darwinian synthesis. There is little bacteria is one obvious example. Similarly, there is some doubt that the functional significance of traits as diverse as evidence that the existence of hypermutable domains the clutch size of great tits and the infanticide behaviour of within the genomes of pathogenic bacteria and viruses male lions can be explained in terms of their effect on an may allow more rapid adaptive responses in the face of individual’s genetic contribution to the future. But are host immune pressure (Moxon et al., 1994; Earl and there characteristics of organisms that function not to Deem, 2004). Second, there are characters that increase increase their fitness, but instead to increase evolvability? genetic variation by mixing genetic material from That is, are there traits that are selected and maintained different lineages. The widespread existence of eukar- because they increase the ability of a population to respond yotic sex has been explained in this context, but genetic to natural selection? Increasingly, the suggestion that many exchange by various mechanisms is also common place features of organismic design can only be understood in among non-eukaryotes (Redfield, 2001). Third, there are this context is taking hold (Kirschner and Gerhart, 1998; characters that increase evolvability by altering the link Earl and Deem, 2004). However, such claims are con- between genotype and phenotype and so modulating the troversial and clear evidence has been hard to come by way in which a given amount of genetic variation is (Poole et al., 2003; Sniegowski and Murphy, 2006). Indeed, expressed at the phenotype level. This category includes even the term evolvability has been defined and used in a diverse array of organismic features that are less easily different ways, from a technical measure of the amount of characterized than the previous two. The modularity of additive genetic variation within a population to the ability organismic design, the structure of gene networks and of a population to generate novel variation or acquire novel genetic architecture and the robustness of developmental functions (Kirschner and Gerhart, 1998; Wagner, 2005; mechanisms are examples of characters that may Sniegowski and Murphy, 2006). Here, we define evolva- increase evolvability in this way (Wagner and Altenberg, bility as the ability of a population to both generate and use 1996; Kirschner and Gerhart, 1998; Wagner, 2005; genetic variation to respond to natural selection and an Hansen, 2006). evolvability trait as a character that is selected because of In theory, there is nothing to preclude the selection of its effects on evolvability. traits that have no other effect than to increase the evolutionary potential of a population. However, in contrast to genes with direct effects on fitness, which Correspondence: Dr N Colegrave, Institute of Evolutionary Biology, respond directly to selection, evolvability genes are School of Biological Sciences, University of Edinburgh, Ashworth Labs, subject to indirect selection. Take as an example a gene King’s buildings, Edinburgh EH9 3JT, UK. E-mail: [email protected] that increases the genomic mutation rate, but has no Received 16 May 2007; revised 12 October 2007; accepted 28 October other effect. Such a gene can potentially increase in 2007; published online 23 January 2008 frequency if it can ‘hitchhike’ with novel beneficial Experimental evolution and evolvability N Colegrave and S Collins 465 mutations that it creates (Sniegowski et al., 2000). Such down, and is unlikely to be a significant force in sexual indirect selection will be weak in comparison to direct organisms. selection, but even weak evolutionary forces can have Naturally occurring mutator strains are found in many considerable importance over the long timescales of bacteria, including Escherichia coli. Loss-of-function mu- evolutionary history. tations in DNA repair systems means that these strains However, while evolvability traits are possible in have a mutation rate 10- to100-fold higher than wild-type theory, demonstrating that a particular trait has been strains. This has allowed the evolutionary dynamics of shaped by such selection pressures is far from straight- mutator genotypes to be examined in the lab. Early forward. First, it is necessary to show that the trait really experiments showed that chemostats inoculated with does increase the rate of adaptation. For example, mixtures of mutator and wild-type E. coli would increasing the supply of beneficial mutations could frequently become dominated by the mutator genotype potentially increase the rate of adaptation, but may have (Cox and Gibson, 1974; Chao and Cox, 1983). The little actual effect if adaptation is not limited by appearance and subsequent spread to fixation of mutator mutational supply. Second, it is also necessary to show alleles within 3 of the 12 long-term selection lines of the that selection favours the trait because it increases Lenski’s group (Sniegowski et al., 1997) showed that such evolvability rather than increased evolvability being an genes can also spread when they arise naturally. Together unselected by-product of selection for some other these results provide direct evidence that genes for function (Sniegowski and Murphy, 2006). This distinction elevated mutation rate can spread through populations, between the adaptive value of a trait and its unselected and that they do so at a rate too high to be explained by consequences is critical if we are to fully understand the genetic drift. However, is increased evolvability the forces that have moulded a trait (Williams, 1996). selective force driving the increase in frequency of these Experimental evolution offers the potential to examine mutators or is their spread due to some other direct directly the evolutionary dynamics of putative evolva- selective benefit? bility traits. Such experiments allow researchers to Further experiments cast doubt on a necessary link observe directly whether a trait does indeed increase between elevated mutation rate and increased evolva- the rate of adaptation of a population, and if so, under bility. When the rate of adaptation of mutator and wild- what circumstances. In addition, it is possible to observe type E. coli was compared directly, elevated mutation whether such traits increase in frequency in situations rate had minimal effect on the rate of adaptation except where the rate of adaptation is limiting (and perhaps in poorly adapted populations with small effective equally importantly, that they do not increase in population sizes (de Visser et al., 1999). The rate of frequency when it is not). In this review, we will examine adaptation of larger populations was limited, not by the some of the ways in which the techniques of experi- rate of mutational supply, but by the efficiency of mental evolution may improve our understanding of selection, and so elevated mutation rates had little effect evolvability. We will not attempt a complete overview of (Gerrish and Lenski, 1998; de Visser et al., 1999). Thus, the field. Instead, we begin by focusing on two areas, the increased mutational supply does not guarantee an evolution of elevated mutation rates in bacteria and the increase in evolvability. evolution of eukaryotic sex. These are areas in which our However, clear evidence that mutator genes can understanding of the evolutionary processes is reason- spread because they increase evolvability came from ably well-developed, and experimental evolution studies detailed examination of the three mutator lines from have been central in aiding that development. We will Lenski’s experiments. In two of the three lines, the then examine the much more controversial question of spread of the mutator genotype was accompanied by an whether aspects of genetic architecture have evolved to increase in the rate of adaptation relative to populations increase evolvability (Wagner, 2005; Hansen, 2006). This that did not substitute mutator alleles (Shaver et al., is an area that has received a great deal of attention 2002). In addition, there was no evidence that the recently, but in which our understanding is much less increase in mutation rate seen was favoured by direct well-developed. We will highlight how experimental selection (Shaver et al., 2002). However, the observed evolution might help to develop this understanding.
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