Maintenance Of Variation Research Articles

Consistency in boldness, activity and exploration at different stages of life

BackgroundAnimals show consistent individual behavioural patterns over time and over situations. This phenomenon has been referred to as animal personality or behavioural syndromes. Little is known about consistency of animal personalities over entire life times. We investigated the repeatability of behaviour in common voles (Microtus arvalis) at different life stages, with different time intervals, and in different situations. Animals were tested using four behavioural tests in three experimental groups: 1. before and after maturation over three months, 2. twice as adults during one week, and 3. twice as adult animals over three months, which resembles a substantial part of their entire adult life span of several months.ResultsDifferent behaviours were correlated within and between tests and a cluster analysis showed three possible behavioural syndrome-axes, which we name boldness, exploration and activity. Activity and exploration behaviour in all tests was highly repeatable in adult animals tested over one week. In animals tested over maturation, exploration behaviour was consistent whereas activity was not. Voles that were tested as adults with a three-month interval showed the opposite pattern with stable activity but unstable exploration behaviour.ConclusionsThe consistency in behaviour over time suggests that common voles do express stable personality over short time. Over longer periods however, behaviour is more flexible and depending on life stage (i.e. tested before/after maturation or as adults) of the tested individual. Level of boldness or activity does not differ between tested groups and maintenance of variation in behavioural traits can therefore not be explained by expected future assets as reported in other studies.

The importance of aboveground-belowground interactions on the evolution and maintenance of variation in plant defense traits.

Over the past two decades a growing body of empirical research has shown that many ecological processes are mediated by a complex array of indirect interactions occurring between rhizosphere-inhabiting organisms and those found on aboveground plant parts. Aboveground–belowground studies have thus far focused on elucidating processes and underlying mechanisms that mediate the behavior and performance of invertebrates in opposite ecosystem compartments. Less is known about genetic variation in plant traits such as defense as that may be driven by above- and belowground trophic interactions. For instance, although our understanding of genetic variation in aboveground plant traits and its effects on community-level interactions is well developed, little is known about the importance of aboveground–belowground interactions in driving this variation. Plant traits may have evolved in response to selection pressures from above- and below-ground interactions from antagonists and mutualists. Here, we discuss gaps in our understanding of genetic variation in plant-related traits as they relate to aboveground and belowground multitrophic interactions. When metabolic resources are limiting, multiple attacks by antagonists in both domains may lead to trade-offs. In nature, these trade-offs may critically depend upon their effects on plant fitness. Natural enemies of herbivores may also influence selection for different traits via top–down control. At larger scales these interactions may generate evolutionary “hotspots” where the expression of various plant traits is the result of strong reciprocal selection via direct and indirect interactions. The role of abiotic factors in driving genetic variation in plant traits is also discussed.

Reduced predation risk for melanistic pygmy grasshoppers in post-fire environments.

The existence of melanistic (black) color forms in many species represents interesting model systems that have played important roles for our understanding of selective processes, evolution of adaptations, and the maintenance of variation. A recent study reported on rapid evolutionary shifts in frequencies of the melanistic forms in replicated populations of Tetrix subulata pygmy grasshoppers; the incidence of the melanistic form was higher in recently burned areas with backgrounds blackened by fire than in nonburned areas, and it declined over time in postfire environments. Here, we tested the hypothesis that the frequency shifts of the black color variant were driven, at least in part, by changes in the selective regime imposed by visual predators. To study detectability of the melanistic form, we presented human “predators” with images of black grasshoppers and samples of the natural habitat on computer screens. We demonstrate that the protective value of black coloration differs between burnt and nonburnt environments and gradually increases in habitats that have been more blackened by fire. These findings support the notion that a black color pattern provides improved protection from visually oriented predators against blackened backgrounds and implicate camouflage and predation as important drivers of fire melanism in pygmy grasshoppers.

Focusing on variation: methods and applications of the concept of beta diversity in aquatic ecosystems

Ecologists usually estimate means, but devote much less attention to variation. The study of variation is a key aspect to understand natural systems and to make predictions regarding them. In community ecology, most studies focus on local species diversity (alpha diversity), but only in recent decades have ecologists devoted proper attention to variation in community composition among sites (beta diversity). This is in spite of the fact that the first attempts to estimate beta diversity date back to the pioneering work by Koch and Whittaker in the 1950s. Progress in the last decade has been made in the development both of methods and of hypotheses about the origin and maintenance of variation in community composition. For instance, methods are available to partition total diversity in a region (gamma diversity), in a local component (alpha), and several beta diversities, each corresponding to one scale in a hierarchy. The popularization of the so-called raw-data approach (based on partial constrained ordination techniques) and the distance-based approach (based on correlation of dissimilarity/distance matrices) have allowed many ecologists to address current hypotheses about beta diversity patterns. Overall, these hypotheses are based on niche and neutral theory, accounting for the relative roles of environmental and spatial processes (or a combination of them) in shaping metacommunities. Recent studies have addressed these issues on a variety of spatial and temporal scales, habitats and taxonomic groups. Moreover, life history and functional traits of species such as dispersal abilities and rarity have begun to be considered in studies of beta diversity. In this article we briefly review some of these new tools and approaches developed in recent years, and illustrate them by using case studies in aquatic ecosystems.

SURVIVAL OF THE WEAKEST IN N- PERSON DUELS AND THE MAINTENANCE OF VARIATION UNDER CONSTANT SELECTION

The persistence of extensive variation in nature seems to stand against the most general principle of evolution by natural selection: in antagonistic interactions, the stronger type is expected to replace the weaker. Game theory shows that, however, in contrast to this intuitive expectation for interactions between two players, strategic considerations on fitness maximization in repeated pairwise interactions between three players (truels) or more (N-person duels) lead to what can be dubbed "survival of the weakest": the weakest individual can have the highest fitness. A paradox arises: competitive skills cannot be improved by natural selection, unless we assume mutations with strong effects or unless we assume that interactions are exclusively between two individuals. The paradox disappears, however, with more realistic assumptions (a mixture of duels and truels; the attacked individual backfires; the contest can end without a winner; defensive and offensive skills are correlated; players not directly involved in the contest suffer collateral damage). An unexpected new result emerges: the weaker types can persist in a population in the absence of recurrent mutations, migration, and fluctuating selection. Game theory and the analysis of N-person duels, therefore, help understand one of the most enduring puzzles in evolutionary biology: the maintenance of variation under constant selection.

Maintenance of variation in sexually selected traits in females: a case study using intrasexual aggression in tree swallows Tachycineta bicolor

A fundamental question in evolutionary biology is how phenotypic variation is maintained in the face of selection that ought to deplete that variation. Much research has investigated this question in traits favored via sexual selection in males, with a common solution implicating the condition dependence of sexually selected phenotypes. Despite growing interest in sexual selection on females, it is not clear if the same mechanisms maintain variation in female ornaments, weaponry or other female behaviors targeted by sexual selection. An important step in testing condition dependence in females is thus to identify whether sexually selected female phenotypes are associated with condition and also with potential costs. Here, I examine these two components of condition dependence for a sexually selected behavior, intrasexual aggression, in female tree swallows Tachycineta bicolor. I asked whether high levels of intrasexual aggression map onto natural variation in female condition and whether aggression is associated with one potential behavioral cost: performance in a vertically challenging test of flight. More aggressive females were heavier for their body size, heavier for their wing size and showed decreased flight ability, relative to less aggressive females. These findings are consistent with condition dependence, where only females in better condition are able to be highly aggressive. The association between high aggression and reduced flight ability may result from the additional lift required to power these relatively heavier birds. These associations between natural variation in aggressive behavior, morphology and flight ability are consistent with condition dependence because they confirm two basic assumptions of condition dependence: a link between aggression and condition, and a link between aggression and a behavioral cost, the speed of escape flight. As the first study to examine these assumptions for a conspicuous behavior favored by intrasexual selection in females, this study suggests broad relevance of condition dependence.

Symbiotic Effectiveness of Rhizobial Mutualists Varies in Interactions with Native Australian Legume Genera

Background and ObjectivesInteractions between plants and beneficial soil organisms (e.g. rhizobial bacteria, mycorrhizal fungi) are models for investigating the ecological impacts of such associations in plant communities, and the evolution and maintenance of variation in mutualisms (e.g. host specificity and the level of benefits provided). With relatively few exceptions, variation in symbiotic effectiveness across wild host species is largely unexplored.MethodsWe evaluated these associations using representatives of several legume genera which commonly co-occur in natural ecosystems in south-eastern Australia and an extensive set of rhizobial strains isolated from these hosts. These strains had been previously assigned to specific phylotypes on the basis of molecular analyses. In the first of two inoculation experiments, the growth responses of each host species was evaluated with rhizobial strains isolated from that species. The second experiment assessed performance across genera and the extent of host specificity using a subset of these strains.ResultsWhile host growth responses to their own (sympatric) isolates varied considerably, rhizobial phylotype was a significant predictor of symbiotic performance, indicating that bacterial species designations on the basis of molecular markers have ecological importance. Hosts responded in qualitatively different ways to sympatric and allopatric strains of rhizobia, ranging from species with a clear preference for their own strains, to those that were broad generalists, through to species that grew significantly better with allopatric strains.ConclusionTheory has focused on trade-offs between the provision of benefits and symbiont competitive ability that might explain the persistence of less beneficial strains. However, differences in performance among co-occurring host species could also drive such patterns. Our results thus highlight the likely importance of plant community structure in maintaining variation in symbiotic effectiveness.

Flyway evolution is too fast to be explained by the modern synthesis: proposals for an ‘extended’ evolutionary research agenda

In this paper, I argue that to fully grasp the generation and maintenance of variation in the migratory phenotypes of (shore-)birds we need to expand our scientific search image and include developmental processes and non-genetic pathways of inheritance in the explanatory frameworks. Traditionally, studies of micro-evolution of migratory phenotypes were restricted to comparative studies on migratory versus non-migratory taxa, and artificial selection and heritability experiments on quantitative behavioural traits related to migration. Such studies had a focus on the genetic axis of inheritance and were restricted to songbirds. In avian groups such as the shorebird families Scolopacidae and Charadriidae, all but a few island species are migrants, which precludes comparative studies at the species level. Like other taxa, shorebirds have geographically separate breeding populations (either or not recognized as subspecies on the basis of morphological differences) which differentiate with respect to the length, general direction and timing of migration, including the use of fuelling at staging sites and the timing of moult. However, their breeding systems preclude artificial selection and heritability experiments on quantitative traits. This would seem to limit the prospects of evolutionary analysis until one realizes that the speed of evolutionary innovation in shorebird migratory life-histories may be so fast as to necessitate other avenues of explanation and investigation. According to our best current estimates based on mitochondrial gene sequence variation, in Red Knots Calidris canutus considerable phenotypic variation has evolved since the Last Glacial Maximum ca. 20,000 years ago, to the extent that six subspecies are currently recognized. This would be too short a time for the origin of the qualitatively and quantitatively distinct and non-overlapping traits to be explained by random point mutations followed by natural selection, although we cannot dismiss the possibility of previously unexpressed (standing) genetic variation followed by selection. I argue that, to understand the flyway evolution of such shorebirds in the ‘extended’ evolutionary framework, we need to give due attention to developmental versatility and broad-sense epigenetic evolutionary mechanisms. This means that experimental studies at the phenotypic level are now necessary. This could involve a combination of observational studies in our rapidly changing world, common garden experiments, and even experiments involving global-scale displacements of particular migratory phenotypes at different phases of development. I provide suggestions on how such experiments could be carried out.

Complex selection on life-history traits and the maintenance of variation in exaggerated rostrum length in acorn weevils

Trophic interactions can trigger the development of exaggerated specialized characters and promote morphological diversification. For example, acorn weevils (genus Curculio) present strikingly long rostrums, which are used by females to perforate oviposition holes through the seed coat. Species exhibiting longer rostrums are known to exploit larger acorns, and therefore rostrum length is thought to be subject to selection to match the preferred acorn type. However, rostrum length is strongly correlated with body size, and morphological divergence could result from either selection on rostrum length for optimal food exploitation or from other pressures acting on body size. We collected infested acorns at oak forests where the large Curculio elephas and the small-bodied Curculio glandium co-occur. There were no interspecific differences in adult female body size to rostrum length allometric relationships, and rostrum length is equally correlated with body size in either species. MtDNA-based species identification showed that C. glandium larvae were present within acorns of all sizes, whereas C. elephas larvae were restricted to acorns above a minimum size, irrespective of oak species. Hence, exploitation of large acorns can hardly have triggered rostrum enlargement, as the small sized C. glandium adults (with short rostrums) could perforate and oviposit in both small and large acorns. Rather, increased rostrum length is probably a by-product of the larger body sizes of individuals emerging from bigger acorns, which allow increased larval size and enhance larval survival likelihood. Summarizing, when exaggerated feeding traits co-vary with other body features, interspecific morphological variability may result from contrasting selective pressures acting on these correlated characters.

Creation and maintenance of variation in allorecognition loci

Allorecognition is the ability of an organism to differentiate self or close relatives from unrelated conspecifics. Effective allorecognition systems are critical to the survival of organisms; they prevent inbreeding and facilitate fusions between close relatives. Where the loci governing allorecognition outcomes have been identified, the corresponding proteins often exhibit exceptional polymorphism. Two important questions about this polymorphism remain unresolved: how is it created, and how is it maintained. Because the genetic bases of several allorecognition systems have now been identified, including alr1 and alr2 in Hydractinia, fusion histocompatibility in Botryllus, the het (vic) loci in fungi, tgrB1 and tgrC1 in Dictyostelium, and self-incompatibility (SI) loci in several plant families, we are now poised to achieve a clearer understanding of how these loci evolve. In this review, we summarize what is currently known about the evolution of allorecognition loci, highlight open questions, and suggest future directions.

Heterogeneous selection on trichome production in Alaskan Arabidopsis kamchatica (Brassicaceae)

Environmental heterogeneity is thought to be one of the primary factors in the evolutionary maintenance of morphological variation. Here, we explore the role of environmental heterogeneity in the maintenance of variation in leaf hair (trichome) production in Arabidopsis kamchatica. • We investigate abiotic correlates of trichome production in A. kamchatica via surveys of both herbarium specimens and wild populations. In addition, we examine patterns of phenotypic selection on trichome production among populations that differ in environmental characteristics. • Trichome-producing herbarium specimens were more likely to occur at lower latitudes and in locations with lower mean annual precipitation and less annual variation in temperature than glabrous specimens. In surveys of wild populations, frequencies of trichome-producing plants were higher in drier habitats than in wetter environments. Using phenotypic selection analysis, we found divergent selection through female fitness (fruit production) on trichome number in populations that differ in environmental characteristics; there was selection for reduced trichome number in one population and selection for increased trichome number in another population. In a population containing both glabrous and trichome-producing plants, glabrous plants produced significantly more fruits than trichome-producing individuals, which indicates selection against the trichome morph. • Our results demonstrate that there is heterogeneity in selection among populations, which could be responsible for the maintenance of trichome variation in Alaskan populations of A. kamchatica.

Interaction between signal timing and signal feature preferences: causes and implications for sexual selection

Females often choose their mates based on features of the male’s advertisement signal, often preferring louder, deeper, longer, or faster signals. Females of chorusing animals also frequently have preferences for signal timing positions, generally preferring the leading signal. Under natural chorus conditions, females must choose among signals that vary in features as well as relative timing. I conducted playback experiments with female Hyla cinerea treefrogs to evaluate whether one type of preference (for attractive signal feature or attractive timing) dominates the overall attractiveness of a signal, or whether different types of preferences can interact. I found that the overall attractiveness of a signal was determined by a combination of timing and feature preferences. The specific signal feature (amplitude, frequency, duration, or rate) was important for the outcome of this interaction effect; signal frequency preferences were very strong and dominated overall signal attractiveness, while signal amplitude, duration or rate preferences were weaker and overall signal attractiveness was largely determined by signal timing preferences. I suggest that preference interactions can result in changes in the dynamics of sexual selection and in the maintenance of variation in signal traits, because males with unattractive signals could gain higher mating success by producing these signals in a leading position, thus weakening or even disrupting feature-based signal selection.

Life history and behavioral type in the highly social cichlid Neolamprologus pulcher

Many studies have found that seemingly unconnected behaviors are correlated into behavioral syndromes. These behavioral syndromes may be the consequence of interindividual variation in life-history strategies. Only few studies have investigated the role of behavioral syndromes in cooperatively breeding species, despite the fact that one would expect particular large variation in behavior due to the wealth of life-history decisions a cooperative breeder faces. In a longitudinal study, we repeatedly tested individualsofthecooperativelybreedingcichlidNeolamprologuspulcherforexploration,boldness,andaggressionandtestedwhether these behaviors were sex specific; whether they were interrelated; and whether they were connected to growth and to 2 major lifehistory decisions, helping, and dispersal. In both sexes, explorative behavior was correlated over time, even though after sexual maturity males increased their exploration rate. In both sexes, exploration, boldness, and aggression correlated when mature, and in females, helping behavior was part of the syndrome. No relationships with growth were detected. Helping and dispersal were related to each other in males, whereas females hardly dispersed. We suggest that the differences in the life histories between males andfemales (maledispersal vs.femalephilopatry)leadto the differences inbehavioraltypes observedand alsoto the differences in the stability of the behavioral syndromes between the sexes. The links between dispersal and helping in males and the behavioral types and helping in females highlight the necessity to study multiple traits to understand the evolution and maintenance of variation in cooperative behavior. Key words: behavioral syndrome, Cichlidae, cooperative breeding, life history, Neolamprologus pulcher, ontogeny. [Behav Ecol]

SEXUAL CONFLICT AND THE MAINTENANCE OF MULTIVARIATE GENETIC VARIATION

Mate choice should erode additive genetic variation in sexual displays, yet these traits often harbor substantial genetic variation. Nevertheless, recent developments in quantitative genetics have suggested that multivariate genetic variation in the combinations of traits under selection may still be depleted. Accordingly, the erosion and maintenance of variation may only be detectable by studying whole suites of traits. One potential process favoring the maintenance of genetic variance in multiple trait combinations is the modification of sexual selection via sexually antagonistic interactions between males and females. Here we consider how interlocus sexual conflict can shape the genetic architecture of male sexual traits in the cricket, Teleogryllus commodus. In this species, the ability of each sex to manipulate insemination success significantly alters the selection acting on male courtship call properties. Using a quantitative genetic breeding design we estimated the additive genetic variation in these traits and then predicted the change in variation due to previously documented patterns of sexual selection. Our results indicate that female choice should indeed deplete multivariate genetic variance, but that sexual conflict over insemination success may oppose this loss of variance. We suggest that changes in the direction of selection due to sexually antagonistic interactions will be an important and potentially widespread factor in maintaining multivariate genetic variation.

Change and maintenance of variation in quantitative traits in the context of the Price equation

The Price equation is a general description of evolutionary change in any character from one generation to the next due to natural selection and other forces such as mutation and recombination. Recently it has been widely utilised in many fields including quantitative genetics, but these applications have focused mainly on the response to selection in the mean of characters. Many different and, in some cases, conflicting models have been investigated by quantitative geneticists to examine the change and maintenance of both genetic and environmental variance of quantitative traits under selection and other forces. In this study, we use the Price equation to derive many such well-known results for the dynamics and equilibria of variances in a straightforward way and to develop them further.

The genetic basis of recessive self-colour pattern in a wild sheep population

Bridging the genotype-phenotype gap for traits of ecological and evolutionary importance in natural populations can provide a novel insight into the origin and maintenance of variation. Here, we identify the gene and putative causal mutations underlying a recessive colour pattern phenotype ('self' or uniform colour) in a wild population of primitive Soay sheep. We targeted the agouti signalling protein (ASIP) gene, a positional candidate based on previous study that mapped the Coat pattern locus to a presumptive region on chromosome 13. We found evidence for three recessive mutations, including two functional changes in the coding sequence and a putative third cis-regulatory mutation that inactivates the promoter. These mutations define up to five haplotypes in Soays, which collectively explained the coat pattern in all but one member of a complex multi-generational pedigree containing 621 genotyped individuals. The functional mutations are in strong linkage disequilibrium in the study population, and are identical to those known to underlie the self phenotype in domestic sheep. This is indicative of a recent (and simultaneous) origin in Soay sheep, possibly as a consequence of past interbreeding with modern domestic breeds. This is only the second study in which ASIP has been linked to variation in pigmentation in a natural population. Knowledge of the genetic basis of self-colour pattern in Soay sheep, and the recognition that several mutations are segregating in the population, will aid future studies investigating the role of selection in the maintenance of the polymorphism.

Frequency-dependent selection by wild birds promotes polymorphism in model salamanders

BackgroundCo-occurrence of distinct colour forms is a classic paradox in evolutionary ecology because both selection and drift tend to remove variation from populations. Apostatic selection, the primary hypothesis for maintenance of colour polymorphism in cryptic animals, proposes that visual predators focus on common forms of prey, resulting in higher survival of rare forms. Empirical tests of this frequency-dependent foraging hypothesis are rare, and the link between predator behaviour and maintenance of variation in prey has been difficult to confirm. Here, we show that predatory birds can act as agents of frequency-dependent selection on terrestrial salamanders. Polymorphism for presence/absence of a dorsal stripe is widespread in many salamander species and its maintenance is a long-standing mystery.ResultsWe used realistic food-bearing model salamanders to test whether selection by wild birds maintains a stripe/no-stripe polymorphism. In experimental manipulations, whichever form was most common was most likely to be attacked by ground-foraging birds, resulting in a survival advantage for the rare form.ConclusionThis experiment demonstrates that frequency-dependent foraging by wild birds can maintain colour polymorphism in cryptic prey.

Evolution of Fitnesses in Structured Populations With Correlated Environments

The outcome of selection in structured populations with spatially varying selection pressures depends on the interaction of two factors: the level of gene flow and the amount of heterogeneity among the demes. Here we investigate the effect of three different levels of spatial heterogeneity on the levels of genetic polymorphisms for different levels of gene flow, using a construction approach in which a population is constantly bombarded with new mutations. We further compare the relative importance of two kinds of balancing selection (heterozygote advantage and selection arising from spatial heterogeneity), the level of adaptation and the stability of the resulting polymorphic equilibria. The different levels of environmental heterogeneity and gene flow have a large influence on the final level of polymorphism. Both factors also influence the relative importance of the two kinds of balancing selection in the maintenance of variation. In particular, selection arising from spatial heterogeneity does not appear to be an important form of balancing selection for the most homogeneous scenario. The level of adaptation is highest for low levels of gene flow and, at those levels, remarkably similar for the different levels of spatial heterogeneity, whereas for higher levels of gene flow the level of adaptation is substantially reduced.

Maternal inheritance, epigenetics and the evolution of polyandry

Growing evidence indicates that females actively engage in polyandry either to avoid genetic incompatibility or to bias paternity in favor of genetically superior males. Despite empirical support for the intrinsic male quality hypothesis, the maintenance of variation in male fitness remains a conundrum for traditional "good genes" models of sexual selection. Here, we discuss two mechanisms of non-Mendelian inheritance, maternal inheritance of mitochondria and epigenetic regulation of gene expression, which may explain the persistence of variation in male fitness traits important in post-copulatory sexual selection. The inability of males to transmit mitochondria precludes any direct evolutionary response to selection on mitochondrial mutations that reduce or enhance male fitness. Consequently, mitochondrial-based variation in sperm traits is likely to persist, even in the face of intense sperm competition. Indeed, mitochondrial nucleotide substitutions, deletions and insertions are now known to be a primary cause of low sperm count and poor sperm motility in humans. Paradoxically, in the field of sexual selection, female-limited response to selection has been largely overlooked. Similarly, the contribution of epigenetics (e.g., DNA methylation, histone modifications and non-coding RNAs) to heritable variation in male fitness has received little attention from evolutionary theorists. Unlike DNA sequence based variation, epigenetic variation can be strongly influenced by environmental and stochastic effects experienced during the lifetime of an individual. Remarkably, in some cases, acquired epigenetic changes can be stably transmitted to offspring. A recent study indicates that sperm exhibit particularly high levels of epigenetic variation both within and between individuals. We suggest that such epigenetic variation may have important implications for post-copulatory sexual selection and may account for recent findings linking sperm competitive ability to offspring fitness.

The Speed of Evolution and Maintenance of Variation in Asexual Populations

The rate at which beneficial mutations accumulate determines how fast asexual populations evolve, but this is only partially understood. Some recent clonal-interference models suggest that evolution in large asexual populations is limited because smaller beneficial mutations are outcompeted by larger beneficial mutations that occur in different lineages within the same population. This analysis assumes that the important mutations fix one at a time; it ignores multiple beneficial mutations that occur in the lineage of an earlier beneficial mutation, before the first mutation in the series can fix. We focus on the effects of such multiple mutations. Our analysis predicts that the variation in fitness maintained by a continuously evolving population increases as the logarithm of the population size and logarithm of the mutation rate and thus yields a similar logarithmic increase in the speed of evolution. To test these predictions, we evolved asexual budding yeast in glucose-limited media at a range of population sizes and mutation rates. We find that their evolution is dominated by the accumulation of multiple mutations of moderate effect. Our results agree with our theoretical predictions and are inconsistent with the one-by-one fixation of mutants assumed by recent clonal-interference analysis.