Fluctuating Selection,Egg Banks and Population Genetic Structure in Cyclically Parthenogenetic Species

Associations between neutral genetic markers and genes under selection have been suggested to explain the population genetic structure of neutral genes in cyclically parthenogenetic freshwater invertebrates. A simulation model was constructed in order to analyse the extrapolated consequences of observed fluctuations in genotype frequencies in Daphnia, in the presence of egg banks. When of sufficient depth and magnitude, egg banks in combination with fluctuating selection were shown to maintain genetic variation within populations indefinitely. The level of equilibrium diversity increased with the depth and magnitude of the banks, and with intensity of selection. The same threshold was responsible for genetic differentiation between populations, which was independent of migration rate, and which was attained very rapidly following initial Hardy–Weinberg equilibrium. In the absence of selection, egg banks increased the effective size of local populations, thereby decreasing genetic differentiation at migration-drift equilibrium. These results suggest that egg banks are crucial to the genetic structure in the presence of fluctuating selective pressures, but more data are needed if this knowledge is to be used in an improved general understanding of the genetic structure of cyclically parthenogenetic species.     

Does local adaptation to resources explain genetic differentiation among Daphnia populations?

Substantial genetic differentiation is frequently observed among populations of cyclically parthenogenetic zooplankton despite their high dispersal capabilities and potential for gene flow. Local adaptation has been invoked to explain population genetic differentiation despite high dispersal, but several neutral models that account for basic life history features also predict high genetic differentiation. Here, we study genetic differentiation among four populations of Daphnia pulex in east central Illinois. As with other studies of Daphnia, we demonstrate substantial population genetic differentiation despite close geographic proximity (<50 km; mean θ = 0.22). However, we explicitly tested and failed to find evidence for, the hypothesis that local adaptation to food resources occurs in these populations. Recognizing that local adaptation can occur in traits unrelated to resources, we estimated contemporary migration rates (m) and tested for admixture to evaluate the hypothesis that observed genetic differentiation is consistent with local adaptation to other untested ecological factors. Using Bayesian assignment methods, we detected migrants in three of the four study populations including substantial evidence for successful reproduction by immigrants in one pond, allowing us to reject the hypothesis that local adaptation limits gene flow for at least this population. Thus, we suggest that local adaptation does not explain genetic differentiation among these Daphnia populations and that other factors related to extinction/colonization dynamics, a long approach to equilibrium F_ST or substantial genetic drift due to a low number of individuals hatching from the egg bank each season may explain genetic differentiation.     

Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitment

In dioecious species with both sexual and asexual reproduction, the spatial distribution of individual clones affects the potential for sexual reproduction and local adaptation. The seaweed Fucus radicans, endemic to the Baltic Sea, has separate sexes, but new attached thalli may also form asexually. We mapped the spatial distribution of clones (multilocus genotypes, MLGs) over macrogeographic (>500 km) and microgeographic (<100 m) scales in the Baltic Sea to assess the relationship between clonal spatial structure, sexual recruitment, and the potential for natural selection. Sexual recruitment was predominant in some areas, while in others asexual recruitment dominated. Where clones of both sexes were locally intermingled, sexual recruitment was nevertheless low. In some highly clonal populations, the sex ratio was strongly skewed due to dominance of one or a few clones of the same sex. The two largest clones (one female and one male) were distributed over 100–550 km of coast and accompanied by small and local MLGs formed by somatic mutations and differing by 1–2 mutations from the large clones. Rare sexual events, occasional long‐distance migration, and somatic mutations contribute new genotypic variation potentially available to natural selection. However, dominance of a few very large (and presumably old) clones over extensive spatial and temporal scales suggested that either these have superior traits or natural selection has only been marginally involved in the structuring of genotypes.     

Frequency‐dependent selection acting on the widely fluctuating sex ratio of the aphid Prociphilus oriens

Frequency‐dependent selection is a fundamental principle of adaptive sex ratio evolution in all sex ratio theories but has rarely been detected in the wild. Through long‐term censuses, we confirmed large fluctuations in the population sex ratio of the aphid Prociphilus oriens and detected frequency‐dependent selection acting on these fluctuations. Fluctuations in the population sex ratio were partly attributable to climatic factors during the growing season. Climatic factors likely affected the growth conditions of host plants, which in turn led to yearly fluctuations in maternal conditions and sex ratios. In the process of frequency‐dependent selection, female proportion higher or lower than ca. 60% was associated with a reduction or increase in female proportion, respectively, the next year. The rearing of aphid clones in the laboratory indicated that mothers of each clone produced an increasing number of females as maternal size increased. However, the mean male number was not related to maternal size, but varied largely among clones. Given genetic variance in the ability to produce males among clones, selection should favour clones that can produce more numerous males in years with a high female proportion. Population‐level sex allocation to females was on average 71%–73% for three localities and more female‐biased when maternal conditions were better. This tendency was accounted for by the hypothesis of competition among foundresses rather than the hypothesis of local mate competition. We conclude that despite consistent operation of frequency‐dependent selection, the sex ratio continues to fluctuate because environmental conditions always push it away from equilibrium.     

Notch signalling inhibits the adipogenic differentiation of single‐cell‐derived mesenchymal stem cell clones isolated from human adipose tissue

ADSCs (adipose‐derived mesenchymal stem cells) are candidate adult stem cells for regenerative medicine. Notch signalling participates in the differentiation of a heterogeneous ADSC population. We have isolated, human adipose tissue‐derived single‐cell clones using a cloning ring technique and characterized for their stem cell characteristics. The role of Notch signalling in the differentiation capacity of these adipose‐derived single‐cell‐clones has also been investigated. All 14 clones expressed embryonic and mesenchymal stem cell marker genes. These clones could differentiate into both osteogenic and adipogenic lineages. However, the differentiation potential of each clone was different. Low adipogenic clones had significantly higher mRNA expression levels of Notch 2, 3 and 4, Jagged1, as well as Delta1, compared with those of high adipogenic clones. In contrast, no changes in expression of Notch signalling component mRNA between low and high osteogenic clones was found. Notch receptor mRNA expression decreased with the adipogenic differentiation of both low and high adipogenic clones. The γ‐secretase inhibitor, DAPT (N‐[N‐(3,5‐difluorophenacetyl)‐l ‐alanyl]‐(S)‐phenylglycine t‐butyl ester), enhanced adipogenic differentiation. Correspondingly, cells seeded on a Notch ligand (Jagged1) bound surface showed lower intracellular lipid accumulation. These results were noted in both low and high adipogenic clones, indicating that Notch signalling inhibited the adipogenic differentiation of adipose ADSC clones, and could be used to identify an adipogenic susceptible subpopulation for soft‐tissue augmentation application.     

A portrait of a sucker using landscape genetics: how colonization and life history undermine the idealized dendritic metapopulation

Dendritic metapopulations have been attributed unique properties by in silico studies, including an elevated genetic diversity relative to a panmictic population of equal total size. These predictions have not been rigorously tested in nature, nor has there been full consideration of the interacting effects among contemporary landscape features, colonization history and life history traits of the target species. We tested for the effects of dendritic structure as well as the relative importance of life history, environmental barriers and historical colonization on the neutral genetic structure of a longnose sucker (Catostomus catostomus) metapopulation in the Kogaluk watershed of northern Labrador, Canada. Samples were collected from eight lakes, genotyped with 17 microsatellites, and aged using opercula. Lakes varied in differentiation, historical and contemporary connectivity, and life history traits. Isolation by distance was detected only by removing two highly genetically differentiated lakes, suggesting a lack of migration–drift equilibrium and the lingering influence of historical factors on genetic structure. Bayesian analyses supported colonization via the Kogaluk's headwaters. The historical concentration of genetic diversity in headwaters inferred by this result was supported by high historical and contemporary effective sizes of the headwater lake, T‐Bone. Alternatively, reduced allelic richness in headwaters confirmed the dendritic structure's influence on gene flow, but this did not translate to an elevated metapopulation effective size. A lack of equilibrium and upstream migration may have dampened the effects of dendritic structure. We suggest that interacting historical and contemporary factors prevent the achievement of the idealized traits of a dendritic metapopulation in nature.     

Linkage Disequilibrium in Subdivided Populations

The linkage disequilibrium in a subdivided populaton is shown to be equal to the sum of the average linkage disequilibrium for all subpopulations and the covariance between gene frequencies of the loci concerned. Thus, in a subdivided population the linkage disequilibrium may not be 0 even if the linkage disequilibrium in each subpopulation is 0. If a population is divided into two subpopulations between which migration occurs, the asymptotic rate of approach to linkage equilibrium is equal to either r or 2(m(1) + m(2)) - (m(1) + m(2))(2), whichever is smaller, where r is the recombination value and m(1) and m(2) are the proportions of immigrants in subpopulations 1 and 2, respectively. Thus, if migration rate is high compared with recombination value, the change of linkage disequilibrium in subdivided populations is similar to that of a single random mating population. On the other hand, if migration rate is low, the approach to lnkage equilibrium may be retarded in subdivided populations. If isolated populations begin to exchange genes by migration, linkage disequilibrium may increase temporarily even for neutral loci. If overdominant selection operates and the equilibrium gene frequencies are different in the two subpopulations, a permanent linkage disequilibrium may be produced without epistasis in each subpopulation.     

Evolution of dispersal in density regulated populations: A haploid model

The evolution of dispersal is explored in a density-dependent framework. Attention is restricted to haploid populations in which the genotypic fitnesses at a single diallelic locus are decreasing functions of the changing number of individuals in the population. It is shown that migration between two populations in which the genotypic response to density is reversed can maintain both alleles when the intermigration rates are constant or nondecreasing functions of the population densities. There is always a unique symmetric interior equilibrium with equal numbers but opposite gene frequencies in the two populations, provided the system is not degenerate. Numerical examples with exponential and hyperbolic fitnesses suggest that this is the only stable equilibrium state under constant positive migration rates (m) less than . Practically speaking, however, there is only convergence after a reasonable number of generations for relatively small migration rates ( ). A migration-modifying mutant at a second, neutral locus, can successfully enter two populations at a stable migration-selection balance if and only if it reduces the intermigration rates of its carriers at the original equilibrium population size. Moreover, migration modification will always result in a higher equilibrium population size, provided the system approaches another symmetric interior equilibrium. The new equilibrium migration rate will be lower than that at the original equilibrium, even when the modified migration rate is a nondecreasing function of the population sizes. Therefore, as in constant viability models, evolution will lead to reduced dispersal.     

Contrasting patterns of selection and drift between two categories of immune genes in prairie‐chickens

Immune‐receptor genes of the adaptive immune system, such as the major histocompatibility complex (MHC), are involved in recognizing specific pathogens and are known to have high rates of adaptive evolution, presumably as a consequence of rapid co‐evolution between hosts and pathogens. In contrast, many ‘mediating’ genes of the immune system do not interact directly with specific pathogens and are involved in signalling (e.g. cytokines) or controlling immune cell growth. As a consequence, we might expect stronger selection at immune‐receptor than mediating genes, but these two types of genes have not been compared directly in wild populations. Here, we tested the hypothesis that selection differs between MHC (class I and II) and mediating genes by comparing levels of population differentiation across the range of greater prairie‐chickens (Tympanuchus cupido). As predicted, there was stronger population differentiation and isolation by distance at immune receptor (MHC) than at either mediating genes or neutral microsatellites, suggesting a stronger role of local adaptation at the MHC. In contrast, mediating genes displayed weaker differentiation between populations than neutral microsatellites, consistent with selection favouring similar alleles across populations for mediating genes. In addition to selection, drift also had a stronger effect on immune receptor (MHC) than mediating genes as indicated by the stronger decline of MHC variation in relation to population size. This is the first study in the wild to show that the effects of selection and drift on immune genes vary across populations depending on their functional role.     

Comparisons of life-history traits between clonal and sexual fish (Poeciliopsis:Poeciliidae) raised in monoculture and mixed treatments

Summary Clonal and sexual co-existence is common in a number of vertebrate taxa, even though the cost of sex makes such co-existence theoretically unlikely. The frozen niche-variation (FNV) model explains this co-existence on the basis of differences in overall niche breadth and competition between clones and sexuals. In the present study I examined two predictions of the FNV model. First, I examined the prediction that genetically variable populations have higher relative fitness when compared with monoclonal populations by comparing the performances of clonal and outcrossed sexual strains ofPoeciliopsis in monocultures at two densities. The prediction of increased overall productivity for the sexuals was verified, with net reproductive rates for the sexuals being between two and four times as high as the clones. Second, I tested the prediction that derived clones will successfully compete with their sexual progenitor(s) in the narrow range to which the clones are adapted, while the sexuals should co-exist because of their ability to use a wider range of resources than any single clone. I examined this prediction by comparing performance variables (e.g. growth, fecundity and survival) of each strain in pure culture with their partitioned performance from the mixed treatments. Clonal performance increased in mixtures compared to monocultures, as expected. However, the expectation that the sexual's performance would be less affected by mixtures than the clones' performance, was not met. The sexuals had reduced growth and fecundity on a par with the increase in both variables in the clones. Therefore, support for the FNV model was mixed. Although the performance in monocultures suggests that the sexuals have a wider niche breadth than the clones, performances in mixtures do not indicate such a relationship. Switching of behaviours or resource-use patterns between mixed and pure cultures may have caused the equivocal results.     

Dynamics of gene differentiation between incompletely isolated populations of unequal sizes

The progress of gene differentiation between two populations of unequal sizes is studied by taking into account the effects of mutation and migration. Explicit formulae for the eventual rate of approach to equilibrium of the probabilities of identity of genes by descent within and between populations are worked out for the case of small migration. Formulae for the equilibrium values of identity probabilities are also given for some special cases. It is shown that in the case of large migration the Nei—Feldman formulae for the temporal changes and equilibrium values of identity probabilities approximately hold, unless the sizes of the two populations are extremely different. The theories developed are applied to study the evolution of cave fish populations in Astyanax mexicanus. The approximate time after divergence between the cave and river populations of this fish has been estimated to be 525,000–710,000 years, which agrees well with the geological data on cave formation.     

Population structure and migration of the Tobacco Blue Mold Pathogen,Peronospora tabacina,into North America and Europe

Tobacco blue mold, caused by Peronospora tabacina, is an oomycete plant pathogen that causes yearly epidemics in tobacco (Nicotiana tabacum) in the United States and Europe. The genetic structure of P. tabacina was examined to understand genetic diversity, population structure and patterns of migration. Two nuclear loci, Igs2 and Ypt1, and one mitochondrial locus, cox2, were amplified, cloned and sequenced from fifty‐four isolates of P. tabacina from the United States, Central America–Caribbean–Mexico (CCAM), Europe and the Middle East (EULE). Cloned sequences from the three genes showed high genetic variability across all populations. Nucleotide diversity and the population mean mutation parameter per site (Watterson's theta) were higher in EULE and CCAM and lower in U.S. populations. Neutrality tests were significant and the equilibrium model of neutral evolution was rejected, indicating an excess of recent mutations or rare alleles. Hudson's S_nn tests were performed to examine population subdivision and gene flow among populations. An isolation‐with‐migration analysis (IM) supported the hypothesis of long‐distance migration of P. tabacina from the Caribbean region, Florida and Texas into other states in the United States. Within the European populations, the model documented migration from North Central Europe into western Europe and Lebanon, and migration from western Europe into Lebanon. The migration patterns observed support historical observations about the first disease introductions and movement in Europe. The models developed are applicable to other aerial dispersed emerging pathogens and document that high‐evolutionary‐risk plant pathogens can move over long distances to cause disease due to their large effective population size, population expansion and dispersal.     

Clonal erosion and genetic drift in cyclical parthenogens – the interplay between neutral and selective processes

The occurrence of alternating phases of clonal and sexual reproduction may strongly impact the interplay between neutral and selective genetic variation in populations. Using a physiologically structured model of the life history of Daphnia, we investigated to what extent clonal erosion associated with selection during the clonal phase affects the genetic structure as observed by neutral markers. Incorporating conservative levels of quantitative genetic variation at 11 physiological and life history traits induces strong clonal erosion, reducing clonal diversity (CD) near the end of the simulations (1000 days) to a level between 1 and 5, even in habitats with high initial CD (10⁸ clones). This strong clonal erosion caused by selection can result in reduced genetic diversity, significant excess of heterozygotes and significant genetic differentiation between populations as observed by neutral markers. Our results indicate that, especially in relatively small habitats, clonal selection may strongly impact the genetic structure and may contribute to the often observed high level of neutral genetic differentiation among natural populations of cyclical parthenogens.     

The consequences of gene flow for local adaptation and differentiation: a two-locus two-deme model

We consider a population subdivided into two demes connected by migration in which selection acts in opposite direction. We explore the effects of recombination and migration on the maintenance of multilocus polymorphism, on local adaptation, and on differentiation by employing a deterministic model with genic selection on two linked diallelic loci (i.e., no dominance or epistasis). For the following cases, we characterize explicitly the possible equilibrium configurations: weak, strong, highly asymmetric, and super-symmetric migration, no or weak recombination, and independent or strongly recombining loci. For independent loci (linkage equilibrium) and for completely linked loci, we derive the possible bifurcation patterns as functions of the total migration rate, assuming all other parameters are fixed but arbitrary. For these and other cases, we determine analytically the maximum migration rate below which a stable fully polymorphic equilibrium exists. In this case, differentiation and local adaptation are maintained. Their degree is quantified by a new multilocus version of $F_\mathrm{ST}$ and by the migration load, respectively. In addition, we investigate the invasion conditions of locally beneficial mutants and show that linkage to a locus that is already in migration-selection balance facilitates invasion. Hence, loci of much smaller effect can invade than predicted by one-locus theory if linkage is sufficiently tight. We study how this minimum amount of linkage admitting invasion depends on the migration pattern. This suggests the emergence of clusters of locally beneficial mutations, which may form ‘genomic islands of divergence’. Finally, the influence of linkage and two-way migration on the effective migration rate at a linked neutral locus is explored. Numerical work complements our analytical results.     

Isolation of early genes expressed in reproductive organs of the dioecious white campion (Silene latifolia) by subtraction cloning using an asexual mutant

The dioecious white campion (Silene latifolia) has been chosen as a working model for sexual development. In this species, sexual dimorphism is achieved through two distinct developmental blocks: inhibition of carpel development in male flowers, and early arrest of anther differentiation in female flowers. The combined advantages of the dioecious system and the availability of a sexual mutant lacking both male and female reproductive organs have been exploited in a molecular subtraction approach using male and asexual flower buds. This resulted in the cloning of 22 cDNA clones expressed in stamens at distinct stages of development. Fourteen of these clones corresponded to genes whose expression was detected in pre-meiotic stamens, a stage of development for which very little information is presently available. Furthermore, the absence of similarities with database sequences for ten clones suggests that they represent novel genes. Functional analysis of each clone will enable their positioning within the reproductive organ developmental pathway(s). In parallel, these clones are being exploited as developmental markers of early differentiation within the flower.     

Population genetics of complex life-cycle parasites: an illustration with trematodes

Accurate inferences on population genetics data require a sound underlying theoretical null model. Organisms alternating sexual and asexual reproduction during their life-cycle have been largely neglected in theoretical population genetic models, thus limiting the biological interpretation of population genetics parameters measured in natural populations. In this article, we derive the expectations of those parameters for the life-cycle of monoecious trematodes, a group comprising several important human and livestock parasites that obligatorily alternate sexual and asexual reproduction during their life-cycle. We model how migration rates between hosts, sexual and asexual mutation rates, adult selfing rate and the variance in reproductive success of parasites during the clonal phase affect the amount of neutral genetic diversity of the parasite (effective population size) and its apportionment within and between definitive hosts (using F-statistics). We demonstrate, in particular, that variance in reproductive success of clones, a parameter that has been completely overlooked in previous population genetics models, is very important in shaping the distribution of the genetic variability both within and among definitive hosts. Within definitive hosts, the parameter F(IS) (a measure of the deviation from random mating) is decreased by high variance in clonal reproductive success of larvae but increased by high adult self-fertilisation rates. Both clonal multiplication and selfing have similar effects on between-host genetic differentiation (F(ST)). Migration occurring before and after asexual reproduction can have different effects on the patterns of F(IS), depending on values of the other parameters such as the mutation rate. While the model applies to any hermaphroditic organism alternating sexual and clonal reproduction (e.g. many plants), the results are specifically discussed in the light of the limited population genetic data on monoecious trematodes available to date and their previous interpretation. We hope that our model will encourage more empirical population genetics studies on monoecious trematodes and other organisms with similar life-cycles.     

A genetic metapopulation model for reef fishes in oceanic islands: the case of the surgeonfish,Acanthurus triostegus

Allozyme data on a surgeonfish, Acanthurus triostegus, were analysed from 10 islands in French Polynesia. We compared estimates of gene flow according to the hypothesis of an equilibrium between genetic drift and migration, and estimated genetic divergence times assuming complete genetic isolation without gene flow since foundation. The significant correlation between genetic divergence and geographic distance, at the within-archipelago level (r = 0.709, P = 0.024) indicates exchange of individuals mainly between neighbouring populations. The correlation was, however, not significant at the among-archipelagoes level (r = 0.325, P = 0.330), suggesting that long distance migrations are more sporadic. This addresses the problem of scale in population biology. According to the spatial scale of analysis, results can change from an island model, with no relation between genetic differentiation and geographical distances between archipelagos, to an isolation-by-distance model within an archipelago. These factors lead us to propose a “patchy population” model, in which all patches are occupied and reproductively active, though with few successful migrations between neighbouring populations. This model describes a subdivided population that is stable through time, with an amount of gene flow small enough to allow significant local differentiation in neutral gene frequency, but high enough to prevent differential fixation in the long term, and therefore preserving the genetic cohesion of the species.     

THE EFFECT OF COLLECTIVE DISPERSAL ON THE GENETIC STRUCTURE OF A SUBDIVIDED POPULATION

Correlated dispersal paths between two or more individuals are widespread across many taxa. The population genetic implications of this collective dispersal have received relatively little attention. Here we develop two‐sample coalescent theory that incorporates collective dispersal in a finite island model to predict expected coalescence times, genetic diversities, and F‐statistics. We show that collective dispersal reduces mixing in the system, which decreases expected coalescence times and increases F_ST. The effects are strongest in systems with high migration rates. Collective dispersal breaks the invariance of within‐deme coalescence times to migration rate, whatever the deme size. It can also cause F_ST to increase with migration rate because the ratio of within‐ to between‐deme coalescence times can decrease as migration rate approaches unity. This effect is most biologically relevant when deme size is small. We find qualitatively similar results for diploid and gametic dispersal. We also demonstrate with simulations and analytical theory the strong similarity between the effects of collective dispersal and anisotropic dispersal. These findings have implications for our understanding of the balance between drift–migration–mutation in models of neutral evolution. This has applied consequences for the interpretation of genetic structure (e.g., chaotic genetic patchiness) and estimation of migration rates from genetic data.     

PHASE III OF WRIGHT'S SHIFTING BALANCE PROCESS AND THE VARIANCE AMONG DEMES IN MIGRATION RATE

Interdemic selection by the differential migration of individuals out from demes of high fitness and into demes of low fitness (Phase III) is one of the most controversial aspects of Wright's Shifting Balance Theory. I derive a relationship between Phase III migration and the interdemic selection differential, S, and show its potential effect on F_ST. The relationship reveals a diversifying effect of interdemic selection by Phase III migration on the genetic structure of a metapopulation. Using experimental metapopulations, I explored the effect of Phase III migration on F_ST by comparing the genetic variance among demes for two different patterns of migration: (1) island model migration and (2) Wright's Phase III migration. Although mean migration rates were the same, I found that the variance among demes in migration rate was significantly higher with Phase III than with island model migration. As a result, F_ST for the frequency of a neutral marker locus was higher with Phase III than it was with island model migration. By increasing F_ST, Phase III enhanced the genetic differentiation among demes for traits not subject to interdemic selection. This feature makes Wright's process different from individual selection which, by reducing effective population size, decreases the genetic variance within demes for all other traits. I discussed this finding in relation to the efficacy of Phase III and random migration for effecting peak shifts, and the contribution of genes with indirect effects to among‐deme variation.     

Neutral processes forming large clones during colonization of new areas

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species’ range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonization starts with an asexual wave. After colonization is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long‐distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a post‐glacially established seaweed species (Fucus radicans) shows that in this case, a neutral mechanism is strongly supported.