Signatures Of Directional Selection Research Articles

Diversity and Molecular Evolution of Antimicrobial Peptides in Caecilian Amphibians.

Antimicrobial peptides (AMPs) are key molecules in the innate immune defence of vertebrates with rapid action, broad antimicrobial spectrum, and ability to evade pathogen resistance mechanisms. To date, amphibians are the major group of vertebrates from which most AMPs have been characterised, but most studies have focused on the bioactive skin secretions of anurans (frogs and toads). In this study, we have analysed the complete genomes and/or transcriptomes of eight species of caecilian amphibians (order Gymnophiona) and characterised the diversity, molecular evolution, and antimicrobial potential of the AMP repertoire of this order of amphibians. We have identified 477 candidate AMPs within the studied caecilian genome and transcriptome datasets. These candidates are grouped into 29 AMP families, with four corresponding to peptides primarily exhibiting antimicrobial activity and 25 potentially serving as AMPs in a secondary function, either in their entirety or after cleavage. In silico prediction methods were used to identify 62 of those AMPs as peptides with promising antimicrobial activity potential. Signatures of directional selection were detected for five candidate AMPs, which may indicate adaptation to the different selective pressures imposed by evolutionary arms races with specific pathogens. These findings provide encouraging support for the expectation that caecilians, being one of the least-studied groups of vertebrates, and with ~300 million years of separate evolution, are an underexplored resource of great pharmaceutical potential that could help to contest antibiotic resistance and contribute to biomedical advance.

Genome-wide SNP analysis of Plasmodium falciparum shows differentiation at drug-resistance-associated loci among malaria transmission settings in southern Mali.

Plasmodium falciparum malaria cases in Africa represent over 90% of the global burden with Mali being amongst the 11 highest burden countries that account for 70% of this annual incidence. The persistence of P. falciparum despite massive global interventions is because of its genetic diversity that drives its ability to adapt to environmental changes, develop resistance to drugs, and evade the host immune system. Knowledge on P. falciparum genetic diversity across populations and intervention landscape is thus critical for the implementation of new strategies to eliminate malaria. This study assessed genetic variation with 12,177 high-quality SNPs from 830 Malian P. falciparum isolates collected between 2007 and 2017 from seven locations. The complexity of infections remained high, varied between sites, and showed a trend toward overall decreasing complexity over the decade. Though there was no significant substructure, allele frequencies varied geographically, partly driven by temporal variance in sampling, particularly for drug resistance and antigen loci. Thirty-two mutations in known drug resistance markers (pfcrt, pfdhps, pfdhfr, pfmdr1, pfmdr2, and pfk13) attained a frequency of at least 2% in the populations. SNPs within and around the major markers of resistance to quinolines (pfmdr1 and pfcrt) and antifolates (pfdhfr and pfdhps) varied temporally and geographically, with strong linkage disequilibrium and signatures of directional selection in the genome. These geo-temporal populations also differentiated at alleles in immune-related loci, including, protein E140, pfsurfin8, pfclag8, and pfceltos, as well as pftrap, which showed signatures of haplotype differentiation between populations. Several regions across the genomes, including five known drug resistance loci, showed signatures of differential positive selection. These results suggest that drugs and immune pressure are dominant selective forces against P. falciparum in Mali, but their effect on the parasite genome varies temporally and spatially. Interventions interacting with these genomic variants need to be routinely evaluated as malaria elimination strategies are implemented.

Candidate genes and signatures of directional selection on fruit quality traits during apple domestication.

During plant domestication, traits can be subject to a variety of types of selection, ranging from strong directional selection for traits such as seed or fruit size to diversifying selection for traits like color or flavor. These types of selection interact with other evolutionary processes including genetic bottlenecks and interspecific gene flow to generate different levels of genetic diversity across the genome and at target genes in domesticated lineages, but little is known about the impacts of these processes in perennial fruit crops. We used sequence capture by hybridization to examine patterns of diversity at a suite of candidate domestication and anonymous background genes in domesticated apple (Malus ×domestica) in comparison to its wild relatives Malus sieversii and Malus orientalis. We found no change in average diversity at these candidate domestication genes across the three species. However, a subset of the genes did exhibit patterns of very high or very low diversity in M. ×domestica compared to its progenitor, M. sieversii. Of the genes with characterized function, the low-diversity genes mainly contributed to fruit quality traits like color and flavor, predicted to be under conscious, directional selection relatively late in the domestication process, while the high-diversity genes included a variety of functions. Overall, these results are consistent with predictions based on the likely timing and nature of selection during domestication and open new avenues for understanding genes with high diversity in a perennial crop compared to its wild relatives.

Heterogeneous Temporal Contrast Adaptation in Drosophila Direction-Selective Circuits.

In visual systems, neurons adapt both to the mean light level and to the range of light levels, or the contrast. Contrast adaptation has been studied extensively, but it remains unclear how it is distributed among neurons in connected circuits, and how early adaptation affects subsequent computations. Here, we investigated temporal contrast adaptation in neurons across Drosophila's visual motion circuitry. Several ON-pathway neurons showed strong adaptation to changes in contrast over time. One of these neurons, Mi1, showed almost complete adaptation on fast timescales, and experiments ruled out several potential mechanisms for its adaptive properties. When contrast adaptation reduced the gain in ON-pathway cells, it was accompanied by decreased motion responses in downstream direction-selective cells. Simulations show that contrast adaptation can substantially improve motion estimates in natural scenes. The benefits are larger for ON-pathway adaptation, which helps explain the heterogeneous distribution of contrast adaptation in these circuits.

Signatures of directional selection in a hybrid yeast population.

Meiotic recombination is a fundamental biological process that leads to crossover and gene conversion. High resolution maps of meiotic recombination have been reported in several model organisms. However, few studies have studied how rapidly selection affects the products of meiotic recombination. Here we constructed and sequenced a yeast population of 38 haploid strains derived from hybridizations of two common used strains S288c and YJM789. We identified 20 regions with strong biased allele frequency across the genome, revealing signatures of selection in a rather short period. These regions harbor ample crossovers and gene conversions, which enable us to trace how selection works on the genomic fragments after meiosis. The total length of such regions under selection accounts for 5% of the entire genome, and those regions contain many functional-related genes. In addition, recombination breaks down linkage disequilibrium to half of its maximum within 42 kb and reduces nucleotide diversity significantly in selected regions. Our study thus provides details of directional selection on the outcomes of meiotic recombination using experimental approaches, and will shed light on our understanding of the fast reshaping of population structure by selection, as well as the important roles of recombination.

Bi-directional Control of Walking Behavior by Horizontal Optic Flow Sensors

Moving animals experience constant sensory feedback, such as panoramic image shifts on the retina, termed optic flow. Underlying neuronal signals are thought to be important for exploratory behavior by signaling unintended course deviations and by providing spatial information about the environment [1, 2]. Particularly in insects, the encoding of self-motion-related optic flow is well understood [1-5]. However, a gap remains in understanding how the associated neuronal activity controls locomotor trajectories. In flies, visual projection neurons belonging to two groups encode panoramic horizontal motion: horizontal system (HS) cells respond with depolarization to front-to-back motion and hyperpolarization to the opposite direction [6, 7], and other neurons havethe mirror-symmetrical response profile [6, 8, 9]. With primarily monocular sensitivity, the neurons' responses are ambiguous for different rotational andtranslational self-movement components. Such ambiguities can be greatly reduced by combining signals from both eyes [10-12] to determine turning and movement speed [13-16]. Here, we explore the underlying functional logic by optogenetic HS cell manipulation in tethered walking Drosophila. We show that de- and hyperpolarization evoke opposite turning behavior, indicating that both direction-selective signals are transmitted to descending pathways for course control. Further experiments reveal a negative effect of bilaterally symmetric de- and hyperpolarization on walking velocity. Our results are therefore consistent with a functional architecture in which the HS cells' membrane potential influences walking behavior bi-directionally via two decelerating pathways.

Signatures of selection in embryonic transcriptomes of lizards adapting in parallel to cool climate.

Populations adapting independently to the same environment provide important insights into the repeatability of evolution at different levels of biological organization. In the 20th century, common wall lizards (Podarcis muralis) from southern and western Europe were introduced to England, north of their native range. Nonnative populations of both lineages have adapted to the shorter season and lower egg incubation temperature by increasing the absolute rate of embryonic development. Here, we tested if this adaptation is accompanied by signatures of directional selection in the transcriptomes of early embryos and, if so, if nonnative populations show adaptive convergence. Embryos from nonnative populations exhibited gene expression profiles consistent with directional selection following introduction, but different genes were affected in the two lineages. Despite this, the functional enrichment of genes that changed their expression following introduction showed substantial similarity between lineages, and was consistent with mechanisms that should promote developmental rate. Moreover, the divergence between nonnative and native populations was enriched for genes that were temperature-responsive in native populations. These results indicate that small populations are able to adapt to new climatic regimes, but the means by which they do so may largely be determined by founder effects and other sources of genetic drift.

Exploring and Harnessing Haplotype Diversity to Improve Yield Stability in Crops.

In order to meet future food, feed, fiber, and bioenergy demands, global yields of all major crops need to be increased significantly. At the same time, the increasing frequency of extreme weather events such as heat and drought necessitates improvements in the environmental resilience of modern crop cultivars. Achieving sustainably increase yields implies rapid improvement of quantitative traits with a very complex genetic architecture and strong environmental interaction. Latest advances in genome analysis technologies today provide molecular information at an ultrahigh resolution, revolutionizing crop genomic research, and paving the way for advanced quantitative genetic approaches. These include highly detailed assessment of population structure and genotypic diversity, facilitating the identification of selective sweeps and signatures of directional selection, dissection of genetic variants that underlie important agronomic traits, and genomic selection (GS) strategies that not only consider major-effect genes. Single-nucleotide polymorphism (SNP) markers today represent the genotyping system of choice for crop genetic studies because they occur abundantly in plant genomes and are easy to detect. SNPs are typically biallelic, however, hence their information content compared to multiallelic markers is low, limiting the resolution at which SNP–trait relationships can be delineated. An efficient way to overcome this limitation is to construct haplotypes based on linkage disequilibrium, one of the most important features influencing genetic analyses of crop genomes. Here, we give an overview of the latest advances in genomics-based haplotype analyses in crops, highlighting their importance in the context of polyploidy and genome evolution, linkage drag, and co-selection. We provide examples of how haplotype analyses can complement well-established quantitative genetics frameworks, such as quantitative trait analysis and GS, ultimately providing an effective tool to equip modern crops with environment-tailored characteristics.

Direct Measurement of Correlation Responses in Drosophila Elementary Motion Detectors Reveals Fast Timescale Tuning

Animals estimate visual motion by integrating light intensity information over time and space. The integration requires nonlinear processing, which makes motion estimation circuitry sensitive to specific spatiotemporal correlations that signify visual motion. Classical models of motion estimation weight these correlations to produce direction-selective signals. However, the correlational algorithms they describe have not been directly measured in elementary motion-detecting neurons (EMDs). Here, we employed stimuli to directly measure responses to pairwise correlations in Drosophila's EMD neurons, T4 and T5. Activity in these neurons was required for behavioral responses to pairwise correlations and was predictive of those responses. The pattern of neural responses in the EMDs was inconsistent with one classical model of motion detection, and the timescale and selectivity of correlation responses constrained the temporal filtering properties in potential models. These results reveal how neural responses to pairwise correlations drive visual behavior in this canonical motion-detecting circuit.

Inhibitory input to the direction-selective ganglion cell is saturated at low contrast.

Direction-selective ganglion cells (DSGCs) respond selectively to motion toward a "preferred" direction, but much less to motion toward the opposite "null" direction. Directional signals in the DSGC depend on GABAergic inhibition and are observed over a wide range of speeds, which precludes motion detection based on a fixed temporal correlation. A voltage-clamp analysis, using narrow bar stimuli similar in width to the receptive field center, demonstrated that inhibition to DSGCs saturates rapidly above a threshold contrast. However, for wide bar stimuli that activate both the center and surround, inhibition depends more linearly on contrast. Excitation for both wide and narrow bars was also more linear. We propose that positive feedback, likely within the starburst amacrine cell or its network, produces steep saturation of inhibition at relatively low contrast. This mechanism renders GABA release essentially contrast and speed invariant, which enhances directional signals for small objects and thereby increases the signal-to-noise ratio for direction-selective signals in the spike train over a wide range of stimulus conditions. The steep saturation of inhibition confers to a neuron immunity to noise in its spike train, because when inhibition is strong no spikes are initiated.

The evolution and adaptive potential of transcriptional variation in sticklebacks--signatures of selection and widespread heritability.

Evidence implicating differential gene expression as a significant driver of evolutionary novelty continues to accumulate, but our understanding of the underlying sources of variation in expression, both environmental and genetic, is wanting. Heritability in particular may be underestimated when inferred from genetic mapping studies, the predominant “genetical genomics” approach to the study of expression variation. Such uncertainty represents a fundamental limitation to testing for adaptive evolution at the transcriptomic level. By studying the inheritance of expression levels in 10,495 genes (10,527 splice variants) in a threespine stickleback pedigree consisting of 563 individuals, half of which were subjected to a thermal treatment, we show that 74–98% of transcripts exhibit significant additive genetic variance. Dominance variance is also prevalent (41–99% of transcripts), and genetic sources of variation seem to play a more significant role in expression variance in the liver than a key environmental variable, temperature. Among-population comparisons suggest that the majority of differential expression in the liver is likely due to neutral divergence; however, we also show that signatures of directional selection may be more prevalent than those of stabilizing selection. This predominantly aligns with the neutral model of evolution for gene expression but also suggests that natural selection may still act on transcriptional variation in the wild. As genetic variation both within- and among-populations ultimately defines adaptive potential, these results indicate that broad adaptive potential may be found within the transcriptome.

A recent local sweep at thePHYAlocus in the Northern European Spiterstulen population ofArabidopsis lyrata

Northern and central European Arabidopsis lyrata ssp. petraea populations are locally adapted to prevailing climatic conditions through differences in timing of life history events. The timing of flowering and, in perennials, the timing of growth cessation influence fitness. Phytochrome A may have an important role in regulating these life history traits as it perceives changes in daylength. We asked whether PHYA has contributed to local adaptation to the northern conditions in A. l. petraea. To search for signals of directional selection at the PHYA locus, we resequenced PHYA and 9 short fragments around PHYA from a 57-kb region from a German (Plech) and a Norwegian (Spiterstulen) population and compared patterns of differentiation and diversity to a set of 19 reference loci around the genome. First, we found that the populations were highly differentiated: there were three nonsynonymous fixed differences at the PHYA locus, which was in stark contrast with the total four fixed differences in the 19 reference loci. Compatible with a sweep hypothesis, variation was almost completely removed from the 9.4-kb region around PHYA in the northern Spiterstulen population. The overall level of linkage disequilibrium (LD) was higher in Spiterstulen, but there was no LD across the PHYA locus in the population, which is also a known consequence of a selective sweep. The sweep has likely occurred after the last glacial maximum, which suggests that it has contributed to adaptation to the northern conditions.

Directional selection against different life histories in the Arctic lamprey (Lethenteron camtschaticum): identification by microsatellite analysis

To understand the selective forces promoting life history evolution in lampreys, footprints of natural selection were identified using the hitchhiking mapping approach for microsatellite loci. To achieve the objective, the Arctic lamprey (Lethenteron camtschaticum), including anadromous and recently derived landlocked-fluvial populations, were targeted. Seven microsatellites were genotyped and subjected to population genetic analyses and outlier tests. The results indicated that a significant signature of directional selection could be detected in one microsatellite locus (Lspn013). Discrepancies in allelic frequencies of Lspn013 were remarkable between anadromous and landlocked-fluvial populations, leading to high population genetic divergence between them. Based on a homology search, Lspn013 was estimated to be located near the vasotocin precursor gene, which plays an important role in osmoregulation. Life history differences in lampreys have resulted from habitat difference in adult phase, which have probably led to recent natural selection on the gene responsible for osmoregulatory functions.

Neuronal representation of visual motion and orientation in the fly medulla

In insects, the first extraction of motion and direction clues from local brightness modulations is thought to take place in the medulla. However, whether and how these computations are represented in the medulla stills remain widely unknown, because electrical recording of the neurons in the medulla is difficult. As an effort to overcome this difficulty, we employed local electroporation in vivo in the medulla of the blowfly (Calliphora vicina) to stain small ensembles of neurons with a calcium-sensitive dye. We studied the responses of these neuronal ensembles to spatial and temporal brightness modulations and found selectivity for grating orientation. In contrast, the responses to the two opposite directions of motion of a grating with the same orientation were similar in magnitude, indicating that strong directional selectivity is either not present in the types of neurons covered by our data set, or that direction-selective signals are too closely spaced to be distinguished by our calcium imaging. The calcium responses also showed a bell-shaped dependency on the temporal frequency of drifting gratings, with an optimum higher than that observed in one of the subsequent processing stages, i.e., the lobula plate. Medulla responses were elicited by on- as well as off-stimuli with some spatial heterogeneity in the sensitivity for “on” and “off”, and in the polarity of the responses. Medulla neurons thus show similarities to some established principles of motion and edge detection in the vertebrate visual system.

GLOBAL ANALYSIS OF GENES INVOLVED IN FRESHWATER ADAPTATION IN THREESPINE STICKLEBACKS (GASTEROSTEUS ACULEATUS)

Examples of parallel evolution of phenotypic traits have been repeatedly demonstrated in threespine sticklebacks (Gasterosteus aculeatus) across their global distribution. Using these as a model, we performed a targeted genome scan--focusing on physiologically important genes potentially related to freshwater adaptation--to identify genetic signatures of parallel physiological evolution on a global scale. To this end, 50 microsatellite loci, including 26 loci within or close to (<6 kb) physiologically important genes, were screened in paired marine and freshwater populations from six locations across the Northern Hemisphere. Signatures of directional selection were detected in 24 loci, including 17 physiologically important genes, in at least one location. Although no loci showed consistent signatures of selection in all divergent population pairs, several outliers were common in multiple locations. In particular, seven physiologically important genes, as well as reference ectodysplasin gene (EDA), showed signatures of selection in three or more locations. Hence, although these results give some evidence for consistent parallel molecular evolution in response to freshwater colonization, they suggest that different evolutionary pathways may underlie physiological adaptation to freshwater habitats within the global distribution of the threespine stickleback.

Genetic (Co)Variation for Life Span in Rhabditid Nematodes: Role of Mutation, Selection, and History

The evolutionary mechanisms maintaining genetic variation in life span, particularly post-reproductive life span, are poorly understood. We characterized the effects of spontaneous mutations on life span in the rhabditid nematodes Caenorhabditis elegans and C. briggsae and standing genetic variance for life span and correlation of life span with fitness in C. briggsae. Mutations decreased mean life span, a signature of directional selection. Mutational correlations between life span and fitness were consistently positive. The average selection coefficient against new mutations in C. briggsae was approximately 2% when homozygous. The pattern of phylogeographic variation in life span is inconsistent with global mutation-selection balance (MSB), but MSB appears to hold at the local level. Standing genetic correlations in C. briggsae reflect mutational correlations at a local scale but not at a broad phylogeographic level. At the local scale, results are broadly consistent with predictions of the "mutation accumulation" hypothesis for the evolution of aging.

Comparative mitochondrial genomics within and among species of killifish

BackgroundThis study was motivated by the observation of unusual mitochondrial haplotype distributions and associated physiological differences between populations of the killifish Fundulus heteroclitus distributed along the Atlantic coast of North America. A distinct "northern" haplotype is fixed in all populations north of New Jersey, and does not appear south of New Jersey except in extreme upper-estuary fresh water habitats, and northern individuals are known to be more tolerant of hyposmotic conditions than southern individuals. Complete mitochondrial genomes were sequenced from individuals from northern coastal, southern coastal, and fresh water populations (and from out-groups). Comparative genomics approaches were used to test multiple evolutionary hypotheses proposed to explain among-population genome variation including directional selection and hybridization.ResultsStructure and organization of the Fundulus mitochondrial genome is typical of animals, yet subtle differences in substitution patterns exist among populations. No signals of directional selection or hybridization were detected. Mitochondrial genes evolve at variable rates, but all genes exhibit very low dN/dS ratios across all lineages, and the southern population harbors more synonymous polymorphism than other populations.ConclusionEvolution of mitochondrial genomes within Fundulus is primarily governed by interaction between strong purifying selection and demographic influences, including larger historical population size in the south. Though directional selection and hybridization hypotheses were not supported, adaptive processes may indirectly contribute to partitioning of variation between populations.

A Motion Aftereffect From Still Photographs Depicting Motion

A photograph of an action can convey a vivid sense of motion. Does the inference of motion from viewing a photograph involve the same neural and psychological representations used when one views physical motion? In this study, we tested whether implied motion is represented by the same direction-selective signals involved in the perception of real motion. We made use of the motion aftereffect, a visual motion illusion. Three experiments showed that viewing a series of static photographs with implied motion in a particular direction produced motion aftereffects in the opposite direction, as assessed with real-motion test probes. The transfer of adaptation from motion depicted in photographs to real motion demonstrates that the perception of implied motion activates direction-selective circuits that are also involved in processing real motion.

Reshaping the binding problem of form and motion vision

The primary visual cortex (or V1) is the first cortical processing stage in primates and receives visual information from the retina via several parallel pathways. Cortical regions beyond V1 are segregated into two major streams: dorsal and ventral. The dorsal route takes information from V1 to the posterior parietal cortex, while the ventral pathway takes a route from V1 to the inferotemporal cortex. The ventral visual stream is tuned for object identification, while the dorsal stream is tuned to detect where objects are located and in which direction they move (Ungerleider & Mishkin, 1982; Ungerleider & Pasternak, 2003). The apparent functional separation between the two paths has created a theoretical challenge. How can the information in the two systems be brought together across cortical space: the visual binding problem (Roskies, 1999). A good example of this binding problem relates to form and motion processing. It is generally believed that motion direction is coded by the dorsal stream and only combined with form processing from the ventral stream at higher cortical levels (e.g. Oram & Perret, 1994). The segregation into form and motion paths begins in V1 where only certain horizontal layers have been shown to have large proportions of direction-selective cells (Hawken et al. 1988). V1 has six layers of neural tissue each being subdivided into functionally distinct sublayers (Fig. 1; Lund, 1988). In primates only layers 4B, 4Cα and 6 have been shown to contain a high proportion of direction-selective cells. Layer 4Cα receives input from the retina via the dorsal lateral geniculate nucleus and transfers signals to layers 4B and 6, which in turn send information to the dorsal stream (Movshon & Newsome, 1996). There has been little evidence of highly direction-selective signals being sent to the ventral pathway. Schematic diagram of the direction-selective (DS) pathways in V1 However, the study by Gur & Snodderly (2007) in this issue of The Journal of Physiology reveals a new pathway for direction-selective neurons that targets the ventral stream (also see Gur et al. 2005). The route begins in the primary visual cortex in sublayer 4Cα (Fig. 1) from where it travels a short distance to a neighbouring sublayer, 4 Cm, which is a thin sheet of cell bodies and dendrites wedged in the middle (m) of layer 4C (Yabuta & Callaway, 1998). The axons of neurons in layer 4Cm travel upwards through V1 and innervate direction-selective neurons in layer 3 (Gur & Snodderly, 2007), from where information is sent to the ventral stream (Ungerleider & Pasternak, 2003). It appears that the ventral visual pathway has no need to extract motion information from the dorsal stream across wide reaches of cortical space. Rather, the ventral path is supplied by its own motion input at source. This is significant because trying to combine form and motion signals that have already travelled through many separate cortical areas, each adding variable transmission delays, is a complex task. Direction-selective neurons in V1 that send motion information to the ventral stream are highly discriminating. They must be activated with a stimulus of exactly the correct orientation, moving in the correct direction and doing so within a very restricted patch of visual space. It is presumed that these cells are involved in fine motion discrimination during fixation, e.g. detecting the movement of facial features. Gur & Snodderly (2007) have also found consistent differences between the directional properties of cells in the other direction-selective V1 layers (4B and 6), which are destined for the dorsal pathway. Taken as a whole it appears that three distinct motion pathways exist in V1, two targeting the dorsal pathway, and one targeting ventral destinations (Fig. 1). Gur & Snodderly (2007) have uncovered a functional pathway by which directional information reaches the ventral stream without first travelling along the dorsal stream. This finding reduces the spatial binding problem by removing the need to combine form and motion information from widely separate brain regions. A binding issue still remains as form and motion need to be united, but at least this can happen in neighbouring cells that have experienced similar transmission delays.

The power of allele frequency comparisons to detect the footprint of selection in natural and experimental situations

Recently, inter-population comparisons of allele frequencies to detect past selection haven gained popularity. Data from genome-wide scans are used to detect the number and position of genes that have responded to unknown selection pressures in natural populations, or known selection pressures in experimental lines. Yet, the limitations and possibilities of these methods have not been well studied. In this paper, the objectives were (1) to investigate the distance over which a signal of directional selection is detectable under various scenarios, and (2) to study the power of the method depending on the properties of the used markers, for both natural populations and experimental set-ups. A combination of recurrence equations and simulations was used. The results show that intermediate strength selection on new mutations can be detected with a marker spacing of about 0.5 cM in large natural populations, 200 to 400 generations after the divergence of subpopulations. In experimental situations, only strong selection will be detectable, while markers can be spaced a few cM apart. Adaptation from standing variation in the base population will be hard to detect, though some solutions are presented for experimental designs.