Generation of a neutral FST baseline for testing local adaptation on gill raker number within and between European whitefish ecotypes in the Baltic Sea basin

Divergent selection at ecologically important traits is thought to be a major factor driving phenotypic differentiation between populations. To elucidate the role of different evolutionary processes shaping the variation in gill raker number of European whitefish (Coregonus lavaretus sensu lato) in the Baltic Sea basin, we assessed the relationships between genetic and phenotypic variation among and within three whitefish ecotypes (sea spawners, river spawners and lake spawners). To generate expected neutral distribution of F_ST and to evaluate whether highly variable microsatellite loci resulted in deflated F_ST estimates compared to less variable markers, we performed population genetic simulations under finite island and hierarchical island models. The genetic divergence observed among (F_CT = 0.010) and within (F_ST = 0.014–0.041) ecotypes was rather low. The divergence in gill raker number, however, was substantially higher between sea and river spawners compared to observed microsatellite data and simulated neutral baseline (P_CT > F_CT). This suggests that the differences in gill raker number between sea and river spawners are likely driven by divergent natural selection. We also found strong support for divergent selection on gill raker number among different populations of sea spawners (P_ST > F_ST), most likely caused by highly variable habitat use and diverse diet. The putative role of divergent selection within lake spawners initially inferred from empirical microsatellite data was not supported by simulated F_ST distributions. This work provides a first formal test of divergent selection on gill raker number in Baltic whitefish, and demonstrates the usefulness of population genetic simulations to generate informative neutral baselines for P_ST–F_ST analyses helping to disentangle the effects of stochastic evolutionary processes from natural selection.     

The effects of selection,drift and genetic variation on life‐history trait divergence among insular populations of natterjack toad,Bufo calamita

Although loss of genetic variation is frequently assumed to be associated with loss of adaptive potential, only few studies have examined adaptation in populations with little genetic variation. On the Swedish west coast, the northern fringe populations of the natterjack toad Bufo calamita inhabit an atypical habitat consisting of offshore rock islands. There are strong among‐population differences in the amount of neutral genetic variation, making this system suitable for studies on mechanisms of trait divergence along a gradient of within‐population genetic variation. In this study, we examined the mechanisms of population divergence using Q_ST–F_ST comparisons and correlations between quantitative and neutral genetic variation. Our results suggest drift or weak stabilizing selection across the six populations included in this study, as indicated by low Q_ST–F_ST values, lack of significant population × temperature interactions and lack of significant differences among the islands in breeding pond size. The six populations included in this study differed in both neutral and quantitative genetic variation. Also, the correlations between neutral and quantitative genetic variation tended to be positive, however, the relatively small number of populations prevents any strong conclusions based on these correlations. Contrary to the majority of Q_ST–F_ST comparisons, our results suggest drift or weak stabilizing selection across the examined populations. Furthermore, the low heritability of fitness‐related traits may limit evolutionary responses in some of the populations.     

Remarkable life history polymorphism may be evolving under divergent selection in the silverleaf sunflower

Substantial intraspecific variation in life history is rare and potentially a signal of incipient ecological speciation, if variation is driven by geographically heterogenous natural selection. We present the first report of extensive life history polymorphism in Helianthus argophyllus, the silverleaf sunflower, and examine evidence for its evolution by divergent selection. In 18 populations sampled from across the species range and grown in a common garden, most quantitative traits covaried such that individuals could be assigned to two distinct life history syndromes: tall and late flowering with small initial flowerheads, or short and early flowering with larger initial flowerheads. Helianthus argophyllus exhibits regional genetic structure, but this population structure does not closely correspond with patterns of phenotypic variation. The early‐flowering syndrome is primarily observed in populations from coastal barrier islands, while populations from the nearby mainland coast, although geographically and genetically close, are primarily late flowering. Additionally, several traits are more differentiated among regions than expected based on neutral genetic divergence (Q_ST > F_ST), including the first principal component score corresponding with life history syndrome. This discordance between patterns of phenotypic and genetic variation suggests that divergent selection is driving genetic differences in life history across the species range. If so, the silverleaf sunflower may be in early stages of ecological speciation.     

Morphometric differentiation across House Sparrow Passer domesticus populations in Finland in comparison with the neutral expectation for divergence

To understand the biology of organisms it is important to take into account the evolutionary forces that have acted on their constituent populations. Neutral genetic variation is often assumed to reflect variation in quantitative traits under selection, though with even low neutral divergence there can be substantial differentiation in quantitative genetic variation associated with locally adapted phenotypes. To study the relative roles of natural selection and genetic drift in shaping phenotypic variation, the levels of quantitative divergence based on phenotypes (P_ST) and neutral genetic divergence (F_ST) can be compared. Such a comparison was made between 10 populations of Finnish House Sparrows (n = 238 individuals) collected in 2009 across the whole country. Phenotypic variation in tarsus‐length, wing‐length, bill‐depth, bill‐length and body mass were considered and 13 polymorphic microsatellite loci were analysed to quantify neutral genetic variation. Calculations of P_ST were based on Markov‐Chain Monte Carlo Bayesian estimates of phenotypic variances across and within populations. The robustness of the conclusions of the P_ST–F_ST comparison was evaluated by varying the proportion of variation due to additive genetic effects within and across populations. Our results suggest that body mass is under directional selection, whereas the divergence in other traits does not differ from neutral expectations. These findings suggest candidate traits for considering gene‐based studies of local adaptation. The recognition of locally adapted populations may be of value in the conservation of this declining species.     

Genetic and Morphological Divergence in Three Strains of Brook Trout Salvelinus fontinalis Commonly Stocked in Lake Superior

Fitness related traits often show spatial variation across populations of widely distributed species. Comparisons of genetic variation among populations in putatively neutral DNA markers and in phenotypic traits susceptible to selection (Q_ST F_ST analysis) can be used to determine to what degree differentiation among populations can be attributed to selection or genetic drift. Traditionally, Q_ST F_ST analyses require a large number of populations to achieve sufficient statistical power; however, new methods have been developed that allow Q_ST F_ST comparisons to be conducted on as few as two populations if their pedigrees are informative. This study compared genetic and morphological divergence in three strains of brook trout Salvelinus fontinalis that were historically or currently used for stocking in the Lake Superior Basin. Herein we examined if morphological divergence among populations showed temporal variation, and if divergence could be attributed to selection or was indistinguishable from genetic drift. Multivariate Q_ST F_ST analysis showed evidence for divergent selection between populations. Univariate analyses suggests that the pattern observed in the multivariate analyses was largely driven by divergent selection for length and weight, and moreover by divergence between the Assinica strain and each of the Iron River and Siskiwit strains rather than divergent selection between each population pair. While it could not be determined if divergence was due to natural selection or inadvertent artificial selection in hatcheries, selected differences were consistent with patterns of domestication commonly found in salmonids.     

Temporal changes in genetic diversity of isolated populations of perch and roach

Genetic drift, together with natural selection and gene flow, affects genetic variation and is the major source of changes in allele frequencies in small and isolated populations. Temporal shifts in allele frequencies at five polymorphic loci were used to estimate the amount of genetic drift in an isolated population of perch (Perca fluviatilis L.) and roach (Rutilus rutilus L.). Here, I used the populations from the Biotest basin at Forsmark, Sweden, to investigate genetic diversity between 1977 and 2000, during which time the population can be considered to be totally isolated from other populations. Microsatellite data reveal stable levels of gene diversity over time for both species. Estimates of genetic differentiation (F _ST) showed a significant divergence between 1977 and 2000 for both perch and roach. A positive correlation between genetic distance and time was found (Mantel test, perch: r = 0.724, P = 0.0112; roach: r = 0.59, P = 0.036). Estimates of effective population size (N _e) differed with a factor six between two different estimators (NeEstimator and TempoFS) applying the temporal method. Ratios of N _e/N ranged between 10⁻² and 10⁻³, values normally found in marine species. Despite low N _e the populations have not lost their evolutionary potential due to drift. But two decades of isolation have lead to isolation by time for populations of perch and roach, respectively.     

Genetic variation and effective population size in isolated populations of coastal cutthroat trout

Following glacial recession in southeast Alaska, waterfalls created by isostatic rebound have isolated numerous replicate populations of coastal cutthroat trout (Oncorhynchus clarkii clarkii) in short coastal streams. These replicate isolated populations offer an unusual opportunity to examine factors associated with the maintenance of genetic diversity. We used eight microsatellites to examine genetic variation within and differentiation among 12 population pairs sampled from above and below these natural migration barriers. Geological evidence indicated that the above-barrier populations have been isolated for 8,000–12,500 years. Genetic differentiation among below-barrier populations (F _ST = 0.10, 95% C.I. 0.08–0.12) was similar to a previous study of more southern populations of this species. Above-barrier populations were highly differentiated from adjacent below-barrier populations (mean pairwise F _ST = 0.28; SD 0.18) and multiple lines of evidence were consistent with asymmetric downstream gene flow that varied among streams. Each above-barrier population had reduced within-population genetic variation when compared to the adjacent below-barrier population. Within-population genetic diversity was significantly correlated with the amount of available habitat in above-barrier sites. Increased genetic differentiation of above-barrier populations with lower genetic diversity suggests that genetic drift has been the primary cause of genetic divergence. Long-term estimates of N _e based on loss of heterozygosity over the time since isolation were large (3,170; range 1,077–7,606) and established an upper limit for N _e if drift were the only evolutionary process responsible for loss of genetic diversity. However, it is likely that a combination of mutation, selection, and gene flow have also contributed to the genetic diversity of above-barrier populations. Contemporary above-barrier N _e estimates were much smaller than long-term N _e estimates, not correlated with within-population genetic diversity, and not consistent with the amount of genetic variation retained, given the approximate 10,000-year period of isolation. The populations isolated by waterfalls in this study that occur in larger stream networks have retained substantial genetic variation, which suggests that the amount of habitat in headwater streams is an important consideration for maintaining the evolutionary potential of isolated populations.     

Divergent selection and drift shape the genomes of two avian sister species spanning a saline–freshwater ecotone

The role of species divergence due to ecologically based divergent selection—or ecological speciation—in generating and maintaining biodiversity is a central question in evolutionary biology. Comparison of the genomes of phylogenetically related taxa spanning a selective habitat gradient enables discovery of divergent signatures of selection and thereby provides valuable insight into the role of divergent ecological selection in speciation. Tidal marsh ecosystems provide tractable opportunities for studying organisms' adaptations to selective pressures that underlie ecological divergence. Sharp environmental gradients across the saline–freshwater ecotone within tidal marshes present extreme adaptive challenges to terrestrial vertebrates. Here, we sequence 20 whole genomes of two avian sister species endemic to tidal marshes—the saltmarsh sparrow (Ammospiza caudacutus) and Nelson's sparrow (A. nelsoni)—to evaluate the influence of selective and demographic processes in shaping genome‐wide patterns of divergence. Genome‐wide divergence between these two recently diverged sister species was notably high (genome‐wide F_ST = 0.32). Against a background of high genome‐wide divergence, regions of elevated divergence were widespread throughout the genome, as opposed to focused within islands of differentiation. These patterns may be the result of genetic drift resulting from past tidal march colonization events in conjunction with divergent selection to different environments. We identified several candidate genes that exhibited elevated divergence between saltmarsh and Nelson's sparrows, including genes linked to osmotic regulation, circadian rhythm, and plumage melanism—all putative candidates linked to adaptation to tidal marsh environments. These findings provide new insights into the roles of divergent selection and genetic drift in generating and maintaining biodiversity.     

Genetic variation and island biogreography: Microsatellite and mitochondrial DNA variation in island populations of the Australian bush rat, Rattus fuscipes greyii

To understand the impact of various factors on the maintenance of genetic variation in natural populations, we need to focus on situations where at least some of these factors are removed or controlled. In this study, we used highly variable, presumably neutral, microsatellite and mtDNA markers to assess the nature of genetic variation in 14 island and two mainland populations of the Australian bush rat, where there is no migration between islands. Thus we are controlling for selection and gene flow. Both marker sets revealed low levels of diversity within the small island populations and extreme differentiation between populations. For six microsatellite loci, all of the small island populations had less genetic variation than the mainland populations; reduction in allelic diversity was more pronounced than loss of heterozygosity. Kangaroo Island, the large island population, had similar levels of diversity to the mainland populations. A 442 base pair (bp) section of the mtDNA control region was screened for variation by outgroup heteroduplex analysis/temperature gradient gel electrophoresis (OHA/TGGE). Only three of the 13 small island populations showed haplotypic diversity: Gambier (2), Waldegrave (2), and Eyere (3). The level of haplotypic diversity in the small island populations was similar to that on the mainland, most likely reflecting a recent population bottleneck on the mainland. In contrast, Kangaroo Island had 9 mtDNA haplotypes. The dominant factor influencing genetic diversity on the islands was island size. No correlation was detected between genetic diversity and the time since isolation or distance form the mainland. The combination of genetic drift within and complete isolation among the small island populations has resulted in rapid and extreme population divergence. Population pair-wise comparisons of allele frequency distributions showed significant differences for all populations for all loci (F _st = 0.11–0.84, R _st = 0.07–0.99). For the mtDNA control region, 92.6% of variation was apportioned between populations; only the Pearson islands shared a haplotype. Mantel tests of pair-wise genetic distance with pair-wise geographic distance showed no significant geographical clustering of haplotypes. However, population substructuring was detected within populations where sampling was conducted over a broader geographical range, as indicated by departures from Hardy-Weinberg equilibrium. Thus substructuring in the ancestral population cannot be ruled out. The dominant evolutionary forces on the islands, after the initial founder event, are stochastic population processes such as genetic drift and mutation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extensive intraspecific genetic diversity of a freshwater crayfish in a biodiversity hotspot

1. The freshwater crayfish Cherax dispar (Decapoda: Parastacidae) inhabits coastal regions and islands of South East Queensland, Australia. We hypothesised that populations of C. dispar on different islands would be more genetically divergent from each other than populations from different drainages within the same island or on the mainland. 2. Phylogenetic and phylogeographic analyses were conducted on two mitochondrial genes (cytochrome oxidase subunit I & 16S ribosomal DNA) and one nuclear gene (Internal Transcribed Spacer region 2). Phylogeographic patterns were compared with those for other freshwater organisms in the area. 3. Deep genetic divergences were found within C. dispar, including four highly divergent (up to 20%) clades. The geographic distribution of each of the clades revealed strong latitudinal structuring along the coast rather than structuring among the islands. The high genetic divergence observed among the C. dispar clades was estimated to have pre‐dated island formation and may represent ancient river drainage patterns. 4. A restricted distribution was observed for the most divergent clade, which was discovered only on two of the sand islands (North Stradbroke Island and Moreton Island). Furthermore, strong phylogeographic structuring was observed within this clade on North Stradbroke Island, where no haplotypes were shared between samples from opposite sides of the island. This low connectivity within the island supports the idea that C. dispar rarely disperse terrestrially (i.e. across watersheds).     

THE EFFECTIVE SIZE OF A HIERARCHICALLY STRUCTURED POPULATION

A knowledge of the effective size of a population (N_e) is important in understanding its current and future evolutionary potential. Unfortunately, the effective size of a hierarchically structured population is not, in general, equal to the sum of its parts. In particular, the inbreeding structure has a major influence on N_e. Here I link N_e to Wright's hierarchical measures of inbreeding, F_IS and F_ST, for an island-structured population (or metapopulation) of size N_T. The influence of F_ST depends strongly on the degree to which island productivity is regulated. In the absence of local regulation (the interdemic model), interdemic genetic drift reduces N_e. When such drift is combined with local inbreeding under otherwise ideal conditions, the effects of F_IS and F_ST are identical: increasing inbreeding either within or between islands reduces N_e, with N_e = N_T/[(1 + F_IS)(1 + F_ST) ? 2F_ISF_ST]. However, if islands are all equally productive because of local density regulation (the traditional island model), then N_e = N_T/[(1 + F_IS)(1 –F_ST)] and the effect of F_ST is reversed. Under the interdemic model, random variation in the habitat quality (and hence productivity) of islands act to markedly decrease N_e. This variation has no effect under the island model because, by definition, all islands are equally productive. Even when no permanent island structure exists, spatial differences in habitat quality can significantly increase the overall variance in reproductive success of both males and females and hence lower N_e. Each of these basic results holds when other nonideal factors are added to the model. These factors, deviations from a 1:1 sex ratio, greater than Poisson variance in female reproductive success, and variation in male mating success due to polygynous mating systems, all act to lower N_e. The effects of male and female variance on N_e have important differences because only females affect island productivity. Finally, it is noted that to use these relationships, F_IS and F_ST must be estimated according to Wright's definition (and corrected to have a zero expectation under the null model). A commonly used partitioning (θ, θ_g) can be biased if either island size or the number of islands is small.     

Divergence at neutral and non-neutral loci in Drosophila buzzatii populations and their hybrids

The impact of intraspecific hybridisation on fitness and morphological traits depends on the history of natural selection and genetic drift, which may have led to differently coadapted gene-complexes in the parental populations. The divergence at neutral and non-neutral loci between populations can be evaluated by estimating F_ST and Q_ST respectively, and hence give an estimate of drift and selection in the populations. Here we investigate (1) whether divergence between populations in quantitative traits (wing size and shape) can be attributed to selection or drift alone, (2) The impact of intraspecific hybridisation on estimators for divergence at neutral (F_ST) and non-neutral loci (Q_ST) in hybrids, (3) If measurement of shape is more informative than size in order to detect divergence in quantitative traits between populations. The aims were addressed by performing two hybridisations between three populations of Drosophila buzzatii, one between populations from Argentina and the Canary Islands (separated for 200 years), and the other between populations from Argentina and Australia (separated for 80 years). We observed the highest divergence at neutral loci between the Argentinean and Canary Island populations, but highest morphological divergence between the Argentinean and Australian populations, indicating that natural selection is acting on the wings. Divergence based on Q_ST measures in the hybrids was sensitive towards increased phenotypic variance (σ²p) within groups and should be used with care when σ²p of populations differ. Our results indicate that measures of shape give a better estimate of divergence at the underlying quantitative traits loci than measures of size.     

Disentangling the effects of recombination,selection, and demography on the genetic variation at a major histocompatibility complex class II gene in the alpine chamois

The major histocompatibility complex (MHC) harbours some of the most polymorphic loci in vertebrate genomes. MHC genes are thought to be subject to some form of balancing selection, most likely pathogen‐mediated selection. Hence, MHC genes are excellent candidates for exploring adaptive processes. In this study, we investigated the genetic variation at exon 2 of the DRB class II MHC locus in 191 alpine chamois (Rupicapra rupicapra) from 10 populations in the eastern Alps of Italy. In particular, we were interested in distinguishing and estimating the relative impact of selective and demographic factors, while taking into account the confounding effect of recombination. The extremely high d_n/d_s ratio and the presence of trans‐species polymorphisms suggest that a strong long‐term balancing selection effect has been operating at this locus throughout the evolutionary history of this species. We analysed patterns of genetic variation within and between populations, and the mitochondrial D‐loop polymorphism patterns were analysed to provide a baseline indicator of the effects of demographic processes. These analyses showed that (i) the chamois experienced a demographic decline in the last 5000–30 000 years, most likely related to the postglacial elevation in temperature; (ii) this demographic process can explain the results of neutrality tests applied to MHC variation within populations, but cannot justify the much weaker divergence between populations implied by MHC as opposed to mitochondrial DNA; (iii) similar sets of divergent alleles are probably maintained with similar frequencies by balancing selection in different populations, and this mechanism is also operating in small isolated populations, which are strongly affected by drift.     

Genetic diversity in micronesian island populations of the tropical tree campnosperma brevipetiolata (anacardiaceae)

Tropical plant species have been the focus of considerable attention in regard to their potential economic and social importance in the face of rapidly diminishing biodiversity in the tropics. Pacific Island species represent an even more fragile resource because different island populations are widely scattered and overall population sizes are small. We examined the distribution of genetic variation in Campnosperma brevipetiolata (Anacardiaceae), an upland rainforest tree species that is of potential use for both lumber and reforestation efforts in the Federated States of Micronesia (FSM). Seeds were collected from multiple populations on four island groups in the Caroline Islands (Kosrae, Pohnpei, and Yap in the FSM; and the Republic of Palau) and subjected to an electrophoretic analysis involving four polymorphic genetic loci. We hypothesized that variation on these islands would decrease with increasing distance from the presumed Indo-Malayan source of these island floras. Indeed, we found a trend of decreasing variation from west to east indicated by the mean number of alleles per locus (A = 1.50-1.33), effective number of alleles per locus (A_e = 1.14-1.12) and mean genetic diversity (H_e = 0.123-0.107). We also found little genetic differentiation among the islands (F_pt = 0.174) and among subpopulations within islands (F_sp = 0.047), indicating that either there are high levels of gene flow among the islands by seed dispersal or that these populations have not been established long enough for divergence to have occurred. The lack of divergence among islands observed for Campnosperma brevipetiolata suggests that germplasm sampled from any one island population would be a suitable starting point for plant breeding or reforestation efforts.     

Contrasting Levels of Variation in Neutral and Quantitative Genetic Loci on Island Populations of Moor Frogs (<Emphasis Type="Italic">Rana arvalis</Emphasis>)

Reduced levels of genetic variability and a prominent differentiation in both neutral marker genes and phenotypic traits are typical for many island populations as compared to their mainland conspecifics. However, whether genetic diversity in neutral marker genes reflects genetic variability in quantitative traits, and thus, their evolutionary potential, remains typically unclear. Moreover, the phenotypic differentiation on islands could be attributable to phenotypic plasticity, selection or drift; something which seldom has been tested. Using eight polymorphic microsatellite loci and quantitative genetic breeding experiments we conducted a detailed comparison on genetic variability and differentiation between Nordic islands (viz. Gotland, Öland and Læsø) and neighbouring mainland populations of moor frogs (Rana arvalis). As expected, the neutral variation was generally lower in island than in mainland populations. But as opposed to this, higher levels of additive genetic variation (V _A) in body size and tibia length were found on the island of Gotland as compared to the mainland population. When comparing the differentiation seen in neutral marker genes (F _ST) with the differentiation in genes coding quantitative traits (Q _ST) two different evolutionary scenarios were found: while selection might explain a smaller size of moor frogs on Gotland, the differentiation seen in tibia length could be explained by genetic drift. These results highlight the limited utility of microsatellite loci alone in inferring the causes behind an observed phenotypic differentiation, or in predicting the amount of genetic variation in ecologically important quantitative traits.     

Allozyme Diversity and Genetic Divergence of the Dolly Varden Salvelinus malma Walbaum from the Kuril Islands

Genetic variation was studied in the southern subspecies of the Asian Dolly Varden Salvelinus malma krascheninnikovi from the Kuril Islands. Thirty-six genetic loci controlling 19 enzyme systems were analyzed in 13 Dolly Varden populations from the Shumshu, Paramushir, Onekotan, Rasshua, Simushir, Urup, Iturup, and Kunashir islands. In the studied populations, the proportion of polymorphic loci was 35 to 85% and the mean heterozygosity was 0.104 to 0.173; populations from the Kunashir Island were characterized by maximum heterozygosity. In the island populations examined, significant interpopulation heterogeneity of allele frequencies was found for all studied population pairs. For the all island populations, the interpopulation diversity (G _ST = 0.188) was comparable to this parameter for the populations from the Kunashir Island only (G _ST = 0.170). Genetic distances between populations did not correlate with the corresponding geographical distances, which indicates the lack of a pronounced gene exchange between the island populations. Cluster analysis and multidimensional scaling based on genetic distances did not reveal clear groups among the studied populations but indicated greater similarity within the Iturup–Simushir–Urup–Paramushir group and a greater genetic divergence of the Kunashir, Onekotan, Rasshua, and especially Shumshu populations. In the Shumshu population, allele frequencies indicate the admixture of genes of the northern Dolly Varden. The observed pattern of genetic differentiation was probably caused largely by genetic drift under the conditions of a limited gene flow because of homing (which is typical of the Dolly Varden) and the presence of isolated nonanadromous populations. The population–genetic analysis of the Dolly Varden from the Kuril Islands does not give grounds to distinguish any other isolated char species in this region than S. malma, which is represented by the southern form S. m. krascheninnikovi with an admixture of the northern form S. m. malma in the Shumshu Island.     

Spatial and ecological population genetic structures within two island‐endemic Aeonium species of different niche width

The Crassulacean genus Aeonium is a well‐known example for plant species radiation on oceanic archipelagos. However, while allopatric speciation among islands is documented for this genus, the role of intra‐island speciation due to population divergence by topographical isolation or ecological heterogeneity has not yet been addressed. The aim of this study was to investigate intraspecific genetic structures and to identify spatial and ecological drivers of genetic population differentiation on the island scale. We analyzed inter simple sequence repeat variation within two island‐endemic Aeonium species of La Palma: one widespread generalist that covers a large variety of different habitat types (Ae. davidbramwellii) and one narrow ecological specialist (Ae. nobile), in order to assess evolutionary potentials on this island. Gene pool differentiation and genetic diversity patterns were associated with major landscape structures in both species, with phylogeographic implications. However, overall levels of genetic differentiation were low. For the generalist species, outlier loci detection and loci–environment correlation approaches indicated moderate signatures of divergent selection pressures linked to temperature and precipitation variables, while the specialist species missed such patterns. Our data point to incipient differentiation among populations, emphasizing that ecological heterogeneity and topographical structuring within the small scales of an island can foster evolutionary processes. Very likely, such processes have contributed to the radiation of Aeonium on the Canary Islands. There is also support for different evolutionary mechanisms between generalist and specialist species.     

Population size is weakly related to quantitative genetic variation and trait differentiation in a stream fish

How population size influences quantitative genetic variation and differentiation among natural, fragmented populations remains unresolved. Small, isolated populations might occupy poor quality habitats and lose genetic variation more rapidly due to genetic drift than large populations. Genetic drift might furthermore overcome selection as population size decreases. Collectively, this might result in directional changes in additive genetic variation (V_A) and trait differentiation (Q_ST) from small to large population size. Alternatively, small populations might exhibit larger variation in V_A and Q_ST if habitat fragmentation increases variability in habitat types. We explored these alternatives by investigating V_A and Q_ST using nine fragmented populations of brook trout varying 50‐fold in census size N (179–8416) and 10‐fold in effective number of breeders, N_b (18–135). Across 15 traits, no evidence was found for consistent differences in V_A and Q_ST with population size and almost no evidence for increased variability of V_A or Q_ST estimates at small population size. This suggests that (i) small populations of some species may retain adaptive potential according to commonly adopted quantitative genetic measures and (ii) populations of varying sizes experience a variety of environmental conditions in nature, however extremely large studies are likely required before any firm conclusions can be made.     

Replicated evolutionary divergence in the cuticular hydrocarbon profile of male crickets associated with the loss of song in the Hawaiian archipelago

Female choice based on male secondary sexual traits is well documented, although the extent to which this selection can drive an evolutionary divergence in male traits among populations is less clear. Male field crickets Teleogryllus oceanicus attract females using a calling song and once contacted switch to courtship song to persuade them to mate. These crickets also secrete onto their cuticle a cocktail of long‐chained fatty acids or cuticular hydrocarbons (CHCs). Females choose among potential mates based on the structure of male acoustic signals and on the composition of male CHC profiles. Here, we utilize two naturally occurring mutations that have arisen independently on two Hawaiian islands and render the male silent to ask whether the evolutionary loss of acoustic signalling can drive an evolutionary divergence in the alternative signalling modality, male CHC profiles. Q_ST‐F_ST comparisons revealed strong patterns of CHC divergence among three populations of crickets on the islands of Hawaii, Oahu and Kauai. Contrasts between wild‐type and flatwing males on the islands of Oahu and Kauai indicated that variation in male CHC profiles within populations is associated with the loss of acoustic signalling; flatwing males had a relatively low abundance of long‐chained CHCs relative to the short‐chained CHCs that females find attractive. Given their dual functions in desiccation resistance and sexual signalling, insect CHCs may be particularly important traits for reproductive isolation and ultimately speciation.