The unusual inheritance pattern of the courtship songs in closely related grasshopper species of the Chorthippus albomarginatus-group (Orthoptera: Gomphocerinae)

Sibling species of the Chorthippus albomarginatus-group are exceptional among all European grasshopper species because they have an extremely elaborate courtship behaviour. Here, we present a genetic analysis of the courtship song differences between two closely related grasshopper species Ch. albomarginatus and Ch. oschei. Measurements of seven courtship song characters and one parameter of courtship visual display were compared among parentals, F(1) and F(2) hybrids, and backcrosses. Variation in one morphological character, the number of stridulatory pegs, was studied as well. The distributions of song phenotypes were more consistent with a type-III genetic architecture, which involves complementary or duplicate pairs of loci. We suggest that this type of architecture may provide the novel elements of the hybrid songs, even more complex than the parental ones, which may offer a new starting point for sexual selection. One parameter of the visual display appeared to be controlled by a single locus, but differences in other characters of visual display could be explained by a more complex inheritance pattern. Inheritance of the peg number differences was consistent with a simple polygenic additive model. Sex linkage and/or maternal effects were not detected for any trait. We found a disproportionate influence of the Ch. albomarginatus parent on most hybrid song characteristics and the visual display, and this may support a previously suggested idea that Ch. albomarginatus is an ancestral form and Ch. oschei is a derived species.     

Two distinct genomic regions, harbouring the period and fruitless genes, affect male courtship song in Drosophila montana

Acoustic signals often have a significant role in pair formation and in species recognition. Determining the genetic basis of signal divergence will help to understand signal evolution by sexual selection and its role in the speciation process. An earlier study investigated quantitative trait locus for male courtship song carrier frequency (FRE) in Drosophila montana using microsatellite markers. We refined this study by adding to the linkage map markers for 10 candidate genes known to affect song production in Drosophila melanogaster. We also extended the analyses to additional song characters (pulse train length (PTL), pulse number (PN), interpulse interval, pulse length (PL) and cycle number (CN)). Our results indicate that loci in two different regions of the genome control distinct features of the courtship song. Pulse train traits (PTL and PN) mapped to the X chromosome, showing significant linkage with the period gene. In contrast, characters related to song pulse properties (PL, CN and carrier FRE) mapped to the region of chromosome 2 near the candidate gene fruitless, identifying these genes as suitable loci for further investigations. In previous studies, the pulse train traits have been found to vary substantially between Drosophila species, and so are potential species recognition signals, while the pulse traits may be more important in intra-specific mate choice.     

The extent of variation in male song, wing and genital characters among allopatric Drosophila montana populations

Drosophila montana, a species of the Drosophila virilis group, has distributed around the northern hemisphere. Phylogeographic analyses of two North American and one Eurasian population of this species offer a good background for the studies on the extent of variation in phenotypic traits between populations as well as for tracing the selection pressures likely to play a role in character divergence. In the present paper, we studied variation in the male courtship song, wing and genital characters among flies from Colorado (USA), Vancouver (Canada) and Oulanka (Finland) populations. The phenotypic divergence among populations did not coincide with the extent of their genetic divergence, suggesting that the characters are not evolving neutrally. Divergence in phenotypic traits was especially high between the Colorado and Vancouver populations, which are closer to each other in terms of their mtDNA genotypes than they are to the Oulanka population. The males of the Colorado population showed high divergence especially in song traits and the males of the Vancouver population in wing characters. Among the male song traits, two characters known to be under sexual selection and a trait important in species recognition differed clearly between populations, implying a history of directional and/or diversifying rather than balancing selection. The population divergence in wing characters is likely to have been enhanced by natural selection associated with environmental factors, whereas the male genitalia traits may have been influenced by sexual selection and/or sexual conflict.     

QTL analysis of a rapidly evolving speciation phenotype in the Hawaiian cricket Laupala

In mate recognition systems, the functional necessity to coordinate traits involved in sexual communication should result in reduced pairing potential for new variants outside the distribution of common reproductive signals. Yet, many closely related, sexual species differ in mate recognition traits, suggesting that directional selection influences the divergence of mate recognition systems. Species of the endemic Hawaiian cricket genus Laupala are morphologically and ecologically cryptic, although both male calling song and female acoustic preference have diverged rapidly between closely related species. These mate recognition phenotypes are therefore often likely to be speciation phenotypes, i.e. traits whose divergence contributes, directly or indirectly, to a reduction of gene flow during speciation, given their frequent participation in early lineage divergence. We conducted a replicated, quantitative trait loci (QTL) mapping study of the genetic basis of differences in male calling song between two closely related species, Laupala paranigra and Laupala kohalensis, allowing us to examine the genetic basis of traits involved in rapid speciation. We found statistical support for eight QTL in one replicate, with at least four of these QTL mapping to the same regions in a second replicate. QTL effects ranged between 3.0% and 10.7% of the difference in pulse rate between L. paranigra and L. kohalensis, and are thus of moderate to small effect. All QTL identified show directional effects consistent with the hypothesis of directional selection. Thus, we conclude that rapid speciation can occur under the influence of many genes of moderate to small effect. This study implicates the role of directional selection in the divergence of mate recognition traits and speciation the Hawaiian cricket genus Laupala.     

Interspecific genetics of speciation phenotypes: song and preference coevolution in Hawaiian crickets

Understanding the genetic architecture of traits involved in premating isolation between recently diverged lineages can provide valuable insight regarding the mode and tempo of speciation. The repeated coevolution of male courtship song and female preference across the species radiation of Laupala crickets presents an unusual opportunity to compare the genetic basis of divergence across independent evolutionary histories. Previous studies of one pair of species revealed a polygenic basis (including a significant X chromosome contribution) to quantitative differences in male song and female acoustic preference. Here, we studied interspecific crosses between two phenotypically less-diverged species that represents a phylogenetically independent occurrence of intersexual signalling evolution. We found patterns consistent with an additive polygenic basis to differentiation in both song and preference (n(E) = 5.3 and 5.1 genetic factors, respectively), and estimate a moderate contribution of the X chromosome (7.6%) of similar magnitude to that observed for Laupala species with nearly twice the phenotypic divergence. Together, these findings suggest a similar genetic architecture underlying the repeated evolution of sexual characters in this genus and provide a counterexample to prevailing theory predicting an association between early lineage divergence and sex-linked 'major genes'.     

Phenotypic variation and covariation indicate high evolvability of acoustic communication in crickets

Studying the genetic architecture of sexual traits provides insight into the rate and direction at which traits can respond to selection. Traits associated with few loci and limited genetic and phenotypic constraints tend to evolve at high rates typically observed for secondary sexual characters. Here, we examined the genetic architecture of song traits and female song preferences in the field crickets Gryllus rubens and Gryllus texensis. Song and preference data were collected from both species and interspecific F1 and F2 hybrids. We first analysed phenotypic variation to examine interspecific differentiation and trait distributions in parental and hybrid generations. Then, the relative contribution of additive and additive‐dominance variation was estimated. Finally, phenotypic variance–covariance ( P ) matrices were estimated to evaluate the multivariate phenotype available for selection. Song traits and preferences had unimodal trait distributions, and hybrid offspring were intermediate with respect to the parents. We uncovered additive and dominance variation in song traits and preferences. For two song traits, we found evidence for X‐linked inheritance. On the one hand, the observed genetic architecture does not suggest rapid divergence, although sex linkage may have allowed for somewhat higher evolutionary rates. On the other hand, P matrices revealed that multivariate variation in song traits aligned with major dimensions in song preferences, suggesting a strong selection response. We also found strong covariance between the main traits that are sexually selected and traits that are not directly selected by females, providing an explanation for the striking multivariate divergence in male calling songs despite limited divergence in female preferences.     

Widespread genetic linkage of mating signals and preferences in the Hawaiian cricket Laupala

The evolution of novel sexual communication systems is integral to the process of speciation, as it discourages gene flow between incipient species. Physical linkage between genes underlying male-female communication (i.e. sexual signals and preferences for them) facilitates both rapid and coordinated divergence of sexual communication systems between populations and reduces recombination in the face of occasional hybridization between diverging populations. Despite these ramifications of the genetic architecture of sexual communication for sexual selection and speciation, few studies have examined this relationship empirically. Previous studies of the closely related Hawaiian crickets Laupala paranigra and Laupala kohalensis have indirectly suggested that many of the genes underlying the difference in pulse rate of male song are physically linked with genes underlying the difference in female preference for pulse rate. Using marker-assisted introgression, we moved 'slow pulse rate' alleles from L. paranigra at five known quantitative trait loci (QTL) underlying male pulse rate into the 'fast pulse rate' genetic background of L. kohalensis and assessed the effect of these loci on female preference. An astounding four out of five song QTL predicted the preferences of female fourth-generation backcrosses, providing direct evidence for the extensive genetic linkage of song and preference in one of the fastest diversifying genera currently known.     

芦鹀种群中的文化、遗传和形态进化的同时比较

我们比较了芦 (Emberizaschoeniclus)两个亚种组 ,即北部的薄喙亚种组和南部的厚喙亚种组的 10个种群中的文化、遗传和形态变异。使用了四个不同的变异标记物 ,其中两个用来测量文化分化 ,一个用来测量遗传分化 ,即微卫星等位基因的频次 ,一个用来测量种群的形态分化 ,即喙的高度。将遗传分化作为进化时间的尺度 ,我们计算了亚种组间和组内分化指标与所估计的进化率之间的相关性 ,发现只有文化定量指标和遗传分化与种群的形态分化相关 ,而两个文化分化指标之间没有关系 ,文化分化与遗传分化之间也没有关系。使用文化 -定量分化指标 ,发现亚种组间的文化进化率高于亚种内的文化进化率 ,提示鸣唱在防止杂交方面只有微弱的、也许是次要的作用。鸣唱定量特征的变异与微卫星频次相同 ,实际上在自然界中更可能是遗传决定的 ,这可以解释由于分析两个文化变异指标所得出的结果的不一致性。鸣唱的声学特性可能由于栖息地的差异或形态上的限制而发生了演变 ,而文化传播单位 (Meme)的特性可能由于学习鸣唱和文化传播而受到了影响     

Patterns of trait divergence between populations of the meadow grasshopper, Chorthippus parallelus

Abstract.— To understand the process of speciation, we need to identify the evolutionary phenomena associated with divergence between populations of the same species. A powerful approach is to compare patterns of trait differences between populations differing in their evolutionary histories. A recent study of genetic divergence between populations of the meadow grasshopper Chorthippus parallelus , from different locations around Europe has allowed us to use this species to investigate which aspects of evolutionary history are associated with divergence in morphology and mating signals. During the last glaciation C. parallelus was confined to a number of refugia in southern Europe and has subsequently recolonized the northern part of the continent. This process of isolation followed by range expansion has created populations differing markedly in their evolutionary pasts–some have been isolated from one another for thousands of years, others have undergone repeated founder events, and others now live in sympatry with a closely related species. Using laboratory-reared grasshoppers from 12 different populations with a range of evolutionary histories, we quantify differences in morphology, chemical signals, and male calling-song. The observed pattern of divergence between these populations is then compared with the pattern predicted by hypotheses about what drives divergence. This comparison reveals that long periods in allopatry and processes associated with repeated founder events are both strongly associated with divergence.     

Paraphyletic species no more – genomic data resolve a Pleistocene radiation and validate morphological species of the Melanoplus scudderi complex (Insecta: Orthoptera)

Rapid speciation events, with taxa generated over a short time period, are among the most investigated biological phenomena. However, molecular systematics often reveals contradictory results compared with morphological/phenotypical diagnoses of species under scenarios of recent and rapid diversification. In this study, we used molecular data from an average of over 29 000 loci per sample from RADseq to reconstruct the diversification history and delimit the species boundary in a short-winged grasshopper species complex (Melanoplus scudderi group), where Pleistocene diversification has been hypothesized to generate more than 20 putative species with distinct male genitalic shapes. We found that, based on a maximum likelihood molecular phylogeny, each morphological species indeed forms a monophyletic group, contrary to the result from a previous mitochondrial DNA sequence study. By dating the diversification events, the species complex is estimated to have diversified during the Late Pleistocene, supporting the recent radiation hypothesis. Furthermore, coalescent-based species delimitation analyses provide quantitative support for independent genetic lineages, which corresponds to the morphologically defined species. Our results also showed that male genitalic shape may not be predicted by evolutionary distance among species, not only indicating that this trait is labile, but also implying that selection may play a role in character divergence. Additionally, our findings suggest that the rapid speciation events in this flightless grasshopper complex might be primarily associated with the fragmentation of their grassland habitats during the Late Pleistocene. Collectively, our study highlights the importance of integrating multiple sources of information to delineate species, especially for a species complex that diversified rapidly, and whose divergence may be linked to ecological processes that create geographic isolation (i.e. fragmented habitats), as well as selection acting on characters with direct consequences for reproductive isolation (i.e. genitalic divergence).     

Elevated yolk androgen levels and the expression of multiple sexually selected male characters

Maternal hormones in bird eggs modulate not only offspring development, but recently it has also been shown that these effects can persist into adult life. A number of long-lasting effects concern traits of which the expression or development is modulated by androgens. This suggests that the nature of yolk hormone-mediated maternal effects may be organizational. Maternal androgens may therefore play an important role in sexual selection, since the expression of sexually selected male characters is often androgen-dependent.We experimentally manipulated the yolk androgen concentrations of European Starling (Sturnus vulgaris) eggs. Subsequently we followed 49 unrelated males from hatching until year of first reproduction. We investigated the expression of multiple sexually selected male characters (song, beak color and throat feather characteristics), taking into account whether a trait is androgen-dependent.Elevated levels of yolk androgens affected the length of the embryonic period, but did not modify the expression of either androgen-dependent or androgen-independent sexually selected male characters including song phenotype at adulthood. Thus the most important function of yolk androgens in starlings and possibly other bird species may relate to the early developmental period. The outcome of our study together with the results of our meta-analysis indicates that the effects of yolk androgens on sexually selected male characters may be comparatively small. Our results suggest that this may relate to the numerous other environmental and/or genetic factors influencing the expression of sexually selected male characters.     

Minor quantitative trait loci underlie floral traits associated with mating system divergence in Mimulus

The genetic basis of species differences provides insight into the mode and tempo of phenotypic divergence. We investigate the genetic basis of floral differences between two closely related plant taxa with highly divergent mating systems, Mimulus guttatus (large-flowered outcrosser) and M. nasutus (small-flowered selfer). We had previously constructed a framework genetic linkage map of the hybrid genome containing 174 markers spanning approximately 1800 cM on 14 linkage groups. In this study, we analyze the genetics of 16 floral, reproductive, and vegetative characters measured in a large segregating M. nasutus x M. guttatus F2 population (N = 526) and in replicates of the parental lines and F1 hybrids. Phenotypic analyses reveal strong genetic correlations among floral traits and epistatic breakdown of male and female fertility traits in the F2 hybrids. We use multitrait composite interval mapping to jointly locate and characterize quantitative trait loci (QTLs) underlying interspecific differences in seven floral traits. We identified 24 floral QTLs, most of which affected multiple traits. The large number of QTLs affecting each trait (mean = 13, range = 11-15) indicates a strikingly polygenic basis for floral divergence in this system. In general, QTL effects are small relative to both interspecific differences and environmental variation within genotypes, ruling out QTLs of major effect as contributors to floral divergence between M. guttatus and M. nasutus. QTLs show no pattern of directional dominance. Floral characters associated with pollinator attraction (corolla width) and self-pollen deposition (stigma-anther distance) share several pleiotropic or linked QTLs, but unshared QTLs may have allowed selfing to evolve independently from flower size. We discuss the polygenic nature of divergence between M. nasutus and M. guttatus in light of theoretical work on the evolution of selfing, genetics of adaptation, and maintenance of variation within populations.     

Mating signal variation and bimodality in a mosaic hybrid zone between Chorthippus grasshopper species

The grasshoppers Chorthippus brunneus and Chorthippus jacobsi are easily distinguished by male calling song and the number of stridulatory pegs on the hind femur, and form a mosaic hybrid zone in northern Spain. In this paper, we fit a two-dimensional cline to variation in male calling song characters, which are of particular interest as they are likely to be involved in mate choice by females. As with variation in peg number, local habitat makes only a small contribution in explaining deviations in mean song score from clinal expectations. However, the fitted width of the cline for song characters is significantly narrower than for peg number, suggesting that mating signals may be associated with reduced hybrid fitness in the field and that recombination rates are sufficient to allow clines for different characters to diverge in width. Despite this, estimates for the overall elevation in linkage disequilibrium at the zone center, based on covariance between peg and song characters, reveal a substantial overrepresentation of parental genotypes at the cline center relative to the expectations of a tension zone of similar width. Examination of covariance at individual sites reveals that this inflated estimate of linkage disequilibrium is caused by several sites where the distribution of phenotypes is effectively bimodal. This substantial variation in linkage disequilibrium at the cline center could result from local variation in the strength of assortative mating or selection against hybrids, or may reflect the long-distance colonization of empty habitat from outside the hybrid zone, which would continually create new contacts between parental genotypes at the cline center. Hybrid zones like this, in which strong linkage disequilibrium occurs in some situations but not in others, are of particular relevance to speciation research and allow investigation of the spread of combinations of alleles through different genetic and ecological backgrounds.     

Elevational differentiation accelerates trait evolution but not speciation rates in Amazonian birds

The importance of ecologically mediated divergent selection in accelerating trait evolution has been poorly studied in the most species‐rich biome of the planet, the continental Neotropics. We performed macroevolutionary analyses of trait divergence and diversification rates across closely related pairs of Andean and Amazonian passerine birds, to assess whether the difference in elevational range separating species pairs – a proxy for the degree of ecological divergence – influences the speed of trait evolution and diversification rates. We found that elevational differentiation is associated with faster divergence of song frequency, a trait important for pre‐mating isolation, and several morphological traits, which may contribute to extrinsic post‐mating isolation. However, elevational differentiation did not increase recent speciation rates, possibly due to early bursts of diversification during the uplift of the eastern Andes followed by a slow‐down in speciation rate. Our results suggest that ecological differentiation may speed up trait evolution, but not diversification of Neotropical birds.     

Sympatry drives colour and song evolution in wood-warblers (Parulidae)

Closely related species often exhibit similarities in appearance and behaviour, yet when related species exist in sympatry, signals may diverge to enhance species recognition. Prior comparative studies provided mixed support for this hypothesis, but the relationship between sympatry and signal divergence is likely nonlinear. Constraints on signal diversity may limit signal divergence, especially when large numbers of species are sympatric. We tested the effect of sympatric overlap on plumage colour and song divergence in wood-warblers (Parulidae), a speciose group with diverse visual and vocal signals. We also tested how number of sympatric species influences signal divergence. Allopatric species pairs had overall greater plumage and song divergence compared to sympatric species pairs. However, among sympatric species pairs, plumage divergence positively related to the degree of sympatric overlap in males and females, while male song bandwidth and syllable rate divergence negatively related to sympatric overlap. In addition, as the number of species in sympatry increased, average signal divergence among sympatric species decreased, which is likely due to constraints on warbler perceptual space and signal diversity. Our findings reveal that sympatry influences signal evolution in warblers, though not always as predicted, and that number of sympatric species can limit sympatry''s influence on signal evolution.     

Geographic variation and the evolution of song in Mesoamerican rufous‐naped wrens Campylorhynchus rufinucha

Speciation may be influenced by geographic variation in animal signals, particularly when those signals are important in reproductive decisions. Here, we describe patterns of geographic variation in the song of rufous‐naped wrens Campylorhynchus rufinucha. This species complex is a morphologically variable taxon confined to tropical dry forest areas from Mexico to northwestern Costa Rica. Morphological and genetic analyses suggest that there are at least three partially isolated groups within the complex, including a secondary‐contact zone in coastal western Chiapas between the subspecies C. r. humilus and C. r. nigricaudatus. Based on recordings throughout their geographic range, we investigate the effects of historical isolation on song structure and analyze whether genetic differences or climatic conditions explain observed patterns of variation. Our findings, based on a culturally‐transmitted and sexually‐selected trait, support the hypothesis that three evolutionary units exist within this taxon. Our results suggest that song differences between genetic groups were influenced by historical isolation. We report a strong relationship between vocal dissimilarity and genetic distance, suggesting that differences in vocal characteristics are probably affected by the same factors that drive genetic divergence. We argue that the evolution of song in this taxon is influenced by vicariant events, followed by accumulation of changes in song structure due to several possible factors: cultural drift in song structure; genetic drift in features related to song production; or natural selection acting on features that influence songs, such as body and beak size.     

Conservation of multivariate female preference functions and preference mechanisms in three species of trilling field crickets

Divergence in mate recognition systems among closely related species is an important contributor to assortative mating and reproductive isolation. Here, we examine divergence in male song traits and female preference functions in three cricket species with songs consisting of long trills. The shape of female preference functions appears to be mostly conserved across species and follows the predictions from a recent model for song recognition. Multivariate preference profiles, combining the pulse and trill parameters, demonstrate selectivity for conspecific pulse rates and high trill duty cycles. The rules for integration across pulse and trill timescales were identical for all three species. Generally, we find greater divergence in male song traits than in associated female preferences. For pulse rate, we find a strong match between divergent male traits and female peak preferences. Preference functions for trill parameters and carrier frequency are similar between species and show less congruence between signal and preference. Differences among traits in the degree of trait–preference (mis)match may reflect the strength of preferences and the potential for linkage disequilibrium, selective constraints and alternative selective pressures, but appear unrelated to selection for mate recognition per se.     

Deep phylogeographic divergence and cytonuclear discordance in the grasshopper Oedaleus decorus

The grasshopper Oedaleus decorus is a thermophilic insect with a large, mostly south-Palaearctic distribution range, stretching from the Mediterranean regions in Europe to Central-Asia and China. In this study, we analyzed the extent of phylogenetic divergence and the recent evolutionary history of the species based on 274 specimens from 26 localities across the distribution range in Europe. Phylogenetic relationships were determined using sequences of two mitochondrial loci (ctr, ND2) with neighbour-joining and Bayesian methods. Additionally, genetic differentiation was analyzed based on mitochondrial DNA and 11 microsatellite markers using F-statistics, model-free multivariate and model-based Bayesian clustering approaches. Phylogenetic analyses detected consistently two highly divergent, allopatrically distributed lineages within O. decorus. The divergence among these Western and Eastern lineages meeting in the region of the Alps was similar to the divergence of each lineage to the sister species O. asiaticus. Genetic differentiation for ctr was extremely high between Western and Eastern grasshopper populations (F(ct)=0.95). Microsatellite markers detected much lower but nevertheless very significant genetic structure among population samples. The nuclear data also demonstrated a case of cytonuclear discordance because the affiliation with mitochondrial lineages was incongruent in Northern Italy. Taken together these results provide evidence of an ancient separation within Oedaleus and either historical introgression of mtDNA among lineages and/or ongoing sex-specific gene flow in this grasshopper. Our study stresses the importance of multilocus approaches for unravelling the history and status of taxa of uncertain evolutionary divergence.     

Phylogenetic signal in the song of crests and kinglets (Aves: Regulus)

Territorial song structures are often the most prominent characters for distinguishing closely related taxa among songbirds. Learning processes may cause convergent evolution of passerine songs, but phylogenetic information of acoustic traits can be investigated with the help of molecular phylogenies, which are not affected by cultural evolutionary processes. We used a phylogeny based on cytochrome b sequences to trace the evolution of territorial song within the genus Regulus. Five discrete song units are defined as basic components of regulid song via sonagraphic measurements. Traits of each unit are traced on a molecular tree and a mean acoustic character difference between taxon pairs is calculated. Acoustic divergence between regulid taxa correlates strongly with genetic distances. Syntax features of complete songs and of single units are most consistent with the molecular data, whereas the abundance of certain element types is not. Whether song characters are innate or learned was interpreted using hand-reared birds in aviary experiments. We found that convergent character evolution seems to be most probable for learned acoustic traits. We conclude that syntax traits of whole verses or subunits of territorial song, especially innate song structures, are the most reliable acoustic traits for phylogenetic reconstructions in Regulus.     

The genetics of speciation: genes of small effect underlie sexual isolation in the Hawaiian cricket Laupala

Sexual behaviours often evolve rapidly and are critical for sexual isolation. We suggest that coordinated sexual signals and preferences generate stabilizing selection, favouring the accumulation of many small‐effect mutations in sexual communication traits. Rapid radiation of a sexual behaviour used in signalling, song pulse rate, has been observed in the Hawaiian cricket genus Laupala. Using marker‐assisted introgression, we isolated five known quantitative trait loci (QTL) influencing species‐level differences in pulse rate from one species, L. paranigra, into a closely related species, L. kohalensis. All five QTL were found to have a significant effect on song and appear to be largely additive in backcross introgression lines. Furthermore, all effect sizes were small in magnitude. Our data provide support for the hypothesis that stabilizing selection on sexual signals in Laupala creates genetic conditions favourable to incremental divergence during speciation, through the evolution of alleles of minor rather than major phenotypic effects.