Rapid evolution of dispersal‐related traits during range expansion of an invasive vine Mikania micrantha

Rapid range expansion of invasive plants provides a unique opportunity to explore evolutionary changes of dispersal‐related traits during the invasion process. Increasing evidence now suggests that a higher dispersal rate is favored at the invasion front. However, little is known about the role of genetic differentiation and phenotypic plasticity on patterns of dispersal ability during the invasion process. In this study, we combined a field survey and a common garden transplant experiment to test for evidence of genetically based dispersal ability in Mikania micrantha, a highly invasive vine, across its invaded range in southern China. Three dispersal‐related traits, plume loading, seed mass and pappus radius, were measured in both natural and common garden populations. We found that in natural conditions, plume loading and seed mass significantly decreased with expanding distance from the source population, but in controlled conditions, these two traits exhibited a significant humped trend against percent field cover, indicating that dispersal ability of M. micrantha was selected for during range expansion and that the related traits were likely to be under genetic control. Furthermore, rebounding dispersal ability was detected in highly competitive sites in the range core, which suggested that this evolutionary process was likely partially driven by intraspecific competition. Because more and more plant species are under spatial nonequilibirum due to climate change, this study can serve to provide hints at the fate of spatially fluctuant populations.     

PHENOTYPIC DIFFERENTIATION AT SOUTHERN LIMIT BORDERS: THE CASE STUDY OF TWO FUCOID MACROALGAL SPECIES WITH DIFFERENT LIFE‐HISTORY TRAITS1

Marginal populations are often geographically isolated, smaller, and more fragmented than central populations and may frequently have to face suboptimal local environmental conditions. Persistence of these populations frequently involves the development of adaptive traits at phenotypic and genetic levels. We compared population structure and demographic variables in two fucoid macroalgal species contrasting in patterns of genetic diversity and phenotypic plasticity at their southern distribution limit with a more central location. Models were Ascophyllum nodosum (L.) Le Jol. (whose extreme longevity and generation overlap may buffer genetic loss by drift) and Fucus serratus L. (with low genetic diversity at southern margins). At edge locations, both species exhibited trends in life‐history traits compatible with population persistence but by using different mechanisms. Marginal populations of A. nodosum had higher reproductive output in spite of similar mortality rates at all life stages, making edge populations denser and with smaller individuals. In F. serratus, rather than demographic changes, marginal populations differed in habitat, occurring restricted to a narrower vertical habitat range. We conclude that persistence of both A. nodosum and F. serratus at the southern‐edge locations depends on different strategies. Marginal population persistence in A. nodosum relies on a differentiation in life‐history traits, whereas F. serratus, putatively poorer in evolvability potential, is restricted to a narrower vertical range at border locations. These results contribute to the general understanding of mechanisms that lead to population persistence at distributional limits and to predict population resilience under a scenario of environmental change.     

Differentiation of movement behaviour in an adaptively diverging salamander population

Dispersal is considered to be a species‐specific trait, but intraspecific variation can be high. However, when and how this complex trait starts to differentiate during the divergence of species/lineages is unknown. Here, we studied the differentiation of movement behaviour in a large salamander population (Salamandra salamandra), in which individual adaptations to different habitat conditions drive the genetic divergence of this population into two subpopulations. In this system, salamanders have adapted to the deposition and development of their larvae in ephemeral ponds vs. small first‐order streams. In general, the pond habitat is characterized as a spatially and temporally highly unpredictable habitat, while streams provide more stable and predictable conditions for the development of larvae. We analysed the fine‐scale genetic distribution of larvae, and explored whether the adaptation to different larval habitat conditions has in turn also affected dispersal strategies and home range size of adult salamanders. Based on the genetic assignment of adult individuals to their respective larval habitat type, we show that pond‐adapted salamanders occupied larger home ranges, displayed long‐distance dispersal and had a higher variability of movement types than the stream‐adapted individuals. We argue that the differentiation of phenotypically plastic traits such as dispersal and movement characteristics can be a crucial component in the course of adaptation to new habitat conditions, thereby promoting the genetic divergence of populations.     

Evolution of dispersal traits along an invasion route in the wind‐dispersed Senecio inaequidens (Asteraceae)

In introduced organisms, dispersal propensity is expected to increase during range expansion. This prediction is based on the assumption that phenotypic plasticity is low compared to genetic diversity, and an increase in dispersal can be counteracted by the Allee effect. Empirical evidence in support of these hypotheses is however lacking. The present study tested for evidence of differentiation in dispersal‐related traits and the Allee effect in the wind‐dispersed invasive Senecio inaequidens (Asteraceae). We collected capitula from individuals in ten field populations, along an invasion route including the original introduction site in southern France. In addition, we conducted a common garden experiment from field‐collected seeds and obtained capitula from individuals representing the same ten field populations. We analysed phenotypic variation in dispersal traits between field and common garden environments as a function of the distance between populations and the introduction site. Our results revealed low levels of phenotypic differentiation among populations. However, significant clinal variation in dispersal traits was demonstrated in common garden plants representing the invasion route. In field populations, similar trends in dispersal‐related traits and evidence of an Allee effect were not detected. In part, our results supported expectations of increased dispersal capacity with range expansion, and emphasized the contribution of phenotypic plasticity under natural conditions.     

Local above‐ground persistence of vascular plants: Life‐history trade‐offs and environmental constraints

Questions: 1. Which plant traits and habitat characteristics best explain local above‐ground persistence of vascular plant species and 2. Is there a trade‐off between local above‐ground persistence and the ability for seed dispersal and below‐ground persistence in the soil seed bank? Locations: 845 long‐term permanent plots in terrestrial habitats across the Netherlands. Methods: We analysed the local above‐ground persistence of vascular plants in permanent plots (monitored once a year for ca. 16 year) with respect to functional traits and habitat preferences using survival statistics (Kaplan‐Meier analysis and Cox’ regression). These methods account for censored data and are rarely used in vegetation ecology. Results: Local above‐ground persistence is determined by both functional traits (especially the ability to form long‐lived clonal connections) and habitat preferences (especially nutrient requirements). Above‐ground persistence is negatively related to the ability for dispersal by wind and to the ability to accumulate a long‐term persistent soil seed bank (‘dispersal through time’) and is positively related to the ability for dispersal by water. Conclusions: Most species have a half‐life expectation over 15 years, which may contribute to time lags after changes in habitat quality or ‐configuration (‘extinction debt’). There is evidence for a trade‐off relationship between local above‐ground persistence and below‐ground seed persistence, while the relationship with dispersal in space is vector specific. The rate of species turnover increases with productivity.     

Environmentally induced dispersal‐related life‐history syndrome in the tropical butterfly,Bicyclus anynana

Dispersal is a key process for understanding the persistence of populations as well as the capacity of organisms to respond to environmental change. Therefore, understanding factors that may facilitate or constrain the evolution of dispersal is of crucial interest. Assessments of phenotypic variation in various behavioural, physiological and morphological traits related to insect dispersal and flight performance are common, yet very little is known about the genetic associations among these traits. We have used experiments on the butterfly Bicyclus anynana to estimate genetic variation and covariation in seven behavioural, physiological and morphological traits related to flight potential and hence dispersal. Our goal was to characterize the heritabilities and genetic correlations among these traits and thus to understand more about the evolution of dispersal‐related life‐history syndromes in butterflies. Using a version of the animal model, we showed that all of the traits varied between the sexes, and most were either positively or negatively (phenotypically and/or genetically) correlated with body size. Heritable variation was present in most traits, with the highest heritabilities estimated for body mass and thorax ratio. The variance in flight activity among multiple measurements for the same individual was high even after controlling for the prevailing environmental conditions, indicating the importance of behavioural switching and/or inherent randomness associated with this type of movement. A number of dispersal‐related traits showed phenotypic correlations among one another, but only a few of these were associated with significant genetic correlations indicating that covariances between these traits in Bicyclus anynana are mainly environmentally induced.     

How river structure and biological traits influence gene flow: a population genetic study of two stream invertebrates with differing dispersal abilities

1. Determined by landscape structure as well as dispersal‐related traits of species, connectivity influences various key aspects of population biology, ranging from population persistence to genetic structure and diversity. Here, we investigated differences in small‐scale connectivity in terms of gene flow between populations of two ecologically important invertebrates with contrasting dispersal‐related traits: an amphipod (Gammarus fossarum) with a purely aquatic life cycle and a mayfly (Baetis rhodani) with a terrestrial adult stage. 2. We used highly polymorphic markers to estimate genetic differentiation between populations of both species within a Swiss pre‐alpine catchment and compared these results to the broader‐scale genetic structure within the Rhine drainage. Landscape genetic approaches were used to test for correlations of genetic and geographical structures and in‐stream barrier effects. 3. We found overall very weak genetic structure in populations of B. rhodani. In contrast, G. fossarum showed strong genetic differentiation, even at spatial scales of a few kilometres, and a clear pattern of isolation by distance. Genetic diversity decreased from downstream towards upstream populations of G. fossarum, suggesting asymmetric gene flow. Correlation of genetic structure with landscape topography was more pronounced in the amphipod. Our study also indicates that G. fossarum might be capable of dispersing overland in headwater regions and of crossing small in‐stream barriers. 4. We speculate that differences in dispersal capacity but also habitat specialisation and potentially the extent of local adaptation could be responsible for the differences in genetic differentiation found between the two species. These results highlight the importance of taking into account dispersal‐related traits when planning management and conservation strategies.     

Expanding population edges: theories,traits, and trade‐offs

Recent patterns of global change have highlighted the importance of understanding the dynamics and mechanisms of species range shifts and expansions. Unique demographic features, spatial processes, and selective pressures can result in the accumulation and evolution of distinctive phenotypic traits at the leading edges of expansions. We review the characteristics of expanding range margins and highlight possible mechanisms for the appearance of phenotypic differences between individuals at the leading edge and core of the range. The development of life history traits that increase dispersal or reproductive ability is predicted by theory and supported with extensive empirical evidence. Many examples of rapid phenotypic change are associated with trade‐offs that may influence the persistence of the trait once expansion ends. Accounting for the effects of edge phenotypes and related trade‐offs could be critical for predicting the spread of invasive species and population responses to climate change.     

Host dispersal as the driver of parasite genetic structure: a paradigm lost?

Understanding traits influencing the distribution of genetic diversity has major ecological and evolutionary implications for host–parasite interactions. The genetic structure of parasites is expected to conform to that of their hosts, because host dispersal is generally assumed to drive parasite dispersal. Here, we used a meta‐analysis to test this paradigm and determine whether traits related to host dispersal correctly predict the spatial co‐distribution of host and parasite genetic variation. We compiled data from empirical work on local adaptation and host–parasite population genetic structure from a wide range of taxonomic groups. We found that genetic differentiation was significantly lower in parasites than in hosts, suggesting that dispersal may often be higher for parasites. A significant correlation in the pairwise genetic differentiation of hosts and parasites was evident, but surprisingly weak. These results were largely explained by parasite reproductive mode, the proportion of free‐living stages in the parasite life cycle and the geographical extent of the study; variables related to host dispersal were poor predictors of genetic patterns. Our results do not dispel the paradigm that parasite population genetic structure depends on host dispersal. Rather, we highlight that alternative factors are also important in driving the co‐distribution of host and parasite genetic variation.     

The role of immigration and local adaptation on fine‐scale genotypic and phenotypic population divergence in a less mobile passerine

Dispersal and local patterns of adaptation play a major role on the ecological and evolutionary trajectory of natural populations. In this study, we employ a combination of genetic (25 microsatellite markers) and field‐based information (seven study years) to analyse the impact of immigration and local patterns of adaptation in two nearby (< 7 km) blue tit (Cyanistes caeruleus) populations. We used genetic assignment analyses to identify immigrant individuals and found that dispersal rate is female‐biased (72%). Data on lifetime reproductive success indicated that immigrant females produced fewer local recruits than their philopatric counterparts whereas immigrant males recruited more offspring than those that remained in their natal location. In spite of the considerably higher immigration rates of females, our results indicate that, in absolute terms, their demographic and genetic impact in the receiving populations is lower than that in immigrant males. Immigrants often brought novel alleles into the studied populations and a high proportion of them were transmitted to their recruits, indicating that the genetic impact of immigrants is not ephemeral. Although only a few kilometres apart, the two study populations were genetically differentiated and showed strong divergence in different phenotypic and life‐history traits. An almost absent inter‐population dispersal, together with the fact that both populations receive immigrants from different source populations, is probably the main cause of the observed pattern of genetic differentiation. However, phenotypic differentiation (P_ST) for all the studied traits greatly exceeded neutral genetic differentiation (F_ST), indicating that divergent natural selection is the prevailing factor determining the evolutionary trajectory of these populations. Our study highlights the importance of integrating individual‐ and population‐based approaches to obtain a comprehensive view about the role of dispersal and natural selection on structuring the genotypic and phenotypic characteristics of natural populations.     

Phenotypic plasticity and genetic isolation-by-distance in the freshwater mussel <Emphasis Type="Italic">Unio pictorum</Emphasis> (Mollusca: Unionoida)

Freshwater mussels (Unionoida) show high intraspecific morphological variability, and some shell morphological traits are believed to be associated with habitat conditions. It is not known whether and which of these ecophenotypic differences reflect underlying genetic differentiation or are the result of phenotypic plasticity. Using 103 amplified fragment length polymorphism (AFLP) markers, we studied population genetics of three paired Unio pictorum populations sampled from two different habitat types (marina and river) along the River Thames. We found genetic differences along the Thames which were consistent with a pattern of isolation by distance and probably reflect limited dispersal via host fish species upon which unionoid larvae are obligate parasites. No consistent genetic differences were found between the two different habitat types suggesting that morphological differences in the degree of shell elongation and the shape of dorso-posterior margin are caused by phenotypic plasticity. Our study provides the first good evidence for phenotypic plasticity of shell shape in a European unionoid and illustrates the need to include genetic data in order properly to interpret geographic patterns of morphological variation.     

Selection on life‐history traits and genetic population divergence in rotifers

A combination of founder effects and local adaptation – the Monopolization hypothesis – has been proposed to reconcile the strong population differentiation of zooplankton dwelling in ponds and lakes and their high dispersal abilities. The role genetic drift plays in genetic differentiation of zooplankton is well documented, but the impact of natural selection has received less attention. Here, we compare differentiation in neutral genetic markers (F_ST) and in quantitative traits (Q_ST) in six natural populations of the rotifer Brachionus plicatilis to assess the importance of natural selection in explaining genetic differentiation of life‐history traits. Five life‐history traits were measured in four temperature × salinity combinations in common‐garden experiments. Population differentiation for neutral genetic markers – 11 microsatellite loci – was very high (F_ST = 0.482). Differentiation in life‐history traits was higher in traits related to sexual reproduction than in those related to asexual reproduction. Q_ST values for diapausing egg production (a trait related to sexual reproduction) were higher than their corresponding F_ST in some pairs of populations. Our results indicate the importance of divergent natural selection in these populations and suggest local adaptation to the unpredictability of B. plicatilis habitats.     

Population structure of an endangered species living in contrasted habitats: Parnassia palustris (Saxifragaceae)

In endangered species, it is critical to analyse the level at which populations interact (i.e. dispersal) as well as the levels of inbreeding and local adaptation to set up conservation policies. These parameters were investigated in the endangered species Parnassia palustris living in contrasted habitats. We analysed population structure in 14 populations of northern France for isozymes, cpDNA markers and phenotypic traits related to fitness. Within population genetic diversity and inbreeding coefficients were not correlated to population size. Populations seem not to have undergone severe recent bottleneck. Conversely to pollen migration, seed migration seems limited at a regional scale, which could prevent colonization of new sites even if suitable habitats appear. Finally, the habitat type affects neither within-population genetic diversity nor genetic and phenotypic differentiation among populations. Thus, even if unnoticed local adaptation to habitats exists, it does not influence gene flow between populations.     

Linking stream ecology with morphological variability in a native freshwater fish from semi‐arid Australia

Environmental variation is a potent force affecting phenotypic expression. While freshwater fishes have provided a compelling example of the link between the environment and phenotypic diversity, few studies have been conducted with arid‐zone fishes, particularly those that occur in geographically isolated regions where species typically inhabit intermittent and ephemeral creeks. We investigated morphological variation of a freshwater fish (the western rainbowfish, Melanotaenia australis) inhabiting creeks in the Pilbara region of northwest Australia to determine whether body shape variation correlated with local environmental characteristics, including water velocity, habitat complexity, predator presence, and food availability. We expected that the geographic isolation of creeks within this arid region would result in habitat‐specific morphological specializations. We used landmark‐based geometric morphometrics to quantify the level of morphological variability in fish captured from 14 locations within three distinct subcatchments of a major river system. Western rainbowfish exhibited a range of morphologies, with variation in body depth accounting for a significant proportion (>42%) of the total variance in shape. Sexual dimorphism was also apparent, with males displaying deeper bodies than females. While the measured local habitat characteristics explained little of the observed morphological variation, fish displayed significant morphological differentiation at the level of the subcatchment. Local adaptation may partly explain the geographic patterns of body shape variation, but fine‐scale genetic studies are required to disentangle the effects of genetic differentiation from environmentally determined phenotypic plasticity in body shape. Developing a better understanding of environment–phenotype relationships in species from arid regions will provide important insights into ecological and evolutionary processes in these unique and understudied habitats.     

Variation in Dispersal Traits in a Narrow-endemic Plant Species, <Emphasis Type="Italic">Centaurea corymbosa</Emphasis> Pourret. (Asteraceae)

The existence of genetic variability for dispersal is a crucial issue for organisms facing increased habitat fragmentation and climate change. We study the genetic basis and evolutionary potential for diaspore traits related to dispersal in Centaurea corymbosa. Using diaspores collected in natural conditions in four of the six extant populations of this narrow-endemic plant species and diaspores produced in a common garden experiment, we study the variation for pappus and achene sizes, and diaspore mass. Using a sample of achenes from the common garden experiment, we find that the best predictor of terminal velocity is a linear combination of pappus length, achene width, and achene weight. We find significant differences among populations for all traits in both conditions, as well as significant differences among families within population. Although the differences among populations for some traits are not exactly the same in controlled conditions compared to natural conditions, the ranking of populations according to their mean trait values is consistent in both conditions. Our study is therefore one of the first to show a correlation between phenotypic differentiation for dispersal traits in natural conditions vs. controlled conditions. We also show evidence of genetic variation for traits commonly thought to be involved in dispersal ability, suggesting the potential for evolutionary changes following environmental change and management actions.Co-ordinating editor: J.F. Stuefer     

Dispersal ability and habitat requirements determine landscape‐level genetic patterns in desert aquatic insects

Species occupying the same geographic range can exhibit remarkably different population structures across the landscape, ranging from highly diversified to panmictic. Given limitations on collecting population‐level data for large numbers of species, ecologists seek to identify proximate organismal traits—such as dispersal ability, habitat preference and life history—that are strong predictors of realized population structure. We examined how dispersal ability and habitat structure affect the regional balance of gene flow and genetic drift within three aquatic insects that represent the range of dispersal abilities and habitat requirements observed in desert stream insect communities. For each species, we tested for linear relationships between genetic distances and geographic distances using Euclidean and landscape‐based metrics of resistance. We found that the moderate‐disperser Mesocapnia arizonensis (Plecoptera: Capniidae) has a strong isolation‐by‐distance pattern, suggesting migration–drift equilibrium. By contrast, population structure in the flightless Abedus herberti (Hemiptera: Belostomatidae) is influenced by genetic drift, while gene flow is the dominant force in the strong‐flying Boreonectes aequinoctialis (Coleoptera: Dytiscidae). The best‐fitting landscape model for M. arizonensis was based on Euclidean distance. Analyses also identified a strong spatial scale‐dependence, where landscape genetic methods only performed well for species that were intermediate in dispersal ability. Our results highlight the fact that when either gene flow or genetic drift dominates in shaping population structure, no detectable relationship between genetic and geographic distances is expected at certain spatial scales. This study provides insight into how gene flow and drift interact at the regional scale for these insects as well as the organisms that share similar habitats and dispersal abilities.     

Long‐term in situ persistence of biodiversity in tropical sky islands revealed by landscape genomics

Tropical mountains are areas of high species richness and endemism. Two historical phenomena may have contributed to this: (i) fragmentation and isolation of habitats may have promoted the genetic differentiation of populations and increased the possibility of allopatric divergence and speciation and (ii) the mountain areas may have allowed long‐term population persistence during global climate fluctuations. These two phenomena have been studied using either species occurrence data or estimating species divergence times. However, only few studies have used intraspecific genetic data to analyse the mechanisms by which endemism may emerge at the microevolutionary scale. Here, we use landscape analysis of genomic SNP data sampled from two high‐elevation plant species from an archipelago of tropical sky islands (the Trans‐Mexican Volcanic Belt) to test for population genetic differentiation, synchronous demographic changes and habitat persistence. We show that genetic differentiation can be explained by the degree of glacial habitat connectivity among mountains and that mountains have facilitated the persistence of populations throughout glacial/interglacial cycles. Our results support the ongoing role of tropical mountains as cradles for biodiversity by uncovering cryptic differentiation and limits to gene flow.     

Role of phenotypic plasticity and population differentiation in adaptation to novel environmental conditions

Species can adapt to new environmental conditions either through individual phenotypic plasticity, intraspecific genetic differentiation in adaptive traits, or both. Wild emmer wheat, Triticum dicoccoides, an annual grass with major distribution in Eastern Mediterranean region, is predicted to experience in the near future, as a result of global climate change, conditions more arid than in any part of the current species distribution. To understand the role of the above two means of adaptation, and the effect of population range position, we analyzed reaction norms, extent of plasticity, and phenotypic selection across two experimental environments of high and low water availability in two core and two peripheral populations of this species. We studied 12 quantitative traits, but focused primarily on the onset of reproduction and maternal investment, which are traits that are closely related to fitness and presumably involved in local adaptation in the studied species. We hypothesized that the population showing superior performance under novel environmental conditions will either be genetically differentiated in quantitative traits or exhibit higher phenotypic plasticity than the less successful populations. We found the core population K to be the most plastic in all three trait categories (phenology, reproductive traits, and fitness) and most successful among populations studied, in both experimental environments; at the same time, the core K population was clearly genetically differentiated from the two edge populations. Our results suggest that (1) two means of successful adaptation to new environmental conditions, phenotypic plasticity and adaptive genetic differentiation, are not mutually exclusive ways of achieving high adaptive ability; and (2) colonists from some core populations can be more successful in establishing beyond the current species range than colonists from the range extreme periphery with conditions seemingly closest to those in the new environment.     

Effects of regional climate and small‐scale habitat quality on performance in the relict species Ramonda myconi

Abstract. The Mediterranean Basin harbours paleo‐endemic species with a highly restricted and fragmented distribution. Many of them might also be of the remnant type, for which the regional dynamics depends on the persistence of extant populations. Therefore, a key issue for the long‐term persistence of these species is to assess the variability and effects of ecological factors determining plant performance. We investigated the spatio‐temporal variability in plant traits and ecological factors of Ramonda myconi, a preglacial relict species with remnant dynamics, in 5 populations over 4–7 yr. Ecological factors contributing to fecundity showed a high degree of between‐year variability. Pre‐dispersal fruit predation had a minor influence on total reproductive output, and most of the variability was found among individuals within populations and years. Spatio‐temporal variability in growth and survival was rather low but significant, whereas recruitment showed important between‐population variability. Among‐year variability in fecundity and growth was related to climatic fluctuations on a regional scale, notably rainfall and temperature in a particular period, while the spatial variability in survival and recruitment was explained by within‐population (patch) habitat quality. Although R. myconi is able to withstand repeated periods of drought, water availability seems to be the most important factor affecting plant performance in all the study populations. These findings suggest that the long‐term persistence of species showing remnant population dynamics in habitats under the influence of Mediterranean climate might be threatened by increased aridity as a result of climate change.     

Host‐associated genomic differentiation in congeneric butterflies: now you see it,now you do not

Ecotypic variation among populations may become associated with widespread genomic differentiation, but theory predicts that this should happen only under particular conditions of gene flow, selection and population size. In closely related species, we might expect the strength of host‐associated genomic differentiation (HAD) to be correlated with the degree of phenotypic differentiation in host‐adaptive traits. Using microsatellite and Amplified Fragment Length Polymorphism (AFLP) markers, and controlling for isolation by distance between populations, we sought HAD in two congeneric species of butterflies with different degrees of host plant specialization. Prior work on Euphydryas editha had shown strong interpopulation differentiation in host‐adapted traits, resulting in incipient reproductive isolation among host‐associated ecotypes. We show here that Euphydryas aurinia had much weaker host‐associated phenotypic differentiation. Contrary to our expectations, we detected HAD in Euphydryas aurinia, but not in E. editha. Even within an E. aurinia population that fed on both hosts, we found weak but significant sympatric HAD that persisted in samples taken 9 years apart. The finding of significantly stronger HAD in the system with less phenotypic differentiation may seem paradoxical. Our findings can be explained by multiple factors, ranging from differences in dispersal or effective population size, to spatial variation in genomic or phenotypic traits and to structure induced by past histories of host‐adapted populations. Other infrequently measured factors, such as differences in recombination rates, may also play a role. Our result adds to recent work as a further caution against assumptions of simple relationships between genomic and adaptive phenotypic differentiation.