Geographic Variation In Selection Research Articles

Pollinator-mediated selection on Krameria oil flowers: a flower-pollinator fit adaptation to an atypical oil-collecting behaviour?

Spatial variation in plant-pollinator interactions is a key driver of floral trait diversification. A so far overlooked qualitative aspect of this variation is the behavioural component on flowers that relates to the pollinator fit. We tested the hypothesis that variation in pollinator behaviour influences the geographical pattern of phenotypic selection across the distribution range of the oil-producing Krameria grandiflora (Krameriaceae). This variation mainly involves the presence or absence of flag petal grasping, which is only performed by representatives of Centris (Centridini, Apidae), an oil-collecting bee group highly associated with Krameriaceae pollination. We quantified variation in floral traits and fitness and estimated pollinator-mediated selection in five populations at a large geographical scale comprising the entire species range. In each population, we sampled individual pollen arrival and germination as a fitness measure, indicating pollination success and pollination performance, which was then relativized and regressed on standardized flower-pollinator fit (flag-stigma distance), advertisement (sepal length) and reward (oil volume) traits. This generated mean-scaled selection gradients used to calculate geographical selection dispersion. Unexpectedly, stronger selection was detected on the flower-pollinator fit trait in populations highly associated with the absence of flag petal grasping. Geographical variation in selection was mainly attributed to differential selection on the flag-stigma distance generating a selection mosaic. This may involve influences of a spatial variation in pollinator behaviour as well as composition and morphology. Our results show the adaptive significance of the specialized flag petals of Krameria in the absence of the grasping behaviour and highlight the contribution of geographical variation in pollinator behaviour on flowers in driving selection mosaics, with implications for floral evolution, adaptation to pollinator fit and phenotypic diversity in specialized systems.

EVIDENCE FOR BATESIAN MIMICRY IN A POLYMORPHIC HOVERFLY

Palatable Batesian mimics are avoided by predators because they resemble noxious or defended species. The striking resemblance of many hoverflies to noxious Hymenoptera is a "textbook" example of Batesian mimicry, but evidence that selection by predators has shaped the evolution of hoverfly patterns is weak. We looked for geographical and temporal trends in frequencies of morphs of the polymorphic hoverfly Volucella bombylans that would support the hypothesis that these morphs are Batesian mimics of different bumblebee species. The frequency of the black and yellow hoverfly morph was significantly positively related to the frequency of black and yellow bumblebees across 52 sites. Similarly, the frequency of the red-tailed hoverfly morph was positively related to the frequency of red-tailed bumblebees. However, the frequencies of hoverfly morphs were positively spatially autocorrelated, and after controlling for this, only one of the two common hoverfly morphs showed a significant positive relationship with its putative model. We conclude that the distribution of V. bombylans morphs probably reflects geographical variation in selection by predators resulting from differences in the frequencies of noxious bumblebee species.

Do highly divergent loci reside in genomic regions affecting reproductive isolation? A test using next-generation sequence data in Timema stick insects

BackgroundGenetic divergence during speciation with gene flow is heterogeneous across the genome, with some regions exhibiting stronger differentiation than others. Exceptionally differentiated regions are often assumed to experience reduced introgression, i.e., reduced flow of alleles from one population into another because such regions are affected by divergent selection or cause reproductive isolation. In contrast, the remainder of the genome can be homogenized by high introgression. Although many studies have documented variation across the genome in genetic differentiation, there are few tests of this hypothesis that explicitly quantify introgression. Here, we provide such a test using 38,304 SNPs in populations of Timema cristinae stick insects. We quantify whether loci that are highly divergent between geographically separated (‘allopatric’) populations exhibit unusual patterns of introgression in admixed populations. To the extent this is true, highly divergent loci between allopatric populations contribute to reproductive isolation in admixed populations.ResultsAs predicted, we find a substantial association between locus-specific divergence between allopatric populations and locus-specific introgression in admixed populations. However, many loci depart from this relationship, sometimes strongly so. We also report evidence for selection against foreign alleles due to local adaptation.ConclusionsLoci that are strongly differentiated between allopatric populations sometimes contribute to reproductive isolation in admixed populations. However, geographic variation in selection and local adaptation, in aspects of genetic architecture (such as organization of genes, recombination rate variation, number and effect size of variants contributing to adaptation, etc.), and in stochastic evolutionary processes such as drift can cause strong differentiation of loci that do not always contribute to reproductive isolation. The results have implications for the theory of ‘genomic islands of speciation’.

Escape from floral herbivory by early flowering in Arabidopsis halleri subsp. gemmifera

Natural selection on flowering phenology has been studied primarily in terms of plant-pollinator interactions and effects of abiotic conditions. Little is known, however, about geographic variation in other biotic factors such as herbivores and its consequence for differential selection on flowering phenology among populations. Here, we examine selection by floral herbivores on the flowering phenology of Arabidopsis halleri subsp. gemmifera using two adjacent populations with contrasting herbivory regimes. Intensive floral herbivory by the leaf beetle Phaedon brassicae occurs in one population, while the beetle is absent in another population. We tested the hypothesis that the two populations experience differential selection on flowering time that is attributable to the presence or absence of floral herbivory. A two-year field study showed that early flowering was favoured in the population under intensive floral herbivory, whereas selection for early flowering was not found in one year in the population where floral herbivory was absent. Selection for early flowering disappeared when the abundance of floral herbivores was artificially decreased in a field experiment. Thus, the heterogeneous distribution of P. brassicae was a major agent for differential selection on flowering time. However, flowering time did not differ between the two populations when plants were grown in the laboratory. The lack of genetic differentiation in flowering time may be explained by ongoing gene flow or recent invasion of P. brassicae. This study illustrates that the role of floral herbivory in shaping geographic variation in selection on flowering phenology may be more important than previously thought.

Habitat-mediated size selection in endangered Atlantic salmon fry: selectional restoration assessment.

Preservation of adaptive variation is a top priority of many species restoration programs, but most restoration activities are conducted without direct knowledge of selection that might foster or impair adaptation and restoration goals. In this study, we quantified geographic variation in selection on fry size of endangered Atlantic salmon (Salmo salar) during the 6-week period immediately following stocking in the wild. We also used a model selection approach to assess whether habitat variables influence patterns of such selection. We found evidence for significant size-selection in five out of six selection trials. Interestingly, the strength and pattern of selection varied extensively among sites, and model selection suggested that this variation in phenotypic selection was related to geographic variation in the presence of large woody debris and the slope of the stream gradient. The strong selection differentials we observed should be a concern for endangered salmon restoration, whether they reflect natural processes and an opportunity to maintain adaptation, or an indicator of the potentially deleterious phenotypic consequences of hatchery practices.

Geographical Variation in Selection, from Phenotypes to Molecules

Molecular technologies now allow researchers to isolate quantitative trait loci (QTLs) and measure patterns of gene sequence variation within chromosomal regions containing important polymorphisms. I develop a simulation model to investigate gene sequence evolution within genomic regions that harbor QTLs. The QTLs influence a trait experiencing geographical variation in selection, which is common in nature and produces obvious differentiation at the phenotypic level. Counter to expectations, the simulations suggest that selection can substantially affect quantitative genetic variation without altering the amount and pattern of molecular variation at sites closely linked to the QTLs. Even with large samples of gene sequences, the likelihood of rejecting neutrality is often low. The exception is situations where strong selection is combined with low migration among demes, conditions that may be common in many plant species. The results have implications for gene sequence surveys and, perhaps more generally, for interpreting the apparently weak connection between levels of molecular and quantitative trait variation within species.

Geographical Variation in Selection, from Phenotypes to Molecules

Geographical Variation in Selection, from Phenotypes to Molecules

Genetic Diversity and the Evolutionary History of Plant Immunity Genes in Two Species of Zea

Plant pathogenesis-related genes (PR genes) code for enzymes, enzyme inhibitors, and other peptides that confer resistance to pathogens and herbivores. Although several PR genes have been the subject of molecular population genetic analyses, a general understanding of their long-term evolutionary dynamics remains incomplete. Here we analyze sequence data from 17 PR genes from two closely related teosinte species of central Mexico. In addition to testing whether patterns of diversity at individual loci depart from expectations under a neutral model, we compared patterns of diversity at defense genes, as a class, to nondefense genes. In Zea diploperennis, the majority of defense genes have patterns of diversity consistent with neutral expectations while at least two genes showed evidence of recent positive selection consistent with arms-race models of antagonistic coevolution. In Zea mays ssp. parviglumis, by contrast, analyses of both defense and nondefense genes revealed strong and consistent departures from the neutral model, suggestive of nonequilibrium population dynamics or population structure. Nevertheless, we found a significant excess of replacement polymorphism in defense genes compared to nondefense genes. Although we cannot exclude relaxed selective constraint as an explanation, our results are consistent with temporally variable (transient or episodic) selection or geographically variable selection acting on parviglumis defense genes. The different patterns of diversity found in the two Zea species may be explained by parviglumis' greater distribution and population structure together with geographic variation in selection.

Pre-dispersal seed predation in Primula veris: among-population variation in damage intensity and selection on flower number.

The geographic mosaic theory of co-evolution states that evolution of interactions is driven by geographical variation in interactions between species. We investigated whether the intensity of pre-dispersal seed predation differed among nine Primula veris populations over 5years, and whether such differences lead to geographical variation in selection on flower number. Seed predation intensity differed significantly among years and populations, and it increased with canopy closure and decreased with the density of the field layer vegetation. Individuals in open habitats also produced the highest number of flowers. Moreover, the phenotypic selection on flower number differed among years and populations. In populations of closed habitats, with high seed predation pressure, the increased number of flowers was often correlated with an increased number of damaged capsules. However, an increased flower number did not result in fewer intact fruits due to seed predation in any population.

The evolution of trade-offs: testing predictions on response to selection and environmental variation.

The concept of phenotypic trade-offs is a central element in evolutionary theory. In general, phenotypic models assume a fixed trade-off function, whereas quantitative genetic theory predicts that the trade-off function will change as a result of selection. For a linear trade-off function selection will readily change the intercept but will have to be relatively stronger to change the slope. We test these predictions by examining the trade-off between fecundity and flight capability, as measured by dorso-longitudinal muscle mass, in four different populations of the sand cricket, Gryllus firmus. Three populations were recently derived from the wild, and the fourth had been in the laboratory for 19 years. We hypothesized that the laboratory population had most likely undergone more and different selection from the three wild populations and therefore should differ from these in respect to both slope and intercept. Because of geographic variation in selection, we predicted a general difference in intercept among the four populations. We further tested the hypothesis that this intercept will be correlated with proportion macropterous and that this relationship will itself vary with environmental conditions experienced during both the nymphal and adult period. Observed variation in the phenotypic trade-off was consistent with the predictions of the quantitative genetic model. These results point to the importance of modeling trade-offs as dynamic rather than static relationships. We discuss how phenotypic models can incorporate such variation. The phenotypic trade-off between fecundity and dorso-longitudinal muscle mass is determined in part by variation in body size, illustrating the necessity of considering trade-offs to be multi factorial rather than simply bivariate relationships.

Managing resistance to the insect growth regulator, pyriproxyfen, inBemisia tabaci

The insect growth regulator pyriproxyfen (a juvenoid) effectively inhibits the hatching of eggs of the tobacco or cotton whitefly, Bemisia tabaci, as well as causing pupal mortality. Since 1991, this insecticide has been one of the main agents for controlling B tabaci on Israeli cotton. Seasonal trends of susceptibility to pyriproxyfen in field populations were monitored from June (prior to treatment) through late summer at different locations in Israel. After seven years of pyriproxyfen use within an insecticide resistance management strategy that limits this insecticide to a single application per season, susceptibility has been maintained in many areas. In other locations where pyriproxyfen had been used against geographically isolated populations of B tabaci, moderate to high levels of resistance have been observed. Ecological and agronomic factors that may contribute to geographical variation in selection for resistance are discussed. The dynamics of pyriproxyfen-susceptible and -resistant populations of B tabaci following a single application of pyriproxyfen were investigated under simulated field conditions in the laboratory. The susceptible population was suppressed very effectively, whereas effects of pyriproxyfen against the resistant population were much more transient. Differences in the productivity of susceptible and resistant strains in the absence of pyriproxyfen treatment could reflect a fitness cost accounting for observed reductions in resistance levels between seasons in the field. They may also explain why, following a recent reduction in the use of pyriproxyfen in cotton fields, resistance in 1998 declined to levels observed in 1995/6. © 1999 Society of Chemical Industry

FITNESS VARIATION ACROSS A SUBDIVIDED POPULATION OF THE ANNUAL PLANT IMPATIENS CAPENSIS.

The evolution of a subdivided population depends on whether subpopulations with high mean fitness make a greater per capita contribution to the next generation than subpopulations with lower mean fitness. I distinguish two different models of ecological population structure, denoted local compensation and global compensation. Local compensation restricts the differential contribution of subpopulations, whereas global compensation allows subpopulations to contribute in direct proportion to their mean fitness. I describe a simple regression-based method that distinguishes these alternatives as points on a continuum of possible population structures. The method is applied to field measurements of local abundance and reproduction in a subdivided population of the annual plant Impatiens capensis. These data suggest that the global compensation model is a more accurate description of the population studied. This result is surprising because local density effects on growth and reproduction occur in I. capensis. The implications of ecological population structure for both geographical variation in selection and kin selection are discussed.

Male predation risk determines female preference in the Trinidad guppy

The 'runaway process of sexual selection' explains the evolution of extreme sexual behaviours and adornments and requires that a preferred secondary sexual character expressed in one sex be genetically correlated with a preference for this character expressed in the other sex1–4. Because of this genetic correlation, individuals selecting the preferred character also select higher levels of preference, producing rapid, self-reinforcing evolution of both characters. Preference for increased development of the secondary sexual character is counterbalanced at equilibrium by natural selection acting against this character. Variation in predation against brightly coloured male guppies, Poecilia reticulata, presents a unique opportunity to test a prediction of this counterbalance between sexual and natural selection, namely that geographical variation in selection against the preferred character should lead to parallel variation in preference5. Here we present evidence that female guppies from high-predation populations show a genetically determined, lower preference for brightly coloured models of male guppies than do females from areas of low predation.

SELECTION INTENSITY IN NORTH AMERICAN HOUSE SPARROWS (PASSER DOMESTICUS).

House sparrows (Passer domesticus) were first introduced at New York City in 1852 and have since colonized most of North America. Present North American populations are mostly descendent from birds from central England (Barrows, 1889). As house sparrows dispersed, populations responded to different selective regimes determined by local conditions. In little more than 100 generations since its introduction, Johnston and Selander ( 1971 ) showed that development of morphological variation has approached the amount seen in this species for the whole of Europe. They related geographic trends in morphological variation to environmental factors, and their interpretations of trends in body size and proportions for North American populations are generally consistent with the ecogeographic rules of Bergmann and Allen. Selection pressures on sparrow populations must vary considerably throughout the continent. A method of measuring such selection intensity is provided by O'Donald (1968a, b, 1970a, 1972, 1973) that uses the proportionate change in fitness, Ar7/rw, as a measure of selection intensity. The model assumed for fitness is a quadratic fitness function, w = 1 a K( -)2. As a character-state, x, deviates from an optimum phenotype, 0, fitness of the individual decreases. Actual fitness is given if the constant a is known, or if a = 0, then fitnesses are relative; K is a constant. From this model, O'Donald (1970b) shows that the change in mean fitness is A?v = Vwl/r, where w7v is mean fitness and Vw is variance in fitness. If the character x is normally distributed, then Afvlfv/ = (x-'C)2/ V + 1/2{ [(X-)2 + (V $-V )V]/ V,}2, where x and V$ are the initial mean and variance of the character and x' and V'$ are these values after some period of selection (O'Donald, 1973). Death can occur for any member of a population through predation, disease, environmental stresses, starvation or accidents. Certain elements of populations may be more susceptible to mortality and in such cases selective deaths will occur. Since Johnston and Selander (1971) have shown that patterns of morphological and climatic variation are highly congruent, suggesting a cause-effect relationship, I felt that any geographic variation in selection on house sparrows was caused primarily by environmental mortality factors. Such influences continuously affect species' populations, and I initially investigated selection in house sparrows with the view of determining relationships between selection intensities and climatic factors. Additionally the questions of when selection initially operated in adapting North American sparrows and to what extent selection still operates on house sparrows were examined.