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Variation
Kinds of Variation Terms modified by Variation Selected AbstractsHERITABILITY OF AND EARLY ENVIRONMENT EFFECTS ON VARIATION IN MATING PREFERENCESEVOLUTION, Issue 4 2010Holger Schielzeth Many species show substantial between-individual variation in mating preferences, but studying the causes of such variation remains a challenge. For example, the relative importance of heritable variation versus shared early environment effects (like sexual imprinting) on mating preferences has never been quantified in a population of animals. Here, we estimate the heritability of and early rearing effects on mate choice decisions in zebra finches based on the similarity of choices between pairs of genetic sisters raised apart and pairs of unrelated foster sisters. We found a low and nonsignificant heritability of preferences and no significant shared early rearing effects. A literature review shows that a low heritability of preferences is rather typical, whereas empirical tests for the relevance of sexual imprinting within populations are currently limited to very few studies. Although effects on preference functions (i.e., which male to prefer) were weak, we found strong individual consistency in choice behavior and part of this variation was heritable. It seems likely that variation in choice behavior (choosiness, responsiveness, sampling behavior) would produce patterns of nonrandom mating and this might be the more important source of between-individual differences in mating patterns. [source] CONFLICTING SELECTION FROM AN ANTAGONIST AND A MUTUALIST ENHANCES PHENOTYPIC VARIATION IN A PLANTEVOLUTION, Issue 4 2010Adam M. Siepielski The raw material for evolution is variation. Consequently, identifying the factors that generate, maintain, and erode phenotypic and genetic variation in ecologically important traits within and among populations is important. Although persistent directional or stabilizing selection can deplete variation, spatial variation in conflicting directional selection can enhance variation. Here, we present evidence that phenotypic variation in limber pine (Pinus flexilis) cone structure is enhanced by conflicting selection pressures exerted by its mutualistic seed disperser (Clark's nutcracker Nucifraga columbiana) and an antagonistic seed predator (pine squirrel Tamiasciurus spp.). Phenotypic variation in cone structure was bimodal and about two times greater where both agents of selection co-occurred than where one (the seed predator) was absent. Within the region where both agents of selection co-occurred, bimodality in cone structure was pronounced where there appears to be a mosaic of habitats with some persistent habitats supporting only the seed disperser. These results indicate that conflicting selection stemming from spatial variation in community diversity can enhance phenotypic variation in ecologically important traits. [source] LATITUDINAL VARIATION IN SPECIATION MECHANISMS IN FROGSEVOLUTION, Issue 2 2010Xia Hua Speciation often has a strong geographical and environmental component, but the ecological factors that potentially underlie allopatric and parapatric speciation remain understudied. Two ecological mechanisms by which speciation may occur on geographic scales are allopatric speciation through niche conservatism and parapatric or allopatric speciation through niche divergence. A previous study on salamanders found a strong latitudinal pattern in the prevalence of these mechanisms, with niche conservatism dominating in temperate regions and niche divergence dominating in the tropics, and related this pattern to Janzen's hypothesis of greater climatic zonation between different elevations in the tropics. Here, we test for latitudinal patterns in speciation in a related but more diverse group of amphibians, the anurans. Using data from up to 79 sister-species pairs, we test for latitudinal variation in elevational and climatic overlap between sister species, and evaluate the frequency of speciation via niche conservatism versus niche divergence in relation to latitude. In contrast to salamanders, we find no tendency for greater niche divergence in the tropics or for greater niche conservatism in temperate regions. Although our results support the idea of greater climatic zonation in tropical regions, they show that this climatic pattern does not lead to straightforward relationships between speciation, latitude, and niche evolution. [source] GEOGRAPHIC VARIATION, FREQUENCY-DEPENDENT SELECTION, AND THE MAINTENANCE OF A FEMALE-LIMITED POLYMORPHISMEVOLUTION, Issue 1 2010Ryan Calsbeek A central problem in evolutionary biology is to understand how spatial and temporal variation in selection maintain genetic variation within and among populations. Brown anole lizards (Anolis sagrei) exhibit a dorsal pattern polymorphism that is expressed only in females, which occur in "diamond,""bar," and intermediate "diamond-bar" morphs. To understand the inheritance of this polymorphism, we conducted a captive breeding study that refuted several single-locus models and supported a two-locus mode of inheritance. To describe geographic variation in morph frequencies, we surveyed 13 populations from two major islands in The Bahamas. Morph frequencies differed substantially between major islands but were highly congruent within each island. Finally, we measured viability selection on each island to test two hypotheses regarding the maintenance of the polymorphism: (1) that spatial variation in selection maintains variation in morph frequencies between islands, and (2) that temporal variation in selection across years maintains variation within islands. Although bar females had relatively lower survival where they were rare, our data do not otherwise suggest that selection varies spatially between islands. However, diamond-bar females were subject to positive frequency-dependent selection across years, and the relative fitness of bar and diamond females alternated across years. We propose that this polymorphism is maintained by temporal variation in selection coupled with the sheltering of alleles via a two-locus inheritance pattern and sex-limited expression. [source] GENES WITH SOCIAL EFFECTS ARE EXPECTED TO HARBOR MORE SEQUENCE VARIATION WITHIN AND BETWEEN SPECIESEVOLUTION, Issue 7 2009Timothy A. Linksvayer The equilibrium sequence diversity of genes within a population and the rate of sequence divergence between populations or species depends on a variety of factors, including expression pattern, mutation rate, nature of selection, random drift, and mating system. Here, we extend population genetic theory developed for maternal-effect genes to predict the equilibrium polymorphism within species and sequence divergence among species for genes with social effects on fitness. We show how the fitness effects of genes, mating system, and genetic system affect predicted gene polymorphism. We find that, because genes with indirect social effects on fitness effectively experience weaker selection, they are expected to harbor higher levels of polymorphism relative to genes with direct fitness effects. The relative increase in polymorphism is proportional to the inverse of the genetic relatedness between individuals expressing the gene and their social partners that experience the fitness effects of the gene. We find a similar pattern of more rapid divergence between populations or species for genes with indirect social effects relative to genes with direct effects. We focus our discussion on the social insects, organisms with diverse indirect genetic effects, mating and genetic systems, and we suggest specific examples for testing our predictions with emerging sociogenomic tools. [source] QUANTITATIVE GENETIC VARIATION IN POPULATIONS OF AMSINCKIA SPECTABILIS THAT DIFFER IN RATE OF SELF-FERTILIZATIONEVOLUTION, Issue 5 2009Magdalena P. Bartkowska Self-fertilization is expected to reduce genetic diversity within populations and consequently to limit adaptability to changing environments. Little is known, however, about the way the evolution of self-fertilization changes the amount or pattern of the components of genetic variation in natural populations. In this study, a reciprocal North Carolina II design and maximum-likelihood methods were implemented to investigate the genetic basis of variation for 15 floral and vegetative traits in four populations of the annual plant Amsinckia spectabilis (Boraginaceae) differing in mating system. Six variance components were estimated according to Cockerham and Weir's "bio" model c. Compared to the three partially selfing populations, we found significantly lower levels of nuclear variance for several traits in the nearly completely self-fertilizing population. Furthermore, for 11 of 15 traits we did not detect nuclear variation to be significantly greater than zero. We also found high maternal variance in one of the partially selfing populations for several traits, and little dominance variance in any population. These results are in agreement with the evolutionary dead-end hypothesis for highly self-fertilizing taxa. [source] RELATIVE CONTRIBUTION OF ADDITIVE, DOMINANCE, AND IMPRINTING EFFECTS TO PHENOTYPIC VARIATION IN BODY SIZE AND GROWTH BETWEEN DIVERGENT SELECTION LINES OF MICEEVOLUTION, Issue 5 2009Reinmar Hager Epigenetic effects attributed to genomic imprinting are increasingly recognized as an important source of variation in quantitative traits. However, little is known about their relative contribution to phenotypic variation compared to those of additive and dominance effects, and almost nothing about their role in phenotypic evolution. Here we address these questions by investigating the relative contribution of additive, dominance, and imprinting effects of quantitative trait loci (QTL) to variation in "early" and "late" body weight in an intercross of mice selected for divergent adult body weight. We identified 18 loci on 13 chromosomes; additive effects accounted for most of the phenotypic variation throughout development, and imprinting effects were always small. Genetic effects on early weight showed more dominance, less additive, and, surprisingly, less imprinting variation than that of late weight. The predominance of additivity of QTL effects on body weight follows the expectation that additive effects account for the evolutionary divergence between selection lines. We hypothesize that the appearance of more imprinting effects on late body weight may be a consequence of divergent selection on adult body weight, which may have indirectly selected for alleles showing partial imprinting effects due to their associated additive effects, highlighting a potential role of genomic imprinting in the response to selection. [source] DOES VARIATION IN SELECTION IMPOSED BY BEARS DRIVE DIVERGENCE AMONG POPULATIONS IN THE SIZE AND SHAPE OF SOCKEYE SALMON?EVOLUTION, Issue 5 2009Stephanie M. Carlson Few studies have determined whether formal estimates of selection explain patterns of trait divergence among populations, yet this is one approach for evaluating whether the populations are in equilibria. If adaptive divergence is complete, directional selection should be absent and stabilizing selection should prevail. We estimated natural selection, due to bear predation, acting on the body size and shape of male salmon in three breeding populations that experience differing predation regimes. Our approach was to (1) estimate selection acting within each population on each trait based on an empirical estimate of reproductive activity, (2) test for trait divergence among populations, and (3) test whether selection coefficients were correlated with trait divergence among populations. Stabilizing selection was never significant, indicating that these populations have yet to attain equilibria. Directional selection varied among populations in a manner consistent with trait divergence, indicating ongoing population differentiation. Specifically, the rank order of the creeks in terms of patterns of selection paralleled the rank order in terms of size and shape. The shortest and least deep-bodied males had the highest reproductive activity in the creek with the most intense predation and longer and deeper-bodied males were favored in the creeks with lower predation risk. [source] INCREASED POSTCOPULATORY SEXUAL SELECTION REDUCES THE INTRAMALE VARIATION IN SPERM DESIGNEVOLUTION, Issue 6 2008Simone Immler Sperm competition is an important force driving the evolution of sperm design and function. Inter- and intraspecific variation in sperm design are strongly influenced by the risk of sperm competition in many taxa. In contrast, the variation among sperm of one male (intramale variation) is less well understood. We investigated intramale variation in sperm design in passerine birds and found that risk of sperm competition is negatively associated with intramale variation. This result is the first clear evidence that variation among sperm within an individual male is influenced by postcopulatory sexual selection. Our finding has important implications for male traits under pre- and postcopulatory sexual selection. [source] PATTERNS OF PHENOTYPIC AND GENETIC VARIATION FOR THE PLASTICITY OF DIAPAUSE INCIDENCEEVOLUTION, Issue 7 2007Wade E. Winterhalter Phenotypic plasticity describes an organism's ability to produce multiple phenotypes in direct response to its environmental conditions. Over the past 15 years empiricists have found that this plasticity frequently exhibits geographic variation and often possesses a significant heritable genetic basis. However, few studies have examined both of these aspects of plasticity simultaneously. Here, we examined both the geographic and genetic variations of the plasticity for diapause incidence (the proportion of eggs that enter an arrested state of development capable of surviving over the winter) relative to temperatures and photoperiods associated with long and short season environments across six populations of the striped ground cricket, Allonemobius socius, using a half-sibling split brood quantitative genetic design. We found that plasticity, as measured by the slope of the reaction norm, was greater in the southern-low altitude region (where populations are bivoltine) relative to the southern-high and northern-low altitude regions (where populations are univoltine). However, the heritability of plasticity was only significantly different from zero in univoltine populations that experienced "intermediate" natal season lengths. These patterns suggest that selection may favor the plasticity of diapause incidence in bivoltine regions, but act against plasticity in regions in which populations are univoltine. Furthermore, our data suggest that under "intermediate" natal season length conditions, the interplay between local adaptation and gene flow may keep the plasticity of diapause incidence low (but still significant) while maintaining its genetic variation. As such, this study not only provides a novel observation into the geographic variation of phenotypic plasticity, but also provides much needed groundwork for tests of its adaptive significance. [source] EVOLUTIONARY ANALYSIS OF A KEY FLORAL TRAIT IN AQUILEGIA CANADENSIS (RANUNCULACEAE): GENETIC VARIATION IN HERKOGAMY AND ITS EFFECT ON THE MATING SYSTEMEVOLUTION, Issue 7 2007Christopher R. Herlihy The mating system of flowering plant populations evolves through selection on genetically based phenotypic variation in floral traits. The physical separation of anthers and stigmas within flowers (herkogamy) is expected to be an important target of selection to limit self-fertilization. We investigated the pattern of phenotypic and genetic variation in herkogamy and its effect of self-fertilization in a broad sample of natural populations of Aquilegia canadensis, a species that is highly selfing despite strong inbreeding depression. Within natural populations, plants exhibit substantial phenotypic variation in herkogamy caused primarily by variation in pistil length rather than stamen length. Compared to other floral traits, herkogamy is much more variable and a greater proportion of variation is distributed among rather than within individuals. We tested for a genetic component of this marked phenotypic variation by growing naturally pollinated seed families from five populations in a common greenhouse environment. For three populations, we detected a significant variation in herkogamy among families, and a positive regression between parental herkogamy measured in the field and progeny herkogamy in the greenhouse, suggesting that there is often genetic variation in herkogamy within natural populations. We estimated levels of self-fertilization for groups of flowers that differed in herkogamy and show that, as expected, herkogamy was associated with reduced selfing in 13 of 19 populations. In six of these populations, we performed floral emasculations to show that this decrease in selfing is due to decreased autogamy (within-flower selfing), the mode of selfing that herkogamy should most directly influence. Taken together, these results suggest that increased herkogamy should be selected to reduce the production of low-quality selfed seed. The combination of high selfing and substantial genetic variation for herkogamy in A. canadensis is enigmatic, and reconciling this observation will require a more integrated analysis of how herkogamy influences not only self-fertilization, but also patterns of outcross pollen import and export. [source] GEOGRAPHIC VARIATION IN THE EVOLUTION AND COEVOLUTION OF A TRITROPHIC INTERACTIONEVOLUTION, Issue 5 2007Timothy P. Craig The geographic mosaic theory of coevolution predicts that geographic variation in species interactions will lead to differing selective pressures on interacting species, producing geographic variation in the traits of interacting species (Thompson 2005). We supported this hypothesis in a study of the geographic variation in the interactions among Eurosta solidaginis and its natural enemies. Eurosta solidaginis is a fly (Diptera: Tephritidae) that induces galls on subspecies of tall goldenrod, Solidago altissima altissima and S. a. gilvocanescens. We measured selection on E. solidaginis gall size and shape in the prairie and forest biomes in Minnesota and North Dakota over an 11-year period. Galls were larger and more spherical in the prairie than in the forest. We supported the hypothesis that the divergence in gall morphology in the two biomes is due to different selection regimes exerted by natural enemies of E. solidaginis. Each natural enemy exerted similar selection on gall diameter in both biomes, but differences in the frequency of natural enemy attack created strong differences in overall selection between the prairie and forest. Bird predation increased with gall diameter, creating selection for smaller-diameter galls. A parasitic wasp, Eurytoma gigantea, and Mordellistena convicta, an inquiline beetle, both caused higher E. solidaginis mortality in smaller galls, exerting selection for increased gall diameter. In the forest there was stabilizing selection on gall diameter due to a combination of bird predation on larvae in large galls, and M. convicta - and E. gigantea- induced mortality on larvae in small galls. In the prairie there was directional selection for larger galls due to M. convicta and E. gigantea mortality on larvae in small galls. Mordellistena convicta- induced mortality was consistently higher in the prairie than in the forest, whereas there was no significant difference in E. gigantea- induced mortality between biomes. Bird predation was nonexistent in the prairie so the selection against large galls found in the forest was absent. We supported the hypothesis that natural enemies of E. solidaginis exerted selection for spherical galls in both biomes. In the prairie M. convicta exerts stabilizing selection to maintain spherical galls. In the forest there was directional selection for more spherical galls. Eurytoma gigantea exerted selection on gall shape in the forest in a complex manner that varied among years. We also supported the hypothesis that E. gigantea is coevolving with E. solidaginis. The parasitoid had significantly longer ovipositors in the prairie than in the forest, indicating the possibility that it has evolved in response to selection to reach larvae in the larger-diameter prairie galls. [source] NATURAL VARIATION FOR A HYBRID INCOMPATIBILITY BETWEEN TWO SPECIES OF MIMULUSEVOLUTION, Issue 1 2007Andrea L. Sweigart Understanding the process by which hybrid incompatibility alleles become established in natural populations remains a major challenge to evolutionary biology. Previously, we discovered a two-locus Dobzhansky,Muller incompatibility that causes severe hybrid male sterility between two inbred lines of the incompletely isolated wildflower species, Mimulus guttatus and M. nasutus. An interspecific cross between these two inbred lines revealed that the M. guttatus (IM62) allele at hybrid male sterility 1 (hms1) acts dominantly in combination with recessive M. nasutus (SF5) alleles at hybrid male sterility 2 (hms2) to cause nearly complete hybrid male sterility. In this report, we extend these genetic analyses to investigate intraspecific variation for the hms1,hms2 incompatibility in natural populations of M. nasutus and M. guttatus, performing a series of interspecific crosses between individuals collected from a variety of geographic locales. Our results suggest that hms2 incompatibility alleles are common and geographically widespread within M. nasutus, but absent or rare in M. guttatus. In contrast, the hms1 locus is polymorphic within M. guttatus and the incompatibility allele appears to be extremely geographically restricted. We found evidence for the presence of the hms1 incompatibility allele in only two M. guttatus populations that exist within a few kilometers of each other. The restricted distribution of the hms1 incompatibility allele might currently limit the potential for the hms1,hms2 incompatibility to act as a species barrier between sympatric populations of M. guttatus and M. nasutus. Extensive sampling within a single M. guttatus population revealed that the hms1 locus is polymorphic and that the incompatibility allele appears to segregate at intermediate frequency, a pattern that is consistent with either genetic drift or natural selection. [source] THE CHANGE IN QUANTITATIVE GENETIC VARIATION WITH INBREEDINGEVOLUTION, Issue 12 2006Josh Van Buskirk Abstract Inbreeding is known to reduce heterozygosity of neutral genetic markers, but its impact on quantitative genetic variation is debated. Theory predicts a linear decline in additive genetic variance (VA) with increasing inbreeding coefficient (F) when loci underlying the trait act additively, but a nonlinear hump-shaped relationship when dominance and epistasis are important. Predictions for heritability (h2) are similar, although the exact shape depends on the value of h2 in the absence of inbreeding. We located 22 published studies in which the level of genetic variation in [source] BALANCING SELECTION, RANDOM GENETIC DRIFT, AND GENETIC VARIATION AT THE MAJOR HISTOCOMPATIBILITY COMPLEX IN TWO WILD POPULATIONS OF GUPPIES (POECILIA RETICULATA)EVOLUTION, Issue 12 2006Cock van Oosterhout Abstract Our understanding of the evolution of genes of the major histocompatibility complex (MHC) is rapidly increasing, but there are still enigmatic questions remaining, particularly regarding the maintenance of high levels of MHC polymorphisms in small, isolated populations. Here, we analyze the genetic variation at eight microsatellite loci and sequence variation at exon 2 of the MHC class IIB (DAB) genes in two wild populations of the Trinidadian guppy, Poecilia reticulata. We compare the genetic variation of a small (Ne, 100) and relatively isolated upland population to that of its much larger (Ne, 2400) downstream counterpart. As predicted, microsatellite diversity in the upland population is significantly lower and highly differentiated from the population further downstream. Surprisingly, however, these guppy populations are not differentiated by MHC genetic variation and show very similar levels of allelic richness. Computer simulations indicate that the observed level of genetic variation can be maintained with overdominant selection acting at three DAB loci. The selection coefficients differ dramatically between the upland (s 0.2) and lowland (s, 0.01) populations. Parasitological analysis on wild-caught fish shows that parasite load is significantly higher on upland than on lowland fish, which suggests that large differences in selection intensity may indeed exist between populations. Based on the infection intensity, a substantial proportion of the upland fish would have suffered direct or indirect fitness consequences as a result of their high parasite loads. Selection by parasites plays a particularly important role in the evolution of guppies in the upland habitat, which has resulted in high levels of MHC diversity being maintained in this population despite considerable genetic drift. [source] GEOGRAPHIC VARIATION OF GENETIC AND BEHAVIORAL TRAITS IN NORTHERN AND SOUTHERN TÚNGARA FROGSEVOLUTION, Issue 8 2006Heike Pröhl Abstract We use a combination of microsatellite marker analysis and mate-choice behavior experiments to assess patterns of reproductive isolation of the túngara frog Physalaemus pustulosus along a 550-km transect of 25 populations in Costa Rica and Panama. Earlier studies using allozymes and mitochondrial DNA defined two genetic groups of túngara frogs, one ranging from Mexico to northern Costa Rica (northern group), the second ranging from Panama to northern South America (southern group). Our more fine-scale survey also shows that the northern and southern túngara frogs are genetically different and geographically separated by a gap in the distribution in central Pacific Costa Rica. Genetic differences among populations are highly correlated with geographic distances. Temporal call parameters differed among populations as well as between genetic groups. Differences in calls were explained better by geographic distance than by genetic distance. Phonotaxis experiments showed that females preferred calls of males from their own populations over calls of males from other populations in about two-thirds to three-fourths of the contrasts tested. In mating experiments, females and males from the same group and females from the north with males from the south produced nests and tadpoles. In contrast, females from the south did not produce nests or tadpoles with males from the north. Thus, northern and southern túngara frogs have diverged both genetically and bioacoustically. There is evidence for some prezygotic isolation due to differences in mate recognition and fertilization success, but such isolation is hardly complete. Our results support the general observation that significant differences in sexual signals are often not correlated with strong genetic differentiation. [source] ABDOMINAL PIGMENTATION VARIATION IN DROSOPHILA POLYMORPHA: GEOGRAPHIC VARIATION IN THE TRAIT, AND UNDERLYING PHYLOGEOGRAPHYEVOLUTION, Issue 5 2005Jennifer A. Brisson AbstractDrosophila polymorpha is a widespread species that exhibits abdominal pigmentation variation throughout its range. To gain insight into this variation we combined phenotypic and genotypic data to test a series of nested hypotheses. First, we tested the null hypothesis that geographic variation in pigmentation is due to neutral factors. We used nested clade analysis to examine the distribution of haplotypes from a nuclear and a mitochrondrial locus. Restricted gene flow via isolation by distance, the primary inference of this phylogeographic analysis, was then used to generate and test the hypothesis of increasing average abdominal pigmentation difference with increasing geographic distance. We found no correlation between geographic distance and phenotypic distance. We then tested the hypothesis that pigmentation is affected by environmental differences among localities. We found a significant effect of habitat type on the average abdominal pigmentation phenotype of different localities. Finally, we tested the hypothesis that pigmentation in D. polymorpha is associated with desiccation resistance. We found that dark individuals of both sexes survived significantly longer in a desiccating environment than light individuals. These patterns combined lead us to hypothesize that abdominal pigmentation variation in D. polymorpha is important in mediating the organism's interactions with local ecological factors. [source] THE CONTRIBUTION OF SPONTANEOUS MUTATION TO VARIATION IN ENVIRONMENTAL RESPONSES OF ARABIDOPSIS THALIANA: RESPONSES TO LIGHTEVOLUTION, Issue 2 2005Christina M. Kavanaugh Abstract It has been hypothesized that new, spontaneous mutations tend to reduce fitness more severely in more stressful environments. To address this hypothesis, we grew plants representing 20 Arabidopsis thaliana mutationaccumulation (M-A) lines, advanced to generation 17, and their progenitor, in differing light conditions. The experiment was conducted in a greenhouse, and two treatments were used: full sun and shade, in which influx of red light was reduced relative to far-red. The shade treatment was considered the more stressful because mean absolute fitness was lower in that treatment, though not significantly so. Plants from generation 17 of M-A developed significantly faster than those from generation 0 in both treatments. A significant interaction between generation and treatment revealed that, counter to the hypothesis, M-A lines tended to have higher fitness on average relative to the progenitor in the shaded conditions, whereas, in full sun, the two generations were similar in fitness. A secondary objective of this experiment was to characterize the contribution of new mutations to genotype x environment interaction. We did not, however, detect a significant interaction between M-A line and treatment. Plots of the line-specific enviromental responses indicate no tendency of new mutations to contribute to fitness trade-offs between environments. They also do not support a model of conditionally deleterious mutation, in which a mutatn reduces fitness only in a particular environment. These results suggest that interactions between genotype and light environment previously documented for A. thaliana are not explicable primarily as a consequence of steady input of spontaneous mutations having environment-specific effects. [source] FROM MICRO- TO MACROEVOLUTION THROUGH QUANTITATIVE GENETIC VARIATION: POSITIVE EVIDENCE FROM FIELD CRICKETSEVOLUTION, Issue 10 2004Mattieu Bégin Abstract . -Quantitative genetics has been introduced to evolutionary biologists with the suggestion that microevolution could be directly linked to macroevolutionary patterns using, among other parameters, the additive genetic variance/ covariance matrix (G) which is a statistical representation of genetic constraints to evolution. However, little is known concerning the rate and pattern of evolution of G in nature, and it is uncertain whether the constraining effect of G is important over evolutionary time scales. To address these issues, seven species of field crickets from the genera Gryllus and Teleogryllus were reared in the laboratory, and quantitative genetic parameters for morphological traits were estimated from each of them using a nested full-sibling family design. We used three statistical approaches (T method, Flury hierarchy, and Mantel test) to compare G matrices or genetic correlation matrices in a phylogenetic framework. Results showed that G matrices were generally similar across species, with occasional differences between some species. We suggest that G has evolved at a low rate, a conclusion strengthened by the consideration that part of the observed across-species variation in G can be explained by the effect of a genotype by environment interaction. The observed pattern of G matrix variation between species could not be predicted by either morphological trait values or phylogeny. The constraint hypothesis was tested by comparing the multivariate orientation of the reconstructed ancestral G matrix to the orientation of the across-species divergence matrix (D matrix, based on mean trait values). The D matrix mainly revealed divergence in size and, to a much smaller extent, in a shape component related to the ovipositor length. This pattern of species divergence was found to be predictable from the ancestral G matrix in agreement with the expectation of the constraint hypothesis. Overall, these results suggest that the G matrix seems to have an influence on species divergence, and that macroevolution can be predicted, at least qualitatively, from quantitative genetic theory. Alternative explanations are discussed. [source] ADAPTIVE REPTILE COLOR VARIATION AND THE EVOLUTION OF THE MCIR GENEEVOLUTION, Issue 8 2004Erica Bree Rosenblum Abstract The wealth of information on the genetics of pigmentation and the clear fitness consequences of many pigmentation phenotypes provide an opportunity to study the molecular basis of an ecologically important trait. The melanocortin-1 receptor (Mc1r) is responsible for intraspecific color variation in mammals and birds. Here, we study the molecular evolution of Mc1r and investigate its role in adaptive intraspecific color differences in reptiles. We sequenced the complete Mc1r locus in seven phylogenetically diverse squamate species with melanic or blanched forms associated with different colored substrates or thermal environments. We found that patterns of amino acid substitution across different regions of the receptor are similar to the patterns seen in mammals, suggesting comparable levels of constraint and probably a conserved function for Mc1r in mammals and reptiles. We also found high levels of silent-site heterozygosity in all species, consistent with a high mutation rate or large long-term effective population size. Mc1r polymorphisms were strongly associated with color differences in Holbrookia maculata and Aspidoscelis inornata. In A. inornata, several observations suggest that Mc1r mutations may contribute to differences in color: (1) a strong association is observed between one Mc1r amino acid substitution and dorsal color; (2) no significant population structure was detected among individuals from these populations at the mitochondrial ND4 gene; (3) the distribution of allele frequencies at Mc1r deviates from neutral expectations; and (4) patterns of linkage disequilibrium at Mc1r are consistent with recent selection. This study provides comparative data on a nuclear gene in reptiles and highlights the utility of a candidate-gene approach for understanding the evolution of genes involved in vertebrate adaptation. [source] EVOLUTION OF COLOR VARIATION IN DRAGON LIZARDS: QUANTITATIVE TESTS OF THE ROLE OF CRYPSIS AND LOCAL ADAPTATIONEVOLUTION, Issue 7 2004Devi M. Stuart-Fox Abstract Many animal species display striking color differences with respect to geographic location, sex, and body region. Traditional adaptive explanations for such complex patterns invoke an interaction between selection for conspicuous signals and natural selection for crypsis. Although there is now a substantial body of evidence supporting the role of sexual selection for signaling functions, quantitative studies of crypsis remain comparatively rare. Here, we combine objective measures of coloration with information on predator visual sensitivities to study the role of crypsis in the evolution of color variation in an Australian lizard species complex (Ctenophorus decresii). We apply a model that allows us to quantify crypsis in terms of the visual contrast of the lizards against their natural backgrounds, as perceived by potential avian predators. We then use these quantitative estimates of crypsis to answer the following questions. Are there significant differences in crypsis conspicuousness among populations? Are there significant differences in crypsis conspicuousness between the sexes? Are body regions "exposed" to visual predators more cryptic than "hidden" body regions? Is there evidence for local adaptation with respect to crypsis against different substrates? In general, our results confirmed that there are real differences in crypsis conspicuousness both between populations and between sexes; that exposed body regions were significantly more cryptic than hidden ones, particularly in females; and that females, but not males, are more cryptic against their own local background than against the background of other populations. Body regions that varied most in contrast between the sexes and between populations were also most conspicuous and are emphasized by males during social and sexual signaling. However, results varied with respect to the aspect of coloration studied. Results based on chromatic contrast ("hue' of color) provided better support for the crypsis hypothesis than did results based on achromatic contrast ("brightness' of color). Taken together, these results support the view that crypsis plays a substantial role in the evolution of color variation and that color patterns represent a balance between the need for conspicuousness for signaling and the need for crypsis to avoid predation. [source] CONDITION DEPENDENCE OF SEXUAL ORNAMENT SIZE AND VARIATION IN THE STALK-EYED FLY CYRTODIOPSIS DALMANNI (DIPTERA: DIOPSIDAE)EVOLUTION, Issue 5 2004Samuel Cotton Abstract We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection. [source] PHYLOGENETIC COMPARATIVE ANALYSIS OF LIFE-HISTORY VARIATION AMONG POPULATIONS OF THE LIZARD SCELOPORUS UNDULATUS:AN EXAMPLE AND PROGNOSISEVOLUTION, Issue 3 2004Peter H. Niewiarowski Abstract Over the past 15 years, phylogenetic comparative methods (PCMs) have become standard in the study of life-history evolution. To date, most studies have focused on variation among species or higher taxonomic levels, generally revealing the presence of significant phylogenetic effects as well as residual variation potentially attributable to adaptive evolution. Recently, population-level phylogenetic hypotheses have become available for many species, making it possible to apply PCMs directly to the level at which experiments are typically used to test adaptive hypotheses. In this study, we present the results of PCMs applied to life-history variation among populations of the widespread and well-studied lizard Sceloporus undulatus. Using S. undulatus (which may represent four closely related species) as an example, we explore the benefits of using PCMs at the population level, as well as consider the importance of several thorny methodological problems including but not limited to nonindependence of populations, lack of sufficient variation in traits, and the typically small sample sizes dictated by the difficulty of collecting detailed demographic data. We show that phylogenetic effects on life-history variation among populations of S. undulatus appear to be unimportant, and that several classic trade-offs expected by theory and revealed by many interspecific comparisons are absent. Our results suggest that PCMs applied to variation in life-history traits below the species level may be of limited value, but more studies like ours are needed to draw a general conclusion. Finally, we discuss several outstanding problems that face studies seeking to apply PCMs below the species level. [source] GENETIC VARIATION IN MALE EFFECTS ON FEMALE REPRODUCTION AND THE GENETIC COVARIANCE BETWEEN THE SEXESEVOLUTION, Issue 6 2003MARY ELLEN CZESAK Abstract., Males of many insect species increase the fecundity and/or egg size of their mates through the amount or composition of their nuptial gifts or ejaculate. The genetic bases of such male effects on fecundity or egg size are generally unknown, and thus their ability to evolve remains speculative. Likewise, the genetic relationship between male and female investment into reproduction in dioecious species, which is expected to be positive if effects on fecundity are controlled by at least some of the same genes in males and females, is also unknown. Males of the seed beetle Stator limbatus contribute large ejaculates to females during mating, and the amount of donated ejaculate is positively correlated with male body mass. Females mated to large males lay more eggs in their lifetime than females mated to small males. We describe an experiment in which we quantify genetic variation in the number of eggs sired by males (mated to a single female) and found that a significant proportion of the phenotypic variance in the number of eggs sired by males was explained by their genotype. Additionally, the number of eggs sired by a male was highly positively genetically correlated with his body mass. The between-sex genetic correlation, that is, the genetic correlation between the number of eggs sired by males and the number of eggs laid by females, was highly positive when eggs were laid on Acacia greggii seeds. This indicates that males that sire many eggs have sisters that lay many eggs. Thus, some of the genes that control male ejaculate size (or some other fecundity-enhancing factor) when expressed in males appear to control fecundity when expressed in females. We found no significant interaction between male and female genotype on fecundity. [source] NONSTOCHASTIC VARIATION OF SPECIES-LEVEL DIVERSIFICATION RATES WITHIN ANGIOSPERMSEVOLUTION, Issue 3 2003Hallie J. Sims Abstract Variations in the origination and extinction rates of species over geological time often are linked with a range of factors, including the evolution of key innovations, changes in ecosystem structure, and environmental factors such as shifts in climate and physical geography. Before hypothesizing causality of a single factor, it is critical to demonstrate that the observed variation in diversification is significantly greater than one would expect due to natural stochasticity in the evolutionary branching process. Here, we use a likelihood-ratio test to compare taxonomic rate heterogeneity to a neutral birth-death model, using data on well-supported sister pairs of taxa and their species richness. We test the likelihood that the distribution of extant species among angiosperm genera and families could be the result of constant diversification rates. Results strongly support the conclusion that there is significantly more heterogeneity in diversity at the species level within angiosperms than would be expected due to stochastic processes. This result is consistent in datasets of genus pairs and family pairs and is not affected significantly by degrading pairs to simulate inaccuracy in the assumption of simultaneous origin of sister taxa. When we parse taxon pairs among higher groups of angiosperms, results indicate that a constant rates model is not rejected by rosid and basal eudicot pairs but is rejected by asterid and eumagnoliid pairs. These results provide strong support for the hypothesis that species-level rates of origination and/or extinction have varied nonrandomly within angiosperms and that the magnitude of heterogeneity varies among major groups within angiosperms. [source] GEOGRAPHIC VARIATION IN ACID STRESS TOLERANCE OF THE MOOR FROG, RANA ARVALIS.EVOLUTION, Issue 2 2003Abstract Spatially varying directional selection together with restricted gene flow among populations is expected to lead to local adaptation. One environmental factor that potentially causes strong directional selection, but is little explored in evolutionary terms, is naturally and anthropogenically induced acidity. We studied local adaptation to acidity in four Swedish populations (two originating from areas that have suffered from severe anthropogenic acidification during the 1900s and two from areas which have remained neutral due to higher buffering capacity) of the moor frog Rana arvalis in a laboratory experiment by investigating whether differences in acid tolerance correspond to population origin. Embryos were raised from fertilization to hatching at three different pH levels (pH 4.0, 4.25 and 7.5), corresponding to levels experienced by these populations in nature, and acid stress tolerance was measured in terms of embryonic survival, hatchling size, and age. Evidence for local adaptation in all of these traits was found, the acid origin embryos having higher survival and less impaired growth performance under acid conditions than the neutral origin embryos. Our estimated rates of divergence (0.007,0.102 haldanes) suggest a rapid adaptation process in response to anthropogenic environmental change, and that the different traits have evolved at relatively similar rates. [source] TEMPORAL VARIATION IN DIVERGENT SELECTION ON SPINE NUMBER IN THREESPINE STICKLEBACKEVOLUTION, Issue 12 2002T. E. Reimchen Abstract., Short-term temporal cycles in ecological pressures, such as shifts in predation regime, are widespread in nature yet estimates of temporal variation in the direction and intensity of natural selection are few. Previous work on threespine stickleback (Gasterosteus aculeatus) has revealed that dorsal and pelvic spines are a defense against gape-limited predators but may be detrimental against grappling insect predators. In this study, we examined a 15-year database from an endemic population of threespine stickleback to look for evidence of temporal shifts in exposure to these divergent predation regimes and correlated shifts in selection on spine number. For juveniles, we detected selection for increased spine number during winter when gape-limited avian piscivores were most common but selection for decreased spine number during summer when odonate predation was more common. For subadults and adults, which are taken primarily by avian piscivores, we predicted selection should generally be for increased spine number in all seasons. Among 59 comparisons, four selection differentials were significant (Bonferroni corrected) and in the predicted direction. However, there was also substantial variability in remaining differentials, including two examples with strong selection for spine reduction. These reversals were associated with increased tendency of the fish to shift to a benthic niche, as determined from examination of stomach contents. These dietary data suggest that increased encounter rates with odonate predation select for spine reduction. Strong selection on spine number was followed by changes in mean spine number during subsequent years and a standard quantitative genetic formula revealed that spine number has a heritable component. Our results provide evidence of rapid morphological responses to selection from predators and suggest that temporal variation in selection may help maintain variation within populations. Furthermore, our findings indicate that variable selection can be predicted if the agents of selection are known. [source] PATHOGEN RESISTANCE AND GENETIC VARIATION AT MHC LOCIEVOLUTION, Issue 10 2002Philip W. Hedrick Abstract., Balancing selection in the form of heterozygote advantage, frequency-dependent selection, or selection that varies in time and/or space, has been proposed to explain the high variation at major histocompatibility complex (MHC) genes. Here the effect of variation of the presence and absence of pathogens over time on genetic variation at multiallelic loci is examined. In the basic model, resistance to each pathogen is conferred by a given allele, and this allele is assumed to be dominant. Given that s is the selective disadvantage for homozygotes (and heterozygotes) without the resistance allele and the proportion of generations, which a pathogen is present, is e, fitnesses for homozygotes become (1 ,s)(n-1)e and the fitnesses for heterozygotes become (1 ,s)(n-2)e, where n is the number of alleles. In this situation, the conditions for a stable, multiallelic polymorphism are met even though there is no intrinsic heterozygote advantage. The distribution of allele frequencies and consequently heterozygosity are a function of the autocorrelation of the presence of the pathogen in subsequent generations. When there is a positive autocorrelation over generations, the observed heterozygosity is reduced. In addition, the effects of lower levels of selection and dominance and the influence of genetic drift were examined. These effects were compared to the observed heterozygosity for two MHC genes in several South American Indian samples. Overall, resistance conferred by specific alleles to temporally variable pathogens may contribute to the observed polymorphism at MHC genes and other similar host defense loci. [source] SHAPE ANALYSIS OF SYMMETRIC STRUCTURES: QUANTIFYING VARIATION AMONG INDIVIDUALS AND ASYMMETRYEVOLUTION, Issue 10 2002Christian Peter Klingenberg Abstract., Morphometric studies often consider parts with internal left-right symmetry, for instance, the vertebrate skull. This type of symmetry is called object symmetry and is distinguished from matching symmetry, in which two separate structures exist as mirror images of each other, one on each body side. We explain a method for partitioning the total shape variation of landmark configurations with object symmetry into components of symmetric variation among individuals and asymmetry. This method is based on the Procrustes superimposition of the original and a reflected copy of each landmark configuration and is compatible with the two-factor ANOVA model customary in studies of fluctuating asymmetry. We show a fully multivariate framework for testing the effects in the two-factor model with MANOVA statistics, which also applies to shapes with matching symmetry. We apply the new methods in a small case study of pharyngeal jaws of the Neotropical cichlid fish Amphilophus citrinellus. The analysis revealed that the symmetric component of variation in the pharyngeal jaws is dominated by the contrast between two alternative trophic morphs in this species and that there is subtle but statistically significant directional asymmetry. Finally, we provide some general recommendations for morphometric studies of symmetric shapes. [source] ON THE ORIGIN OF MODULAR VARIATIONEVOLUTION, Issue 8 2002Hod Lipson Abstract We study the dynamics of modularization in a minimal substrate. A module is a functional unit relatively separable from its surrounding structure. Although it is known that modularity is useful both for robustness and for evolvability (Wagner 1996), there is no quantitative model describing how such modularity might originally emerge. Here we suggest, using simple computer simulations, that modularity arises spontaneously in evolutionary systems in response to variation, and that the amount of modular separation is logarithmically proportional to the rate of variation. Consequently, we predict that modular architectures would appear in correlation with high environmental change rates. Because this quantitative model does not require any special substrate to occur, it may also shed light on the origin of modular variation in nature. This observed relationship also indicates that modular design is a generic phenomenon that might be applicable to other fields, such as engineering: Engineering design methods based on evolutionary simulation would benefit from evolving to variable, rather than stationary, fitness criteria, as a weak and problem-independent method for inducing modularity. [source] | ||||||||||||||||||||||||||||||||||||||||||||||||||||