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If the direction of selection changes from generation to generation, the ability to respond to selection is maladaptive: the response to selection in one generation leads to reduced fitness in the next. Because the response is determined by the amount of genetic variance expressed at the phenotypic level, rapidly fluctuating selection should favor modifier genes that reduce the phenotypic effect of alleles segregating at structural loci underlying the trait. Such reduction in phenotypic expression of genetic variation has been named “genetic canalization.” I support this argument with a series of two- and multilocus models with alternating linear selection and Gaussian selection with fluctuating optimum. A canalizing modifier gene affects the fitness of its carriers in three ways: (1) it reduces the phenotypic consequences of genetic response to previous selection; (2) it reduces the genetic response to selection, which is manifested as linkage disequilibrium between the modifier and structural loci; and (3) it reduces the phenotypic variance. The first two effects reduce fitness under directional selection sustained for several generations, but improve fitness when the direction of selection has just been reversed. The net effect tends to favor a canalizing modifier under rapidly fluctuating selection regimes (period of eight generations or less). The third effect improves fitness of the modifier allele if the fitness function is convex and reduces it if the function is concave. Under fluctuating Gaussian selection, the population is more likely to experience the concave portion of the fitness function when selection is stronger. Therefore, only weak to moderately strong fluctuating Gaussian selection favors genetic canalization. This paper considerably broadens the conditions that favor genetic canalization, which so far has only been postulated to evolve under long-term stabilizing selection.
Adaptation is characterized by the movement of a population toward a many-character optimum, movement that results in an increase in fitness. Here I calculate the rate at which fitness increases during adaptation and describe the curve giving fitness versus time as a population approaches an optimum in Fisher's model of adaptation. The results identify several factors affecting the speed of adaptation. One of the most important is organismal complexity—complex organisms adapt more slowly than simple ones when using mutations of the same phenotypic size. Thus, as Fisher foresaw, organisms pay a kind of cost of complexity. However, the magnitude of this cost is considerably larger than Fisher's analysis suggested. Indeed the rate of adaptation declines at least as fast as n−1, where n is the number of independent characters or dimensions comprising an organism. The present results also suggest that one can define an effective number of dimensions characterizing an adapting species.
The occurrence of reinforcement is compared when premating isolation is caused by the spread of a gene causing females to prefer to mate with males carrying a population-specific trait (a “preference” model) and by a gene that causes females to prefer to mate with males that share their own trait phenotype (an “assortative mating” model). Both two-island models, which have symmetric gene flow, and continent-island models, which have one-way gene flow, are explored. Reinforcement is found to occur much more easily in a two-island assortative mating model than in any of the other three models. This is due primarily to the fact that in this model the assortative mating allele will automatically become genetically associated in each population with the trait allele that is favored by natural selection on that island. In contrast, natural selection on the trait both favors and opposes the evolution of premating isolation in the two-island preference model, depending on the particular population. These results imply that species recognition in the context of mating may evolve particularly easily when it targets cues that are favored by natural selection in each population. In the continent-island models, reinforcement is found to occur more often under the preference model than the assortative mating model, thus reversing the trend from the two-island models. Patterns of population subdivision may therefore play a role in determining what types of premating isolation may evolve.
Sexual selection (defined as the change in genotypic or phenotypic frequencies of mated versus total population frequencies) and sexual isolation (defined as the deviation from random mating in mated individuals) show different evolutionary consequences and partially confounded causes. Traditionally, the cross-product estimator has been used to quantify sexual selection, whereas a variety of indexes, such as Yule V, Yule Q, YA, joint I, and others have been used to quantify sexual isolation. Because the two types of estimators use different scales, the effects of both processes cannot be monitored simultaneously. We describe three new related statistics that quantify both sexual selection (PSS) and sexual isolation (PSI) effects for every mating pair combination in polymorphic traits, as well as measure their combined effects (PTI = PSI × PSS). The new statistics have the advantage of providing information on every mating pair combination, quantifying the effects of sexual selection and isolation in the same units, and detecting asymmetry in sexual isolation. The ability of the new statistics to ascertain the biological causes of sexual selection and sexual isolation are investigated under different models involving distinct marginal frequencies, mate propensity, and mate choice coefficients. We also studied the use of classical isolation indexes applied on PSI coefficients, instead of on raw data. The use of the classical indexes applied to PSI coefficients considerably reduces the statistical bias of the estimates, revealing the good estimation properties of the new statistics.
Although the developmental and genetic mechanisms underlying sex differences are being elucidated in great detail in a number of species, there remains a breach between proximate and evolutionary studies of sexual dimorphism. More precisely, the evolution of sex-limited gene expression at autosomal loci has not been well reasoned using either theoretical or empirical methods. Here, I show that a Mendelian genetic model including elementary details of sexual differentiation provides novel insight into the evolution of sex differences via sex limitation. This model indicates that the nature of allelic effects and the pattern of selection must be known in both sexes to predict the evolution of sex differences. That is, selection interacts with genetic variation for sexual dimorphism to produce unanticipated patterns of trait divergence or convergence between the sexes. Ultimately, this model may explain why previous models for the evolution of sexual dimorphism do not predict the erratic behavior of the sex difference during artificial selection experiments.
We develop a Monte Carlo–based likelihood method for estimating migration rates and population divergence times from data at unlinked loci at which mutation rates are sufficiently low that, in the recent past, the effects of mutation can be ignored. The method is applicable to restriction fragment length polymorphisms (RFLPs) and single nucleotide polymorphisms (SNPs) sampled from a subdivided population. The method produces joint maximum-likelihood estimates of the migration rate and the time of population divergence, both scaled by population size, and provides a framework in which to test either for no ongoing gene flow or for population divergence in the distant past. We show the method performs well and provides reasonably accurate estimates of parameters even when the assumptions under which those estimates are obtained are not completely satisfied. Furthermore, we show that, provided that the number of polymorphic loci is sufficiently large, there is some power to distinguish between ongoing gene flow and historical association as causes of genetic similarity between pairs of populations.
Host organisms can respond to the threat of disease either through resistance defenses (which inhibit or limit infection) or through tolerance strategies (which do not limit infection, but reduce or offset its fitness consequences). Here we show that resistance and tolerance can have fundamentally different evolutionary outcomes, even when they have equivalent short-term benefit for the host. As a gene conferring disease resistance spreads through a population, the incidence of infection declines, reducing the fitness advantage of carrying the resistance gene. Thus genes conferring complete resistance cannot become fixed (i.e., universal) by selection in a host population, and diseases cannot be eliminated solely by natural selection for host resistance. By contrast, as a gene conferring disease tolerance spreads through a population, disease incidence rises, increasing the evolutionary advantage of carrying the tolerance gene. Therefore, any tolerance gene that can invade a host population will tend to be driven to fixation by selection. As predicted, field studies of diverse plant species infected by rust fungi confirm that resistance traits tend to be polymorphic and tolerance traits tend to be fixed. These observations suggest a new mechanism for the evolution of mutualism from parasitism, and they help to explain the ubiquity of disease.
There is a large body of theoretical studies that investigate factors that affect the evolution of virulence, that is parasite-induced host mortality. In these studies the host population is assumed to be genetically homogeneous. However, many parasites have a broad range of host types they infect, and trade-offs between the parasite virulence in different host types may exist. The aim of this paper is to study the effect of host heterogeneity on the evolution of parasite virulence. By analyzing a simple model that describes the replication of different parasite strains in a population of two different host types, we determine the optimal level of virulence in both host types and find the conditions under which strains that specialize in one host type dominate the parasite population. Furthermore, we show that intrahost evolution of the parasite during an infection may lead to stable polymorphisms and could introduce evolutionary branching in the parasite population.
There has been wide disagreement as to whether sperm competition among animals can produce a soldier class of sperm to fight against other males. Utilizing mathematical models, we analyze the appropriate conditions for the evolution and maintenance of a soldier sperm class. We conclude that: (1) soldier sperm evolve even if one soldier sperm can kill or block less than one competing sperm; (2) soldier sperm evolve faster when there is a large variance in the number of competing sperm; (3) soldier ratio increases until reproductive sperm are too scarce to fertilize all ova or a sperm intensely refuses to become a soldier; and (4) soldier sperm are more likely to be smaller than reproductive sperm. Our models suggest that the conditions for the evolution of a soldier sperm class are not stringent.
Can the evolution of plant defense lead to an optimal primary production? In a general theoretical model, Loreau (1995) and de Mazancourt et al. (1998, 1999) have shown that herbivory could increase primary production up to a moderate rate of grazing intensity through recycling of a limiting nutrient, provided several conditions are fulfilled. In the present paper, we assume: (1) grazing intensity is controlled by plants through their level of palatability; and (2) plant fitness is determined by its productivity. We explore the conditions under which such an optimal production may be reached through natural selection. We model two competing plant types that differ only in palatability and are distributed in a patchy landscape determined by the plant-herbivore interaction. Patch size is determined by herbivore behavior: herbivores recycle nutrient homogeneously within patches, but recycle nutrient proportionally to consumption between patches. The model shows that a strategy of intermediate palatability can be adaptive in response to a small herbivore that lives on and recycles nutrient around one or a few individual plants. For moderately small herbivores, plant palatability may evolve towards one of two local convergent strategies, depending on the initial conditions. For medium- to large-sized herbivores, the nonpalatable strategy is always selected. We discuss the functional and evolutionary implications of these results, and suggest that the traditional dichotomy describing antagonistic and mutualistic interactions may be misleading.
Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life-history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near-optimal conditions (control). Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size- and growth-related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress- avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life-history strategies.
The present study explored phenotypic selection on phenological and morphological reproductive traits in hawkmoth-pollinated Platanthera bifolia (Orchidaceae), a Eurasian perennial herb displaying bisexual, long-spurred flowers. The work was carried out during three flowering seasons (1993–1995) in a Swedish population. Fitness was estimated as the number of pollinia removed (male fitness) and fruits produced (female fitness). Targets and patterns of selection were compared between years and sex functions by the use of multiple linear regression (including correlational selection estimates, i.e., of combination of traits), analysis of covariance, and projection pursuit regression (PPR). Results from the nonparametric surface-fitting-method PPR showed that selection was mostly linear, thus justifying the use of the parametric methods. In all study years, male and female fitness were highest in plants with many flowers. This reflects that flower number sets an upper limit to fitness and that a large inflorescence attracts more pollinators. In 1994, the summer was dry and the average spur length of P. bifolia was shorter than in the other years. In this year, male and female fitness were positively related to spur length, apparently because the spur of short-spurred plants was somewhat too short relative to the tongue length of the local pollinator for optimal pollen export and import. Additionally, the dry weather in 1994 caused a tendency for correlational selection, which was not found in the other years of study. Among small individuals (apparently more sensitive to drought than large ones), early-flowering plants had higher male and female fitness. The results show that patterns of selection may vary both between years and between sex functions in perennial hermaphroditic plants. The present study is one of the first to consider correlational selection in plants, which probably is of common occurrence and deserves to be investigated more.
Under competitive conditions, stem elongation in plants is thought to enhance fitness by increasing light interception. However, the onset of competition should vary with the species of competitor due to interspecific differences in timing of emergence and plant growth form. The fitness benefits of elongation may therefore depend on the timing of this plastic response. Phenotypic selection analyses and path analysis were used to evaluate selection acting on stem elongation at early and late life-history stages and the combination of germination timing and elongation in an annual plant. Velvetleaf (Abutilon theophrasti) were raised in one of three environments experienced by natural populations (cornfields; soybean fields; and disturbed, weedy sites). Due to the rapid growth rate and high density of plants in disturbed areas, selection to increase seedling-stage elongation was expected in weedy sites. Due to the wide spacing of crop plants, competition for light is initially low in cultivated fields, but intensifies as the season progresses. Selection for increased elongation at later nodes was expected in soybean fields because velvetleaf can often overtop soy and thereby increase leaf exposure. In contrast, selection against late elongation was expected in cornfields because velvetleaf are incapable of overtopping corn. Individuals that elongate would experience the carbon cost of allocating to structural tissue, but fail to experience a carbon return through increased light interception. The phenotypic selection analyses were consistent with these predictions and therefore support the role of stem elongation as an adaptation to interspecific competition. Selection also acted on the combination of germination timing and elongation. In the weedy environment, early emergence in conjunction with enhanced stem elongation conveyed the highest fitness. Reduced elongation was favored among individuals that emerged late, potentially because these individuals were unable to overtop neighbors. The results of this study demonstrate that the timing of stem elongation strongly affects competitive success. Environments that differ in the timing of competition for light select for elongation at different life-history stages, and this selection depends on the timing of emergence.
The plant genera in which natural hybridization is most prevalent tend to be outcrossing perennials with some mechanism for clonal (i.e., asexual) reproduction. Although clonal reproduction in fertile, sexually reproducing hybrid populations could have important evolutionary consequences, little attention has been paid to quantifying this parameter in such populations. In the present study, we examined the frequency and spatial patterning of clonal reproduction in two Louisiana iris hybrid populations. Allozyme analysis of both populations revealed relatively high levels of genotypic diversity. However, a considerable amount of clonality was apparent. Nearly half of all genets (47%) in one population and more than half (61%) in the other had multiple ramets. Furthermore, both populations exhibited relatively high levels of genetic structuring, a pattern that resulted from the aggregation of clonal ramets. The occurrence of clonal reproduction in hybrid populations could not only facilitate introgression through an increase in the number of flowering ramets per genet and/or the survivorship of early generation hybrids, but might also influence the mating system of such populations. Any potential increase in the selfing rate due to cross-pollination among ramets of the same genet may, in turn, increase the likelihood of homoploid hybrid speciation.
A persistent question in the evolution of life histories is the fitness trade-off between reproducing only once (semelparity) in a lifetime or reproducing repeated times in different seasons (iteroparity). The problem can be formulated into a research agenda by assuming that one reproductive strategy is resident (has already evolved) and by asking whether invasion (evolution) of an alternative reproductive strategy is possible. For a spatially nonstructured system, Bulmer (1994) derived the relationship vPA < 1 (PA is adult survival; vbS and bS are offspring numbers for iteroparous and semelparous breeding strategies, respectively) at which semelparous population cannot be invaded by an iteroparous mutant. When the inequality is changed to vPA > 1, invasion of a semelparous mutant is not possible. From the inequalities, it is easy to see that possibilities for evolutionary establishment of a novel reproductive strategy are rather narrow. We extended the evolutionary scenario into a spatially structured system with dispersal linkage among the subunits. In this domain, a rare reproductive strategy can easily invade a population dominated by a resident reproductive strategy. The parameter space enabling invasion is far more generous with spatially structured evolutionary scenarios than in a spatially nonstructured system.
Understanding the genetic and environmental bases of phenotypic variation and how they covary on local and broad geographic scales is an important goal of evolutionary ecology. Such information can shed light on how organisms adapt to different and changing environments and how life-history trade-offs arise. Surveys of phenotypic variation in 25 Littorina obtusata populations across an approximately 400-km latitudinal gradient in the Gulf of Maine revealed pronounced clines. The shells of snails from northern habitats weighed less and were thinner and weaker in compression than those of conspecifics from southern habitats. In contrast, body size (as measured by soft tissue mass) followed an opposite pattern; northern snails weighed more than southern snails.
A reciprocal transplant between a northern and southern habitat revealed substantial plasticity in shell form and body mass and their respective measures of growth. Southern snails transplanted to the northern habitat produced lighter, thinner shells and more body mass than controls raised in their native habitat. In contrast, northern snails transplanted to the southern site produced heavier, thicker shells and less body mass than controls raised in their native habitat. Patterns of final phenotypic variation for all traits were consistent with cogradient variation (i.e., a positive covariance between genetic and environmental influences). However, growth in shell traits followed a countergradient pattern (i.e., a negative covariance between genetic and environmental influences). Interestingly, body growth followed a cogradient pattern, which may reflect constraints imposed by cogradient variation in final shell size and thickness. This result suggests the existence of potential life-history trade-offs associated with increased shell production.
Differences in L. obtusata shell form, body mass, and their respective measures of growth are likely induced by geographic differences in both water temperature and the abundance of an invading crab predator (Carcinus maenas). Water temperatures averaged 6.8°C warmer during the transplant experiment and C. maenas abundance is greater in the southern Gulf of Maine. Because both increased water temperature and crab effluent affect shell form in the same way, future experiments are needed to determine the relative importance of each. Nevertheless, it is clear that phenotypic plasticity has an important role in producing geographic variation in L. obtusata shell form. Moreover, the evolution of phenotypic plasticity in L. obtusata and other marine gastropods may be driven by architectural constraints imposed by shell form on body mass and growth.
Two theories for the maintenance of sexual reproduction, the Red Queen hypothesis and mutation accumulation, suggest that the dispersal rates of sexuals and asexuals may determine the elimination or persistence of asexuals. Under higher dispersal rates of asexuals, asexuals may temporarily escape virulent parasites and reduce the effects of deleterious mutations. In the present study, I examine the population structure, parasite loads, and juvenile survivorship of Campeloma limum sexuals and autodiploid parthenogens from the southeastern U.S. Atlantic coastal plain. Using mtDNA sequence variation, it is shown that parthenogenetic haplotypes with limited sequence divergence are geographically widespread throughout this region and there is no significant population differentiation over a broad geographical scale. Sexual C. limum populations show significant mtDNA differentiation among and within river drainages and there is significant isolation by distance. These patterns are consistent with a recent origin and range expansion of parthenogens. Prevalence of infection by digenetic trematodes is significantly higher in autodiploid parthenogens, and the variance of prevalence is also higher in autodiploid parthenogens. I argue that the latter pattern indicates that unparasitized parthenogens have temporarily escaped these virulent parasites, but recolonization of these populations by trematodes results in high infection levels (> 40%), possibly due to reduced variation in resistance genes. I also examined whether the survivorship of juvenile sexuals and parthenogens varied under different stress levels. Sexual juveniles had twofold higher survivorship in all environments. Compared to polyploid parthenogens, autodiploid parthenogens may be less buffered against the effects of deleterious recessive alleles. I propose that the combined effects of higher parasitism and reduced juvenile survivorship of these autodiploid parthenogens accounts for the spatial distribution of sexual and parthenogenetic C. limum in the Atlantic coastal plain. Parthenogens may persist by higher dispersal rates into marginal habitats where there is a temporary escape from digenetic trematodes and competition with sexuals.
We analyzed a hybrid zone between two chromosome races (2n = 16 and 2n = 22) of a Japanese harvestman, Gagrellopsis nodulifera Sato and Suzuki (Arachnida: Opiliones: Phalangiidae). The hybrid zone is located in the eastern part of Tottori Prefecture, western Honshu. The width of the zone is approximately 5 to 15 km. Three independent tandem fusions/fissions seem to be the main cause of the karyotypic differences between the parental races. Ten karyotypic variants were found in the hybrid zone. They differed by numbers of diploid chromosomes and trivalents detected in meiosis. In most of the collecting sites, karyotypic heterozygotes were less common than expected. A positive correlation was found between number of trivalents in a karyotype and its deficiency rate. In some sites, the deficit of heterozygous individuals was accompanied by an excess of the intermediate homozygotes. One of the three transects across the zone was studied in detail. We found that three types of single heterozygotes (2n = 17, 2n = 19 and 2n = 21) formed a series of successive, spatially separated peaks along the transect. Two types of intermediate homozygotes (2n = 18 and 2n = 20) were also spatially separated. The most parsimonious explanation of such a structure is the staggering of clines of three tandem (or Robertsonian) fusion/fission variants that differentiate the parental races caused by selection against multiple heterozygotes. Analysis of nondisjunction in single heterozygotes demonstrated that there was a strong interindividual variation in nondisjunction rate. The mean frequency of aneuploid MII in single heterozygotes was 0.10 ± 0.03. Crossover exchanges in some critical regions of trivalents result in abnormal chromosomal configurations: chromosomes with unequal chromatids and dicentric chromosomes. Frequency of crossover-induced chromosomal abnormalities was low in single heterozygotes (≃ 4%), and was unexpectedly high in the double heterozygotes (≃ 15%). Selection against karyotypic heterozygotes is considered as a main evolutionary force responsible for the structuring of the hybrid zone. A positive association between diploid chromosome number and altitude was found. The race 2n = 16 tended to occupy lower altitudes than the 2n = 22 parental race. Differences in ecological preferences may be a result of previous adaptations to different environments in allopatry. A hypothesis concerning the origin and evolution of the hybrid zone is proposed.
Until now, only two Wolbachia-mediated cytoplasmic incompatibility (CI) types have been described in haplodiploid species, the first in Nasonia (Insect) and the second in Tetranychus (Acari). They both induce a male-biased sex ratio in the incompatible cross. In Nasonia, CI does not reduce fertility since incompatible eggs develop as haploid males, whereas in Tetranychus CI leads to a partial mortality of incompatible eggs, thus reducing the fertility of females. Here, we study Wolbachia infection in a Drosophila parasitoid, Leptopilina heterotoma (Hymenoptera: Figitidae). A survey of Wolbachia infection shows that all natural populations tested are totally infected. Crosses between infected males and cured females show complete incompatibility: almost no females are produced. Moreover, incompatible eggs die early during their development, unlike Nasonia. This early death allows the parasitized Drosophila larva to achieve its development and to emerge. Thus, uninfected females crossed with infected males have reduced offspring production consisting only of males. Evidence of this CI type in insects demonstrates that the difference in CI types of Nasonia and Tetranychus is not due to specific factors of insects or acari. Using theoretical models, we compare the invasion processes of different strategies of Wolbachia: CI in diploid species, the two CI types in haplodiploid species, and parthenogenesis (the classical effect in haplodiploid species). Models show that CI in haplodiploid species is less efficient than in diploid ones. However, the Leptopilina type is advantageous compared to the Nasonia type. Parthenogenesis may be more or less advantageous, depending on the infection cost and on the proportion of fertilized eggs. Finally, we can propose different processes of Wolbachia strategy evolution in haplodiploid species from Nasonia CI type to Leptopilina CI type or parthenogenesis.
Many organisms show latitudinal variation for various genetically determined traits. Such clines may involve neutral variation and originate from historical events or their maintenance may be explained by selection. For Drosophila melanogaster, latitudinal variation for allozymes, inversions, and quantitative traits has been found on several continents. We sampled D. melanogaster populations in Panama and along a transect of 40 latitudinal degrees on the west coast of South America. Negative correlations with latitude were found for AdhS and αGpdhF allele frequencies and for the frequency of the cosmopolitan inversion In(2L)t in AdhS αGpdhF chromosomes. A positive correlation existed between wing length and latitude. Significant correlations were found between these traits and climatic variables like temperature and rainfall. The observed clines show considerable resemblance to those found on other continents. Gametic disequilibrium between AdhS and αGpdhF occurred predominantly at higher latitudes and was caused by the presence of In(2L)t. The reasons for the clinal distributions are discussed and it is argued that selection is the most likely explanation. However, the exact nature of the selective force and the interactions of allozymes with each other and with In(2L)t are complex and not fully understood. In tropical regions In(2L)t-containing genotypes have higher fitness than ST/ST and Adh and αGpdh hitchhike with the inversion, but there is also evidence for balancing selection at the Adh locus.
The role of reinforcement in speciation can be explained by two distinct models. In model I, two diverged populations hybridize and produce fertile hybrids that successfully backcross (hybridization with gene flow). In model II, two populations hybridize but succeeding backcrosses are unproductive (hybridization without gene flow). Using Drosophila persimilis and D. pseudoobscura, we have tested model I by comparing the extent of heterospecific introgression in sympatric versus allopatric populations. We show that certain expectations of this particular model of reinforcement, which is based on hybridization and gene flow between divergent populations after secondary contact, are not realized in these two species. The evidence consists of the similarity of genetic distances as well as proportions of unique/rare alleles between sympatric and allopatric heterospecific populations and a negative correlation between genetic distance and geographical distance between heterospecific populations, which suggests ecological differentiation. This approach in quantifying differential gene flow has important consequences to studies that compare sympatric and allopatric isolation using genetic distance. Following model I, one would expect a pattern of higher prezygotic isolation in sympatric species compared to allopatric species of the same genetic distance simply as a result of an underestimation of genetic distance due to introgression between sympatric populations. We suggest more parsimonious explanations such as reinforcement without genetic exchange (model II) and ecological differentiation, which require high levels of preexisting reproductive isolation between populations.
We investigated the effects of inbreeding on various fitness components and their genetic load in laboratory metapopulations of the butterfly Bicyclus anynana. Six metapopulations each consisted of four subpopulations with breeding population sizes of N = 6 or N = 12 and migration rate of m = 0 or m = 0.33. Metapopulations were maintained for seven generations during which coancestries and pedigrees were established. Individual inbreeding coefficients at the F7 were calculated and ranged between 0.01 and 0.51. Even though considerable purging had occurred during inbreeding, the genetic load remained higher than that of many outbreeding species: approximately two lethal equivalents were detected for egg sterility, one for zygote survival, one for juvenile survival, and one for longevity. Severe inbreeding depression occurred after seven generations of inbreeding, which jeopardized the metapopulation survival. This finding suggests that the purging of genetic load by intentional inbreeding cannot be recommended for the genetic conservation of species with a high number of lethal equivalents.
Marine species generally show high dispersal capabilities, which should be accompanied by high levels of gene flow and low speciation rates. However, studies that focused on the relationship between dispersal and gene flow in marine fishes have been inconclusive. This study focuses on the black surfperch, Embiotoca jacksoni, a temperate reef fish that lacks a pelagic larval stage and lives on almost continuous reefs along the California and Baja California coasts. Mitochondrial control-region sequences from 240 individuals were obtained, and phylogeographic patterns were analyzed. A major phylogeographic break was found at Santa Monica Bay, a sandy expanse that prevents adult dispersal. Deep water separating the southern California Channel Islands was also found to be a major barrier to gene flow. Minor phylogeographic breaks were also detected in the Big Sur/Morro Bay and in the Punta Eugenia/Guerrero Negro regions, but none in the Point Conception region. Gene flow levels in E. jacksoni were found to be almost identical to those of another species with limited dispersal, Acanthochromis polyacanthus, thus indicating that the lack of a pelagic larval stage combined with barriers to adult dispersal may have had similar effects on these two species.
Tradeoffs in performance or fitness across environments have important implications regarding the nature of evolutionary constraints. It remains controversial whether tradeoffs such as these reflect genetic correlations that are genuine evolutionary constraints. However, if such long-term genetic constraints do exist, they must be due to underlying pleiotropy such that alleles that confer high performance in one environment invariably confer low performance in another. The distribution of genetic correlations within and among populations can provide insight about the existence of such pleiotropic tradeoffs.
The long-term association of certain teleost fish taxa with particular abiotic environments suggests that tradeoffs in performance across environments have constrained the geographic distribution of those taxa. Here we report the results of an experiment in which we artificially selected on acute heat- and cold-stress tolerance in two stocks of the poeciliid fish Heterandria formosa from source populations with different thermal histories. Unexpectedly, we observed no direct responses to selection. Under certain conditions, fish from the different source populations differed significantly in cold tolerance, but not in heat tolerance. The results suggest there are no strong pleiotropic tradeoffs between heat- and cold-stress tolerance in these populations.
The larger islands of the Lesser Antilles are ecologically and geologically complex and are inhabited by single, but morphologically variable, Anolis species. Although earlier work has indicated that a large part of the morphological variation in Anolis oculatus from Dominica can be attributed to selection, a history of recurrent volcanic activity over the last few million years suggests that vicariance may have also played a significant role. We report a study of variation in the cytochrome b gene of mitochondrial DNA across the island to address this issue. We uncovered a very high degree of polymorphism, with an overall gene diversity of 0.97 and a nucleotide diversity of 0.04. Sequences, on average, differ by 3.82% and the maximum pairwise divergence (corrected for multiple hits) is 9.29%. Most haplotypes are restricted to single localities (a pattern not changed by increasing the sample size). Phylogenetic analysis revealed the presence of two distinct lineages on the island with strong phylogeographic structure. One of these is geographically restricted to a relatively small part of the central Caribbean coast. Sublineages were also discernible within the other more widely distributed lineage, but resolution within and support for these sublineages was poor. The phylogeographic pattern is not congruent with generalized body shape and scalation, but is significantly correlated with color pattern. Even when correcting for this lineage effect with partial Matrix correspondence tests, the relationship between color pattern and vegetation is reaffirmed, suggesting that although both vicariance and selection have played a role in the morphological differentiation of this species, selection for current environmental conditions has been more important. We discuss the causes of the phylogeographic structure in light of the volcanic history of the island and highlight the exceptional instance of congruence between all morphological character systems and lineage boundaries, which occurs at the transition between the northern and southern Caribbean ecotypes.
Sexual size dimorphism (SSD) is the evolutionary result of selection operating differently on the body sizes of males and females. Anolis lizard species of the Greater Antilles have been classified into ecomorph classes, largely on the basis of their structural habitat (perch height and diameter). We show that the major ecomorph classes differ in degree of SSD. At least two SSD classes are supported: high SSD (trunk-crown, trunk-ground) and low SSD (trunk, crown-giant, grass-bush, twig). Differences cannot be attributed to an allometric increase of SSD with body size or to a phylogenetic effect. A third explanation, that selective pressures on male and/or female body size vary among habitat types, is examined by evaluating expectations from the major relevant kinds of selective pressures. Although no one kind of selective pressure produces expectations consistent with all of the information, competition with respect to structural habitat and sexual selection pressures are more likely possibilities than competition with respect to prey size or optimal feeding pressures. The existence of habitat-specific sexual dimorphism suggests that adaptation of Anolis species to their environment is more complex than previously appreciated.
Sexual selection driving display trait divergence has been suggested as a cause of rapid speciation, but there is limited supporting evidence for this from natural populations. Where speciation by sexual selection has occurred in newly diverged populations, we expect that there will be significant differences in female preferences and corresponding male display traits in the absence of substantial genetic and other morphological differentiation. Two allopatric populations of the Vogelkop bowerbird, Amblyornis inornatus, show large, qualitative differences in a suite of display traits including bower structure and decorations. We experimentally demonstrate distinct male decoration color preferences within each population, provide direct evidence of female preferences for divergent decoration and bower traits in the population with more elaborate display, and show that there is minimal genetic differentiation between these populations. These results support the speciation by sexual selection hypothesis and are most consistent with the hypothesis that changes in male display have been driven by divergent female choice.
DNA sequence variation at the hypervariable 5′ end of the mitochondrial control region was examined in 247 individuals to detect genetic divergence among 14 populations of red grouse (Lagopus lagopus scoticus) in northeastern Scotland. Ten haplotypes were resolved, several of which were shared among populations. Analysis of molecular variance, Nei's γST, and a cladistic estimate of the amount of gene flow indicated a lack of overall population differentiation. Patterns of overall panmixia are in stark contrast to previous reports of localized subdivision among the same set of populations detected using hypervariable microsatellite markers. Because grouse cocks are territorial and show extreme natal philopatry and females are the dispersing sex, such discordance could be explained by sex-biased dispersal, with extensive female-mediated gene flow preventing mitochondrial DNA divergence. However, it is difficult to reconcile how effective dispersal of females would not homogenize both mitochondrial and nuclear structure simultaneously. We use a model that examines the spatial and temporal dynamics of diparentally and uniparentally inherited genes to show that, under realistic ecological scenarios and with specific differences in the dispersal of males and females, the local effective size of the nuclear genome can be less than that of the mitochondrial and the patterns of structuring we observe are meaningful.
A biallelic viability model based on human data for maternal-fetal interactions reported by Hedrick (1997) gives the interesting result of neutral stability at all gene frequencies. I show that there are two levels of selection, within and among families, acting in opposing directions in this model and that the neutral stability occurs when the two levels of selection exactly balance one another, as they do in a randomly mating population. Deviations from random mating disrupt the balance and consequently destroy the neutral stability. However, with inbreeding avoidance, which characterizes the human histocompatibility loci, within-family selection is strengthened and among-family selection is weakened. This favors the invasion of new alleles and contributes to a high equilibrium level of genetic diversity at loci with maternal-fetal interactions affecting offspring viability in the pattern described by Hedrick. This pattern of selection is remarkably similar to that observed for the maternal effect selfish genes, Medea in flour beetles and scat in the mouse, and the Gp-9 gene in the fire ant.
Here I study a kin selection model of reproductive effort, the allocation of resources to fecundity versus survival, in a patch-structured population. Breeding females remain in the same patch for life. Offspring have costly, partial long-distance dispersal and compete for breeding sites, which become vacant upon the death of previous occupants. The main result is that the evolutionarily stable reproductive effort decreases as offspring dispersal rate increases. The result can be understood as follows: In a well-mixed population with global competition, neither adults nor juveniles compete with relatives, but in a patch-structured population with dispersal restricted to the juvenile phase, juveniles experience relatively less competition with relatives than adults, thus making juveniles relatively more valuable. Because this asymmetry between adults and juveniles decreases with the dispersal rate, so does the evolutionarily stable level of allocation to fecundity.
Using data from three years (1994–1996), I tested whether differential migration occurs from demes of high mean fitness in the shining fungus beetle, Phalacrus substriatus. The results show evidence for differential migration, thus providing evidence from a natural population for a critical demographic assumption of many interdemic selection models. To predict the evolutionary response to interdemic selection through differential migration, the genetic basis of the variation among demes in mean fitness must be known because the observed patterns could also be explained by some demes having an intrinsically favorable habitat. Thus, the importance of differential migration through interdemic selection in natural populations cannot be unequivocally answered without experiments specifically addressing the question of what causes differences in mean fitness among demes.
Species of Anolis lizards that use broad substrates have long legs, which provide enhanced maximal sprint speed, whereas species that use narrow surfaces have short legs, which permit careful movements. We raised hatchling A. sagrei in terraria provided with only broad or only narrow surfaces. At the end of the experiment, lizards in the broad treatment had relatively longer hindlimbs than lizards in the narrow treatment. These results indicate that not only is hindlimb length a plastic trait in these lizards, but that this plasticity leads to the production of phenotypes appropriate to particular environments. Comparison to hindlimb lengths of other Anolis species indicates that the range of plasticity is limited compared to the diversity shown throughout the anole radiation. Nonetheless, this plasticity potentially could have played an important role in the early stages of the Caribbean anole radiation.
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