A TEST OF THE ROLE OF EPISTASIS IN DIVERGENCE UNDER UNIFORM SELECTION

Five populations of Drosophila melanogaster have previously been shown to be replicably different in their responses to artificial selection for knockdown resistance to ethanol fumes (Cohan and Hoffmann, 1986). The present study tests whether this divergence could be attributed to the epistatic mechanism assumed by Wright's shifting-balance model of evolution, in which alleles favored in the genetic background of one population are not favored in that of another. If this were the mechanism of divergence, crosses between selected lines from different populations would be expected to yield an epistatic loss of the selected phenotype. However, all such crosses showed a good fit to an additive model with dominance. Divergence by an epistatic mechanism may also be associated with epistatic variance within populations, but no evidence for such epistasis was found. The populations therefore appear to have responded in different ways to selection not because of epistasis but because knockdown-resistance alleles that were common in some populations were absent (or at least less common) in others.     

A ROLE FOR LEARNING IN POPULATION DIVERGENCE OF MATE PREFERENCES

Learning and other forms of phenotypic plasticity have been suggested to enhance population divergence. Mate preferences can develop by learning, and species recognition might not be entirely genetic. We present data on female mate preferences of the banded demoiselle (Calopteryx splendens) that suggest a role for learning in population divergence and species recognition. Populations of this species are either allopatric or sympatric with a phenotypically similar congener (C. virgo). These two species differ mainly in the amount of wing melanization in males, and wing patches thus mediate sexual isolation. In sympatry, sexually experienced females discriminate against large melanin wing patches in heterospecific males. In contrast, in allopatric populations within the same geographic region, females show positive (“open‐ended”) preferences for such large wing patches. Virgin C. splendens females do not discriminate against heterospecific males. Moreover, physical exposure experiments of such virgin females to con‐ or hetero‐specific males significantly influences their subsequent mate preferences. Species recognition is thus not entirely genetic and it is partly influenced by interactions with mates. Learning causes pronounced population divergence in mate preferences between these weakly genetically differentiated populations, and results in a highly divergent pattern of species recognition at a small geographic scale.     

THE EVOLUTIONARY HISTORY OF DROSOPHILA BUZZATII. VIII. EVIDENCE FOR ENDOCYCLIC SELECTION ACTING ON THE INVERSION POLYMORPHISM IN A NATURAL POPULATION

The pattern of selection acting in nature on the chromosomal polymorphism of the cactophilic species Drosophila buzzatii was investigated by comparing inversion and karyotypic frequencies through four different life-cycle stages: adult males, eggs, third-instar larvae, and immature adults. All population samples were obtained in June 1981 at an old Opuntia ficus-indica plantation near Carboneras, Spain. The analysis rests on several assumptions which are explicitly set forth and discussed. The results, if these assumptions prove true, indicate strong directional selection for larval viability acting on the second-chromosome karyotypes and also suggest selective differences in fecundity and longevity. Heterotic selection, however, cannot be ruled out for other fitness components such as male mating success. This kind of selection could be operating on the fourth-chromosome polymorphism as well. Some gene arrangements showed significant and opposite changes in frequency at different parts of the life cycle, thus demonstrating endocyclic selection.     

A TEST AND REVIEW OF THE ROLE OF EFFECTIVE POPULATION SIZE ON EXPERIMENTAL SEXUAL SELECTION PATTERNS

Experimental evolution, particularly experimental sexual selection in which sexual selection strength is manipulated by altering the mating system, is an increasingly popular method for testing evolutionary theory. Concerns have arisen regarding genetic diversity variation across experimental treatments: differences in the number and sex ratio of breeders (effective population size; N_e ) and the potential for genetic hitchhiking, both of which may cause different levels of genetic variation between treatments. Such differences may affect the selection response and confound interpretation of results. Here we use both census-based estimators and molecular marker-based estimates to empirically test how experimental evolution of sexual selection in Drosophila pseudoobscura impacts N_e and autosomal genetic diversity. We also consider effects of treatment on X-linked N_e s, which have previously been ignored. Molecular autosomal marker-based estimators indicate that neither N_e nor genetic diversity differs between treatments experiencing different sexual selection intensities; thus observed evolutionary responses reflect selection rather than any confounding effects of experimental design. Given the increasing number of studies on experimental sexual selection, we also review the census N_e s of other experimental systems, calculate X-linked N_e , and compare how different studies have dealt with the issues of inbreeding, genetic drift, and genetic hitchhiking to help inform future designs.     

MUTATION,SELECTION, AND THE MAINTENANCE OF LIFE-HISTORY VARIATION IN A NATURAL POPULATION

In an effort to provide insight into the role of mutation in the maintenance of genetic variance for life-history traits, we accumulated spontaneous mutations in 10 sets of clonal replicates of Daphnia pulex for approximately 30 generations and compared the variance generated by mutation with the standing level of variation in the wild population. Mutations for quantitative traits appear to arise at a fairly high rate in this species, on the order of at least 0.6 per character per generation, but have relatively small heterozygous effects, changing the phenotype by less than 2.5% of the mean. The mean persistence time of a new mutation affecting life-history/body-size traits is approximately 40 generations in the natural population, which requires an average selection coefficient against new mutations of approximately 3% in the heterozygous state. These data are consistent with the idea that the vast majority of standing genetic variance for life-history characters may be largely a consequence of the recurrent introduction of transient cohorts of mutations that are at least conditionally deleterious and raise issues about the meaning of conventional measures of standing levels of variation for fitness-related traits.     

SELECTION ON LOCOMOTOR PERFORMANCE CAPACITY IN A NATURAL POPULATION OF GARTER SNAKES

Selection on locomotor performance was determined for a series of marked and recaptured individuals from a population of garter snakes (Thamnophis sirtalis fitchi) in Northern California. We measured snake length and mass, burst speed, endurance on a treadmill, and the distance crawled around a stationary circular track. Size-corrected values (residuals) of mass and locomotor performance were generated from the scaling equations of S–V length (SVL). Randomization tests and regressions were used to determine the probability that a trait was a significant predictor of survivorship, and a nonparametric, cubic spline estimate of the fitness function was used to facilitate detection of the patterns of selection. From 275 (“cohort”) snakes measured and tested within 8 days of birth in 1985, 79 were recaptured in the spring–summer of 1986 and subsequent years. Birth SVL was the only significant (randomization P = 0.022) predictor of neonatal survival from 1985 to 1986 with directional selection favoring larger individuals. In addition to the lab-born cohort, 382 field-born snakes from all ages in the population were captured, tested, and released during spring–summer 1986. Similar to the 1985 cohort, the survivorship of 37 of 86 neonates from 1986 to 1987 showed no significant relationship with any residual value using any statistical test. Survivorship from 1986 to 1987 for 127 of 250 yearlings (including 32 lab-born cohort snakes) analyzed with the randomization test showed that greater values of both speed (P = 0.007) and distance residual (P = 0.008) significantly favored survival, whereas intermediate values of mass residual (P = 0.006) were significantly more likely to survive. Univariate regressions predicting the survival of yearlings from 1986 to 1987 gave similar results to the randomization test, but in a multiple regression with yearling burst speed residual, distance capacity residual, and a quadratic term of mass residual, distance capacity residual was the least important predictor variable. For the survivorship of 37 of the 113 older snakes, greater burst speed residual significantly favored survival (randomization P = 0.001).     

THE DUAL ROLE OF SELECTION AND EVOLUTIONARY HISTORY AS REFLECTED IN GENETIC CORRELATIONS

The patterns of genetic correlations between a series of eye and antenna characters were compared among two sets of spring-dwelling and cave-dwelling populations of Gammarus minus. The two sets of populations originate from different drainages and represent two separate invasions of cave habitats from surface-dwelling populations. Matrix correlations, using permutation tests, indicated significant correlations both between populations in the same basin and from the same habitat. The technique of biplot, which allows for the simultaneous consideration of relationships between different genetic correlations and different populations, was used to further analyze the correlation structure. A rank-3 biplot indicated that spring and cave populations were largely differentiated by eye-antennal correlations, whereas basins were differentiated by both eye-antennal and antennal-antennal correlations. Eye-antennal correlations, which are likely to be subject to selection, were most similar within habitats, which are likely to have similar selective regimes.     

PATTERN OF PHENOTYPIC VIABILITY AND FECUNDITY SELECTION IN A NATURAL POPULATION OF IMPATIENS PALLIDA

Estimates of viability and fecundity selection of 13 phenotypic characters for 1,536 individuals of Impatiens pallida growing in 24 locations within a single natural population were compared. Directional viability selection of cotyledon area, day of initial leaf production, number of leaves, and stem length was detected throughout this population. Directional fecundity selection of cotyledon area, day of initial flower production, number of leaves present on day of initial flower production, stem length on day of initial flower production, number of leaves, and stem length was also detected. Phenotypic selection of these characters was strong in some cases, and the strength of selection was significantly heterogeneous among locations. For several of the characters, directional phenotypic selection within the population was significantly positive in some locations and significantly negative in others separated by only a few meters. Fecundity selection was stronger than viability selection for some characters, implying that fecundity selection was at least as important as viability selection within this population. Soil moisture levels and light intensities played a larger role than soil nutrient levels in determining the patterns of both viability and fecundity selection, and differences in directional viability selection were more strongly related to environmental variation than were differences in fecundity selection. The pattern of phenotypic selection could not be reliably inferred from the patterns of mortality and reproduction alone.     

LANDSCAPE GENOMICS OF POPULUS TRICHOCARPA: THE ROLE OF HYBRIDIZATION,LIMITED GENE FLOW,AND NATURAL SELECTION IN SHAPING PATTERNS OF POPULATION STRUCTURE

Populus trichocarpa is an ecologically important tree across western North America. We used a large population sample of 498 accessions over a wide geographical area genotyped with a 34K Populus SNP array to quantify geographical patterns of genetic variation in this species (landscape genomics). We present evidence that three processes contribute to the observed patterns: (1) introgression from the sister species P. balsamifera, (2) isolation by distance (IBD), and (3) natural selection. Introgression was detected only at the margins of the species’ distribution. IBD was significant across the sampled area as a whole, but no evidence of restricted gene flow was detected in a core of drainages from southern British Columbia (BC). We identified a large number of F_ST outliers. Gene Ontology analyses revealed that F_ST outliers are overrepresented in genes involved in circadian rhythm and response to red/far‐red light when the entire dataset is considered, whereas in southern BC heat response genes are overrepresented. We also identified strong correlations between geoclimate variables and allele frequencies at F_ST outlier loci that provide clues regarding the selective pressures acting at these loci.     

MALE GAMETOPHYTE SELECTION IN A NATURAL POPULATION OF HAPLOPAPPUS GRACILIS (COMPOSITAE)

Distributions of two forms of the A chromosome of Haplopappus gracilis in a hybrid natural population and in artificial crosses are analyzed. Pollen bearing the standard type A chromosome is generally superior to that bearing the Mexican type A when they compete on homozygous pistils of either type, but there is no selection on heterozygous pistils. The distribution of sporophyte karyotypes in the natural population is not in equilibrium, and it is surmised that the standard type A chromosome is increasing in frequency in the population.     

FINE‐SCALE GENETIC STRUCTURE IN A WILD BIRD POPULATION: THE ROLE OF LIMITED DISPERSAL AND ENVIRONMENTALLY BASED SELECTION AS CAUSAL FACTORS

Individuals are typically not randomly distributed in space; consequently ecological and evolutionary theory depends heavily on understanding the spatial structure of populations. The central challenge of landscape genetics is therefore to link spatial heterogeneity of environments to population genetic structure. Here, we employ multivariate spatial analyses to identify environmentally induced genetic structures in a single breeding population of 1174 great tits Parus major genotyped at 4701 single‐nucleotide polymorphism (SNP) loci. Despite the small spatial scale of the study relative to natal dispersal, we found multiple axes of genetic structure. We built distance‐based Moran's eigenvector maps to identify axes of pure spatial variation, which we used for spatial correction of regressions between SNPs and various external traits known to be related to fitness components (avian malaria infection risk, local density of conspecifics, oak tree density, and altitude). We found clear evidence of fine‐scale genetic structure, with 21, seven, and nine significant SNPs, respectively, associated with infection risk by two species of avian malaria (Plasmodium circumflexum and P. relictum) and local conspecific density. Such fine‐scale genetic structure relative to dispersal capabilities suggests ecological and evolutionary mechanisms maintain within‐population genetic diversity in this population with the potential to drive microevolutionary change.     

NATURAL SELECTION ON PLEIOTROPIC QUANTITATIVE TRAIT LOCI AFFECTING A LIFE-HISTORY TRADE-OFF IN AVENA BARBATA

We applied QTL mapping to fitness variation of Avena barbata under well-watered greenhouse conditions. One hundred eighty recombinant inbred lines were assayed for flowering time, total size, mass allocation, and fitness. Composite Interval Mapping identified two to five loci affecting these traits. These were well supported in more powerful Multiple and Bayesian interval mapping analyses that indicated that additional QTL, as well as epistatic interactions also affect the traits. The posterior distribution of the number of QTL peaked at five to eight additive loci and one to two interactions, but the specific locations of the additional loci could not be determined with certainty. In most cases in which loci for separate traits mapped to similar locations, explicit tests supported pleiotropy over close linkage of separate loci. Alleles that hastened first flowering generally reduced vegetative mass, increased reproductive mass, and were associated with high fitness. Because effects on mass allocation generally cancelled one another, few loci affected total plant size. Only one QTL affected vegetative mass independent of reproductive mass and this locus had little effect on fitness. Thus selection acts to shift the mass allocation toward greater reproductive allocation, because the correlated decrease in vegetative mass poses only a minor fitness cost.