Selection In Experimental Populations Research Articles

An experimental test of density-dependent selection on temperament traits of activity, boldness and sociability.

Temperament traits are seen in many animal species, and recent evolutionary models predict that they could be maintained by heterogeneous selection. We tested this prediction by examining density-dependent selection in juvenile common lizards Zootoca vivipara scored for activity, boldness and sociability at birth and at the age of 1 year. We measured three key life-history traits (juvenile survival, body growth rate and reproduction) and quantified selection in experimental populations at five density levels ranging from low to high values. We observed consistent individual differences for all behaviours on the short term, but only for activity and one boldness measure across the first year of life. At low density, growth selection favoured more sociable lizards, whereas viability selection favoured less active individuals. A significant negative correlational selection on activity and boldness existed for body growth rate irrespective of density. Thus, behavioural traits were characterized by limited ontogenic consistency, and natural selection was heterogeneous between density treatments and fitness traits. This confirms that density-dependent selection plays an important role in the maintenance of individual differences in exploration-activity and sociability.

Strong parasitoid-mediated selection in experimental populations of aphids.

Clonal diversity in asexual populations may be maintained if different clones are favoured under different environmental conditions. For aphids, parasitoids are an important variable of the biotic environment. To test whether parasitoids can mediate selection among host clones, we used experimental populations consisting of 10 clones of the peach-potato aphid, Myzus persicae, and allowed them to evolve for several generations either without parasitoids or in the presence of two species of parasitoid wasps. In the absence of parasitoids, strong shifts in clonal frequencies occurred, mostly in favour of clones with high rates of increase. The parasitoid Diaeretiella rapae hardly affected aphid densities but changed the outcome of competition by favouring one entirely resistant clone and disfavouring a highly susceptible clone. Aphidius colemani, the more infective parasitoid, strongly reduced aphid densities and dramatically changed host clonal frequencies. The most resistant clone, not a successful clone without parasitoids, became totally dominant. These results highlight the potential of temporal or spatial variation in parasitoid densities to maintain clonal diversity in their aphid hosts.

Distribution of fitness effects among beneficial mutations before selection in experimental populations of bacteria

The extent to which a population diverges from its ancestor through adaptive evolution depends on variation supplied by novel beneficial mutations. Extending earlier work, recent theory makes two predictions that seem to be robust to biological details: the distribution of fitness effects among beneficial mutations before selection should be (i) exponential and (ii) invariant, meaning it is always exponential regardless of the fitness rank of the wild-type allele. Here we test these predictions by assaying the fitness of 665 independently derived single-step mutations in the bacterium Pseudomonas fluorescens across a range of environments. We show that the distribution of fitness effects among beneficial mutations is indistinguishable from an exponential despite marked variation in the fitness rank of the wild type across environments. These results suggest that the initial step in adaptive evolution--the production of novel beneficial mutants from which selection sorts--is very general, being characterized by an approximately exponential distribution with many mutations of small effect and few of large effect. We also document substantial variation in the pleiotropic costs of antibiotic resistance, a result that may have implications for strategies aimed at eliminating resistant pathogens in animal and human populations.

Effects of genetic background on response to selection in experimental populations of Arabidopsis thaliana.

The extent to which genetic background can influence allelic fitness is poorly understood, despite having important evolutionary consequences. Using experimental populations of Arabidopsis thaliana and map-based population genetic data, we examined a multigeneration response to selection in populations with differentiated genetic backgrounds. Replicated experimental populations of A. thaliana with genetic backgrounds derived from ecotypes Landsberg and Niederzenz were subjected to strong viability and fertility selection by growing individuals from each population at high density for three generations in a growth chamber. Patterns of genome-wide selection were evaluated by examining deviations from expected frequencies of mapped molecular markers. Estimates of selection coefficients for individual genomic regions ranged from near 0 to 0.685. Genomic regions demonstrating the strongest response to selection most often were selected similarly in both genetic backgrounds. The selection response of several weakly selected regions, however, appeared to be sensitive to genetic background, but only one region showed evidence of positive selection in one background and negative selection in another. These results are most consistent with models of adaptive evolution in which allelic fitnesses are not strongly influenced by genetic background and only infrequently change in sign due to variation at other loci.

Genetic Divergence and Fitness Convergence Under Uniform Selection in Experimental Populations of Bacteria

Replicate populations of bacteria were propagated for 1000 generations in the laboratory. The growth substrate was periodically renewed, so that during most generations (cell doublings) it was not limiting. The final clones demonstrated about a 40% fitness increase when competed against their common ancestor. This increase was uniform both among and within populations despite extensive differentiation in correlated traits: cell size, resistance to starvation and dry mass of culture. It is suggested that genetic diversity developed because selection promoted any changes directing cell activity toward a higher maximum growth rate. Evolution of this trait halted at a similar level when some basic constraints on bacterial metabolism were met. The selective values of emerging mutations must have depended on the genetic background. They would be beneficial early in evolution but ineffective near the limit of adaptation. This hypothesis was tested for one mutation that affected both fitness and colony morphology. In some clones it was the first adaptive mutation and provided a third of the total fitness increase, but it was not assimilated by the clones that reached the adaptive ceiling in some other way. Near the limit of adaptation, epistasis levels off the fitnesses of genetically variable clones.

Multivariate Analysis of Selection in Experimental Populations Derived from Hybridization of Two Ecotypes of the Annual Plant Diodia teres W. (Rubiaceae)

Multivariate Analysis of Selection in Experimental Populations Derived from Hybridization of Two Ecotypes of the Annual Plant Diodia teres W. (Rubiaceae)

Natural Selection in Experimental Populations of Reef-Building Corals (Scleractinia)

Natural Selection in Experimental Populations of Reef-Building Corals (Scleractinia)

Natural selection in experimental populations of the landsnail Cepaea nemoralis (L.)

A field experiment with Cepaea nemoralis is described. Twenty cages, designed to exclude vertebrate predators, divided equally between five sites at different altitudes in a single valley, were populated by adult snails taken from a 180 m population sampled in adjacent 10 m “donors”. The population showed a steep cline in morph frequencies. Each morph from each donor was apportioned equally between the cages. Between May and October 1974 more than half the snails died. There were position-dependent differences in mortality, and survival was greater where the morph frequencies in the original population were clinal. Repeated differences in survival between the morphs occurred at four sites, and these were opposite in direction between two of them. Brown-shelled snails survived best at an apparently cool site and worst at an exposed one. Overall, yellow five-banded snails survived better than the other morphs. It is suggested that overcrowding is a primary source of stress for all the morphs, and that temperature is important in determining the selective outcome between them. Alternative interpretations are given. It is concluded that the population cage provides a useful tool for the study of natural selection in Cepaea.

Natural Selection in Experimental Populations of Cepaea hortensis

(1971). Natural Selection in Experimental Populations of Cepaea hortensis. Journal of Biological Education: Vol. 5, No. 1, pp. 25-34.

Selection in experimental populations. II. Components of selection and their fluctuations in two populations of Drosophila melanogaster.

Selection in experimental populations. II. Components of selection and their fluctuations in two populations of Drosophila melanogaster.

SELECTION IN EXPERIMENTAL POPULATIONS. I. LETHAL GENES

THE main concerns of population genetics are the frequencies of genes in populations and the forces that alter them. Central in importance among these forces is natural selection. The analysis of selection in experimental populations began with the work of L’H~RITIER and TEISSIER in 1934 and is today a standard laboratory procedure. Despite the attention of geneticists for thirty years, however, certain important aspects of selection remain undemonstrated. The purpose of this article is to investigate in detail two particular aspects of selection: how it varies, and how it is partitioned among its more important components. Lethal genes have been chosen for this inquiry into the mechanisms of selection, since for them the algebra of selection is greatly simplified. We shall consider selection which acts on two alleles, one of them lethal, at a single locus on one of the autosomes. A lethal allele is one which, when homozygous, causes death before reproduction. The selection against the homozygotes for the lethal allele is thus complete; our task is to estimate the selection on heterozygotes for the lethal allele (LETH) and a non-lethal allele (NL) . COMPONENTS OF SELECTION We shall first develop an algebraic model for the study of lethal genes. The model will involve discrete generations; that is, it will be constructed with specific, non-overlapping periods for reproduction. Let us call the total selective value of organisms carrying two non-lethal alleles, W. “W” is assigned to the heterozygotes in accordance with the usual convention in discussions o€ lethal genes. It measures the average number of off spring produced by heterozygotes relative to the number produced by organisms carrying two non-lethal alleles. The homozygotes for a lethal produce no offspring, so their selective value is 0. The number of offspring, of course, determines the number of alleles present in the next generation. Thus the selective values determine the frequencies of the alleles. In order to reproduce, a fly must live to reproductive age; thus, the total selective value W is composed of all the various types of selection which occur from the formation of zygotes in one generation to the formation of zygotes in the next. The terms “fitness” and “adaptive value” are frequently used as synonyms for selective value. Consider a population in which we determine the frequencies of the genotypes among the adults of each generation before they are transferred to fresh food and

THE ANALYSIS OF SELECTION IN EXPERIMENTAL POPULATIONS

THE ANALYSIS OF SELECTION IN EXPERIMENTAL POPULATIONS

SELECTION AND INVERSION POLYMORPHISM IN EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA INITIATED WITH THE CHROMOSOMAL CONSTITUTIONS OF NATURAL POPULATIONS.

Natural populations of Drosophila pseudoobscura, with an area of distribution ranging from British Columbia to Guatemala and the highlands of Colombia, are widely polymorphic for structural rearrangements of the third chromosome. Sturtevant and Dobzhansky (1936) first showed how the different gene arrangements could be related by a stepwise process of inversion from one ancestral chromosome pattern. Thus a phylogeny of chromosomal types was constructed (Dobzhansky and Sturtevant, 1938). The maintenance of the inversion polymorphisms by natural selection was inferred from the seasonal cyclic changes which occur in some localities (Dobzhansky, 1943), and was confirmed in studies of experimental populations maintained in the laboratory (Wright and Dobzhansky, 1946). The chromosome frequencies are delicately balanced, shifting with changes in the environment and with changes in the genetic composition of the populations. There have been long term (of the order of years) changes in chromosomal frequencies in some parts of the distribution area of the species, changes which show a pattern over thousands of miles of habitat (see Dobzhansky, Anderson, and Pavlovsky, 1966 and references therein). Pavlovsky and Dobzhansky (1966) recently studied experimental populations containing gene arrangements from a locality in California; gene arrangements from this same locality had been used to establish similar experimental populations twenty years previously. The different courses of selection in the two sets of populations, observed twenty years apart, reflect in a broad way the changes in the gene arrangements in nature during the time between the samples. The process of selection in experimental populations with chromosomes from a single locality is repeatable. For instance, four replicate populations were set up, carrying the ST and CH gene arrangements from California (Dobzhansky and Pavlovsky, 1953). Seven samples were taken over a period slightly longer than a year, at which time the gene arrangements had approached the equilibrium frequencies. There was no statistically significant heterogeneity among the samples from the replicate populations. But experimental populations initiated with chromosomes of mixed geographic origin behave quite differently. The course of selection is not predictable, and replicate populations may diverge widely. This led Dobzhansky (1949) to conclude that the gene contents of the chromosomes within each population are coadapted, i.e., mutually adjusted by natural selection. There are broad patterns of distribution of the third chromosome gene arrangements over the territory inhabited by Drosophila pseudoobscura (Dobzhansky, 1944). With the demonstration of the selective differences among the various gene arrangements, it became probable that the chromosomal races are products of natural selection. This selection occurs over large areas, and is on a geographically greater scale than the coadaptation within local populations. 1 Present address: Department of Biology, Yale University, New Haven, Connecticut. 2 Present address: Department of Anatomy, The University of Texas, Southwestern Medical School, Dallas, Texas.

Responses of esterase 6 alleles of Drosophila melanogaster and D. simulans to selection in experimental populations.

Responses of esterase 6 alleles of Drosophila melanogaster and D. simulans to selection in experimental populations.

Natural Selection in Experimental Populations of Tribolium. III. Characteristics of Tribolium confusum Populations in Equilibrium

Two groups of Tribolium confusum populations were observed in population cages for 80 weeks. Upon reaching equilibrium the numbers and sizes of adults and pupae were greater in one group than in the other. In the larger populations, natalities were estimated to be higher and adult life spans shorter, and, in separate experiments, the two population groups were also found to differ in the interaction between fecundity and adult density. The latter response was proposed as a possible cause of the other population differences. The presence of characteristics leading to density dependent control of population size, was construed as an adaptation to a stable, undisturbed environment. Variations in these characteristics, generated initially by genetic differences and possibly amplified by population interactions, suggest retention by the species of the adaptive variability necessary for the maintenance of population fitness.

Genetic drift and natural selection in experimental populations of Drosophila pseudoobscura.

Genetic drift and natural selection in experimental populations of Drosophila pseudoobscura.

Interallelic selection in experimental populations of Drosophila melanogaster: White and Satsuma.

Interallelic selection in experimental populations of Drosophila melanogaster: White and Satsuma.

Natural selection in experimental populations of Tribolium

Natural selection in experimental populations of Tribolium

A Comparison of Productivity in a Mutant and Wild Strain of Tribolium castaneum Herbst

Previous articleNext article No AccessLetters to the EditorsA Comparison of Productivity in a Mutant and Wild Strain of Tribolium castaneum HerbstAnn L. Phillips, and Daniel J. McDonaldAnn L. Phillips Search for more articles by this author , and Daniel J. McDonald Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 92, Number 867Nov. - Dec., 1958 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/282050 Views: 2Total views on this site Citations: 3Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Charles E. King, Peter S. Dawson Population Biology and the Tribolium Model, (Jan 1972): 133–227.https://doi.org/10.1007/978-1-4757-0256-9_5 Alexander Sokoloff , Robert R. Shrode , and James H. Bywatters Productivity in Tribolium castaneum, Physiological Zoology 38, no.22 (Sep 2015): 165–173.https://doi.org/10.1086/physzool.38.2.30152823Daniel J McDonald, Nancy Jane Peer Natural selection in experimental populations of Tribolium, Heredity 16, no.33 (Aug 1961): 317–330.https://doi.org/10.1038/hdy.1961.37