Selection In Natural Populations Research Articles

AN ESTIMATE OF THE RELATIVE OPPORTUNITIES FOR NATURAL AND SEXUAL SELECTION IN A POPULATION OF MILKWEED BEETLES.

In a recent paper Wade and Arnold (1980) provide an elegant framework for the empirical study of sexual selection. They point out that an evolutionary response to sexual selection is dependent on both the intensity of selection and the heritability of fitness. While the heritability of fitness or its components would be very difficult to measure in most field populations, the parameters necessary for an estimation of the intensity of selection can often be obtained. Their paper focuses on measures of the intensity of, or opportunity for, sexual selection on males and shows this intensity to be equal to the variance in number of mates per individual divided by the square of the mean number of mates. Though variation among individuals in mating success is an obvious requirement for the operation of sexual selection, surprisingly few studies have succeeded in documenting the extent of such individual differences in natural populations. While empirical studies of sexual selection have been quite successful in documenting those morphological or behavioral characteristics that determine mating success in a variety of species, actual measures of variance in mating success or the intensity of sexual selection in natural populations are rarer. This is particularly true when mating success is considered over the lifetime of the organism rather than a single breeding season. Considerations of variance in mating success measured over the lifetime of the study organisms must take into account both the intraand intersexual interactions commonly associated with sexual selection and any variation in survivorship during the reproductive period that is usually associated with natural selection. Because the expression of an individual's mating ability is conditional on surviving to a given mating opportunity, a long-lived individual could accrue more mates than a shorter-lived competitor simply because he experiences more opportunities for encountering mates. Thus, the intensity of both natural and sexual selection must be considered when one uses the total number of mates acquired in a lifetime as a measure of fitness. The question then arises as to whether differences between males in their ability to find and court females (mating efficiency) or differences between males in survivorship after onset of reproduction contribute more to the total variance in the fitness of males. In understanding the evolution of life-history or mating strategies it would also be important to know if there are any phenotypic or genetic correlations between these two components of variance; such correlations have been identified by Arnold and Wade (unpubl.) as cointensities or the covariance between fitnesses at sequential episodes of selection. In order to estimate variance among individuals in number of mates one must be able to recognize individuals, identify their whereabouts during the mating season or seasons, and identify mating events. These criteria are most often met in vertebrate mating systems and accurate measures of mating success have been made for several such systems (e.g., Howard, 1979; Payne, 1979; Downhower and Brown, 1980). Few studies follow individuals through more than one breeding season, however, and thus measures of variance in lifetime mating success or fitness are virtually non-existent. Studies of invertebrates rarely examine

The Natural Selection of Populations and Communities.

The Natural Selection of Populations and Communities.

The Natural Selection of Populations and Communities.

The Natural Selection of Populations and Communities.

The natural selection of populations and communities, by David Sloan Wilson [Review

The natural selection of populations and communities, by David Sloan Wilson [Review

SELECTION COMPONENTS OF FOUR ALLOZYMES IN NATURAL POPULATIONS OF PEROMYSCUS MANICULATUS.

Because of the complex manner in which selection may occur, its measurement is a formidable problem. Adequate measurements require characterization of all selection components, age-dependence of fertility and mortality, immigration and emigration, and the temporal constancy and geographic variation of these patterns (Lewontin, 1974). One of the most comprehensive methods to detect selection in natural populations is based on analyses of selection components of natural polymorphisms in population samples which include mother-offspring combinations. The Christiansen-Frydenberg selection component analysis (SCA) (Christiansen and Frydenberg, 1973, 1976) consists of a hierarchy of hypotheses and tests which together characterize the modes of selection acting on particular chromosomal segments. Because it is based on one randomly selected progeny per mother-offspring combination, the hypotheses are interdependent. As a consequence, testing along the sequence is terminated at the first significant deviation from a null hypothesis. In addition, tests for gametic and sexual selection are confounded (Christiansen and Frydenberg, 1973; Nadeau et al., in press). Nadeau et al. (in press) showed that by analyzing all progeny of each mother-offspring combination, all tests except gametic selection in females are independent, gametic selection in males can be unambiguously detected, and even with analyses of entire litters,

FEMALE DRY WEIGHT AND FEMALE CHOICE IN CHAULIOGNATHUS PENNSYLVANICUS.

JOHNSON, M. S. 1976. Allozymes and area effects in Cepaea nemoralis on the western Berkshire Downs. Heredity 36:105-121. 1979. Inheritance and geographic variation of allozymes in Cepaea nemoralis. Heredity 43: 137-141. JONES, J. S., B. H. LEITH, AND P. RAWLINGS. 1977. Polymorphism in Cepaea: a problem with too many solutions? Ann. Rev. Ecol. Syst. 8:109143. JONES, J. S., R. K. SELANDER, AND G. D. SCHNELL. 1980. Patterns of morphological and molecular polymorphism in the land snail Cepaea nemoralis. Biol. J. Linn. Soc. In press. LANGLEY, C. H. 1977. Nonrandom associations between allozymes in natural populations of Drosophila melanogaster, p. 265-273. In F. B. Christiansen and T. M. Fenchel (eds.), Measuring Selection in Natural Populations. SpringerVerlag, N.Y. LEWONTIN, R. C. 1974. The Genetic Basis of Evolutionary Change. Columbia Univ. Press, N.Y. OXFORD, G. S. 1978. The nature and distribution of food-induced esterases in helicid snails. Malacologia 17:331-339. SELANDER, R. K., M. H. SMITH, S. Y. YANG, W. E. JOHNSON, AND J. B. GENTRY. 1971. Biochemical polymorphism and systematics in the genus Peromyscus. I. Variation in the old-field mouse (Peromyscus polionotus). Stud. Genet. 6:49-90.

Group Selection and Evolution The Natural Selection of Populations and Communities David Sloan Wilson

Group Selection and Evolution The Natural Selection of Populations and Communities David Sloan Wilson

Group Selection, Population Growth Rate, and Competitive Ability in the Flour Beetles, Tribolium Spp.

This is a report of an empirical study of competition between a laboratory strain of Tribolium confusum and populations of T. castaneum selected for increased and decreased productivity by group and individual selection. A pronounced inverse relationship between productivity and competitive ability was observed: fast growing T. castaneum populations had the smallest effect on the ability of T. confusum to recruit adults and slow—growing populations had the largest effect. The average duration of fast—growing T. castaneum populations was prolonged by competition but slow—growing populations were not affected in this regard. Approximately half of the slow—growing populations were facilitated in the production of adults by the presence of a competitor. The data suggest that individual fitness, measured as relative reproductive rate, and population fitness, measured as persistence, may be in conflict. The relevance of these findings to group and individual selection in natural populations is discussed.

GROWTH RATE IN OYSTERS: AN OVERDOMINANT PHENOTYPE AND ITS POSSIBLE EXPLANATIONS.

It is a well established phenomenon that homozygosity induced either by inbreeding or by especially designed experiments (such as, e.g., the technique of Sved and Ayala [1970] for comparing overall performance of chromosomal heterozygotes) generally results in low fitness. Because in all such cases a large part of the genome is made homozygous, the reduction in fitness can be equally well attributed to either reduced heterozygosity at heterotic loci, or to homozygosity for detrimental recessive genes. The problem of distinguishing between the two explanations is one of major importance in population genetics and is thoroughly discussed by Lewontin (1974). Another approach to studying the effect of heterozygosity is to correlate heterozygosity with a quantifiable character in individuals drawn randomly from a population. This approach does not rely on inbreeding, and although some inbreeding may occur in the population, if reduced homozygote performance is observed, it may well be attributed to other factors. Chromosomal inversions and electrophoretic variants provide suitable means for such an approach, the latter having the additional advantage of allowing one to study the effect of homozygosity or heterozygosity at a rather small segment of the genome. It appears, however, that most researchers have preferred to search for balancing selection in natural populations by comparing the observed levels of heterozygosity in the populations with that expected under the assumption of no selection (e.g., Marshall and Allard, 1970; Richmond and Powell, 1970; Allard et al., 1972). Only rarely has the individual's state in regard to homozygosity or heterozygosity been compared with its score for a quantitative character. Most of these studies have shown that heterozygotes perform differently than homozygotes (Koehn et al., 1973; Chaisson et al., 1976; Schaal and Levin, 1976; Stalker, 1976; Mitton, 1978). In a previous paper (Singh and Zouros, 1978) we reported the main findings from a study of a cohort of oysters. The cohort consisted of individuals of same age, drawn as spat (settling larvae) from a natural population, and grown under uniform conditions. We observed that the number of heterozygous individuals in the population was much lower than predicted from Hardy-Weinberg equilibrium. We also observed that there was a correlation between an individual's weight and the number of loci at which the individual was heterozygous. The first observation is common for populations of marine molluscs (for references see Singh and Zouros, 1978, and Skibinski et al., 1978). The second observation is less common in the literature and has an obvious bearing on the question of adaptive significance of enzyme heterozygosity. Here we report the findings of a second study. Our main objective was to see whether the patterns observed in the first study are reproducible. We have employed more loci and analyzed a much larger sample. This allowed us to firmly establish the correlation between heterozygosity and growth rate. It also provided us with a sizeable body of informati6n on which to evaluate certain hypotheses about the nature of the correlation, and ' Present address: Department of Zoology, University of Western Ontario, London, Ontario, Canada N6A 5B7.

Models of r and d

The Natural Selection of Populations and Communities. By D.S. Wilson. Pp.186. (Benjamin Cummings: Menlo Park, California, 1980.) $12.95.

Units of Natural Selection: The Natural Selection of Populations and Communities . David Sloan Wilson. Benjamin/Cummings, Menlo Park, Calif., 1980. xviii, 186 pp., illus. $12.95. Series in Evolutionary Biology.

Units of Natural Selection: <i>The Natural Selection of Populations and Communities</i> . David Sloan Wilson. Benjamin/Cummings, Menlo Park, Calif., 1980. xviii, 186 pp., illus. $12.95. Series in Evolutionary Biology.

SELECTIVE MORTALITY AND REPRODUCTION IN A NATURAL POPULATION OF BUFO BOREAS.

Controversy surrounding the evolutionary relevance of enzyme variation has added new impetus to theoretical investigations into the forces responsible for genetic polymorphism (e.g., Christiansen and Feldman, 1975; King and Ohta, 1975; Ohta and Kimura, 1975; Hedrick et al., 1976; Gillespie, 1977; Lewontin et al., 1978). These considerations have traditionally centered upon various forms of natural selection or, in its absence, random processes. It is, then, particularly important to seek and study selection in natural populations if we hope to understand more fully its role in the maintenance of genetic variation. Survivorship, as a component of fitness, must not be regarded as the sole determinant of reproductive success, yet it is clearly important. Consequently, there is reason to seek genotype-related differences in rates of mortality in natural populations. A simple first step is the search for changes in the genetic composition of a population over time. Literature addressing the question of genetic change in natural populations is restricted mainly to chromosomal analysis (Dobzhansky, 1970; Dobzhansky et al., 1976) and the monitoring of simple morphological differences (Ford, 1975). Surprisingly few studies have addressed the problem utilizing the electrophoretic technique although it has been successfully employed to monitor genetic changes in laboratory populations (Kojima and Yarbrough, 1967; Yarbrough and Kojima, 1967; Kojima, 1971; Powell, 1971; MacDonald and Ayala,

GENETIC VARIATION AND HOST PLANT RELATIONS IN A PARTHENOGENETIC MOTH.

Ecological genetics is the investigation of the influence of ecological factors on the genetic properties of populations, and the influence of genetic structure on their ecological properties. Ideally, these studies should determine how genetic variation is affected by selection and by the size, dynamics, and degree of subdivision of populations; what ecological factors determine the relative fitness of genotypes; and what effect the genetic composition of a population has on such ecological parameters as its density, stability, and ecological amplitude. Many studies have demonstrated selection in natural populations; indeed, the widespread existence of selection is perhaps the only generalization that can be made about ecological genetics. But most studies are incomplete. A few, such as the analysis of inversion karyotypes in Drosophila pseudoobscura by Dobzhansky

Measuring Selection in Natural Populations.

Measuring Selection in Natural Populations.

Measuring Selection in Natural Populations.

1. Study of Selection.- Estimation of mating cycle components of selection in plants.- Population genetics of Zoarces viviparus (L.), a review.- A study of sexual selection in natural populations of the milkweed beetle, Tetraopes tetraophthalmus.- How does the genome congeal?.- Attempts to measure selection by altering gene frequencies in natural populations.- Genetic variance for viability and linkage disequilibrium in natural populations of Drosophila melanogaster.- Sexual selection and the evolution of territoriality in birds.- Functional aspects of genetic variation.- On conditional inference for deviation from Hardy-Weinberg distribution.- 2. Study of Polymorphism.- Selection and neutrality.- Protein evolution: Nonrandom patterns in related species.- Polymorphism, selection, and multi-locus heterozygosity in the plaice, Pleuronectes platessa L..- Hidden heterogeneity among electrophoretic alleles.- Natural selection and the ?-GPDH locus in Drosophilidae.- Nonrandom associations between allozymes in natural populations of Drosophila melanogaster.- Polymorphism for the number of genes coding for salivary amylase in the bank vole, Clethrionomys glareola.- A geometric formulation of the stability condition at a triallelic locus.- A general model to account for enzyme variation in natural populations. IV The quantitative genetics of viability mutants.- 3. Sex and Evolution.- The sex habit in plants and animals.- Evolution of the sex ratio in the wood lemming, Myopus schisticolor.- Population genetics, demography and the sex ratio.- A comparative study on enzyme polymorphisms in sympatric diploid and polyploid populations of Lumbricillus lineatus (O.F.M.), Enchytraeidae, Oligochaeta.- Selection and genetic differentiation in parthogenetic populations.- 4. Ecology and Evolution.- Cactus-breeding Drosophila - A system for the measurement of natural selection.- Test of the hypothesis that migration balances selection in differentiated subpopulations of Spirorbis borealis.- The selection regime of Philaneus spumarius (L.) (Homoptera).- Effects of a virus on competition and selection in barley.- Selection and interspecific competition.- Coevolution in ecological systems: Results from loop analysis for purely density-dependent coevolution.- 5. Human Evolution.- Selection and non-Mendelian variability.- Estimation of the characteristics of rare variants.- The genetics of HLA and disease associations.

A DARWINIAN INTERPRETATION OF HINDLIMB VARIABILITY IN FROG POPULATIONS.

In the absence of detailed knowledge about many of the n dimensions of the environment-organism interaction, the best hope of evaluating the effects of natural selection in natural populations lies in comparative studies of variability. Briefly, all other factors being equal, the more intense selection has been, the less variable will a character be (Bumpus, 1898; Weldon, 1901; O'Donald, 1973; Salthe, 1975). Some care must be exercised in choosing characters for this kind of study in that polymorphism, canalization, allometry, and other relationships can interfere with the simple proposition just stated. We will attempt to evaluate these factors in our work and thereby neutralize them as contributors of noise in the study (see Materials and Methods). Given the validity of the above proposition, the following interpretations follow from the general principles of evolutionary population biology. We may discern both withinand between-population variability as we sample populations from any given species. Given a high withinpopulation variability, signifying relatively low selective pressure towards the trait we are measuring, no further information is gleaned from knowing whether the between-population variability is low or high because genetic drift could produce either situation under some sets of conditions. Thus, if that variability is high, the different populations have drifted towards different means, while, if it is low, temporal or physical constraints

Multiple Insemination and Male Sexual Selection in Natural Populations of Drosophila pseudoobscura

Data are presented which indicate a high frequency of concurrent multiple paternity in the progeny of females in a natural population of Drosophila pseudoobscura. A probability model is presented and used to obtain maximum-likelihood estimates of parameters describing the pattern of sperm use and the amount of concurrent multiple paternity in several population samples. The probability that two randomly selected offspring from a single randomly selected female have the same father was found to be .79. Estimates of effective mating frequency of male genotypes suggest that one of the samples came from a population in which there was nonrandom mating. Some evolutionary consequences of multiple matings in Drosophila are discussed.

Population structure, gene flow and natural selection in populations of Euphydryas phaeton.

An examination of seven proteins, presumably encoded by seven structural gene loci, in three local populations of the supposedly sedentary and colonial butterfly, Euphydryas phaeton revealed that three (43 per cent) were polymorphic with three to five alleles each. In addition to this high level of heterozygosity, no statistically significant differences in allele frequencies were found at two of the three polymorphic loci. Since the effective breeding size in each population was estimated to range from as few as 20 to 200 individuals, it appears that some level of gene flow between populations must be invoked to explain the high levels of genetic variability maintained in local populations of this butterfly, despite its apparently colonial nature.

The effect of experience on the selective behaviour of song thrushes feeding on artificial populations ofCepaea (held)

Song thrushes fed on populations of bread-stuffedCepaea shells. The results indicate that experience influences the selective behaviour of thrushes for both shell colour and shell size. Such behaviour could lead to frequency dependent selection in natural populations. Within the ranges of size and colour offered, thrushes learned from a colour rather than a size experience when both were varied simultaneously.

Genetic load in natural populations: is it compatible with the hypothesis that many polymorphisms are maintained by natural selection?

Recent studies of genetically controlled enzyme variation lead to an estimation that at least 30 to 60% of the structural genes are polymorphic in natural populations of many vertebrate and invertebrate species. Some authors have argued that a substantial proportion of these polymorphisms cannot be maintained by natural selection because this would result in an unbearable genetic load. If many polymorphisms are maintained by heterotic natural selection, individuals with much greater than average proportion of homozygous loci should have very low fitness. We have measured in Drosophila melanogaster the fitness of flies homozygous for a complete chromosome relative to normal wild flies. A total of 37 chromosomes from a natural population have been tested using 92 experimental populations. The mean fitness of homozygous flies is 0.12 for second chromosomes, and 0.13 for third chromosomes. These estimates are compatible with the hypothesis that many (more than one thousand) loci are maintained by heterotic selection in natural populations of D. melanogaster.