Role In Selection Research Articles

Possible role of natural selection in the formation of tandem-repetitive noncoding DNA.

A simulation model of sequence-dependent amplification, unequal crossing over and mutation is analyzed. This model predicts the spontaneous formation of tandem-repetitive patterns of noncoding DNA from arbitrary sequences for a wide range of parameter values. Natural selection is found to play an essential role in this self-organizing process. Natural selection which is modeled as a mechanism for controlling the length of a nucleotide string but not the sequence itself favors the formation of tandem-repetitive structures. Two measures of sequence heterogeneity, inter-repeat variability and repeat length, are analyzed in detail. For fixed mutation rate, both inter-repeat variability and repeat length are found to increase with decreasing rates of (unequal) crossing over. The results are compared with data on micro-, mini- and satellite DNAs. The properties of minisatellites and satellite DNAs resemble the simulated structures very closely. This suggests that unequal crossing over is a dominant long-range ordering force which keeps these arrays homogeneous even in regions of very low recombination rates, such as at satellite DNA loci. Our analysis also indicates that in regions of low rates of (unequal) crossing over, inter-repeat variability is maintained at a low level at the expense of much larger repeat units (multimeric repeats), which are characteristic of satellite DNA. In contrast, the microsatellite data do not fit the proposed model well, suggesting that unequal crossing over does not act on these very short tandem arrays.

Population Aggregation in the Prehistoric American Southwest: A Selectionist Model

The settlement practices of the prehistoric Anasazi of the North American Southwest exhibited shifts from the occupation of dispersed settlements to aggregated villages in many locales both concurrently and at different times. Explaining the nature and timing of these shifts has long been a focus of interest to researchers working in the Southwest. We present a model that outlines the relations among population growth, population dispersion and aggregation, regional abandonments, the nature of specialized systems of production, labor organization, climatic change, and the role of natural selection in producing evolutionary explanations. We offer the hypothesis that aggregation is the product of changes in the organization of corporate labor related to the stabilization of specialized strategies of resource production in response to changes in environmental conditions.

Latitudinal allozymic variability in natural populations ofZaprionus indianus from India

Twelve natural populations of Z. indianus from India, collected along lines of latitude, were analyzed electrophoretically and the geographical pattern of variation was compared at 15 allozyme loci. Except for Mdh-2, all the loci revealed extensive polymorphism. The allele frequency variation revealed a statistically significant correlation as well as regression coefficients with the latitude. The populations revealed extensive clinical variation at all the polymorphic loci, which depicted moderate to higher FST values. Thus, the observed genetic divergence in colonizing populations of Z. indianus shows the role of natural selection that varies spatially along the north-south axis of the Indian subcontinent.

Evolutionary responses of stomatal density to global CO2 change

Stomatal density is known to respond to CO2 levels during leaf development. Current interest in the increasing concentration of atmospheric CO2 has stimulated much experimentation on the responses of plants to relatively short-term exposure in artificially high CO2 levels. Attempts to extrapolate from short-term to long-term responses raise fundamental questions concerning evolutionary change in response to rising global CO2 levels. We consider the improved water use efficiency observed under elevated CO2 levels to be the main driving force of natural selection affecting the genotypic component controlling stomatal density. Whether a response is merely phenotypic or becomes incorporated into the genotype depends on two factors: (i) the time scale of exposure and (ii) the generation time of a species. Measurements of stomatal density on fossil leaves of Salix herbacea through a glacial cycle covering the last 140 000 years have shown a decrease in stomatal density in response to the rising CO2 levels of this period. This accords with the shorter-term observations on leaves of trees seen in herbarium specimens where the stomatal density has decreased in response to the rising CO2 levels of the last 200 years. The results indicate that natural selection over the 140000-year period may have favoured a similar response to that shown by trees phenotypically over the last 200 years. Since there is now some evidence for the genetic control of stomatal density, the role of natural selection affecting it must be considered when translating responses from short-term experiments to predict how stomatal density will be affected by long-term climatic and atmospheric change.

The Role of Natural Selection in the Evolution of Culture

The Role of Natural Selection in the Evolution of Culture

Structural diversity of the classical H-2 genes: K, D, and L.

Twenty-three class I DNA sequences, representing alleles of the H-2K, D, and L loci, were analyzed to assess patterns of nucleotide and amino acid diversity. Comparisons of the allelic and nonallelic sequences revealed locus specificity in regions encoding the leader peptides and the carboxyl-terminal segments of the Ag presenting molecules. Analyses focusing on the sequences that determine the Ag binding domains revealed weak or insignificant allelic associations, a finding that is in sharp contrast to previously observed relationships among the homologous human sequences. The amino acid positions exhibiting high diversity in the encoded glycoproteins in both mice and humans are localized primarily to the Ag binding site. In the mouse, diverse amino acids were positioned similarly in the K and D/L glycoproteins, although in humans, the A and B glycoproteins exhibit distinctive differences in their locations within the Ag binding site. The absence of locus specificity among the sequences that determine the Ag binding domains of the mouse is consistent with the hypothesis that ectopic gene conversion leads to interlocus exchange of class I sequences. Comparable interlocus exchanges among human class I genes have not played a similar role in shaping human A and B sequences. The basis of this difference between mice and humans is not clear. The nature of amino acid substitutions distinguishing class I loci in mice and humans are comparable, and the role of natural selection in determining diversity appears to be similar in the two species.

Universal constraint on evolution of all coding sequences

What often escapes notice is the inherent paradox in the role of natural selection in evolution. Inasmuch as a coding sequence encoding a functionless protein shall be ignored by natural selection, the initial acquisition of a function by a protein has to be intrinsic in the construction principle of coding sequences. The very fact that many proteins of very divergent functions tend to have rather similar amino acid compositions suggests that there must be a universal rule in the construction of all coding sequences, and this rule was previously defined as the TA/CG-deficiency—TG/CT-excess rule. When manifesting itself in coding sequences of rather balanced base composition, this rule establishes C T G as one of the most numerous base trimers. While Leu is the major residue of most proteins; C T G is the most frequently utilized of 6 Leu codons in E. coli to man. Yet, the two most common base tetramers, G C T G and C C T G, need not be translated in their second reading frame to yield Leu. Their preferential translation in the first reading frame increases Ala and Pro at the expense of Leu, while that in the third reading frame yielded extremely Cys-rich metallothionein.

What Selection: Herbivory or Competition?

Paige and Whitham (1987) showed that browsed plants of Ipomopsis aggregate had greater seed production than unbrowsed plants. This study has been hailed as the first unequivocal support for Owen and Wiegert's (1976) prediction that consumers can increase plant fitness (Owen 1990). These surprising results touched off a number of recent commentaries about the interpretation of selective forces exerted by herbivores on plants (Crawley 1987, Leather 1988, Fitter 1989, Westoby 1989, Owen 1990). Most of this discussion has centered on the interpretation of yield increase or compensation in response to herbivory as an adaptive response that has evolved as a consequence of natural selection due to herbivory. In wrestling with this interpretation, Paige and Whitham (1987) write: That plants should evolve to negate the deleterious effects of herbivory is obvious; that they should evolve to depend on herbivores to achieve their greatest fitness through overcompensation is less clear (p. 414). In addressing this quandary, Crawley (1987) suggested that, the predictability of herbivore attack on the primary shoots is sufficiently high, it is not difficult to imagine that an evolutionary advantage might accrue to a strategy of restrained early reproduction, leading to the production of single, primary shoots, followed by vigorous multistemmed regrowth following herbivore attack. However, this argument requires that attack on the regrowth shoots is predictably low. Even with this plethora of 'ifs' the story fails to explain why ungrazed plants do not put on a second burst of shooting once the risk of herbivory has passed, for, if it is advantageous to produce new flowering shoots following defloration of the primary shoot, then surely it is just as advantageous to produce them when ungrazed? (p. 168). According to Crawley (1987), one of the most puzzling questions here is, Why do we not witness the prospering of plants that branch from the base when they are not grazed? (p. 168). We think that the answer to this question requires a shift in focus concerning the role of natural selection. Several studies have shown that plants which have had the shoot apex removed produce greater yields than plants with the shoot apex left intact (Harris 1974, Clifford 1979, Binnie and Clifford 1980, Tayo 1980, 1982, Inouye 1982, Argall and Stewart 1984, Sheldon 1986, Benner 1988, Wein and Minotti 1988). The reason for these yield increases appears simply to involve release from the constraints of apical dominance which, when left uninterupted, can be regarded as an adaptive consequence of selection from competition for light. In a recent paper, Maschinski and Whitham (1989) recognized the important role of the disruption of apical dominance in producing the overcompensatory response in Ipomopsis. The critical question then becomes, does yield stimulation following herbivory represent an adaptive consequence of selection pressures from herbivory or an indirect consequence of selection from competition? The mechanism of apical dominance has been studied extensively by physiologists (Phillips 1975, Hillman 1984). Surprisingly however, the ecological and evolutionary implications of apical dominance have been scarcely addressed. Presumably, the evolutionary advantage of apical dominance is conferred by a disproportionate meristem commitment to vertical extension because, under crowded conditions, dominance is commonly achieved by those neighbours that can get the tallest, the fastest. Since lateral meristems are suppressed however, apical dominance may incur an overall 'cost' in terms of reduced maximum potential yield for plants growing free from the suppressive effects of neighbours. Harris (1974) writes, dominant plants remain programmed for apical dominance even when growing without competition and with non-limiting resources. The consequence is that in these situations, which are mostly man-made, the growth of the apically dominant plant is restricted to the maximum that can be achieved by the terminal. Removal of the terminal releases the lateral buds from correlative inhib-

Whither the Genome Project?

The Human Genome Project represents the convergence of three billion years of biological evolution and ten thousand or more years of cultural evolution-and their interaction is bound to change both, profoundly. From now on, their futures will be indissolubly linked. Actually, these streams, coursing through time, have already unwittingly made contact. With the invention of sanitation and the introduction of modem medicine, the role of natural selection in the perpetuation of the species was markedly reduced, with inevitable influence on the human gene pool. Now we foresee a more conscious interaction. Those who fear, even dread, its uncertain consequences, argue that we should prevent or abort this meeting. To do so would, of course, consign humanity forever to the cruelties and opportunities of the genetic lottery-statistical cruelties that mean blighted lives for those afflicted; statistical opportunities that provide the rare genius who, appearing at the right time, forever changes our civilization. Must we accept the former to have the latter? And though rarely noted, to do so implies that our present genetic endowment-with all of its admixed ancient baggage and recent innovation-is beyond human grasp and appreciation, much less improvement, an acme, or the expression of a divine will. To some, even increased knowledge of the genetic basis of human characteristics is an assault upon human dignity. But is human dignity best affirmed by willful ignorance, or by increasing human self-knowledge? Some, seeking an intermediate course, argue that biology and culture should meet at arms length. They fear the consequence of unrestrained interaction. They seek a decorous relationship in which the culture may pick and choose from the biological knowledge acquired to alleviate a few grievous genetic ailments, but avoid large-scale biological, and inevitably cultural, change. Others would set no a priori limits to this interaction, but would advocate great caution. We know so little about the complexity and tolerances of either our biological or cultural organization, they argue, that either or both could easily be destabilized with catastrophic consequence. In their view any experiments should be of small scale and slow pace. Still others, no doubt, full of confidence, would simply permit free enterprise to reign. The human genome project per se is intended solely to acquire knowledge. Yet in the ethos of modem society, it is feckless to suppose that knowledge, once available, will not be used. While ethicists may properly debate the morality of experimentation with sentient beings-in either the biological or cultural mode-heuristically, in a secular age dominated by the twin pillars of power-commerce and politics-what in fact is likely to happen? …

Metabolic Scaling: A New Perspective Based on Scaling of Glycolytic Enzyme Activities

The decline of mass specific aerobic metabolic rates with increasing animal size has a long history of study in zoology. Attempts to explain this phenomenon have generally been concerned only with aerobic metabolism and with estimators of muscle and skeletal strength. Our finding of tremendous increases in mass-specific glycolytic enzyme activity in locomotory muscle with size in some species of pelagic fishes indicates that this approach has been too narrow. It is necessary to consider total metabolic power in any consideration of metabolic scaling in relation to skeletal strength or muscle power, since the anaerobic component of muscle power is usually greater than the aerobic and often scales differently. We show that scaling of glycolytic power appears to be much more variable among species than is scaling of aerobic power, and we suggest that the different glycolytic power scaling patterns reflect selection for different sprint swimming abilities in fishes of different habits. The rather narrow range of variation in aerobic scaling patterns suggests that they are the result of natural selection acting in the context of geometric constraints on maximum aerobic gas uptake and transport. The glycolytic scaling data emphasize that the role of natural selection has usually been neglected in considerations of scaling of metabolism while the role of the scaling of solids has been overemphasized.

Evolution of Nonvisual Communication and Photoperiodic Perception in Speciation and Adaptation of Blind Subterranean Mole Rats

AbstractThe evolution of vocal and vibrational communications and photoperiod perception in the blind subterranean mole rat of the Spalax ehrenbergi superspecies, are reviewed in the context of adaptation and active speciation in the subterranean ecotype. Progressive evolution of the acoustic system and regressive evolution of the atrophied eye coupled with progressive evolution in photoperiodic perception and biorhythmicity systems, are discussed. Prospects are suggested for future research at the molecular, anatomical and organismal levels, in the twin evolutionary processes of adaptation and speciation. Spalax appears to be a uniquely productive evolutionary model for the study of evolutionary structural reductions and hypertrophies as well as functional shifts in evolution. It provides particularly a rich observational and experimental material to critically evaluate the challenges recently levelled against the synthetic theory of evolution concerning gradualism in speciation, the "adaptationist programme", and the role of natural selection as a creative force in evolution.

Growth and Size Variation in Contrasting Populations of Trout Salmo trutta: An Experimental Study on the Role of Natural Selection

Growth and Size Variation in Contrasting Populations of Trout Salmo trutta: An Experimental Study on the Role of Natural Selection

Evolving the improbable

The debate about the evolutionary roles of natural selection and molecular drive cannot be resolved by assigning probabilities with hind-sight to unique events in the evolution of complex adaptations. Hoyle dismisses natural selection by calculating, erroneously, the improbability of assembling by chance a string of amino acids in the order in which they occur in a given protein. Dawkins dismisses molecular drive on the same false grounds of the improbability of making the right decision, by chance, at each step in a long series of steps in the evolution of a given trait. Calculation of the probability, at any given step, of a successful match between fortuitous genetic variation and fortuitous environmental heterogeneity requires detailed knowledge of all parameters at that step. Such information is an essential requirement for quantifying the roles of natural selection and molecular drive in the evolution of one actual series of steps out of many potential series.

Connective tissue responses in blacks in relation to disease: further observations.

Additional information is presented in support of the hypothesis (Polednak: Am. J. Phys. Anthropol. 41:49-58, 1974) that in some black populations certain connective-tissue responses, which are involved in protection against infection and repair after injury, also may predispose to specific chronic diseases. These diseases include some autoimmune disorders (i.e., systemic lupus erythematosus, sarcoidosis, and scleroderma) and various benign and malignant tumors involving connective-tissue cells. Complex interactions between genetic factors (HLA and non-HLA loci) and environmental agents may be involved both in the etiology of these autoimmune diseases and in population differences in the incidence of these diseases. A framework is reviewed whereby cellular responses to infectious agents, involving chiefly immunoglobulin-producing cells and macrophages, may have consequences in terms of pathogenesis of specific chronic diseases more common in some black populations. The possible role of natural selection in maintaining some of these diseases is also considered, along with the need for involvement of biomedical anthropologists in their investigation.

BEHAVIORAL REPRODUCTIVE ISOLATION AMONG POPULATIONS OF THE ROTIFER BRACHIONUS PLICATILIS.

Several theories have been proposed to explain how reproductive isolation develops between populations (see Mayr, 1970; Bush, 1975; Carson, 1975; Templeton, 1980, 1981). Different mechanisms have been suggested for allopatric, sympatric, and parapatric populations, and there is some consensus about the importance of geographical isolation. Considerable disagreement persists, however, concerning the role of selection and genetic drift in the formation of reproductive isolation, as well as whether pre-mating or post-mating barriers develop first. Muller (1942) was the first to argue that reproductive barriers appear as by-products of adaptations to different environments. Populations adapting to different environments accumulate genetic differences that eventually lead to reproductive isolation. Dobzhansky (1940) placed more emphasis on the role of natural selection in the formation of barriers to gene flow. He supposed that slight post-mating barriers appear in allopatric populations, but that pre-mating barriers arise when the two populations come into contact once again. At this time selection against hybrids produces the preponderance of reproductive isolation between the incipient species. Carson (1968, 1971) has emphasized the role of genetic drift through founder effects in the formation of reproductive isolation between populations with his population flush principle. While there is still disagreement about the evolutionary dynamics of how reproductive isolation develops, the existence of reproductive isolation in many groups is well documented. Numerous examples of both preand post-mating reproductive isolating mechanisms can be found in any modern textbook on evolutionary biology. Furthermore, it is generally agreed that behavioral reproductive isolation plays a premier role in animal speciation (Dobzhansky, 1970). In certain groups, the type of reproductive isolating mechanisms that operate and how they develop is still largely unknown. This is true of zooplankton in general and rotifers in particular. For rotifers, ecologists and systematists have provided some initial insights into how pre-mating mechanisms might contribute to reproductive isolation. Allopatry probably plays a major role, at least in fresh water rotifers, because of the patchy nature of lake environments. This discrete type of environment is likely to pose substantial barriers to gene flow between populations inhabiting different lakes. Temporal isolation among populations also appears to be important. In the cyclical parthenogenetic breeding system of rotifers (Birky and Gilbert, 1971; King and Snell, 1977), sexual reproduction is an ephemeral event. Observations in natural populations suggest that rotifer species which co-occur in a habitat, often do not overlap in their periods of sexual reproduction (WesenbergLund, 1930; Carlin, 1943; reviewed by King, 1980). This lack of temporal correspondence in sexual periods could be an effective reproductive isolating mechanism. Only a single work (Gilbert, 1963) has provided any detail about the operation of behavioral reproductive isolating mechanisms in rotifers. In reciprocal tests using the freshwater rotifers Brachionus calycifiorus and Brachionus angularis, Gilbert found the males of both of these species completely species-specific in their mating reactions. When B. calycifiorus males were tested with females of three additional ro-

The Existential Game of Individual Self-Maintaining Units: Selection and Defence Tactics of Trees

An outline for natural selection is presented by (1) redefining the concept of the individual as a self-maintaining unit provided with an intrinsic organization for maintenance and reproduction, (2) describing the elementary processes of natural selection as an existential game where the maintenance ability of each individual is separately tested by the environment, and (3) comparing persistence and abundance measures of fitness. The tactics minimizing the risk that the individual dies without leaving surviving progeny are feasible short-term tactics in the existential game whether or not they maximize the numerical contribution to future generations. Maximizing abundance measures of fitness is not the primary long-term strategy in evolution. The tactics with the highest short-term reproductive success in one generation do not necessarily, in the long run, allow the continuation of the existential game through generations in fluctuating environments. The role of natural selection in the evolution of defence tactics of trees is discussed on this theoretical basis. From the functional viewpoint, trees are individual self-maintaining units in which reproduction may be decentralized to a number of sub-units organized around the same maintenance system. Defence mechanisms in such organizations can, in principle, evolve under selection between and/or within individual trees. They can evolve also under negative mass selection during periodical outbreaks of herbivores. In such cases, rare events instead of adaptation to the average environment are driving forces of evolutionary change.

Selection at the alpha-Gpdh locus in experimental populations of Drosophila melanogaster.

Three sets of experiments have been conducted in order to evaluate the role of natural selection at the alpha-Gpdh locus in Drosophila melanogaster. (1) The evolution of the F-allele frequency has been followed for many generations in 13 experimental populations having different genetic backgrounds. (2) Egg-to-adult viability has been measured in synthetic populations derived from one locality (Brouilly) and the results have been compared with those of a previous experiment involving a different local population (Tostes). (3) The effects of sodium octanoate on egg-to-adult viability have been measured on the genotypes FF, FS, SF, and SS. The results demonstrate that selection operates on a small block of genes which includes the alpha-Gpdh locus.

Cytogerontology since 1881: a reappraisal of August Weismann and a review of modern progress.

Cytogerontology, the science of cellular ageing, originated in 1881 with the prediction by August Weismann that the somatic cells of higher animals have limited division potential. Weismann's prediction was derived by considering the role of natural selection in regulating the duration of an organism's life. For various reasons, Weismann's ideas on ageing fell into neglect following his death in 1914, and cytogerontology has only reappeared as a major research area following the demonstration by Hayflick and Moorhead in the early 1960s that diploid human fibroblasts are restricted to a finite number of divisions in vitro. In this review we give a detailed account of Weismann's theory, and we reveal that his ideas were both more extensive in their scope and more pertinent to current research than is generally recognised. We also appraise the progress which has been made over the past hundred years in investigating the causes of ageing, with particular emphasis being given to (i) the evolution of ageing, and (ii) ageing at the cellular level. We critically assess the current state of knowledge in these areas and recommend a series of points as primary targets for future research.

The Role of Functional Analysis in Phylogenetic Inference: Examples from the History of the Xiphosura

SYNOPSIS. Conventional cladistic analyses of phylogeny can be interpreted as operating at the level of phylogenetic trees. They assume that all “evolutionary steps” (transitions from one character state to the next, along a morphocline) are independent and equal, and, on that basis, select the cladogram which is consistent with the most parsimonious trees. Evaluation of the assumptions of independence and equality requires consideration of hypotheses at the levelof scenarios. In some cases, arguments based on functional analysis can suggest revised interpretations of either homology or polarity. If properly formulated, these arguments can alter the evaluation of parsimony for trees to the extent that even the choice of cladogram is affected. The structure of scenario level arguments is identical to that of arguments operating at tree level. Examples of phylogenetic inference in the context of xiphosurans (horseshoe crabs), using both comparative morphological and functional analysis, illustrate this approach. In different cases, orthodox interpretations of relationship are either challenged or corroborated. Although the introduction of functional analysis into the process of phylogenetic inference may appear to compromise the usefulness of the reconstructed phylogeny for testing hypotheses concerning the role of natural selection in evolution, it actually increases the strength of such tests.

Population Biology of Avena. VIII. Colonization Experiment as a Test of the Role of Natural Selection in Population Divergence

Population Biology of Avena. VIII. Colonization Experiment as a Test of the Role of Natural Selection in Population Divergence