The minimum number of genes contributing to quantitative variation between and within populations
TL;DR: The minimum number of genes involved in producing a large difference between populations in a quantitative trait is typically estimated to be about 5 or 10, with occasional values up to 20, which strongly supports the neo-Darwinian theory that large evolutionary changes usually occur by the accumulation of multiple genetic factors with relatively small effects.
Abstract: A procedure is outlined for estimating the minimum number of freely segregating genetic factors, nE, contributing to the difference in a quantitative character between two populations that have diverged by artificial or natural selection. If certain simple criteria are satisfied approximately on an appropriate scale of measurement, nE can be estimated by comparing the phenotypic means and variances in the two parental populations and in their F1 and F2 hybrids (and backcrosses). This generalizes the method of Wright to genetically heterogeneous (or wild) parental populations, as well as inbred lines. Standard errors of the estimates are derived for large samples. The minimum number of genes involved in producing a large difference between populations in a quantitative trait is typically estimated to be about 5 or 10, with occasional values up to 20. This strongly supports the neo-Darwinian theory that large evolutionary changes usually occur by the accumulation of multiple genetic factors with relatively small effects.
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TL;DR: This work derives selection indices that maximize the rate of improvement in quantitative characters under different schemes of MAS combining information on molecular genetic polymorphisms (marker loci) with data on phenotypic variation among individuals (and their relatives).
Abstract: Molecular genetics can be integrated with traditional methods of artificial selection on phenotypes by applying marker-assisted selection (MAS). We derive selection indices that maximize the rate of improvement in quantitative characters under different schemes of MAS combining information on molecular genetic polymorphisms (marker loci) with data on phenotypic variation among individuals (and their relatives). We also analyze statistical limitations on the efficiency of MAS, including the detectability of associations between marker loci and quantitative trait loci, and sampling errors in estimating the weighting coefficients in the selection index. The efficiency of artificial selection can be increased substantially using MAS following hybridization of selected lines. This requires initially scoring genotypes at a few hundred molecular marker loci, as well as phenotypic traits, on a few hundred to a few thousand individuals; the number of marker loci scored can be greatly reduced in later generations. The increase in selection efficiency from the use of marker loci, and the sample sizes necessary to achieve them, depend on the genetic parameters and the selection scheme.
1,405 citations
Cites background from "The minimum number of genes contrib..."
...…importance are quantitative traits, influenced by numerous loci throughout the genome that often have individually small effects (BREESE and MATHER 1957; THODAY 1961; WRIGHT 1968 Ch. 15; LANDE 1981; EDWARDS, STUBER and WENDEL 1987; WELLER, SOLLER and BRODY 1988; SHRIMPTON and ROBERTSON 1988a, b)....
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...Most quantitative traits are influenced by numerous genes (WRIGHT 1968 Ch. 15; LANDE 1981), and typically a few loci have relatively large effects with many others having smaller effects (SPICKETT and THODAY 1966; GREGORY 1965, 1966; THOMPSON 1975; ED- WARDS, STUBER and WENDEL 1987; PATERSON et al.…...
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...A different but related measure can also be defined within a single random mating population (LANDE 1981), and evaluated for the geometric series of variance contributions in formula 10 as 12E = (i i a2i = (1 + a)/(l -a)....
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...At these rates, spontaneous mutation has been judged to be important in long-term pp. 41-44; WRIGHT 1968 Ch. 15; LANDE 1981,1983)....
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TL;DR: It is suggested that rapid evolution in response to changing environmental stress may allow many short-lived species to respond to human-induced environmental change and provide opportunities to develop stress-resistant crops.
Abstract: Traits that enable plants to exploit low-resource environments (e.g., slow tissue turnover, low transpiration rate, high root: shoot ratio, and high concentrations of plant defenses against pathogens and herbivores) are physiologically linked to key growth-related traits (low rates of photosynthesis, nutrient uptake, and growth). Similar suites of traits occur as both phenotypically plastic and evolutionary responses to stress. We suggest that a genetic change in a switch or underlying trait that turns on this stress resistance syndrome (SRS), which causes it to be expressed over a wider range of environmental circumstances, would effectively convert a high-resource genotype into one that is more stress-tolerant. Because of physiological linkages between growth-related traits and the SRS, any heritable change in a key growth-related trait will pleiotropically affect the SRS. Therefore, heritable changes in these key growth-related traits could be accompanied by evolution of the entire SRS. Evidence for this hypothesis comes from single-gene mutants that differ in many stress-related traits, rapid evolution of metal- tolerant populations that are broadly stress-resistant, and consistent patterns of traits in species along gradients in resource availability. Similar evolutionary patterns occur in many animal taxa, which suggests that it is a general evolutionary phenomenon. We suggest that rapid evolution in response to changing environmental stress may allow many short-lived species to respond to human-induced environmental change and provide opportunities to develop stress-resistant crops. However, the time lag between generations of long-lived species that dominate most natural vegetation may not allow mature individuals of these species to keep pace with rapid global change.
842 citations
TL;DR: It is shown that in constant or unpredictable environments genetic variability reduces population mean fitness and increases the risk of extinction, and in predictable, highly variable environments genetic variance may be essential for adaptive evolution and population persistence.
Abstract: Much recent conservation effort has focused on genetic events in small populations, such as threatened or endangered species on the verge of extinction. However, the overwhelming causes of population reductions and extinctions worldwide are habitat destruction and the introduction of exotic species of parasites, predators, and competitors. The restoration and maintenance of healthy habitats and ecosystems should be of great concern to a mature science of conservation biology. The long-term preservation of biodiversity requires understanding not only the demography and genetics of small populations but also the ecology and evolution of abundant species. Here we show that in constant or unpredictable environments genetic variance reduces population mean fitness and increases the risk of extinction. In predictable, highly variable environments genetic variance may be essential for adaptive evolution and population persistence. Most of the characters of interest to ecologists and evolutionary biologists are quantitative characters influenced by many genes and environmental factors. Meristic and threshold characters also are amenable to analysis using quantitative genetic methods (Wright 1968, ch. 15; Falconer 1989). As examination of the fossil record attests, quantitative characters are of great importance in adaptive evolution (Simpson 1953; Carroll 1988). Although adaptive evolution can occur by mutations of large effect, the divergence in the quantitative traits that distinguish both different populations within a species and closely related species usually has a polygenic basis (Wright 1968, ch. 15; Lande 1981; Coyne 1985). No comprehensive evaluation of the importance of genetic variability in quantitative traits to population persistence and adaptation exists currently. In the short-term, genetic variability is often less critical than other determinants of population persistence (Lande 1988), but in the long-term, it can play the decisive role in allowing a population to persist and adapt in a changing environment. The rate of evolution in the mean phenotype in response to selection on a single quantitative character is proportional to the product of the additive genetic variance in the character and the intensity of directional selection (Lande 1976; Falconer 1989). However, genetic variability is thought not to be the rate-limiting factor in long-term evolution. Instead, long-term rates of evolution and adaptive radiation are constrained by ecological opportunity (Simpson 1953, pp. 77-80; Wright 1968, p. 520). That the shortand the long-term views are not inconsistent can be seen in a model of the common situation in which natural selection acting on a quantitative character (other than fitness itself) favors an intermediate phenotype. In this situation the rate of evolution in the character is limited not only by the magnitude of the additive genetic variance in the character but also by the rate of change in the optimum phenotype as the environment changes. An intermediate-optimum model such as that which follows also demonstrates that genetic variability may be either beneficial or detrimental, depending on the pattern of environmental change.
712 citations
TL;DR: This review indicates that few experiments approach basic issues such as the number of loci that contribute to within-population variation, the rate of polygenic mutation, the extent of pleiotropy, the mechanisms that maintain additive variance, and the reasons for reduced fitness of extreme phenotypes.
Abstract: Our emphasis is on the genetic basis of quantitative variation, and its effects on the evolution of quantitative. Our review indicates that few experiments approach basic issues such as the number of loci that contribute to within-population variation, the rate of polygenic mutation, the extent of pleiotropy, the mechanisms that maintain additive variance, and the reasons for reduced fitness of extreme phenotypes
665 citations
TL;DR: The findings suggest that current recovery goals for many threatened and endangered species are inadequate to ensure long-term population viability.
Abstract: Mutation can critically affect the viability of small populations by causing inbreeding depression, by maintaining potentially adaptive genetic variation in quantitative characters, and through the erosion of fitness by accumulation of mildly detrimental mutations. I review and integrate recent empirical and theoretical work on spontaneous mutation and its role in population viability and conservation planning. I analyze both the maintenance of potentially adaptive genetic variation in quantitative characters and the role of detrimental mutations in increasing the extinction risk of small populations. Recent experiments indicate that the rate of production of quasineutral, potentially adaptive genetic variance in quantitative characters is an order of magnitude smaller than the total mutational variance because mutations with large phenotypic effects tend to be strongly detrimental. This implies that, to maintain normal adaptive potential in quantitative characters under a balance between mutation and random genetic drift (or among mutation, drift, and stabilizing natural selection), the effective population size should be about 5000 rather than 500 (the Franklin-Soule number). Recent theoretical results suggest that the risk of extinction due to the fixation of mildly detrimental mutations may be comparable in importance to environmental stochasticity and could substantially decrease the long-term viability of populations with effective sizes as large as a few thousand. These findings suggest that current recovery goals for many threatened and endangered species are inadequate to ensure long-term population viability.
La mutacion puede afectar criticamente la viabilidad de poblaciones pequenas al causar la depresion de endocria, mantener la variacion genetica potencialmente adaptativa en caracteres cuantitativos, y por medio de la erosion de la condicin por acumulacion de mutaciones levemente perjudiciales. En el presente estudio revise e integre trabajos empiricos y teoricos recientes sobre mutaciones espontaneas y su papel en la viabilidad de las poblaciones y la planificacion para la conservacion. Se analizo tanto el mantenimiento de la variabilidad genetica potencialmente adaptativa en los caracteres cuantitativos como el papel de las mutaciones perjudiciales en el incremento de riesgo de extincion de las poblaciones pequenas. Experimentos recientes indican que la tasa de produccion de varianza genetica cuasineutral y potencialmente adaptativa en los caracteres cuantitativos es de un orden de magnitud menor que la varianza mutacional total debido a que las mutaciones con efectos fenotipicos pronunciados tienden a ser fuertemente perjudiciales. Esto implica que a efecto de mantener el potencial adaptativo normal en los caracteres cuantitativos bajo un balance entre mutacion y deriva genica al azar (o entre mutacion, deriva genica y seleccion natural estabilizadora), el tamano poblacional efectivo debe ser de aproximadamente 5000 y no 500 (numero de Franklin-Soule). Resultados teoricos recientes sugieren que el riesgo de extincion debido a la fijacion de mutaciones levemente perjudiciales podria ser comparable en importancia a la estocasticidad ambiental y podria reducir substancialmente la viabilidad a largo plazo de las poblaciones con un tamano poblacional efectivo de solo unos pocos miles. Estos descubrimientos sugieren que las metas de recuperacion para muchas especies en peligro y amenazadas son inadecuadas para asegurar la viabilidad poblacional a largo plazo.
611 citations
References
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TL;DR: It is concluded that large amounts of genetic variation can be maintained by mutation in polygenic characters even when there is strong stabilizing selection.
Abstract: It is assumed that a character under stabilizing selection is determined genetically by n linked, mutable loci with additive effects and a range of many possible allelic effects at each locus. A general qualitative feature of such systems is that the genetic variance for the character is independent of the linkage map of the loci, provided linkage is not very tight. A particular detailed model shows that certain aspects of the genetic system are moulded by stabilizing selection while others are selectively neutral. With reference to experimental data on characters of Drosophila flies, maize, and mice, it is concluded that large amounts of genetic variation can be maintained by mutation in polygenic characters even when there is strong stabilizing selection. The properties of the model are compared with those of heterotic models with linked loci.
735 citations
TL;DR: The genic character and degree of variability in troglobitic and epigean populations of the characid fish Astyanax mexicanus in Mexico are compared to show the impact of climatic changes associated with glaciation on population genetics.
Abstract: Attempts to understand evolutionary processes in cave organisms have been largely limited to speculation on the causes of loss of photoreceptor organs and pigmentation, increase in size and complexity of tactile sensory structures, and certain modifications in physiology (see review in Barr, 1968). Little is known of the population genetics of cave organisms or of the genetic changes accompanying the transformation of epigean (surface-dwelling) forms to troglobites (obligate cavernicoles). Yet genetic information is essential to the confident development of theories of troglobite evolution. For this reason, we have compared the genic character and degree of variability in troglobitic and epigean populations of the characid fish Astyanax mexicanus in Mexico. Students of cave biology have generally accepted the thesis that the ancestors of many troglobites entered caves as troglophiles (facultative cavernicoles) before the end of the Pleistocene, and became isolated with the local extinction of surface populations as a result of climatic changes associated with glaciation (Barr, 1968). If this is true, many troglobites have been living
337 citations
TL;DR: It is stressed that the study of the metrical characters is not only of utilitarian interest, but also that when individual factors cannot be recognized the analytic method of genetic study cannot be commenced, and the question arises as to whether genetics as a science has any further resource to offer.
Abstract: For the past 20 years genetical methods have gradually been made more and more familiar to the practical breeders of plants and animals, upon whom the improvement for human use of the domesticated animals and cultivated plants finally depends. During this period it has become increasingly clear that the hereditary mechanism is well represented by the Mendelian scheme, as extended mainly by the work of the Drosophilists. It has been equally clear, however, that in all the practical problems of animal or plant improvement we are invariably faced with quantitative characters, which have shown themselves to be entirely intractable by the familiar genetical methods. These methods rest primarily upon the recognition of the effects of different single factors, and when these can be recognized the study of their effects in combination follows as a matter of routine. When individual factors cannot be recognized the analytic method of genetic study cannot even be commenced, and the question arises as to whether genetics as a science has any further resource to offer. The successes of analytic genetics have been obtained mainly with the numerous deleterious recessives which are abundant in most species, with certain easily recognizable characters of practical importance to the plant or animal breeder and with fancy characters such as the crest, or silky plumage in the fowl, which, however attractive to fanciers, cannot be regarded as of general utility to mankind. The development of the quantitative characters on which practical utility is founded owes very little to genetic analysis except in so far as i t has been demonstrated that i t depends on a definite gene complex. This is implicit in the study of the individual effects of Mendelian factors without the means of evaluating the mass effects of a large number of minor factors severally influencing the utility character. We would stress, however, that the study of the metrical characters is not only of utilitarian interest. The nature of the heritable
208 citations