Natural Selection and Evolution (Genetics & Inherited Conditions)
In 1859, English naturalist Charles Darwin published On the Origin of Species by Means of Natural Selection, in which he made two significant contributions to the field of biology: First, he proposed that biological evolution can occur by “descent with modification,” with a succession of minor inherited changes in a lineage leading to significant change over many generations; and second, he proposed natural selection as the primary mechanism for such change. (This was also proposed independently by Alfred R. Wallace and was presented with Darwin in the form of a joint research paper some years earlier.) Darwin reasoned that if an individual organism carried traits that allowed it to have some advantage in survival or reproduction, then those traits would be carried by its offspring, which would be better represented in future generations. In other words, the individuals carrying those traits would be “naturally selected” because of the advantages of the traits. For example, if a small mammal happened to have a color pattern that made it more difficult for predators to see, it would have a better chance of surviving and reproducing. The mammal’s offspring would share the color pattern and the advantage over differently patterned members of the same species. Over many generations, the proportion of individuals with the selected pattern would increase until it was present in every member of the...
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Population GeneticsPopulation genetics and Natural Selection (Genetics & Inherited Conditions)
Population geneticists explore the actual and theoretical changes in the genetic composition of natural or hypothetical populations. Not surprisingly, a large part of the theoretical and empirical work in the field has concentrated on the action of natural selection on genetic variation in a population. Ronald A. Fisher and J. B. S. Haldane were the primary architects of selection theory beginning in the 1930’s, and Theodosius Dobzhansky was a pioneer in the detection of natural selection acting on genetic variants in populations of Drosophila melanogaster (fruit flies).
The most basic mathematical model of genes in a population led to the Hardy-Weinberg law, which predicts that there would be no change in the genetic composition of a population in the absence of any evolutionary forces such as natural selection. However, models that include selection show that it can have specific influences on a population’s genetic variation. In such models, the fitness of an organism’s genotype is represented by a fitness coefficient (or the related selection coefficient), in which the genotype with the highest fitness is assigned a value of 1, and the remaining genotypes are assigned values relative to the highest fitness. A fitness coefficient of 0 represents a lethal genotype (or, equivalently, one that is incapable of reproduction).
The simplest models of selection...
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Detecting and Measuring Fitness (Genetics & Inherited Conditions)
Although a great amount of theoretical work on the effects of selection has been done, it is also important to relate theoretical results to actual populations. Accordingly, there has been a substantial amount of research on natural and laboratory populations to measure the presence and strength of natural selection. In practice, selection must be fairly strong for it to be distinguished from the small random effects that are inherent in natural processes.
Ideally, a researcher would measure the total selection on organisms over their entire life cycles, but in some cases this may be too difficult or time-consuming. Also, a researcher may be interested in discovering what specific parts of the life cycle selection influences. For these reasons, many workers choose to measure components of fitness by breaking down the life cycle into phases and looking for fitness differences among individuals at some or all of them. These components can differ with different species but often include fertility selection (differences in the number of gametes produced), fecundity selection (differences in the number of offspring produced), viability selection (differences in the ability to survive to reproductive age), and mating success (differences in the ability to successfully reproduce). It is often found in such studies that total lifetime fitness is caused primarily by fitness in one of these components, but not all. In...
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Units of Selection (Genetics & Inherited Conditions)
Darwin envisioned evolution by selection on individual organisms, but he also considered the possibility that there could be forms of selection that would not favor the survival of the individual. He noted that in many sexual species, one sex often has traits that are seemingly disadvantageous but may provide some advantage in attracting or competing for mates. For instance, peacocks have a large, elaborately decorated tail that is energetically costly to grow and maintain and might be a burden when fleeing from predators. However, it seems to be necessary to attract and secure a mate. Darwin, and later Fisher, described how such a trait could evolve by sexual selection if the female evolves a preference for it, even if natural selection would tend to eliminate it.
Other researchers have suggested that in some cases selection may act on biological units other than the individual. Richard Dawkins’s The Selfish Gene (1976) popularized the idea that selection may be acting directly on genes and only indirectly on the organisms that carry them. This distinction is perhaps only a philosophical one, but there are specific cases in which genes are favored over the organism, such as the “segregation distorter” allele in Drosophila that is overrepresented in offspring of heterozygotes but lethal in homozygous conditions.
The theory of kin selection was developed to explain the evolution of altruistic...
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Impact and Applications (Genetics & Inherited Conditions)
The development of theories of selection and the experimental investigation of selection have always been intertwined with the field of evolutionary biology and have led to a better understanding of the history of biological change in nature. More recently, there have been medical applications of this knowledge, particularly in epidemiology. The specific mode of action of a disease organism or other parasite is shaped by the selection pressures of the host it infects. Selection theory can aid in the understanding of cycles of diseases and the response of parasite populations to antibiotic or vaccination programs used to combat them.
Although the idea of natural selection as a mechanism of biological change was suggested in the nineteenth century, artificial selection in the form of domestication of plants and animals has been practiced by humans for many thousands of years. Early plant and animal breeders recognized that there was variation in many traits, with some variations being more desirable than others. Without a formal understanding of genetics, they found that by choosing and breeding individuals with the desired traits, they could gradually improve the lineage. Darwin used numerous examples of artificial selection to illustrate biological change and argued that natural selection, while not necessarily as strong or directed, would influence change in much the same way. It is important to make a clear...
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Further Reading (Genetics & Inherited Conditions)
Bell, Graham. Selection: The Mechanism of Evolution. 2d ed. New York: Oxford University Press, 2008. Examines the concept of selection within the context of advanced discoveries in genetics, genomics, molecular biology, and other fields.
Dawkins, Richard. Extended Phenotype: The Long Reach of the Gene. Rev. 2d ed. Afterword by Daniel Dennett. New York: Oxford University Press, 1999. Argues that the selfish (individual) gene extends to making artifacts, such as birds’ nests, and to manipulative, persuasive behavior for survival. Bibliography, index.
_______. The Selfish Gene. New York: Oxford University Press, 1989. Argues that the world of the selfish gene revolves around competition and exploitation, yet acts of apparent altruism do exist in nature. A popular account of sociobiological theories that revitalized Darwin’s natural selection theory.
Fisher, Ronald Aylmer. The Genetical Theory of Natural Selection: A Complete Variorum Edition. Edited with a foreword and notes by J. H. Bennett. New York: Oxford University Press, 1999. Facsimile of the 1930 edition. Illustrated.
Godfrey-Smith, Peter. Darwinian Populations and Natural Selection. Oxford, England: Oxford University Press, 2009. Godfrey-Smith’s central concept is that a “Darwinian population” is a collection of things with the capacity to undergo change by natural selection. Based on this...
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Web Sites of Interest (Genetics & Inherited Conditions)
The Complete Work of Charles Darwin Online. http://darwin-online.org.uk. Includes online versions of the six editions of Darwin’s On the Origin of Species by Means of Natural Selection.
Early Theories of Evolution, Darwin and Natural Selection. http://anthro.palomar.edu/evolve/evolve_2.htm. Dennis O’Neil, a professor at Palomar College in San Marcos, California, includes a page about Darwin’s theories of natural selection in his online exploration of evolutionary theory.
Understanding Evolution, Natural Selection. http://evolution.berkeley.edu/evolibrary/article/evo_25. Understanding Evolution, created by the University of California Museum of Paleontology and the Nature Center for Science Education, aims to provide information about evolution for students and teachers. The site includes a page on natural selection.
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