26_an_introduction_to_genetic_analysis - An Introduction to...

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An Introduction to Genetic Analysis Chapter 26 Evolutionary Genetics Key Concepts Evolution consists of continuous heritable change of organisms within a single line of descent (phyletic evolution) and the differentiation between different lines of descent to form different species (diversification). The Darwinian mechanism of evolution rests on three principles: (1) organisms within a species vary from one another, (2) the variation is heritable, and (3) different types leave different numbers of offspring in future generations. Both phyletic change and diversification are the result of the interaction between the directional force of natural selection and random events. Natural selection is the differential reproduction of different genotypes that is a consequence of their different physiological, morphological, and behavioral traits. Random effects include the sampling of gametes each generation in finite populations and the random occurrence of mutations. A consequence of the random factors in evolution is that the same forces of natural selection do not lead to the same evolutionary result in independent lines of descent. Species are reproductively isolated populations of organisms that can exchange genes within the group but not with other species, because the groups are physiologically, behaviorally, or developmentally incompatible. Evolutionary novelties are possible because new DNA is acquired either by duplication and subsequent differentiation of DNA already present in the species or by the introduction of novel DNA from other species. Introduction The modern theory of evolution is so completely identified with the name of Charles Darwin (1809–1882) that many people think that the concept of organic evolution was first proposed by Darwin, but that is certainly not the case. Most scholars had abandoned the notion of fixed species, unchanged since their origin in a grand creation of life, long before publication of Darwin's The Origin of Species in 1859. By that time, most biologists agreed that new species arise through some process of evolution from older species; the problem was to explain how this evolution could occur. Darwin's theory of the mechanism of evolution begins with the variation that exists among organisms within a species. Individuals of one generation are qualitatively different from one 勇勇勇勇勇勇勇勇, 勇勇勇勇勇勇勇勇勇勇勇勇勇勇勇. 1
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An Introduction to Genetic Analysis another. Evolution of the species as a whole results from the differential rates of survival and reproduction of the various types, so the relative frequencies of the types change over time. Evolution, in this view, is a sorting process. For Darwin, evolution of the group resulted from the differential survival and reproduction of individual variants already existing in the group—variants arising in a way unrelated to the environment but whose survival and reproduction do depend on the environment.
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