Theory Of Evolution Essay Research Paper Theory — страница 2

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growth. Darwin After more than 20 years of observation and experiment,Charles Darwinproposed his theory of evolution through natural selection to theLinnaean Societyof London in 1858. He presented his discovery along with another Englishnaturalist, Alfred Russel Wallace, who independently discovered naturalselection atabout the same time. The following year Darwin published his fulltheory,supported with enormous evidence, in On the Origin of Species. Genetics The contribution of genetics to the understanding ofevolution hasbeen the explanation of the inheritance in individuals of the samespecies. GregorMendel discovered the basic principles of inheritance in 1865, but hiswork wasunknown to Darwin. Mendel’s work was “rediscovered” by other scientistsaround1900. From that

time to 1925 the science of genetics developed rapidly,and manyof Darwin’s ideas about the inheritance of variations were found to beincorrect.Only since 1925 has natural selection again been recognized as essentialin evolution. The modern theory of evolution combines the findings ofmoderngenetics with the basic framework supplied by Darwin and Wallace,creating thebasic principle of Population Genetics. Modern population genetics wasdevelopedlargely during the 1930s and ’40s by the mathematicians J. B. S. Haldaneand R. A.Fisher and by the biologists Theodosius Dobzhansky , Julian Huxley,Ernst Mayr ,George Gaylord SIMPSON, Sewall Wright, Berhard Rensch, and G. LedyardStebbins. According to the theory, variability among individuals in apopulation ofsexually reproducing

organisms is produced by mutation and geneticrecombination. The resulting genetic variability is subject to naturalselection in theenvironment. POPULATION GENETICS The word population is used in a special sense to describeevolution. Thestudy of single individuals provides few clues as to the possibleoutcomes ofevolution because single individuals cannot evolve in their lifetime. Anindividualrepresents a store of genes that participates in evolution only whenthose genes arepassed on to further generations, or populations. The gene is the basicunit in thecell for transmitting hereditary characteristics to offspring. Individuals are unitsupon which natural selection operates, but the trend of evolution can betracedthrough time only for groups of interbreeding individuals,

populationscan beanalyzed statistically and their evolution predicted in terms of averagenumbers. The Hardy-Weinberg law, which was discovered independentlyin 1908 bya British mathematician, Godfrey H. Hardy, and a German physician,WilhelmWeinberg, provides a standard for quantitatively measuring the extent ofevolutionary change in a population. The law states that the genefrequencies, orratios of different genes in a population, will remain constant unlessthey arechanged by outside forces, such as selective reproduction and mutation. Thisdiscovery reestablished natural selection as an evolutionary force. Comparing theactual gene frequencies observed in a population with the frequenciespredicted, bythe Hardy-Weinberg law gives a numerical measure of how far thepopulationdeviates

from a nonevolving state called the Hardy-Weinberg equilibrium. Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium willholdtrue, because it depends on the laws of probability. Changes areproduced in thegene pool through mutations, gene flow, genetic drift, and naturalselection. Mutation A mutation is an inheritable change in the character of agene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutationin humans isextremely low; nevertheless, the number of genes in every sex cell, isso large thatthe probability is high for at least one gene to carry a mutation. Gene Flow New genes can be introduced into a population through newbreedingorganisms or gametes from

another population, as in plant pollen. Geneflow canwork against the processes of natural selection. Genetic Drift A change in the gene pool due to chance is called geneticdrift. Thefrequency of loss is greater the smaller the population. Thus, in smallpopulationsthere is a tendency for less variation because mates are more similargenetically. Natural Selection Over a period of time natural selection will result inchanges in thefrequency of alleles in the gene pool, or greater deviation from thenonevolvingstate, represented by the Hardy-Weinberg equilibrium. NEW SPECIES New species may evolve either by the change of one speciesto another orby the splitting of one species into two or more new species. Splitting,thepredominant mode of species formation, results from the