Untitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION — страница 2

and disease limited population growth. Darwin After more than 20 years of observation and experiment, Charles Darwinproposed his theory of evolution through natural selection to the Linnaean Societyof London in 1858. He presented his discovery along with another Englishnaturalist, Alfred Russel Wallace, who independently discovered natural selection atabout the same time. The following year Darwin published his full theory,supported with enormous evidence, in On the Origin of Species. Genetics The contribution of genetics to the understanding of evolution hasbeen the explanation of the inheritance in individuals of the same species. GregorMendel discovered the basic principles of inheritance in 1865, but his work wasunknown to Darwin. Mendel’s work was "rediscovered"

by other scientists around1900. From that time to 1925 the science of genetics developed rapidly, and manyof Darwin’s ideas about the inheritance of variations were found to be incorrect.Only since 1925 has natural selection again been recognized as essentialin evolution. The modern theory of evolution combines the findings of moderngenetics with the basic framework supplied by Darwin and Wallace, creating thebasic principle of Population Genetics. Modern population genetics was developedlargely during the 1930s and ’40s by the mathematicians J. B. S. Haldane and 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 a population ofsexually reproducing organisms is produced by mutation and geneticrecombination. The resulting genetic variability is subject to natural selection in theenvironment. POPULATION GENETICS The word population is used in a special sense to describe evolution. Thestudy of single individuals provides few clues as to the possible outcomes ofevolution because single individuals cannot evolve in their lifetime. An individualrepresents a store of genes that participates in evolution only when those genes arepassed on to further generations, or populations. The gene is the basic unit in thecell for transmitting hereditary characteristics to offspring. Individuals are unitsupon which natural selection operates, but the trend of evolution can be tracedthrough

time only for groups of interbreeding individuals, populations can beanalyzed statistically and their evolution predicted in terms of average numbers. The Hardy-Weinberg law, which was discovered independently in 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 gene frequencies, orratios of different genes in a population, will remain constant unless they 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 frequencies predicted, bythe Hardy-Weinberg law

gives a numerical measure of how far the populationdeviates from a nonevolving state called the Hardy-Weinberg equilibrium. Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium will holdtrue, because it depends on the laws of probability. Changes are produced in thegene pool through mutations, gene flow, genetic drift, and natural selection. Mutation A mutation is an inheritable change in the character of a gene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutation in humans isextremely low; nevertheless, the number of genes in every sex cell, is so 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 new breedingorganisms or gametes from another population, as in plant pollen. Gene flow canwork against the processes of natural selection. Genetic Drift A change in the gene pool due to chance is called genetic drift. Thefrequency of loss is greater the smaller the population. Thus, in small populationsthere is a tendency for less variation because mates are more similar genetically. Natural Selection Over a period of time natural selection will result in changes in thefrequency of alleles in the gene pool, or greater deviation from the nonevolvingstate, represented by the Hardy-Weinberg equilibrium. NEW SPECIES New species may evolve either by the change of one species to another orby the splitting of one species into two or more new