Mutation Theory PDF

1 Unit V Mutation Theory and Synthetic Theory of Evolution 5. MUTATION THEORY Hugo de Vries (1848—1935), a Dutch botanist, one of the independent rediscoveries of Mendelism, put forward his views regarding the formation of new species in 1901. He also met some of the objections found in Darwin’s...

1 Unit V Mutation Theory and Synthetic Theory of Evolution 5. MUTATION THEORY Hugo de Vries (1848—1935), a Dutch botanist, one of the independent rediscoveries of Mendelism, put forward his views regarding the formation of new species in 1901. He also met some of the objections found in Darwin’s theory. According to him, new species are not formed by continuous variations but by sudden appearance of variations, which he named as mutations. Hugo de Vries stated that mutations are heritable and persist in successive generations. Experiments Conducted by Hugo de Vries: He conducted his experiments on Oenothera Lamarckian, (Evening Primrose) and found several aberrant types. When O. Lamarckian was self-pollinated and its seeds were allowed to grow, majority of F1 plants were similar to the parents, but a few were different plants. The different plants were also self-pollinated and when their seeds were sown, the majority of the plants were similar to the parents while a few were still more different plants and this continued generation after generation. These plants appeared to be new species, Hugo de Vries suggested from his experiments that new types of inherited characteristics may appear suddenly without any previous indication of their presence in the race. Hugo de Vries believed that mutation causes evolution and not the minor heritable varia- tions which was mentioned by Darwin. Mutations are random and directionless while Darwin’s variations are small and directional. According to Darwin, evolution is gradual while Hugo de Vries believed that mutation caused species formation and hence known as saltation (single step large mutation). On the basis of above observations, Hugo de Vries (1901) put forward a theory of evolution, called mutation theory. The theory states that evolution is a jerky process where new varieties and species are formed by mutations (discontinuous variations) that function as raw material of evolution. The salient features of mutation theory 1. Mutations or discontinuous variations are the raw material of evolution.  2 2. Mutations appear all of a sudden. They become operational immediately. 3. Unlike Darwin’s continuous variations or fluctuations, mutations do not revolve around the mean or normal character of the species. 4. The same type of mutations can appear in a number of individuals of a species. 5. Accumulation of variations produce new species. Sometimes a new species is produced from a single mutation. 6. Useful mutations are selected by nature. Lethal mutations are eliminated. However, useless and less harmful ones can persist in the progeny. 7. Mutations appear in all conceivable directions. 8. All mutations are inheritable. Points in Favour of the Mutation Theory (1) Mutations are actually the source of all variations and hence fountain head of evolution. (2) Mutation theory can explain both progressive and retrogressive evolution. (3) As the ratio of mutations is not the same in all individuals and their parts, mutation theory can explain the occurrence of both changed and unchanged forms. (4) A number of mutations have appeared in the past. They have given rise to new varieties. (a) Ancon Sheep is a short legged variety which appeared suddenly in Massachusetts in 1791. (b) Hornless Cattle developed as mutation from the homed cattle in 1889. (c) A single mutation can give rise to a new variety and even species of plants, e.g., Delicious Apple, Novel Orange, Red Sunflower (d) Hairless cats and double-toed cats have developed through mutations. Criticism of the Mutation Theory (1) Oenothera lamarckiana of Hugo de Vries was not a normal plant but a complex heterozygous form with chromosome aberrations. (2) Natural mutations are not common as Hugo de Vries thought them to occur. (3) Most of the mutations are negative or retrogressive. (4) Mutations are generally recessive while traits taking part in evolution are usually dominant. (5) Mutation theory cannot satisfactorily explain the development of mimicry, mutual dependence of flowers and pollinating insects. (6) This theory does not explain the role of nature.  3 Significance of Mutation Theory This Theory gives direct attention to the mutations. But later on it was thought that evolution cannot occur by mutations alone. Natural selection and isolation of mutants were also essential for evolution. 6. Synthetic Theory of Evolution The synthetic theory of evolution describes the evolution in terms of genetic variations in a population that leads to the formation of a new species. It explains the contribution of factors such as genetic variations, reproductive and geographical isolation, and natural selection. It is a modern concept that emerged from subsequent development of Neo-Darwinism from Darwinism.  The modern synthetic theory of evolution is the result of the work of a number of scientists namely T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Swall Wright, Ernst Mayr, and G.L. Stebbins.  It includes the following factors, gene mutations, variation (recombination), heredity, natural selection and isolation.  It is coupled with the knowledge of evolution at the molecular level (DNA, RNA, Genes, and Genome etc.) Development of Synthetic Theory of Evolution Darwin’s theory of natural selection was accepted. The strong supporters of Darwinism are Wallace, Huxley, Haeckel, and Weismann. Darwin’s theory lacked an input of modern concepts of genetics and the mechanisms how characters appear and persist in a population. In the light of recent researches the theory was modified. Several experimental evidences have gone in favour of Darwinism. Based on those facts and statistical data a synthetic theory of evolution was proposed. This is the modified theory of Darwinism which is also called as Neo-Darwinism. There were 3 groups of supporters of Darwinian evolution  Field naturalist.  Paleoontologist (These two groups concentrated only on observation).  Geneticists: Unlike the other two, they were dependent on experiment and these people didn’t agree with natural selection as the cause of evolution. The field naturalists thought that graphical isolation was the cause of evolution. The geneticists proved that “Blending theory of Darwin was wrong – Meanwhile De Vries discovered mutation in the plant Oenothera lamarckiana. They believed that mutation was very necessary for evolution. Mutation brought a big and discontinuous variation. This idea was just opposite to Darwinism. Then the Synthetic theory of evolution appeared where all the questions of scientific criticism of Darwinism were answered.  4 The seven basic postulates of the synthetic theory The Cold Spring Harbour Symposium, California (1959) identified the following seven postulates of the Synthetic Theory of evolution. Of these postulates, 1st to 4th is concerned with the origin of genetic variability, from 5th to 6th are concerned with the origin of species and the 7th one highlights how evolution is a continuous process. 1. At least in higher animals and plants, evolution proceeds principally as the result of the interaction between four indispensable processes like Mutation, Gene Recombination, Isolation and Natural Selection. 2. Mutation neither directs evolution as the earlier evolutionists believed, nor even serves as an immediate source of variability upon which selection may act. It is rather a reserve or potential resource of variability, which serves to replenish the gene pool as it becomes depleted through the action of selection. 3. The mutations which are most likely to be accepted by selection and so to form the basis of new types of organisms and those which individually have relatively slight effect on the phenotype, and collectively for the basis of multi-factorial inheritance. 4. The role of natural selection is much more than the purely negative one of eliminating unfit types. By greatly increasing the frequency of gene combination which otherwise has a very low chance of appearing, selection has an essentially creative and progressive effect. 5. The continued separation of new adaptive lines of evolution from related lines with different adaptations requires the origin of barriers of reproductive isolation, preventing or greatly restricting gene flow between them. This separation is essential for maintaining the diversity of adaptation which exists in any one habitat and so should be regarded as the basis of species formation. 6. The origin of reproductive isolation like that of new adaptive types requires the establishment of many new genetic changes, including structural alternatives of the chromosome and cytoplasmic changes as well as gene mutation. 7. The Origin of Genera and other higher categories, as well as the long-time trends, which have given, rise to increasingly complex and highly organized forms of life. Factors involved in synthetic theory a) Variations b) Mutations c) Natural selection d) Genetic drift e) Isolation of species. Over production, struggle for existence, and universal occurrence of variation will take place as usual. But in the synthetic theory, the formation of variations and mutations were discussed with experimental evidence for evolution which Darwin was unable to explain.  5 a) Variations During Darwin’s time little was known about genetic variations. During Meiosis and crossing over, synapsis will take place. Because of this, regrouping of genes will take place. Because of which genetic variation will appear or chromosomal aberrations will take place. The chromosomes may lose a bit or gain in a bit or order may be changed, or chromosomal bits may be exchanged between two chromosomes. These aberrations will become heritable variations. Now and then the sets of chromosomes will increase or decrease. This is called ploidy. Because of this polyploidy, heritable variations will arise they will be carried to number of generations. This may result in the origin of new species. b) Mutations Any change in the nucleotide sequence of DNA and if one pair of nucleotides is replaced mutations will arise. These mutations are called point mutations. These are caused spontaneously in nature. They can also be brought by induction. They are sudden and heritable. They may be harmful or beneficial. Most of the mutant genes are recessive. They can be expressed only in homozygous state. Because of these sudden mutations, new species are formed. For evolution, variations and mutations will be the raw material. c) Natural Selection Natural selection includes both physical and biotic factors and determine how and in what direction an organism is to change. Natural selection has no favouritism. But it is obvious that the organisms which are suited for environmental conditions will survive over power in the force of competition. Because of this, better survivors are retained in the nature. d) Genetic Drift In small inter breeding population, heterozygous gene pairs will tend to become homozygous. Because of this, disadvantage characters may be expressed and those organisms will be weeded out. Such genetic drifts are not theoretical. They operate in small populations of Islands. This genetic drift will provide a way to determine the line of evolution. e) Isolation In Darwin’s time, nothing was known about isolation. Isolation is very important part in evolution. Usually the organisms of a population will be segregated into several populations because of physiological or geographical Isolation. Mutations, large stretches of water may separate a population and in the separated groups, one group may change. Because of this, new species will be developed. Thus geographical isolation will bring evolution. The effects of natural selection in different environments will give different species. Thus the old Darwin’s concept is re-organised with experimental proofs, New Darwinism was proposed. Examples of natural selection 1. The industrial melanic moth Biston betularia, the industrial melanic moth, is a gray-colored moth that perfectly camouflages on tree trunks covered with lichen in England and escapes predation by birds. With industrial revolution in England in the middle of 19th century, lichens on tree trunks  6 got killed due to smoke belching out of factories. Tree trunks were now bare and dark and made the light gray moth prominent to the predatory birds. Now natural selection favoured dark coloured moths, which could camouflage on bare tree trunks. Since the moth has only one generation in a year, in less than 50 generations, the natural selection replaced gray population with black population. 2. Resistance in mosquitoes and houseflies DDT was used extensively, sometimes by airplanes over large areas. Initially it killed 99% of mosquito population but at the same time put a lot of pressure on the surviving individuals to mutate. Mutant resistant strains survived DDT application and became the parents of the next generation. Natural selection preserved the resistant populations and eliminated the susceptible ones. This can be called an artificial selection by man, due to which today not only mosquito and housefly but also many agricultural pests have become resistant to most of the available insecticides. 3. Liederberg’s replica plating experiment Liederberg (1952) conducted experiment on Escherichia coli by exposing the susceptible strains to penicillin repeatedly. As the generation time of the bacterium is 20-30 minutes only, hundreds of generations could be cultured and exposed to penicillin within a short time. He found that mutations for resistance appeared instantly and quickly replaced the susceptible populations by natural selection.  7 Synthetic theory according to Stebbins (1950) George Ledyard Stebbins Jr. (January 6, 1906 – January 19, 2000) was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Along with Dobzhansky (1900 – 1975), animal systematist Ernst Mayr, and paleontologist George Gaylord Simpson (1902 – 1984), Stebbins is considered one of the “architects” of the modern evolutionary synthesis of the 1930s and 1940s, an intellectual watershed and historic turning point that brought together research in cytology, genetics, systematics, palaeontology into a common evolutionary framework. He compared the synthetic theory with a motor car. According to him, Main factors of evolution 1. Gene mutation – Petrol The changes that occur in the gene due to phenotypic effect is known as mutation. This produces a variety of changes that may be harmful. It causes variations in the offspring. 2. Chromosome mutation – Transmission or, acceleration Chromosomal mutations like deletion, inversion, duplication, translocation, polyploidy result in the recombination. 3. Genetic recombination – Engine Mainly occurs as crossing over during meiotic pachytene phase between homologous chromosomal pairs. It produces a new mix up of genes that creates offspring with a new combination of characters. 4. Natural selection – Driver Natural selection produces a change in the frequency of the genes from one generation to the other favoring the differential form of the reproduction. The natural selection process creates an adaptive relation between the environment and the population through various combinations of genes. It does not produce genetic change, but once it has occurred it acts to encourage some genes over others. 5. Sexual isolation – Speed limit or direction of road. It is one of the significant factors responsible for the synthetic theory of evolution. Isolation may be caused by physical barriers like rivers, oceans, high mountains, sometimes even the differential water current within a river system. The isolation helps in preventing the interbreeding of related organisms. 6. Migration, Introgressive hybridization and Genetic drift - Accessory power or accessory factors of evolution. These factors cause the evolution to happen faster. According to him, evolution process on the basis of Natural selection occurs in two steps  8 1. Origin of heritable variants An isolated population of a species independently develops different types of mutations. The latter accumulate in its gene pool and creates variants within species. 2. Selection among variants and origin of heritable variants It is actually triggered by natural selection. The variants within a species are not equally adapted to the change of environments. So, naturally, the environment acts as a filter that results in the survival of the fittest ones and the removal of the unfit members. After several generations, through selection and mutations, the variants increase many folds and cause to originate a different species that is significantly different from the original one. Criticism of Synthetic Theory of Evolution The main criticism labelled against synthetic theory are as follows-  This theory is helpful in explaining previous evolutionary theory but it fails in providing the clue of future evolutionary changes.  This theory treats convergence and hybridization as secondary factors in evolution. But both these factors play a valuable role in human evolution.  Migration cannot be treated as a different factor because migration can be explained by convergence and hybridization.  This theory is unable to explain the adaptation of new variations.