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Home , Lamarckism

B.Sc.I Evolutionary Theories Dr. K.A. Gajare

EVOLUTIONARY THEORIES

I) Theory of inheritance of acquired characters () Postulates of Lamarckism: 1. Changing Environment: The environment is not constant. It goes on changing. As the environment changes it force the to change accordingly. 2. Internal Force of : Though changing environment force the change, the internal urge of the is also essential to carry out the change. This force causes increase in size of an . 3. Use or Disuse: Various organs became improved by continuous use and reduce to vestigial due to disuse. The organs may become stronger or larger by continuous use or weaker by disuse. This is called acquired characters. 4. Inheritance of acquired characters: Such acquired characters are inherited to the next generation. The individuals of next generation continue further the use or disuse making the organ stronger or weaker accordingly and transmitted to next generation. Thus after many generations a new with new characters develops. Examples of Lamarckism: 1. of : The ancestor of giraffe was a horse like organism, feeds on grass. Along with the change in environment the grass became scanty. The food available was in the trees. So to get the food the ancestor stretches its neck and legs. This results in somewhat longer neck and legs. These characters are inherited to next generation. The stretching is continuous in next generation increases the length of neck and legs. Thus after many generations an organism with very long neck and legs is developed which is nothing but the modern day Giraffe. 2. Evolution of Snake: Ancestors of snakes were lizard like creatures with elongated body with two pairs of limbs. While escaping from enemy it has to creep through holes and crevices. During these movements limbs become obstacles many of the time. So it stopped using its limbs and lost after many generations. 3. Other examples: a. Whales lost their hind limbs. b. Ducks and other aquatic has webbed foot. c. Eyes are reduced in moles d. Dorsally located eyes of flat fish. e. Vestigial organs like wisdom teeth and Appendix in human. Critics or Objections of Lamarckism: 1. Lamarck proposed increase in size of an organ. But it is not true every time. In many cases evolution takes place without increase in size. 2. As per Lamarck new organ resulted from new need, then in similar environmental conditions all organisms should evolve in similar way. 3. Inheritance of acquired characters was not accepted by the scientists. As the acquired characters are somatic. Those cannot be transmitted to next generation. a. E.g. Scars on the body are not transmitted to next generation. b. Muscles of a body builder are not transmitted to next generation.

II) Theory of () Charles Robert Darwin In 1859 solved the mystery of mechanism of organic evolution by proposing the theory of natural selection in his book ‘The origin of species’. The evidences that influence Darwin’s thoughts for development of the theory are as follows,

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B.Sc.I Evolutionary Theories Dr. K.A. Gajare

1. Charles Lyell’s Book, ‘Principles of geology’ : In this book Lyell asserted that, the mountains, valleys, rivers and coastlines were shaped by the action of rains, winds, earth quakes, volcanoes and other natural forces and not by Noah’s flood. This make Darwin to conclude that, characteristics of organisms and earths face can change over a vast span of time. 2. Living and extinct organisms: In the flat plains of Argentine pampas Darwin Dag up many bony remains of extinct mammals from the old river beds. By observing those fossils he came to certain conclusions, a. Study of Thoatherium: Fossil study of Thoatherium proved that a native horse was an ancient inhabitant of the continent and disappeared. This indicated that, species gradually became modified with time, and not all species survived through age. b. Study of Giant Armadillos and Sloths: By this study in South America Darwin came to know that, Living species have ancestors. Also he concluded that, Species change not only with time but also with geographical distance. c. Study on Galapagos Islands: This is a cluster of volcanic Islands in east Pacific near equator. Darwin studied unusual animals on these islands. One of those was land dwelling tortoise named galapago. Those were about 275 kg in weight and 183 cm in length and can attain 200 to 250 years of age. Darwin noticed that the tortoises were clearly different from island to island. Darwin gives the explanation as; in isolation each population develops its own distinctive characters. Still they show basic resemblance with each other and with mainland tortoise. Similarly Darwin also observed the small birds known as Darwin’s finches. They were different on various islands yet were closely related to each other. Darwin concluded that a single ancestral group can give rise to several different species. d. Pangenesis Hyothesis: Darwin’s pangenesis hypothesis assumed that every organism produces its miniature in its body called gemmules or pangenes. They take part in fertilisation and gave rise to next generation. There by the character developed by the organism under the influence of environmental factors are transmitted to next generation. Postulates of Darwin’s theory: 1. The universal occurrence of variation: Every group of organisms show variation in many ways. The variation may be based on geographical regions, time period, food, habitat etc. 2. An excessive ability of reproduction: In absence of environmental check, every species tends to increase in geometrical manner. If a population doubles in one year, it will be quadruple in next year and so on. A common oyster shed 80 million eggs in a season. A salmon fish produce 28000000 eggs in a season. A pair of elephant, the slowest breeder could have 29 million decedents in 800 years. Though each organism has tremendous power of reproduction, the number of individuals in a species remains almost constant due to environmental check. 3. Struggle for existence: More individuals are born than can survive. So it is clear that most of the individuals cannot attend their reproductive age. This is because struggle for natural resources which are limited. The struggle may be intraspecific, interspecific or environmental. a. Intraspecific struggle: i.e. struggle among the members of same species. Since all the members share common food and shelter, there is struggle for them. Apart from them struggle for mate for reproduction is one of the major cause of intraspecific struggle.

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B.Sc.I Evolutionary Theories Dr. K.A. Gajare

b. Interspecific struggle: i.e. struggle among the members of different species. If two or more species are sharing common food or shelter then there is struggle for those natural resources. Apart from that struggle between pray and predator, parasites and hosts etc are also interspecific struggle. c. Environmental struggle: i.e. struggle with environmental conditions like drought, flood, extreme temperatures, infections and diseases, earthquake, land sliding, volcanic eruptions ect. 4. : In the struggle for existence, the unfit individuals are eliminated and those which have adapted satisfactorily can survive. The weaker individual cannot sustain the struggle for existence and die before they attain reproductive age. 5. Inheritance of the successful variation: Thus selects the fittest individual, which will give rise to next generation. Thus the more fit variation is transmitted to the next generation and less fit variation is eliminated. The successive generations are better adapted for the environment change. As the environment goes on changing, further take place and subjected to natural selection. Thus after many generations the dependence differs from their ancestor and forms a separate species. The variation which may be fit for one environmental condition may be unfit fir other. So in different regions the organisms adapt in different way. Thus from single ancestor two or more species may evolved. Critics or demerits of Darwin’s theory: 1. Tooth and claws selection: Many of the workers critic Darwin’s ‘survival of fittest’ concept, as it directs only tooth and claws force for the fitness and for selection by nature. are also evolved without any bloody competition. 2. Arrival of fittest is not explained: Darwin explained how the fittest individual is selected by the nature and how that individual is able to produce next generation. But Darwin is unable to explain how the fittest arrive. How the character which make the organism well adapted is not explained by Darwin. 3. Over specialisation cannot be explained: some organisms attained over specialisations, like Irish deer has huge antlers which overweight its skeleton. Evolution of huge or mimicry in Kallima insect or Siberian mammoths with large tusks cannot be explained by Darwin’s theory. 4. New variation can be lost by dilution: new variation when arrive initially, it may be with one or few individuals. Most of the population is without that variation. Though it is a beneficial variation it is very dilute and there are more chances that it may loss after generations. 5. Indirect acceptance of Lamarkism: By proposing pangenesis Darwin accepted indirectly the theory proposed by Lamarck. As he also accept the inheritance of acquired characters.

III) Neo- Darwinism: Modern synthetic theory Neo-Darwinism is reinterpretation of Natural selection and theory with modern knowledge of . According to Neo-Darwinism the unit of evolution is population. Therefore in Neo-Darwinism is combined with with Darwin’s theory. The term Neo-Darwinism was coined by G.J. Romanes (1848-1894). Huxley in 1942 called it as synthetic theory. Neo-Darwinism is also called biological theory of evolution or genetical theory of evolution. Many scientists viz. Dobzhansky F.A., Fisher S.B., Haldane S., Sewell Wright, Mayer Ernst and Stibbons G.L. contributed to the neo-Darwinism theory.

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B.Sc.I Evolutionary Theories Dr. K.A. Gajare

Hardy Weinberg Principle: Hardy Weinberg gives the basic principle of population genetics. According to this principle, ‘the frequency of a population does not change after generations if the population is extremely large, if it is randomly matting population and there is no mutation, migration or natural selection.’ The five condition of Hardy Weinberg principle cannot be fulfilled by real population. So the gene frequencies go on changing and the forces which lead to change in frequencies are as follows, 1. Mutation: defined, Mutation as sudden heritable change. These are random with respect to the need of organism to adapt in positive direction. The mutations may be harmful, useful or neutral. Harmful or neutral mutations may not help the organisms to adapt for the changing environment. The mutations which are harmful may become useful in changing environment. The rate of mutation is very slow but it is sufficient to create genetic variations. Mutations introduce new in gene pool and causes change in gene frequencies. 2. Recombination: Recombinations are formed during meiosis. In meiosis I prophase I crossing over takes place. This results in new combinations of genes and alleles. Recombinations when occur within genes results in formation of new alleles. This leads to change in gene frequencies. 3. Gene migration: Migration of individuals from one population to other genetically different but interbreeding population causes . It involves spread of genes from one population to other. This causes change in gene frequencies. 4. : Random change in gene frequencies is called genetic drift. Its effect is less in large populations but more pronounced in small or isolated populations. Loss of some genes may occur due to interbreeding for generations or by of a particular sector of population in natural calamity. The original population is founders. Elimination of some alleles and fixation of others is called bottle neck effect. When a few individuals of a population are separated from it and establish as a new successful population in new habitat, the bottle neck effect can be seen. E.g. Parsis who came to India from Persia established their own population in India. 5. Natural selection: Natural selection in neo-Darwinism is driven differential reproduction and comparative reproductive success and not by the survival of fittest as in Darwinism. The best adapted member of the population has better chance to survive and to reproduce than the less adapted one. So greater percentage of the genes for best adaptations will be there in the gene pool of next generation. Thus those are better adapted are fevered. If such differential reproduction continues for generations gradually leads to change in gene frequencies. 6. and isolation: Species is defined as a group of interacting and interbreeding individuals which closely resemble each other and breed to produce fertile progeny. Speciation is formation of one or more new species from existing one. It is initiated by isolation. The isolation may be geographical or reproductive. If some individuals separated by geographical barriers like mountain, ocean etc. Isolated population has its own gene pool and they adapt differently in different geographical conditions. Afters many generations give rise to new species. This is called geographical isolation. A sub population if become reproductively isolated from the parent population then it is called . It may cause by mutation or polyploidy. E.g. Ancon sheep.

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