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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , CHAPTER NO: 05, , ORIGIN AND, EVOLUTION OF LIFE,  Modern Synthetic Theory Of Evolution:, o Statement: This theory states that organic evolution was due to, genetic change in population leading to origin of new species. It is a, result of true synthesis of all biological discipline., o Dobzhansky :Book- Genetics and the origin of species, o Julian Huxley: Proposed term modern synthesis., o Stebbins : Stebbins in his book discussed five key factors contributed, in the evolution of new species:, 1. Gene mutations, 2. Mutations in the chromosome structure, and number, 3. Genetic recombination, 4. Natural selection, 5. Reproductive isolation., o Genetic/Mendelian population: A small group of similar, individuals interbreeding among themselves occupying a geographical, area called as Mendelian population. The Mendelian population is, unit of variation or evolution., o Gene pool: The sum total of all genes of all individuals of, interbreeding population called as gene pool or The total genetic, information encoded in sum total of genes in a Mendelian population, is called gene pool., Simply, gene pool means the total number of genes of all, individuals in a population. The gametes produced by individual, 1 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE

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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , furnish a pool of genes of next generation from which the genes will, be selected., The migration of population effectively alters the gene pool., , The gene pool also changes due to replacement of one generation by, another in the Mendelian population. Thus any change in the gene, pool affects population., o Gene frequency: The proportion of an allele in the gene pool, compared with other allele at the same locus c/a gene frequency., Genes are arranged linearly on the chromosome having, definite positions. According to Mendel, every gene that influences a, trait has two alleles. The proportion of an allele in the gene pool, to the, total number of alleles at a given locus, is called gene frequency, , , , Main features/Concepts/Basic principles:, A. Genetic variation, B. Natural selection, C., , A. Genetic variations:, , Isolation, , The change in gene and gene, , frequencies is called as genetic variations. Genetic variation is the raw, material for evolution., , o Factors responsible for genetic variations:, 1. Gene mutations: Sudden permanent change in chemical/genetic, make-up of a gene is called gene mutation or point mutation. Gene, mutation introduces variations in gene pool and causes change in, gene frequency., Mutation can occur in the gene, in the chromosome and, in chromosome number. Mutation that occurs within the single gene,, is called point mutation or gene mutation. This leads to the change in, the phenotype of the organism., , 2 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE

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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , 2. Gene flow: Transfer of genes between populations that differ, genetically from one another. Migration leads to gene flow and causes, change in gene frequency causes evolutionary changes., Gene flow is movement of genes into or out of a, population. Gene movement may be in the form of migration of, organism, or gametes (dispersal of pollens) or segments of DNA, (transformation). Gene flow also alters gene frequency causing, evolutionary changes., , 3. Genetic recombination: Non-sister chromatids of homologous, chromosomes exchange genetic material by crossing over during, gametogenesis and produces new genetic combinations of genes, called as genetic recombination., It results in variation. Fertilization between opposite, mating gametes leads to various recombinations resulting into the, phenotypic variations causing change in the frequencies of alleles., , 4. Genetic drift: Any random fluctuations /alteration in allele, frequency in the natural phenomenon by pure chance is called as, genetic drift., o Sewall Wright Effect: The small populations have greater chances, for genetic drift is called as Sewall Wright effect. It will result in the, change in the gene frequency, [Example: when the size of a population is severely reduced due to, natural disasters like earthquakes, floods, fires, etc. cause elimination, of particular alleles from a population], , 5. Chromosomal aberrations: The structural and morphological, changes in gene arrangement leads to, structural alterations in chromosomes called as chromosomal, aberrations. It changes the genes arrangement (order or sequence), that results in the variation. Chromosomal aberrations occur due to Due to depletion/deletion , duplication, translocation and, inversion ,, , there is variations in the Mendelian population., , 3 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE

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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , a. Deletion: Loss of genes from chromosome., , b. Duplication: Genes are repeated or doubled in number on, chromosome., , c. Inversion: A particular segment of chromosome is broken and, gets reattached to the same chromosome in an inverted position, due to 1800 twist. There is no loss or gain of gene complement of, the chromosome., , d. Translocation: Transfer (transposition) of a part of chromosome, or a set of genes to a non-homologous chromosome is called, translocation. It is effected naturally by the transposons present, in the cell., , B. Natural selection: Natural selection encourages genes with, highest degree of a adaptive efficiency. It is fundamental and operative, mechanism of evolution., Natural selection brings evolutionary changes, by favoring differential reproduction of genes which leads to change in, gene frequency from one generation to another. Natural selection is, the process by which better adapted organisms grow and produce, more number of offspring’s in the population. ., According to Darwin, natural selection is the main driving, force behind the evolution. This holds that genetic variations rise, within the population. The ‘fittest’ able to reproduce and continues to, enjoy greater survival and reproductivity., , 4 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE

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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , Natural selection encourages those genes or traits that, assure highest degree of adaptive efficiency between population and, its environment., , o Industrial melanism [Kettle well]:, , Great Britain, Before Industrial Revolution, Nocturnal in habitat and rest, tree trunks covered by lichens, , Bison betularia, Light colored, Escape from, predatory birds, , After industrial Revolution, , Release of black sooty smoke, Killing of lichens & tree, Peppered moths, trunks with sooty deposits, , Bison carbonaria, , Bison betularia, , Black colored, , Light colored, , Easy victim for, predatory birds, , Easy victim for, predatory birds, , Bison carbonaria, Black colored, Escape from, predatory birds, , High population, Low population, Low population, High population, -----------------------------------------------------------------------------------------------------, , B. Isolation: Separation of a single population into several sub units, , called as isolation., Isolation is the separation of the population of a particular species, into smaller units which prevents interbreeding between them., Each isolated group may develop a set of new traits by out breeding, leads to evolutionary changes and new species occurred., ,  Isolating mechanism: The barriers which prevents gene flow or, exchange of genes between isolated populations, is called isolating, mechanism., Number of isolating mechanisms is operated in nature and, therefore divergence and speciation may occur. geographical isolation, and reproductive isolation., , o Types of isolating mechanisms: A. Geographical isolation, , A.Geographical isolation:, , B. Reproductive isolation., , 1. Geographical barriers [rivers, mountains, ocean, islands formation,, forests] separate original population., 2. Separated population acquires variations by developing mutations and, recombination by exposure to different kinds of environmental factors., 5 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE

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SURYA CLASSES, , LECTURE NOTES : BIOLOGY [PAPER-II], , XII SCIENCE, , 3. The variations processed by natural selection leads to progressive, divergence., 4. The separated population develops distinct gene pool and interbreeding, avoided., 5. The geographical isolation causes formation of new species, 6. Example: Darwin’s finches differentiated in size, color ,beak and food, habits from main land finches., , B.Reproductive isolation:, , 1. Change in genetic material, gene pool and structure of genital organs, leads to reproductive isolation., 2. Reproductive isolation prevents interbreeding between, population[geographically isolated or living close together ], , o Subtypes of reproductive isolation:, a. Premating /Pre zygotic isolating mechanism:, 1. Habitat/Ecological isolation: Members of population living in the, same region occupies different habitats., 2. Seasonal/Temporal isolation: Members of population attain, sexual maturity at different times and prevent interbreeding, 3. Ethological isolation: Members of two populations have different, mating behaviors., 4. Mechanical isolation: Members of two populations have, differences in the structure of reproductive organs., b. Post mating/Post zygotic isolating mechanisms:, 1. Gamete mortality: Sperm transfer takes place but fertilization, avoided., 2. Zygote mortality: Fertilization takes place but zygote dies or, development of zygote into embryo avoided, 3. Hybrid mortality: Hybrids are sterile unable to reproduce., , 6 ORIGIN & EVOLUTION OF LIFE, , | SIGN OUT BY DR. VIRESH SHEKE