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Maharashtra State BoardHSC Science (General) 12th Standard Board Exam
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टाइम टेबल३२
अभ्यासक्रम

Overview of Origin and Evolution of Life

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Maharashtra State Board: Class 12

Key Points: Origin of life

Living organisms show basic characteristics such as metabolism, growth, reproduction, energy transformation and responsiveness.

The origin of life is still a mystery and many scientific fields have tried to explain how life began on Earth.

  1. The theory of special creation states that all life was created by a supernatural power, but it has no scientific evidence.
  2. The cosmozoic (panspermia) theory suggests that life came to Earth from outer space in the form of spores or microorganisms, but it does not explain the origin of life elsewhere.
  3. The theory of spontaneous generation (abiogenesis) proposed that life arose from non-living matter, but this idea was disproved by Louis Pasteur.
  4. The theory of biogenesis states that life arises only from pre-existing life, explaining continuity of life but not the origin of the first living organism.
Maharashtra State Board: Class 12

Key Points: Chemical Evolution of Life

Stage Process Main Events Key Scientists / Terms
Chemical evolution Origin of life by chemical reactions Life arose from non-living chemicals through gradual reactions over long time Haeckel, Oparin, Haldane
Origin of Earth & atmosphere Formation of primitive Earth Earth formed ~4.6 billion years ago; atmosphere was reducing and lacked free oxygen Big Bang – Lemaitre
Formation of simple gases Chemical combination of elements Formation of CH₄, NH₃, H₂O, H₂S from H, C, N, S Reducing atmosphere
Simple organic molecules Abiotic synthesis Amino acids, sugars, fatty acids formed using UV rays, lightning, heat “Primordial soup” – Haldane
Complex organic molecules Polymerisation Formation of proteins, fats, polysaccharides, nucleotides Proteins acted as enzymes
Nucleic acids Self-replication RNA and DNA formed; gained ability to replicate Basis of heredity
Protobionts / Procells Pre-cellular structures Coacervates and microspheres formed showing growth and division Oparin (coacervates), Fox (microspheres)
First living cell Origin of true cells First cells were anaerobic, heterotrophic and chemo-heterotrophic Protocells / Eobionts
Maharashtra State Board: Class 12

Key Points: Organic Evolution

  • Meaning of evolution – Evolution means gradual and irreversible change in organisms, resulting in present-day complex forms from simpler ancestral forms.
  • Organic evolution – Organic evolution is a slow, continuous process through which descendants become different from their ancestors over generations.
  • Darwin’s view – According to Charles Darwin, evolution is “descent with modification”, where variations are inherited and lead to formation of new species.
  • Weismann’s contribution – August Weismann disproved inheritance of acquired characters and proposed the theory of continuity of germplasm, stating that only germ cell variations are inherited.
Maharashtra State Board: Class 12

Key Points: Modern Synthetic Theory of Evolution

  1. Meaning – Modern Synthetic Theory of Evolution is a combined explanation of evolution based on genetics, ecology, paleontology, anatomy and natural selection.
  2. Major contributors – Important contributors include R. A. Fisher, J. B. S. Haldane, T. Dobzhansky, Julian Huxley, Ernst Mayr, G. G. Simpson and Stebbins.
  3. Population concept – Evolution occurs in populations; a small interbreeding group within a species is called a Mendelian population.
  4. Gene pool and gene frequency – The total genes present in a population constitute the gene pool, and the proportion of an allele in the gene pool is called gene frequency.
  5. Factors causing evolution – Gene mutations, chromosomal changes, genetic recombination, migration and natural selection alter the gene pool.
  6. Speciation – Reproductive isolation along with genetic variation and natural selection leads to the formation of new species.
Maharashtra State Board: Class 12

Key Points: Genetic variations

  1. Genetic variation refers to changes in genes and gene frequencies within a population, leading to differences among individuals.
  2. Gene mutation is a sudden, permanent and heritable change in a gene or chromosome, producing new traits and phenotypic variation.
  3. Genetic recombination occurs during sexual reproduction due to crossing over and fertilization, resulting in new gene combinations.
  4. Gene flow is the movement of genes into or out of a population through migration, gamete transfer or DNA transfer, altering gene frequencies.
  5. Genetic drift is a random change in allele frequencies due to chance events, especially significant in small populations.
  6. Chromosomal aberrations involve changes in chromosome number or structure (deletion, duplication, inversion and translocation), causing genetic variation and evolutionary change.
 
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