Notes-Part-1-Class-12-Biology-Chapter-5-Origin and Evolution of Life -Maharashtra Board

Chapter-5-Origin and Evolution of Life

Maharashtra Board-Class-12th-Biology-Chapter-5


Topics to be learn in in Part-1

  • Origin of life (Protobiogenesis)
  • Chemical Evolution
  • Organic Evolution
  • Darwinism
  • Mutation Theory of Life (Self-assembly theory of origin of life)
  • Modern synthetic theory of evolution
  • Mechanism of organic evolution

Topics to be learn - Part-2 

  • Hardy-Weinberg’s principle
  • Adaptive radiation
  • Evidences of organic evolution
  • Speciation
  • Geological time scale
  • Human Evolution

Origin of life (Protobiogenesis) : The living matter shows attributes or characters like responsiveness, growth, metabolism, energy transformations and reproduction.

Various theories and hypotheses to propose origin of life :

(i) Theory of special creation : All living organisms are created by a supernatural power.

  • Oldest theory. No scientific proof. Only religious beliefs.

(ii) Cosmozoic theory/Theory of Panspermia : Life descended to the earth from other planets in the form of cosmozoa/panspermia.

  • NASA has reported fossils of bacteria-like organisms on a piece of Martian rock recovered from Antarctica.

(iii) Theory of spontaneous generation (Abiogenesis) : Life originated from non-living material

  • Disproved by Louis Pasteur.

(iv) Theory of biogenesis : Living organisms produced from pre-existing living forms, by process called reproduction.

  • Explains only the continuity of life.

Chemical Evolution of Life (Self assembly theory of origin of life ) :

Theory Of biochemical origin of life : Life originated on earth by combinations of several chemicals through Constant chemical reactions over a long period of time.

  • Formulated by Haeckel, developed by Alexander I. Oparin (1924) and J. B. S. Haldane (1929).

The steps in the process of chemical evolution :

  • Origin of Earth and Primitive atmosphere.
  • Formation of ammonia, water and methane.
  • Formation of simple organic molecules.
  • Formation of complex organic molecules.
  • Formation of Nucleic acids.
  • Formation of Protobionts or Procells.
  • Formation of first cell.

(i) Origin of Earth and Primitive atmosphere : Big-Bang theory of Georges Lemaitre (1931). Formation of reducing atmosphere.

Explanation :

Explanation :

  • Earth originated about 4.6 billion years ago as a part of the solar system.
  • When it was formed, it was a rotating cloud of hot gases and cosmic dust. It was then appearing like a nebula.
  • Later the condensation and cooling started which resulted in stratification.
  • Heavier elements like nickel and iron settled to the core. Lighter elements like helium, hydrogen, nitrogen, oxygen, carbon, etc. remained on the surface and they formedthe primitive atmosphere.
  • This atmosphere of the earth was of a reducing type, devoid of free oxygen and very hot.


(ii) Formation of ammonia, water and methane : The early atmosphere was rich in hydrogen,carbon, nitrogen and sulphur. Hydrogen was most active and hence it reacted with other elements to form chemicals on earth like CH4, NH3, H2O and H2S.

(iii) Formation of simple organic molecules : Formation of monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc. Formation of water bodies resulting into ‘hot dilute soup’ or ‘primitive broth’.

Explanation :

Explanation :

  • Initially earth’s temperature was very high but as the cooling process started, lighter elements reacted chemically with each other.
  • The early atmosphere was rich in hydrogen, carbon, nitrogen and sulphur. Hydrogen was most active and hence it reacted with other elements to form chemicals on earth like CH4, NH3, H2O and H2S.
  • With decreasing temperature of the earth, steam condensed into water that resulted in heavy rainfall. This constantly falling rainwater got accumulated on the land to form different water bodies and especially oceans. It also cooled down the earth.
  • The early molecules of hydrocarbons, ammonia, methane and water underwent reactions like condensation, polymerisation, oxidation and reduction due to different energy sources such as ultra-violet rays, radiations, lightning and volcanic activities.
  • These reactions resulted in formation of simple organic molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc.


(iv) Formation of complex organic molecules: Formation of complex organic molecules like polysaccharides, fats, proteins, nucleosides and nucleotides. Protoproteins —>proteins. Formation of protein molecules : Landmark in the origin of life.

Explanation :

Explanation :

  • The primitive broth in which simple organic molecules were suspended, was neutral and free from oxygen.
  • In this broth polymerisation took place and simple organic molecules aggregated to form new complex organic molecules like polysaccharides, fats, proteins, nucleosides and nucleotides.
  • Protoproteins were formed by polymerization of amino acids. These protoproteins later formed proteins.
  • Formation of protein molecules is considered as landmark in the origin of life. Later the enzymes were formed which accelerated the rate of other chemical reactions.


(v) Formation of Nucleic acids: Formation of Nucleotides —>nucleic acids (RNA, DNA) —> acquired self-replicating ability -> fundamental property of living form.

(vi) Formation of Protobionts or Procells : First form of life called protobionts was formed from nucleic acids by coacervation.

  • Protobionts : Prebiotic chemical aggregates having some properties of living system.
  • Protobionts also called Coacervates (Oparin) and pi-otenoids or microspheres
  • (Sidney Fox)
  • Coacervates and microspheres were non-living colloidal aggregations of lipids and proteinoids respectively. They turned into eobionts or protocell.
  • Showed growth and division and hence considered as first primitive living system.

Explanation :

Explanation :

  • By the reaction between phosphoric acid, sugar and nitrogenous bases (purines and pyrimidines), nucleotides may have been formed.
  • These nucleotides joined together to form nucleic acids such as RNA and DNA.
  • Nucleic acids acquired self-replicating ability which is a fundamental property of living form.
  • They later formed protobionts. They were the first form of life formed by nucleic acids along with inorganic and organic molecules.
  • Protobionts were the prebiotic chemical aggregates having some properties of living system. Aggregation of organic molecules due to coacervation formed these protobionts.


(vii) Formation of first cell : First cell developed by formation of RNA and DNA system. First cell was anaerobic, heterotrophic and obtained energy by chemoheterotrophic processes.

Stanley L. Miller and Harold C. Urey provided the first experimental evidence for Oparin's chemical evolution theory.

Urey and Miller’s experiment :

Urey and Miller’s experiment :

Urey and Miller performed an experiment to prove Oparin’s theory of chemical evolution.

  • They selected a spark discharge apparatus that consisted of closed system of glass having tungsten electrodes, flask for water boiling, a side tube connected to a vacuum pump, a cooling jacket and U-shaped trap.
  • The entire apparatus was first evacuated and made sterile and pre-biotic atmosphere was created in it.
  • The flask was filled with some water and mixture of methane, ammonia and hydrogen in the ratio of 1 : 2 : 2 were slowly passed through the stopcock, without allowing air.
  • Heat was supplied to the flask at very low temperature causing water to boil. The flask simulated the ocean present on primitive earth. Process of evaporation and precipitation was simulated by using heating mantle and condenser respectively.
  • Water vapours along with other gases were circulated continuously through continuous electric sparks. These sparks were given to the mixture for several days causing the gases to interact. This too simulated lightning.
  • Mixture of CH4, NH3 and H2 gases passed through a condenser and was condensed to liquid.
  • The liquefied mixture was collected in the U-shaped trap, present at the bottom of the apparatus. It was found that variety of simple organic compounds (urea, amino acids, lactic acid and sugars) were formed in the apparatus.

This experiment provides the evidence in support to the fact that simple molecules present in the earth's early atmosphere combined to form the organic building blocks of life.


RNA World Hypothesis :

RNA World hypothesis is based on discovery of catalytic RNA or ribozymes. It was proposed by Carl Woese, Francis Crick and Leslie Orgel in 1960 whereas Ribozymes were discovered by Sidney Altman and Thomas Cech in 1980.

According to this hypothesis, early life must have been based most probably on RNA.

Factors supporting this hypothesis :

Factors supporting this hypothesis are :

  • RNA is found abundantly in all living cells.
  • It is structurally related to DNA.
  • Chains of RNA can evolve or undergo mutations, replicate and catalyse reactions.
  • Biomolecules like Acetyl-Co-A have a nucleotide in their molecular structure.
  • Ribosome acts as a protein assembly unit in the cell and is seen in many types of cells.
  • In ribosomes, translation process is catalysed by RNA.

The primitive molecules underwent repeated replication and mutation forming varieties of RNA molecules with varying sizes and catalytic properties.

They later developed their own protein coats and machinery to survive the assembly of primitive cell.

From them DNA was developed which was double stranded stable structure.

It further kept on evolving giving rise to rich biodiversity on earth.


Organic Evolution :

Evolution (Latin word, e=from; volvere=to roll) : The act of unrolling or unfolding of nature.

  • Organic evolution : Slow, gradual, continuous and irreversible changes through which the present-day complex forms of the life developed (or evolved) from their simple pre-existing forms.
  • Charles Darwin’s definition of evolution: ‘Descent with modification’.
  • Lamarck’s theory (Theory of origin of acquired characters/Inheritance of acquired characters and use and disuse of organs) : The traits are acquired due to internal force, changes in environment, new needs and the use and disuse of organs. This gives rise to new species after several generations. Lamarckism was disproved by August Weismann.
  • Weismann’s theory of Germplasm : Variations produced in somatic cells (somatoplasm) are not inherited while variations produced in germ cells (germplasm) are inherited to next generation.

Darwinism :

  • Darwinism or theory of origin of species by Natural Selection.
  • Darwin’s book: ‘The Origin of species by Natural Selection‘ wrote in 1859 after observations of variations between the tortoises and finches on Galapagos islands.
  • C. Lyell’s viewpoint on which Darwin’s theory was based : The natural forces that existed in the past are same as those existing at present.
  • Wallace also made similar observations.
  • R. Malthus provided the idea that increase in human population leads to competition and struggle for existence of human species.

Darwinism and its five main postulates :

Darwinism and its five main postulates :

Darwinism means theories of natural selection and speciation as put forth by Charles Darwin. The five main postulates of his theories are as follows : Overproduction or prodigality, Struggle for existence, Organic variations, Natural selection, Origin of new species (speciation).

  • Overproduction (Prodigality of nature) :There is a natural tendency to produce more number of progeny in geometric ratio for continuing the species. E.g. Salmon fish produces about 28 lakh eggs in a single season. Single pair of elephants would produce 19,000,000 elephants. But the size of given species in a given area remains relatively constant because of fluctuations that occur seasonally.
  • Struggle for existence : Due to over-production there is struggle for existence between the members of population for limited supply of food or to overcome adverse environmental conditions or for a space or to escape from enemies, etc.
  • Organic variations : There are differences in morphology, physiology, nutrition, habit, behavioural patterns, etc., among the members of same species or members of different species. These variations act as raw material for evolution.
  • Natural selection : Some organisms possess better variations to get adapted and survive under existing environmental conditions, while some do not have. Better adapted organisms are selected by the nature while those with unfavourable variations perish. The principle by which useful variations are preserved by nature, is called ‘Natural Selection’. It is also called ‘survival of fittest’ by H. Spencer.
  • Origin of new species (speciation) : Favourable variations are transmitted from generation to generation, resulting into better adapted generations. Gradually these adaptations with few new modifications become fixed in the life cycle, forming a new species.


Evidences of Darwinism :

Evidences of Darwinism:

  • Height of neck of Giraffe : Long-necked Giraffe came into existence in the following way. Long-necked Giraffe could pluck and eat more leaves from tall trees and woody climbers. So it was well adapted to the environment. Short-necked one could not get food and thus perished in the struggle. This adaptation was transmitted to their offspring.
  • Black colour peppered moths : The example of industrial melanism seen in U.K. is an excellent example of natural selection in action. Black coloured moths evolved gradually as new species from the previous white coloured forms.
  • DDT resistance in mosquitoes: Intensive DDT spraying destroyed all types of mosquitoes. Some mosquitoes developed resistance to DDT and survived in spite of DDT spray. They reproduced more and were thus selected naturally.


Drawbacks and Objections to Darwinism :

Drawbacks and Objections to Darwinism :

  • Darwin took into consideration minute fluctuating variation as principal factors. But these are neither heritable nor are part of evolution.
  • Darwin did not distinguish somatic and germinal variation and considered all variations are heritable.
  • Arrival of the fittest’ was not explained by him.
  • Darwin was unable to explain the cause, origin and inheritance of variations and of vestigial organs. He also could not explain extinction of species.
  • Gradual accumulation of useful variations forms the new species, but their intermediate forms were not recognised.
  • Darwin could not explain existence of neutral flowers and the sterility of hybrids.


Mutation Theory :

Hugo de Vries proposed mutation theory based on his observations on Oenothera lamarckiana.

Though offspring resemble their parents in many characters, some sudden and spontaneous variations are seen in them, which is said to be mutations or discontinuous variations.

Main features of mutation theory :

  • Mutations are large, sudden and discontinuous variations in a population.
  • These changes are inheritable.
  • Mutations provide the raw material for organic evolution. ‘
  • Mutation may be useful or harmful. Useful mutations are selected by nature.
  • Accumulation of these mutations over a period of time leads to the origin and establishment of new species.
  • Harmful mutation may persist or get eliminated by nature.

Objections to Mutation Theory :

  • The large and discontinuous variations were chromosomal aberrations which bring about minor changes.
  • Rate of mutation is very slow.
  • Chromosomal aberrations are unstable and hence not important in evolution.

Speciation (Formation of new species) :

  • Small Darwinian variations are directional.
  • Variations due to mutations are large, sudden, random.
  • Darwin’s opinion : Gradual, inheritable variations over a long period of time, lead to speciation.
  • De Vries’s opinion : Mutations cause speciation.
  • Saltation : A single step large mutation.

 Modern Synthetic Theory of Evolution :

  • Modern synthetic theory of evolution is the result of modification of Darwinism and theory of mutations by taking into consideration studies of genetics, ecology, anatomy, geography and palaeontology.
  • Five key factors of modern synthetic theory are gene mutations, mutations in the chromosome structure and number, genetic recombinations, natural selection and reproductive isolation. All these finally contribute in the evolution of new species or process of speciation.
  • Population or Mendelian population is the small group of ‘interbreeding populations".
  • For every Mendelian population there is a gene pool which is constituted by total number of genotypes in it. The genotype of an organism in a population is constant, but the gene pool constantly undergoes change due to different factors such as mutations, recombination, gene flow, genetic drift, etc.
  • Every gene has two alleles. The proportion of a particular allele in the gene pool, to the total number of alleles at a given locus, is called gene frequency. Thus any change in the gene frequency in the gene pool affects population.

The five main factors are broadly divided into three main concepts as follows :

(i) Genetic variations :

(i) Genetic variations : The change in gene and gene frequencies is known as genetic variation. Genetic variations are caused by following factors :

  • Mutations : Sudden permanent heritable change is called mutation. Mutation can occur in the gene, in the chromosome structure 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, causing variations.
  • Genetic recombination : In sexually reproducing organisms, during gamete formation, exchange of genetic material occurs between non-sister chromatids of homologous chromosomes. This is called crossing over. It produces new genetic combinations which result in variation. Fertilization between opposite mating gametes leads to various recombinations resulting into the phenotypic variations. These result in change in the frequencies of alleles.
  • Gene flow : 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.
  • Genetic drift : Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift. For example, when the size of a population is severely reduced due to natural disasters like earthquakes, floods, fires, etc. elimination of particular alleles from a population becomes possible. Smaller populations have greater chances for genetic drift. It results in the change in the gene frequency. Genetic drift is also an important factor for evolutionary change.
  • Chromosomal aberrations : The structural, morphological change in chromosome due to rearrangement of genes is called chromosomal aberrations. Due to changes in the gene arrangement or gene sequence variations are caused.


(ii) Natural selection :

(ii) Natural selection is said to be the main driving force in evolution. It brings about evolutionary changes by selecting favourable gene combinations by differential reproduction of genes. This brings about changes in gene frequency from one generation to next generation.


(iii) Isolation :

(iii) Isolation means the separation of the population of a particular species into, smaller units which prevents interbreeding between them. This over a long time period leads to speciation or formation of new species.


Isolating mechanisms : Barrier which prevents gene flow or exchange of genes between isolated populations.

Due to isolating mechanisms in nature the divergence among organisms takes place gradually leading to speciation. The isolating mechanisms are of two types namely, geographical isolation and reproductive isolation.

Types of isolating mechanisms :

(i) Geographical Isolation : The barrier in the form of physical distance or geographical barrier is called geographical isolation.

  • The original population gets divided into two or more groups by geographical barriers such as river, ocean, mountain, glacier, etc.
  • Organisms cannot cross the barriers on their own and hence interbreeding is prevented between isolated groups.
  • The separated groups experience different environmental factors and they acquire new traits by mutations.
  • The separated populations develop distinct gene pool and they do not interbreed. Each subgroup then evolves differently which results into formation of new species. E.g. Darwin’s Finches, African elephant, Loxodonta and Indian elephant, Elephas.

(ii) Reproductive Isolation : Two populations may be occupying the same area, they may not be separated by geographical barrier, but then also they are reproductively isolated.

  • Such reproductive isolation occurs due to change in genetic material, gene pool and structure of genital organs.
  • Such differences prevent interbreeding between population. Such isolation later leads to speciation.


Different types of reproductive isolations :

Reproductive isolation is of two types, viz., pre-zygotic and post-zygotic isolating mechanisms.

Pre-zygotic isolating mechanisms :

Pre-zygotic isolating mechanisms :

Pre-zygotic or Pre-mating isolating mechanisms do not allow individuals to mate with each other at all.

By various mechanisms the two groups remain isolated. These mechanisms are of following types :

  • Habitat isolation : Habitat isolation is the phenomenon in which members of a population living in the same region occupy different habitats. Hence the potential mates do not interbreed among themselves.
  • Seasonal isolation : In seasonal isolation, members of a population share the same region but attaining sexual maturity at the different times of the year. They thus remain isolated reproductively preventing interbreeding among themselves.
  • Ethological isolation : Ethological isolation is seen when members of two populations have different mating behaviours. This prevents interbreeding.
  • Mechanical isolation : Mechanical isolation is seen when the members of two populations have differences in the structure of reproductive organs. Due to such differences interbreeding is not possible.


Post-zygotic isolating mechanisms :

Post-zygotic isolating mechanisms :

In post-zygotic or post-mating isolating mechanisms, the two individuals can mate but the result of mating is not favourable.

Thus the populations remain isolated without the actual genetic exchange.

Post-mating isolating mechanisms are divided into the following categories :

  • Gamete mortality :ln gamete mortality there is death of gametes. Sperm transfer may take place but the egg is not fertilized due to gamete mortality.
  • Zygote mortality : In zygote mortality, the zygote is formed but it fails to thrive. Though the egg is fertilized the zygote does not survive.
  • Hybrid sterility : In this isolation, there is the formation of hybrid as the gametes or zygotes do not die but the hybrid formed is sterile. Sterile hybrid cannot contribute genetically to further generations.


Mechanism of organic evolution :

It is the population that evolves and not its individual members. Individual’s role is to pass its genetic variation to its offspring.

The following are the basic processes which bring about evolution viz. Mutations, gene recombination, gene flow (migration), genetic drift, natural selection, isolation and speciation.

Meaning and Action on Population :

Meaning and Action on Population :

Mutations : Permanent heritable changes in the genetic material of an organism.

  • New alleles are added to gene pool through mutations.

Gene recombination : Alleles combine during sexual reproduction producing recombinations.

  • Due to random union of gametes, anaphasic separation of chromosomes and crossing over and the recombinations, variations are produced.

Gene flow : Emigration and immigration causing transfer of gene during interbreeding of genetically different populations.

  • Brings about changes in the allele frequency.

Genetic drift Or Sewall wright effect Or Founder effect : Alteration in allelic frequency in the natural population by chance.

  • Random genetic drifts cause significant effect in small population. The original drifted population becomes ‘founders’.

Natural selection : 3 types Stabilizing -Directional –Disruptive : Better adapted individuals with useful variations are selected by nature and leave greater number of offspring.

  • Differential reproduction. Adaptation and then speciation.

Isolation : Separation of a single interbreeding population into subunits.

  • Restricts gene flow between discrete population due to different barriers.

Speciation : The subunits of single interbreeding population are broken clown into species.

  • Prevention of interbreeding in groups of population later forms species.


Type of Natural selection :

Type of Natural selection :

(i) Stabilizing selection : (Balancing selection)

  • Stabilizing selection is the type of natural selection which balances the population, hence it is also known as balancing selection.
  • In such population more individuals acquire a mean character value.
  • Such selection tends to favour the intermediate forms and eliminate both the phenotypic extremes.
  • E,g. More number of infants with intermediate weight survive better as compared to overweight or underweight infants.
  • Stabilizing selection reduces variations.
  • It tends to maintain phenotypic stability within population, and does not bring about drastic evolutionary changes.
  • A population showing stabilizing selection is well-adapted to its environment.

(ii) Directional selection :

  • Natural selection bringing about directional change Without disrupting the balance is called directional selection.
  • In a population when more individuals acquire characters which are other than the mean character value, then it is called directional selection.
  • Natural selection usually acts to eliminate one of the extremes of the phenotypic range and favour the other. E.g. systematic elimination of homozygous recessives.
  • Directional selection operates for many generations, it results in an evolutionary trend within a population and shifting a peak in one direction.
  • E.g. Industrial melanism, DDT resistant mosquito, etc.

(iii) Disruptive Natural selection :

  • Here more number of individuals acquire peripheral character value at both ends of the distribution curve.
  • Nature select extreme phenotypes and eliminate intermediate. Hence two peaks are formed in distribution of traits.
  • This kind of selection is rare.
  • It ensures the effect on the entire gene pool of a population, considering all mating types or systems.

Example of disruptive selection :

  • African seed cracker finches are types of seed-feeder birds which have different sizes of beak. The seeds available to them were of small and large sized.
  • Large beak sized birds feeds on large seeds While small beak sized birds feed on small seeds. Such large and small birds thus thrive well.
  • However, intermediate beak sized birds are unable to feed on either type of seeds so they starve and their population was decreased gradually.
  • Natural selection eliminated them and thus the population of finches appear disrupted.


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  1. Good notes but concepts should be explained brodly for understanding it.

    1. Thanks for your valuable suggestion. We will surely think about it.

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