Notes of CHP 5 XII Biology Maharashtra board Origin and evolution of life

1. Indeed Biology by Vivek Sir
CHP – 5 Origin and Evolution Of Life
Origin of life (Protobiogenesis)
1. Attributes of living organisms : Responsiveness, growth, metabolism, energy transformations and
reproduction.
2. Various theories and hypotheses to propose origin 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 : Big-Bang theory of Georges Lemaitre (1931).
Formation of reducing atmosphere.
 Formation of ammonia, water and methane : Formation of CH4, NH3, H2O and H2S.
 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’.
 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.
 Formation of Nucleic acids : Formation of Nucleotides→nucleic acids (RNA, DNA) →acquired self-
replicating ability→ fundamental property of living form.
 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 protenoids or microspheres (Sidney Fox)
 Coacervates and microspheres were non-living colloidal aggregations of lipids and proteinoids
respectively.
 They turned into eobionts or protocell.
Name of the theory Characteristic point What does the theory say?
Theory of special creation Oldest theory. No scientific proof.
Only religious beliefs.
All living organisms are created by a
supernatural power.
Cosmozoic theory/Theory of
Panspermia
NASA has reported fossils of
bacteria-like organisms on a piece
of Martian rock recovered from
Antarctica.
Life descended to the earth from other
planets in the form of
cosmozoa/panspermia.
Theory of spontaneous generation
(Abiogenesis)
Disproved by Louis Pasteur. Life originated from non-living material
Theory of biogenesis Explains only the continuity of
life.
Living organisms produced from pre-
existing living forms, by process called
reproduction.

2. Indeed Biology by Vivek Sir
 Showed growth and division and hence considered as first primitive living system.
 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.
Urey and Miller’s Experiment:-
 Stanley L. Miller and Harold C. Urey provided the first experimental evidence for Oparin’s chemical
evolution theory.
 The experiment of Urey and Miller was based on the concept of Oparin and Haldane on the origin of life.
 Urey and Miller created the early Earth conditions in a flask in the laboratory.
 They created the primitive earth's atmosphere in the flask having gases such as ammonia, hydrogen, water
vapour, and methane that existed on the primitive earth.
 The mixture of gases was provided with electric discharges for some days which later got condensed and
accumulated in another chamber.
 They observed the presence of simple organic molecules like sugars and amino acids in the condensed
liquid-containing chamber which supported the origin of life from pre-existing simple molecules.
Note:- For diagram refer textbook page no. 96
RNA World Hypothesis :
 First proposed by Carl Woese, Francis Crick and Leslie Orgel in 1960.
 Sidney Altman and Thomas Cech independently found out that RNA can also act as biocatalyst which is
termed as Ribozyme.
 Early life must have been based exclusively on nucleic acids, most probably RNA.
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.

3. Indeed Biology by Vivek Sir
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.
Five main postulates of Darwinism :
 Overproduction (Prodigality of nature)
 It is the natural tendency to produce more number of progeny in geometric ratio for survival of the species.
 Darwin observed potential of many species of plats and animals.
 Eg. Salmon fish produces about 28 lakh eggs in a single season.
 Struggle for existence
 Tendency of production leads to struggle for existence.
 This is due to limited supply of food or space or to escape from enemies etc.
 Organic variations
 Various kinds of differences that occur in morphology, physiology, nutrition, habit behavioural patterns etc
is known as variation.
 Darwin recognizes these variations as raw material for evolution.
 Natural selection (H. Spencer termed natural selection as ‘survival of fittest’)
 Organic variation serves as evidence that for some organisms have better adapted to survive exixting
environmental conditions than others.
 Organisms with favourable variations are selected by the nature while those with unfavourable variations
perish.
 Hence this principle by Darwin is known as Natural selection and named this process as ‘Survival of the
fittest’
 Origin of new species
 As favourable variations are transmitted from generation to generation, successive generations are
become better adapted.
 Gradually these adaptations with few new modifications become fix in the life cycle and finally give rise to
new species.
Evidences Darwinism include :
 Evolution of long-necked Giraffe came to existence.
 Black colour peppered moths evolved gradually as new species.
 DDT resistance.
 DDT resistant mosquitoes.
Drawbacks and Objections to Darwinism :
 Darwin wrongly considered minute non- heritable fluctuating variation as principal factors. They do not
form part of evolution.
 He did not distinguish somatic and germinal variation and considered all variations are heritable.

4. Indeed Biology by Vivek Sir
 He did not explain the ‘arrival of the fittest’. He did not explain the cause, origin and inheritance of
variations and of vestigial organs.
 He could not explain the extinction of species.
 Intermediate form during evolution were not recognised.
 He 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 for 7
generations.
 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 :
 Large, sudden and discontinuous variations, inheritable changes in a population which provide the raw
material for organic evolution.
 Mutation may be useful or harmful. Useful mutations are selected by nature.
 Accumulation of useful mutations over a period of time leads to the origin and establishment of new
species, while harmful mutations are eliminated by nature or can remain in population.
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-
R. Fischer, J. B. S. Haldane, T. Dobzhansky, J. Huxley, E. Mayr, Simpson, Stebbins, Fisher, Sewall Wright,
Medel, T. H. Morgan, etc. together have synthesised the modern theory of evolution.
Stebbins suggested five key factors for evolution : Gene mutations, chromosomal mutations, genetic
recombinations, natural selection and reproductive isolation → together bring about evolution of new species.
Population : All individuals of the same species form population.
‘Mendelian population’ : Small groups of interbreeding populations.
Gene pool : Gene pool is the total genetic information or sum total of genes of all individuals in a Mendelian
population.
Factors affecting gene pool : Migration, replacement of one generation by another in the Mendelian population.
Gene frequency : The proportion of an allele in the gene pool, to the total number of alleles at a given locus.
Factors of Modern synthesis theory:

5. Indeed Biology by Vivek Sir
Gene Mutation :
 Sudden permanent heritable change in the genetic material is called mutation.
 Single gene mutations are called point mutations.
 Chromosomal aberrations and ploidy too cause mutations.
 All mutations lead to variations.
Genetic recombination : Crossing over in sexually reproducing organisms cause variations , during gamete
formation. Crossing over also create recombination.
Crossing over : Exchange of genetic material occurs between non-sister chromatids of homologous chromosomes.
Gene flow : Movement of genes into or out of a population. Migration of organism, or gametes (dispersal of
pollens) or segments of DNA (transformation).
Genetic drift : Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure
chance.
Chromosomal aberrations : The structural, morphological change in chromosome due to rearrangement cause
chromosomal aberrations.
Types of aberrations :
 Deletion : Loss of genes from chromosome.
 Duplication : Repetitions of genes or doubling of chromosome number.
 Inversion : Sequence of the genes get inverted due to 180° twist.
 Translocation : Transfer (transposition) of a part of chromosome or a set of genes to a non-homologous
chromosome.
Natural selection :
 Main driving force behind the evolution.
 It brings about evolutionary changes by favouring differential reproduction of genes that bring about
changes in gene frequency from one generation to next generation.
 The ‘fittest’ (well adapted) survives and leaves more progeny.
 E.g. Industrial melanism in peppered moth (Biston betularia) is example of natural selection.
Isolation :
The separation of the population of a particular species into smaller units which prevents interbreeding between
them is isolation.
Isolating mechanisms : Barrier which prevents gene flow or exchange of genes between isolated populations.
Type of isolating mechanisms :
 Isolation is the separation of the population of a species or different species due to some barriers that can
be natural or genetic.
 Whenever the isolation occurs due to geographical barriers then it is called geographical isolation.
 When the isolation occurs due to genetic barriers or reproductive barriers then it is called reproductive
isolation.
There are two types of reproductive isolation:
 Pre-zygotic or premating reproductive isolation
 Whenever there is no fertilisation due to reproductive isolation, then it is called prezygotic reproductive
isolation

6. Indeed Biology by Vivek Sir
The common reasons for pre-zygotic isolation are:-
 Habitat Isolation :- Different habits of individuals in a population
 Seasonal or Temporal Isolation :- Individuals sexually mature at different times
 Mechanical Isolation :- Different reproductive structures
 Ethological Isolation :- Different mating behavior.
 Post-zygotic or post-mating reproductive isolation
 When the isolation occurs after the formation of the zygote or post-fertilization, then it is called postzygotic
reproductive isolation.
The common reasons for post-zygotic isolation are:-
 Gamete Mortality :- Death of Gamete.
 Zygote mortality :- Death of Zygote.
 Hybrid Sterility :- The sterility of hybrids is also a reason for this type of isolation.
Mechanism of organic evolution-
 During evolution population evolves and not the individual.
 Basic processes which bring about evolution :
Mutations, gene recombination, gene flow (migration), genetic drift, natural selection, isolation and speciation.
Genetic Drift:-
 Any alteration in allele frequency in the natural population by chance is called genetic drift.
 Eg. Elimination of a particular allele from a population due to events like accidental death prior to mating
of an organism.
 The concept of genetic drift was first given by Sewall Wright, and hence also called Sewall Wright effect.
 Genetic drift are random and directionless.
 The effect of genetic drift is more significant in small population than in large population.
 Sometimes, a few individuals become isolated from the large population and they produce a small new
population in new area.
 The original members that drifted and established the new population becomes the ‘founders’ and the
effect is called founder effect.
 The bottle neck phenomenon is also a type of genetic drift and hence known as bottle neck effect.
Hardy-Weinberg’s principle-
 Hardy-Weinberg’s equilibrium/principle/law states that ‘at equilibrium point both the gene (allele)
frequency and genotypic frequency remain constant from generation to generation.’
 This law hold true in diploid, sexually reproducing , large free interbreeding population.
 Representation of Hardy-Weinberg equilibrium:-
 A single locus has two alleles(A and a).
 The frequency of these alleles are p and q respectively.
 The allele frequency for any locus is one. i.e. p + q = 1.
 The genotypic frequency of both the alleles are represented by (p +q)2 = 1
 The binomial expansion of this is p2 + 2pq + q2 = 1
 This can be explained by Punnet Square as follows:-
Hybrid Aa
A (p) a (q)
Hybrid Aa
A (p) AA (P2) Aa (pq)
a (q) Aa (pq) aa (q2)

7. Indeed Biology by Vivek Sir
Adaptive Radiation-
 When many species are evolved or radiated from a single species that live in a geographical area then it is
called adaptive radiation.
 All the radiated species have evolved into different species but they have a common ancestor. The main
reason for adaptive radiation is natural selection.
 Darwin's finches on Galapagos island are the best example of adaptive radiation.
 There are different species of birds having different feeding habits but all of them evolved from seed-eating
species.
 This was observed by Darwin by identifying their beaks.
Evidences of organic evolution-
 Evolutionary evidences are based on following types of disciplines : Palaeontology, comparative anatomy,
embryology and molecular biology
Note:- Refer 12th biology textbook page no’ 106-109
Significance of Palaeontology:-
 It is useful in reconstruction of phylogeny.
 It is helps in studying various forms and structure of extinct animals.
 It provides records of missing links.
 It helps in study of habits of extinct organisms.
Speciation-
Speciation : The process of formation of a new species from the pre-existing species.
Species : A group of similar organisms that can interbreed and produce a fertile offspring in nature.
 Types of speciation:
 Allopatric speciation
 Formation of new species due to separation of a segment of population by geographical isolation.
 Migration of individual also cause allopatric speciation.
 The mode of evolution here is called adaptive radiation.
 Eg. 14 different species of finches in Galapagos islands and several masupials in the Australian continent.
 Sympatric speciation
 Formation of new species due to reproductive isolation.
 Eg. Cichlid fishes in Lake Victoria.
 Mutations are also helpful in sympatric speciation.
Geological time scale –
 It is used to understand the sequence of events that took place on the earth in different ages over a period
of time.
 It is divided into sic major ‘Eras’.
 Each era ended with major environmental changes on earth resulting into extinction of some species and
emergence of new species.
 The eras are further divided into periods and epochs based on minor but landmark events in each era.
Note:- Refer table in 12th Biology textbook page no’ 110, 111

8. Indeed Biology by Vivek Sir
Human Evolution-
1. Major changes in evolutionary development of man :
 Increase in size and complexity of brain and enhanced intelligence with increased cranial capacity.
 Bipedal locomotion, erect posture.
 Opposable thumb.
 Shortening of forelimbs and lengthening of hind limbs.
 Development of chin. Orthognathous face.
 Broadening of pelvic girdle, development of lumbar curvature.
 Articulated speech, art, development of tools, social and cultural development.
Classification of mammals:
(1) Order Primates is divided into two suborders – Prosimii (including lemurs, lorises : and tarsiers) and
Anthropoidea (including : New world monkeys – Ceboidea, Old world monkeys-Cercopithecoidea, Apes and :
Man – Hominoidea).
(2) Hominoidea evolved in Miocene in three separate lines are shown as under :
Hyalobatidae – Gibbons
Pongidae – Gorilla, Chimpanzee and Orangutan
Hominidae – Primates with human characteristics.
4. Palaeontological evidences of human evolution :
The available fossils are skulls, • mandibles, teeth, bones like humerus, femur and stone implements.
5. Important stages in the origin of man :
Ape like stage : Dryopithecus
Men-like stage : Ramapithecus
Connecting link between ape and man : Australopithecus.
Handy man : Homo habilis
Ape man : Homo erectus
Advanced prehistoric man : Homo neanderthalensis (Neanderthal man)
Modern man : Homo sapiens