II PUC BIOLOGY EVOLUTION

CHAPTER 7
EVOLUTION
Evolution is an orderly change from one form to another.
Evolutionary Biology is the study of evolutionary history of life forms.
ORIGIN OF LIFE:
The Big Bang theory attempts to explain the origin of universe.
The Big Bang theory:
 Big Bang Theory states that universe originated about 20 billion years ago by
a singular huge explosion.
 The universe expanded and hence the temperature came down.
 Hydrogen and Helium formed sometime later.
 The gases condensed under gravitation and formed the galaxies of the
present day universe.
 In the solar system of the Milky Way galaxy, earth was supposed to have been
formed about 4.5 billion years back.
Condition of early earth:
 Earth formed 4.5 billion years back.
 There was no atmosphere on early earth. Water vapour, methane, carbon
dioxide and ammonia released from molten mass covered the surface.
 The UV rays from the sun broke up water into Hydrogen and oxygen and
lighter H2 escaped.
 Oxygen combined with ammonia and methane to form water, CO 2 and others.
 The ozone layer was formed. As it cooled, the water vapour fell as rain, to fill
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all the depressions and form oceans.

 Life appeared 500 Million years after the formation of earth i.e, 4 billion years
ago.
THEORIES OF ORIGIN OF LIFE:
1. Panspermia theory: Early Greek thinkers thought units of life
called spores were transferred to different planets including earth from outer
space.
2. Theory of spontaneous generation (Abiogenesis): This theory was
proposed by Greek philosophers and was supported by Aristotle. According to
this theory, life was originated from non-living materials spontaneously and
repeatedly. It was believed that life arose from the decaying and rotting
matter like straw, mud etc. This was the theory of spontaneous
generation.
3. Theory of biogenesis/Louis Pasteur experiment: Careful
experimentation demonstrated that life comes only from pre-existing life. He
showed that in pre-sterilized flasks, life did not come from killed yeast while in
another flask open to air, new living organism arose from ‘killed yeast’. This
disproved the theory of spontaneous generation.
4. Theory of organic evolution/ Oparin – Haldane theory of origin of
life:
 Oparin of Russia and Haldane of England proposed that
1. The first form of life could have come from pre- existing non-living organic
molecule (e.g. RNA, protein etc.)
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2. Formation of life was preceded by chemical evolution i.e. formation of
diverse organic molecule from inorganic constituents.
Urey and Miller experiment:
 In 1953, S. L. Miller, an American Scientist created similar conditions in a
laboratory scale.
 He created electric discharge in a closed flask to raise temperature up to
8000C as it was in primitive earth.
 He used CH4 H2, NH3 and water vapour inside the flask. (The conditions on
earth were –High temperature, volcanic storms, reducing atmosphere
containing CH4, NH3 etc).
 He observed the formation of amino acids.

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Acceptance of chemical evolution theory: (evidences)

 In similar experiments others observed, formation of sugars, nitrogen bases,
pigment and fats.
 Analysis of meteorite content also revealed similar compounds indicating that
similar processes are occurring elsewhere in space.
Theory of chemical evolution:
 The first non-cellular forms of life could have originated 3 billion years back.
 They would have been giant molecules (RNA, proteins, Polysaccharides, etc).
 These capsules reproduced their molecules perhaps, named as coacervates.
 The first cellular forms of life were probably unicellular.
 All life forms were in water environment only.
EVOLUTION OF LIFE FORMS – A THEORY:
 The conventional religious literature proposed the theory of special
creation to explain the evolution of life forms. This theory has three
connotations as follows:
1. The earth is about 4,000 years old.
2. All the existing organisms have been created at the beginning along the
creation of earth.

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3. The diversity was always the same since creation will be the same in
future.
 Darwin's contribution to evolution:
 Charles Darwin made a keen observations during his sea voyage in a ship
called HMS Beagle round the world and he made the following
conclusions.
1. Evolution of living organisms is a gradual process and nature plays a
major role.
2. Existing life forms share the similarities not only among themselves but
also life forms that are existed in million years ago.
3. The extinction of different life forms in different periods of earth history
is due to arrival of new forms
4. Any population with variation in characters enable the some to survive
better and other to less endowed to survive in natural conditions.
5. The individuals with better reproductive fitness in an environment
produce more progeny and survive more than the others.
 Alfred Wallace contribution to evolution:
 Alfred Wallace a naturalist who worked in Malay Archipelago had
concluded the same ideas as Darwin's natural selection, he given the
following conclusions.
1. All the existing life forms share similarities and share common
ancesters.
2. These ancestors were found at different periods in the history of earth.
3. The geological history of earth is closely correlates with the biological
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history of earth.
[Note: Malay Archipelago: An extensive group of Islands in the Indian and
Pacific oceans, between southeast Asia and Australia.]
EVIDENCES FOR EVOLUTION
1. Palaeontological evidences:
 Palaeontology is the study of fossils.
 Fossils are remnants of life forms found in rocks. Fossils are written documents
of evolution.
Significance of fossils:
a. To study phylogeny (evolutionary history or race history). E.g. Horse
evolution.
b. To study the connecting link between two groups of organisms. E.g.
Archaeopteryx.
c. To study about extinct animals. E.g. Dinosaurs.
d. To study about geological period by analysing fossils in different
sedimentary rock layers. The study showed that life forms varied over
time and certain life forms are restricted to certain geological time spans.
2. Comparative anatomy and morphological evidence:
Comparative anatomy and morphology shows that different forms of animals
have some common structural features. This can be explained as follows:.
a. Homologous organs:

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 Homologous organs are the organs having fundamental similarity in
structure and origin but different functions. This phenomenon is called
Homology.
 Homology indicates common ancestry.
E.g., 1. Homology in animals: Human hand, Whale’s flippers, Bat’s wing
& Cheetah’s foot. These forelimbs have different functions but similar
anatomical structures such as bones (e.g. humerus, radius, ulna, carpals,
metacarpals & phalanges).
E.g., 2. Homology is also seen in hearts or brains of vertebrates.
E.g., 3. Homology in plants: Thorns of Bougainvillea and tendrils of
Cucurbita.
 The origin of homologous organs is due to Divergent evolution. It is the
evolution by which related species become less similar to survive and
adapt in different environmental condition.
b. Analogous organs
 These are the organs having similar function but different structure &
origin. This phenomenon is called Analogy.
E.g., 1. ▪ Wings of insects (formed of a thin flap of chitin) and wings of
birds (modified forelimbs).
2. Eyes of Octopus (retina from skin) and mammals (retina from embryonic
brain).
3. Flipper of Penguins and Dolphins.
4. Sweet potato (modified root) & Potato (modified stem).
5.Trachea of insects (from ectoderm) and lungs of vertebrates (from
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endoderm).
 Origin of analogous organs is due to Convergent evolution. It is the
evolution by which unrelated species become more similar to survive and
adapt in similar environmental condition.
3. Biochemical evidences/evidences from similarities in proteins and
genes.
Similarities in proteins and genes performing a given function among
diverse organisms give clues to common ancestry. These biochemical
similarities point to the same shared ancestry.
4. Embryological support for evolution:
 Proposed by Ernst Haeckel.
 He observed that all vertebrate embryos have some common features that
are absent in adult. –
 E.g. all vertebrate embryos (including human) develop vestigial gill slits just
behind the head. But it is functional only in fish and not found in other adult
vertebrates.
 This was disproved on careful study performed by Karl Ernst von Baer.
He noted that embryos never pass through the adult stages of other
animals.
5. Evidences from artificial selection: Man has bred selected plants and
animals for agriculture, horticulture, sport or security. This intensive breeding

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program has created breeds that differ from other breeds but still are of the
same group.
6. Evolution by natural selection:
 Natural selection is the process of selection of better organisms with
favourable variations by the nature is called natural selection.
 The organisms with favourable variations are supported by the nature to
survive and produce more offspring than the organisms with unfavourable
variations.
Industrial melanism:
 In England, before industrialization (1850s), there were more white-winged
moths (Biston betularia) on trees than dark winged or melanised moths
(Biston carbonaria). After industrialization (1920), more dark-winged moths
and less white winged moths were developed.
Reason:
 Before industrialization: There was white lichens covered the trees. In
that background, white winged moths survived but dark winged moths
were picked out by predators.
 After industrialization: The tree trunks became dark due to industrial
smoke and soot. No growth of lichens. Under this condition the white
winged moth did not survive because the predators identified them easily.
Dark winged moth survived because of suitable dark background.
7. Evolution by anthropogenic action:
 Excess use of herbicides, pesticides etc., has only resulted in selection of
resistant varieties in a much lesser time scale. This is also true for microbes
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against which we employ antibiotics or drugs against eukaryotic

organisms/cell.
 Hence resistance organisms/cells are appearing in a time scale of months or
years and not in centuries.
 Evolution is a stochastic process based on chance events in nature and chance
mutation in the organisms.
ADAPTIVE RADIATION:
The process of evolution of different species in a given geographical area
and radiating to other new habitats is called adaptive radiation.
Ex.1: Darwin finches (Birds): The finches of different islands of Galapagos
are evolved in their beaks from the original seed eating finches of main land to
insectivorous and vegetarian finches. The environment and availability of food
on the various islands of Galapagos were different from those of main land. As
result, variations appeared in their beak to adapt with their habitats.
Ex.2: Australian Marsupials: Different kinds of marsupials are evolved from
a common ancestor, but all are within the Australian island continent.
Ex.3: Placental mammals: Placental mammals in Australia also exhibit
adaptive radiation, where placental mammal appears to be corresponding
marsupial. Ex: Placental wolf and Tasmanian wolf (Marsupial).
BIOLOGICAL EVOLUTION:
1. Lamarckism (Theory of Inheritance of Acquired characters):

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 It was proposed by Lamarck. It states that evolution of life forms occurred by
the inheritance of acquired characters.
 Acquired characters develop by use and disuse of organs.
o Evolution by use of organs: E.g. Long neck of giraffe is due to
continuous elongation to forage leaves on trees. This acquired character was
inherited to succeeding generations.
o Evolution by disuse: E.g. Disappearance of limbs in snakes. This theory
was eliminated out because it is proved that the characters are inherited
only through genes.
2. Thomas Malthus, who worked on populations influenced by the Darwin and
given the conclusions on Natural selection based on certain observations, as
follows.
a. The population size is stable, when natural resources are limited, except
for seasonal fluctuations.
b. Members of a population exhibits variations even though they are
superficially similar.
c. These variations are heritable, which enable some organisms to utilise
resources effectively and produce more progeny.
d. This results in the change in population characters, thus new life forms are
appeared.
MECHANISM OF EVOLUTION
Darwinism (Theory of Natural selection)
 Proposed by Charles Darwin.
 This theory has following conclusions TJP

1. New species arises by natural selection

2. Heritable variations are the sources for evolution.
3. These variations are small and directional.
4. Evolution is slow and gradual process.
 Darwinism is based on 2 key concepts:
o Branching descent
o Natural selection
[Note: Branching descent: The process by which new species originated from
a single common species. One of the two organisms get adapted to a
particular environment results in the formation of new species is called
branching descent.]
MUTATION THEORY:
 This was proposed by Hugo de Vries based on his work on evening
primrose. This theory has following conclusions.
1. New species arises from the existing species due to mutations.
2. Mutations are the sources for evolution and not the minor variations
(Heritable).
3. These mutations are random and directionless.
4. The evolution is sudden process i.e., The origin of new species due to
single step large mutations is called saltation.
HARDY – WEINBERG PRINCIPLE

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 It states that allele frequencies in a population are stable and constant from
generation to generation in the absence of other evolutionary influences. The
gene pool (total genes and their alleles in a population) remains a constant.
This is called genetic equilibrium (Hardy-Weinberg equilibrium).
 Sum total of all the allelic frequencies = 1
 E.g. In a diploid, p and q are the frequencies of alleles A & a respectively.
The frequency of AA = p2
The frequency of aa = q2
The frequency of Aa = 2pq
Hence p2 + 2pq + q2 = 1 [binomial expansion of (p+q) 2 ]
 Change of frequency of alleles in a population causes disturbance in genetic
equilibrium. This is due to evolution.

Factors affecting Hardy-Weinberg equilibrium:

1. Gene migration or gene flow
2. Genetic drift
3. Mutation
4. Genetic recombination
5. Natural selection
1. Gene migration: Gene flow from one population to another. Here gene
frequencies change in both populations. Gene flow occurs if migration
happens multiple times.
2. Genetic drift: The accidental gene flow causing change in frequency.
Sometimes, the change in frequency is so different in the new sample of
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population that they become a different species. The original drifted

population becomes founders and the effect is called founder effect.
3. Mutation: Mutations result in formation of new phenotypes. Over few
generations, this leads to speciation.
4. Genetic recombination: The recombination of genes during gametogenesis
and random fusion of gametes during fertilization leads to the change in allelic
frequency of a population. This results in speciation.
5. Natural selection: The heritable variations in a population enable the
organisms to survive and produce more number of progeny. This bring about
the change in allelic frequency of a favoured variations in a population.
It is of 3 types.
i. Stabilizing selection: Here, more individuals acquire mean character
value and variation is reduced. E.g. consider the body size of organisms.

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ii.Directional selection: Here, individuals of one extreme are more
favoured.
iii. Disruptive selection: Here, individuals of both extremes are more
favoured.

A BRIEF ACCOUNT OF EVOLUTION

The geological time scale includes 4 eras: Proterozoic, Palaeozoic, Mesozoic
& Cenozoic.
1. Proterozoic era: 2500 - 541 million years ago (mya)
 First cellular forms of life appeared (2000 mya).
 Some of the cells had the ability to release O 2 as the light reaction in
photosynthesis.
 Single celled organisms became multicellular organisms.
2. Palaeozoic era (540 - 252 mya) TJP

 It has 6 periods: Cambrian (540 - 490 mya), Ordovician (490 - 443 mya),
Silurian (425 mya), Devonian (405 mya), Carboniferous (360 mya) &
Permian (285 mya).
 500 mya: Invertebrates were formed.
 450 mya: First land organisms (plants) appeared.
 400 mya: Arthropods invaded the land.
 350 mya: Jawless fishes were evolved. Fishes with stout and strong fins
could move on land and go back to water. In 1938, a Coelacanth fish
(lobefins) was caught in South Africa which was thought to be extinct. This
fish was evolved into first amphibians (ancestors of modern day frogs and
salamanders).
 320 mya: Sea weeds and few plants were existed.
 Amphibians evolved to reptiles. They lay thick-shelled eggs (do not dry up
in sun).
 Giant ferns (Pteridophytes) were present but they all fell to form coal
deposits slowly.
3. Mesozoic era (252 - 66 mya) Age of reptiles
 It has 3 periods: Triassic (230 mya), Jurassic (208 mya) & Cretaceous
(144 mya).
 200 mya: Some of the land reptiles went back into water to evolve into
fish-like reptiles (E.g. Ichthyosaurs).

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 The land reptiles were Dinosaurs (Tyrannosaurus rex, Triceratops,
Pteranodon, Stegosaurus, Brachiosaurus etc.) T. rex was the largest
dinosaur (20 feet in height, huge fearsome dagger-like teeth).
 First toothed birds were emerged.
4. Cenozoic era (66 - 0 mya)
 Age of Mammals & Angiosperms. –
 It has 2 periods: Tertiary (66 mya) & Quaternary (2 mya - Age of man).
 65 mya: Dinosaurs suddenly disappeared. Some say climatic changes killed
them. Some say most of them evolved into birds.
 First mammals were shrew-like. Their fossils are small sized.
 In South America, there were mammals resembling horse, hippopotamus,
bear, rabbit etc. Due to continental drift, when South America joined North
America, these animals were overridden by North American fauna.
 Due to continental drift, Australian marsupials survived because of lack of
competition from any other mammals.

ORIGIN AND EVOLUTION OF MAN

1. About 15 mya primates called Dryopithecus and Ramapithecus were
existing. They were hairy and walked like gorillas and chimpanzees.
 Ramapithecus was more man like while Dryopithecus was more ape-
like.
2. Few fossils of man-like bones have been discovered in Ethiopia and Tanzania.
3. Australopithecus:
 Two mya Australopithecines probably lived in East African grasslands.
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 They hunted with stone weapons. Essentially ate fruit.

4. Homo habilis:
 The first human-like being the hominid and was called Homo habilis.
 Brain capacity was between 650 – 800 cc. They did not eat meat.
5. Homo erectus:
 Fossils discovered in Java in 1891 revealed the next stage i.e. Homo
erectus about 1.5 mya.
 Had large brain around 900 cc. They probably ate meat.
6.Neanderthal man:
 Brain size 1400 cc, Lived in east and central Asia between 1, 00,000-
40,000 years back.
 They used hides to protect their body. Buried their dead.
7. Homo sapiens:
 Arose in Africa and moved across continents and developed distinct races.
 During ice age between 75,000-10,000 years ago modern Homo sapiens
arose.
 Pre historic cave art developed about 18,000 years ago.
 Agriculture came around 10,000 years back and human settlement
started.
IMPORTANT DEFINITIONS
1. Origin of life: The appearance of life for the first time on the earth.

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2. Palaeontology: Palaeontology is the branch of science that deals with study
of fossils.
3. Fossils: Fossils are the remains and/or impressions of organisms that lived in
the remote past.
4. Homologous Organs: Homologous organs are organs in different groups of
organisms, which have similar basic structural plan and origin but perform
different functions.
5. Analogous Organs: Analogous organs are those organs in different groups
of organisms, which perform the same function, but are dissimilar in their
basic structural plan and developmental origin.
6. Convergent Evolution: It is the evolutionary process where anatomically
different structures in different groups of organisms evolve towards the
same function.
7. Divergent Evolution: It is the evolutionary process where the same
structure develops along different directions in different groups of
organisms as adaptations to different needs in new habitats.
8. Speciation: Speciation is the process of formation of new species from the
pre-existing species.
9. Gene Pool: The sum total of different kinds of genes (alleles) pooled by all
the members of a population, is called gene pool.
10. Adaptive Radiation: It is an evolutionary process in which a common
stock/ancestor gives rise to new species in a given geographical area
starting from a point and literally radiating to other geographical areas
(habitats). TJP

11. Natural Selection: Natural selection is the process occurring in nature that
acts over a number of generations and slowly increases proportion of those
individuals which are well adapted to the environment due to their heritable
characteristics.
12. Artificial Selection: Artificial selection is the process carried out by man to
select better breeds of plants and animals, which are advantageous to
human beings.
13. Hardy-Weinberg principle: Allelic frequencies in a population are stable
and remain constant from one generation to the next. The sum of allelic
frequencies is always one.
14. Genetic drift: Changes in allelic frequencies due to chance factors.
QUESTIONS FROM PREVIOUS YEAR PAPERS

Year Question Mark

of s
exam
March 1. What is gene pool? 1
2014 2. What are Homologous organs? Mention any two examples. 2
3. List the period, brain capacity and probable food of Homo 3
erectus stage of human evolution.
July 1. What is adaptive radiation? 1
2014 2. What is a homologous structure? Give an example. 2
3. Draw a labelled diagram of diagrammatic representation of 3

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Miller’s experiment.
March 1. Give an example for adaptive radiation. 1
2015 2. Give the diagrammatic representation of Miller’s 5
experiment.
July 1. What is saltation? 1
2015 2. Name the six stages of human evolution in correct 3
sequence.
March What are analogous organs? Give an example. 2
2016 2. State Hardy-Weinberg principle. Mention any two factors 3
that affect Hardy-Weinberg principle.
July 1. Name the theory which attempts to explain ‘Origin of 1
2016 universe’. 2
2. What is adaptive radiation? Give an example. 3
3. Differentiate between homologous and analogous organs
by giving plant example.
March 1. Write any two factors effecting Hardy-Weinberg principle. 2
2017 2. What are analogous organs? Give any two examples 3
July 1. With the help of neat labelled diagram explain Stanly 5
2017 Miller’s experiment in support of chemical evolution.
March 1. Define saltation. 1
2018 2. Mention any two examples of evolution by anthropogenic 2
action. 3
3. State Hardy-Weinberg principle of genetic equilibrium.
Write any two factors affecting the equilibrium.
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July 1. What are analogous organs? Give an example. 2

2018 2. Define adaptive radiation. Give two examples. 3
March 1. Enumerate on convergent and divergent evolution with 2
2019 suitable examples in plants.
2. (a) Draw a neat labelled diagram of Miller’s experiment. 3
(b) Mention two assumptions of Oparin and Haldane with 2
reference to origin of life.
July 1. State Hardy-Weinberg principle. Mention any four factors 5
2019 affecting Hardy-Weinberg equilibrium.
March 1. Name the two primates that were existing 15 mya. 2
2020 2. Sketch and label Miller’s experiment 3
Sept 1. Name the primate who lived in East African grasslands. 1
2020 2. State Hardy-Weinberg principle. Explain any four factors 5
affecting Hardy-Weinberg equilibrium.
April 1. Write any two analogous organs in animals 2
2022 2. Write a detailed note on adaptive radiations 5
July – 1. What is convergent Evolution? 1
2022 2. Describe Millers experiment with a labelled diagram. 5
March 1. Which of the following is used as industrial pollution 1
- 2023 indicator?
a. Lepidoptera b. Lichens c. Lycopersicon d. Lycopodium
2. Mention any two examples of evolution by anthrogenic 2

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action. 3
3. Draw a neat labelled diagram of Miller’s experiment.
May 1. The factor that effects Hardy Weinberg equilibrium is gene 1
2023 flow. 2
2. What are analogous organs? Give an Example. 3
3. Draw a neat labelled diagram of Millers Experiment.
Aug 1. The work of Thomas Malthus on Population influence 1
2023 drawing to explain the concept of natural selection.
2. What is adaptive radiation? Give an example. 2
3. Draw a neat labelled diagram of Millers Experiment. 3
March 1. Which is the correct statement regarding Founder effect? 1
2024 a) Named after the scientist John founder b) No large
change in frequency c) The original drifted population
become founders d) Formation of no species
2. Natural indicator od Industrial pollution is _______. 1
3. List the characteristics of Neanderthal man. 2
4. List the difference between Homologous and Analogous 3
organs. Write one example for each in animals

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