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(chapter-7) Evolution  Special creation theory does not have scientific evidences because of which
it is not accepted.
• Evolutionary biology is the study of history of life forms on the earth.
• Origin of life is believed to be a unique event in the history of universe. 2. Theory of Panspermia or cosmozoic Theory:
• The Big Bang theory attempts to explain to us the origin of universe. - According to Richter, life has originated on earth from other planets or
outer space.
The Big Bang Theory - The theory states that protoplasm reached the earth in form of ‘spores’
• A singular huge explosion of very dense matter from a point of singularity. or ‘pansperms’ from some unknown part of universe with some cosmic
• This caused the universe to expand and the temperature to fall drastically. dust and subsequently evolved into various forms of life.
• After some time hydrogen and helium are formed.
• The gases condense due to gravitation. 3. Theory of spontaneous generation or abiogenesis:
• They later formed the galaxies in the universe. - It was also believed that living things originated abiogenetically from
non-living materials like straw.
The Earth and its origin - Louis Pasteur demonstrated the origin of life from pre-existing life.
• Earth was considered to be formed about 4.5 billion years ago, in the solar - He demonstrated that no life originates when yeast broth, which was
system of Milky-way galaxy. heat, killed kept in pre-sterilised close flask.
• Water vapours, methane, carbon dioxide and ammonia released from the - New living organisms originated from the same heat killed yeast that
molten mass covered the surface. was kept in flask left open in air.
• The UV rays from sun broke up water into hydrogen and oxygen and the
lighter hydrogen particles escaped while oxygen combined with methane and 4. Theory of chemical evolution:
ammonia to form water, carbon dioxide etc. - Oparin and Heldane proposed this theory, which is also called as
• The cooled water vapour fell as rain and formed oceans. chemical theory of evolution.
• Oxygen also formed the ozone layer. - This theory states that first life originated from pre-existing, non-living
• Life originated about 500 million years after formation of earth so it organic molecules by chemical evolution.
happened about four billion years ago.
- The conditions on earth favouring chemical evolution were-very high
Theories of origin of life temperature, volcanic storms, reducing atmosphere containing methane,
1. Theory of special creation ammonia, water vapours etc.
- This theory is talk about the creation of life by supernatural powers and it
Experimental proof for chemical evolution of life:
includes three cannotations:
 Stanley Miller and Harold Urey created conditions similar to primitive
i. All living organisms that we see today were created as such.
atmosphere using glass apparatus and tubes in laboratory.
ii. The ,diversity of life was also same since creation and will remain same
 The electric discharge was created by using electrodes in a closed flask
in future also.
containing methane, ammonia, hydrogen and water vapour at 800°C.
iii. The earth is about 4000 years old.
 The chamber containing water was heated to provide water vapour.
 He passes the mixture through a condenser.
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 He circulated the gases continuously in the way for one week and then 1. PALAEONTOLOGY (EVIDENCES FROM FOSSILS)
analysed the chemical  Study of fossils is called Palaeontology and fossils support organic
composition of the liquid inside evolution.
the apparatus.  Study of fossils in different sedimentary layers indicates the geological
period in which they exist.
 He observed a large number of
 The fossils of extinct animals are different from the living forms like in
simple organic compounds and case of dinosaurs and present reptiles.
some amino acids like alanine,  The fossil records have helped in building the broad historical sequence
glycine and aspartic acid. of biological evolution.
 Miller proved that organic  Petrification is the most common method of fossil formation in which
compounds were basis of life, hard parts like bones, shells, teeth and trunk of trees are preserved as
which supported Oparin’s rocks.
 Scientists have developed radioactive disintegration method to
theory.
determine the age of such rocks and fossils.

Origin of Earliest Cell 2. EVIDENCES FROM COMPARATIVE ANATOMY AND MORPHOLOGY

 The first non-cellular form of life was supposed to be originated about  This branch is known as Tectology, it shows similarities and differences
three billion years before. among the present day organisms, and those existed long ago.
 They would have been giant molecules like RNA, proteins or
Homologous organs
polysaccharides.
- The organs having common origin but performing different functions are
 These capsules have the capacity to reproduce and replicate. known as homologous organs and the phenomenon is called homology.
 All forms of life arose in water bodies only. - Homology of organs indicates
 These single cellular forms evolve into complex biodiversity that are seen their common ancestry.
today. - e.g., forelimbs of whale, bat,
birds, amphibians and human
Evolution have basic anatomical
 Evolution simply means the gradual change from earlier simpler forms to structures developed with
the present day complex form. bones humerus, radio ulna,
carpals, meta carpals and
 Organic evolution/Biological evolution is described as ‘descent with
phalanges.
modification’ in organisms of a population over a number of generations. - Heart and brain of vertebrates
is another example for
Evidences of evolution
1. Palaeontology homology.
2. Comparative anatomy and morphology - Among plants, the thorns of Bougainvillea are
3. Molecular homology homologous to tendrils of cucurbita, both arising
4. Biogeography in the axillary position.
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- Homology is the result of Divergent Evolution. (It is The process by which • Human DNA differs in only 1.8% of its base pair from chimpanzee DNA,
different organisms having common ancestors develop different traits or and there is no difference between the amino acid sequence of their
characteristics to adapt to the changing environmental conditions and cytochrome.
needs)
• The molecular structure of actin and tubulin proteins is similar in all
animals, which indicates the common ancestry.
Analogous organs
- The organs having different origin but 4. BIOGEOGRAPHY
performing same function are called • Common ancestry is indicated by the differential geographical
analogous organs and the phenomenon disturbance of different group of organisms in a restricted region.
is called analogy. • Habitat isolation has restricted these organisms to particular geographical
- e.g., i. Eyes of octopus and those of area of earth.
mammals
ii. Wings of butterfly (insects) Connecting links
and birds • The organisms which posses the characters of two different groups are
iii. Flippers of whales and those called connecting links.
of penguins e.g., Euglena and other chlorophyll-containing green protozoans are
iv. Tubers of sweet potato (modified root) and of potato connect the animal kingdom with the plant kingdom.
(modified stem)
Atavism
- Analogy is the result of convergent evolution.
• It is the reappearance of certain ancestral characters which had either
- convergent evolution occurs when species occupy similar ecological
disappeared or were reduced.
niches and adapt in similar ways in response to similar selective pressures.
e.g., In human being some persons have the power to move muscle
pinna, have greatly developed canine teeth, short tail in some babies
Vestigial organs
etc.
- The organs which are present in reduced form and do not perform any
function in the body but correspond to the fully developed functional Evidence from embryology (embryological evidence)
organs of related animals are called vestigial organs. • These evidences are bases on the comparative study of the embryos of
In human – Muscles of pinna, vermiform appendix etc. various animals.
In animals – Wings of flightless birds It states that all the multicellular animals are similar, which indicates their
In plants – Pistilloides in the male flowers of cucurbitaceae. common ancestry. e.g.,
i. The embryos of all vertebrates including human develop a row of
3. MOLECULAR HOMOLOGY
vestigial gill slit but it is a functional organ only in fishes and not found in any
• The similarities in the biomolecules of different groups of organisms is
other adult vertebrates.
called molecular homology.
ii. The development of heart of a mammal or bird is initially two chambered, then
• Molecular homology indicates common ancestry among diverse
three chambered and ultimately four chambered, which shows that they have
organisms.
originated from fishes through amphibians and reptiles.
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Recapitulation Theory and Biogenetic Law: 5. Survival of the fittest

• This theory was proposed by Von Bear, which was renamed as Biogenetic - Nature select only those individuals which are with favourable variations and
Law by Ernst Haeckel. are best adapted to the environment.
• According to this theory, every organism during development repeats the - The less fit and unfit organisms are destroyed.
evolutionary history of its race. 6. Formation of new species
• According to Haeckel, ontogony (development of the embryo) is - Useful variations are transmitted to the offspring and appear prominently in
recapitulation of phylogeny (development of race). further generations.
e.g., The tadpole larva of frog is aquatic and has fish like characters, which - Thus variations keep on accumulating.
indicates that frog has evolved from fishes. - After a number of generations, the offspring attain certain character to form
a new species.
Darwinism: (Theory of natural selection) - Origin of new species by gradual modification of older ones is called
• Darwin formulated the theory of natural selection to explain the speciation.
evolution.
• The main features of the theory of natural selection are Examples of Natural selection
1. Over production (Rapid multiplication) 1. Industrial Melanism
- All living beings have enormous fertility and they multiply in geometric 2. Resistance of Insects to pesticides
ratio. 1. Industrial Melanism
2. Limited food and space - It is an adaptation where the moths living in an industrial area develops
- The source of food and space for plants and animals remains constant. melanin pigment to match their body colour to tree trunks.
- Therefore, the carrying capacity of the nature does not allow the population - Before industrialisation in England it was observed that there were
to grow indefinitely. more white winged moths when almost white coloured lichen covered
3. Struggle for existence the tree trunks.
- The struggle for existence can be of three types - But after industrialisation there were more dark winged moths when
i. Intraspecific – The struggle between the members of the same species. the tree trunks became dark due to industrial smoke and soot.
ii. Interspecific – This is the struggle between the members of different - It was explained that when the soot
species. covered the trees the ark winged
iii. Envirnmental – This is the struggle between the organisms and the moths escaped the predators by
environment such as heat, cold etc. camouflage.
4. Variations - While white coloured moths
- Variation is the law of the nature. survived better in pre-industrial
- The variations are gradual and those which are helpful in the period because white coloured
adaptations of an individual towards its surrounding would be inherited to lichens covered the trees.
the next generation.
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2. Resistance of Insects to pesticides

- Resistant varieties of organism in less time are resulted due to use of
insecticides/pesticides. e.g., DDT resistance in mosquitoes.
- It is also correct for microbes as many as antibiotic resistant varieties
of disease causing bacteria are appearing in a very short period.

Adaptive Radiation
• An evolutionary process in which an ancestral form gives rise to new species
adapted to new habitats and new ways of life is called adaptive radiation.
1. Darwin’s finches in Galapagos island Lamarckism
- Darwin observed many small black birds in the Galapagos island. • Lamarck stated that evolution of life forms occurs by the use and disuse
- Many other varieties of such birds were found on same island. of organs.
- Darwin gave the reason that these birds have originated from a common • More used organs develop while those, which are not used, become
ancestral seed eating stock and then radiated to different geographical vestigeal in the organisms.
areas and have undergone adaptive radiation, especially in their types • The characters, which an organism develops during its lifetime, are
of beak. passed on the progeny.
2. Australian Marsupials • He gave the example of long neck of Giraffe, which according to him is an
- Many marsupials each different outcome of an attempt to stretch their neck continuously to eat leaves
From the other have evolved form long trees.
from a common ancestral form • Over a long period time, Giraffes came to acquire long neck by passing on
within Australia. this acquired character of neck to succeeding generation.
- Parallel evolution is shown by
placental mammals as they have Theory of continuity of Germplasm
evolved from other marsupial • This theory was given by August Weismann.
mammals and the resemblance • According to him, the characters influencing the germ cells are inherited
of each of them is with a only but protoplasm of somatic cells is not transmitted to the next
corresponding marsupial. generation.
• Weismann cut off the tails of rats for about 22 generations; nut tailless
rats were never born.

Mutation theory
- Mutation theory of evolution was proposed by Hugo de Varies, which stated
that the new species arise from pre-existing ones in a single generation by sudden
appearance of variation called saltation (single step large mutation).
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- This theory was base on the observation on Oenothera lamarckiana. Factors essential for Hardy-Weinberg equilibrium
- The salient features of mutation theory are • No mutation – there should not be either gene or chromosomal
i. Mutations, discontinuous variations are the raw material of evolution. mutation.
ii. Mutations appear all of a sudden. • No gene migration – there must be no exchange of genes between the
iii. They become operational immediately. populations.
iv. Unlike Darwin’s continuous variations or fluctuations, mutations do • No genetic drift – the population must be very large.
not revolve around the mean or normal character of the species. • No random mating – The population must reproduce sexually and mating
v. The same type of mutations can appear in a number of individuals of a must be random.
species. • No natural selection pressure - There must be no natural selection
vi. All mutations are inheritable and appear in all conceivable directions. pressure with respect to the alleles in question.
vii. Useful mutations are selected by nature while lethal mutations are
eliminated.
viii. Accumulation of variations produces new species.
ix. Sometime a new species is produced from a single mutation.
x. Evolution is a jerky and discontinuous process.

Hardy-weinberg principle
• Hardy-Weinberg principle states that allele frequency in a population is
stable and is constant from generation to generation.
• The gene pool (total genes and their alleles in a population) remains a
constant that is known as genetic equilibrium.
• The genetic variation in a population will remain constant from one
generation to the next in the absence of disturbing factors. Natural selection and its effects:
• The equilibrium is expressed as p2 + 2qp + q2 = 1 1. Directional selection (Progressive selection)
• p and q represent the frequency of allele A and a - The population changes towards one particular direction in respect to
• AA in a population is p2, aa is q2 and Aa is 2pq. certain characteristics.
• p2 represents the frequency of the homozygous dominant genotype (AA) 2. Stabilizing selection ( Normalization selection)
• q2 represents the frequency of the homozygous recessive genotype (aa) - It favors the intermediate variants instead of favoring individuals with
• 2pq represents the frequency of the heterozygous genotype (Aa). extreme phenotypes.
• Sum total of all the allelic frequencies is 1 - It reduces variation, however, it maintains the mean value from
• If the gene frequency remains constant over several generations, it generation to generation.
indicates that evolution is not taking place. 3. Disruptive selection (Diversifying selection)
• Changing gene frequencies would refer that evolution is in progress. - It favours individuals of both extreme of the distribution.
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Brief Account of evolution: • After the origin of ostracoderms

• About 2000 millions of years ago the first cellular forms of life originated acanthodians (the earliest known
on earth. vertebrates with lower jaws)
• Some of these cells could release oxygen. appeared in Silurian.
• The reaction could have been similar to the light reaction in • Placoderms (jawed vertebrates)
photosynthesis, where water and carbon dioxide were utilised with the arose in Silurian.
help of solar energy in the presence of certain pigments. • Origin of amphibians occurred in
• Slowly unicellular organisms became multicellular. Devonian .
• Invertebrates were formed and active by 500 mya. • Reptiles appeared in Carboniferous.
• Cyclostomes (Jawless fishes) probably evolved about 350 mya. • Origin of dinosaurs and mammals
• Seaweeds and few plants developed by 350 mya. took place in Triassic.
• The first organisms that existed on land were plants. • Toothed birds (first bird) appeared
• There were plenty of plants when animals came to land. in jurassic.

Evolution of plants Human evolution

• The patterns of evolution of • Dryopithecus that lived about 15 million years ago supposed to be the
major groups of plants are common ancestor of apes and man.
different from those of • In late Miocene epoch Dryopithecus gave rise to Ramapithecus, which
vertebrates. was the earliest man like primate.
• Different kinds of algae • The early human stock gave rise to Austrolopithecus (African ape man).
were present in Cambrian, • It’s fossils were recovered from Tanzania and Ethoipia.
• Its cranial capacity ranged from 450-600 c.c (cubic centimeter)
whereas Marine algae were
abundant in Ordovician. • Austrolopithecus can be described as an with ape like brain.
• Origin of bryophytes took • These were intermediate between Ramapithecus and genus Homo.
place before the appearance of vascular plants, the origin of which took
place in silurian.
• First gymnosperm appeared in Devonian and seed plants first occurred in
Carboniferous.
• Angiosperms appeared in Cretaceous, which diversified in Miocene and
their adaptive radiation occurred in Pliocene.

Evolution of vertebrates
• Origin of vertebrates took place in Ordovician period in the form of
ostracoderms (Jawless vertebrates)
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Prehistoric man • They buried their dead with ceremonies and believed in immortality of
• These were a number of species of Homo used to be appeared and soul.
became extinct from time to time on the evolutionary scene before the • They used animal skin for clothing and constructed hut like dwelling
origin of modern man. structures.
• They probably live in near east and central Asia between 1,00,000 –
1. Australopithecus (First ape man) 40,000 years back.
- Their fossils were discovered from Tuang in Africa.
- It was about 1.5 m high and had human as well as ape characters. Homo sapiens fossilis (Cro-Magnon Man)
- They have omnivorous diet and had erect posture.
• Its fossils were found from Cro-Magnon rocks of France.
- Their brain capacity was about 500 c.c. Similar to that of an ape. • Its cranial capacity was more than ours (About 1600 c.c.)
2. Homo habilis (Handy man or tool maker) • They used ivory ornaments and their tools included spears, bows and
- It lived in open grassy land, moved erect and their cranial capacity arrows.
ranges between 650-800 c.c. • They did some excellent cave painting.
- They probably did not eat meat
Homo sapiens sapiens (The living Modern Man)
- It was the first tool maker and used stone weapons. • The Cro-Magnon man was the direct ancestor of modern man.
3. Homo erectus • Evolution of modern man includes cultural evolution rather than
- It was the first hominid who knew the use of fire and more advanced anatomical evolution.
tools. • They learnt to cultivate plants and domesticate animals of economic
- They had erect posture and their cranial capacity was about 900 c.c.. importance.
- They probably ate meat. • The average cranial capacity of modern man is 1450 c.c.
- They were about 1.5-1.8 m tall and males were probably larger than • They probably arose during ice-age between 15,000- 10,000 years ago.
females.

Modern man:
• The modern man (Homo sapiens) includes the following subspecies
a. Homo sapiens neaderthalensis (Neanderthal man) ***
b. Homo sapiens fossils (Cro-Magnon Man)
c. Homo sapiens sapiens (The Living Modern Man)

Homo sapiens neanderthalensis (Neanderthal Man)

• Its fossils were first recovered from Neander Valley (Germany).
• The cranial capacity was about 1400 c.c.
• Speech centre had developed in them and they were capable of
communicating with each other.