Philosophers and scientists have been busy to solve the riddle as to how the universe and our earth were formed and how and when ‘life’ originated on earth. The branch of life science for the study of ‘Origin of life’ and evolution of different forms of life on earth was called Bioevolution or Evolutionary Biology by Mayer, (1970).
The study of universe or cosmos is called Cosmology. Our earth belongs to the Solar system having nine stars called planets constantly rotating around a common Sun. On the basis of the order of the distance from the sun these planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto while moon is a satellite of earth. The universe is made up of matter and energy and it was formed about 10 to 13 billion years ago as a red hot, dense, rotating gaseous cloud of cosmic dust called Ylem or primaeval matter. The Ylem consisted of particles of matter (like neutron, proton and electron) and antiparticles of antimatter. Scientists like Lemaitre (1931), Gamow (1948), Dicke (1964) etc. supported the Big-Bang Hypothesis which explains that collision between these particles and antiparticles caused a tremendous explosion to form atoms of hydrogen. Cosmic evolution began with the fusion of hydrogen atoms with progressively heavy atoms of different elements. Stellar systems and stars were formed by spreading of original gaseous cloud into the space and divided into smaller and larger masses. Most of the stars are masses of red hot gases even today.
Kant (1755) and Laplace (1796) supported Nebular Hypothesis which explains our solar system to have evolved about 4.5 to 5 billion years ago from a rotating red-hot gaseous cloud containing millions of free atoms of different varieties. First of all sun was formed when this cloud condensed, next the planets were thrown off from the sun and later on, in turn satellites were thrown off by planets. However, Nebular hypothesis was rejected by astronomers of present century like Weizsacker (1944), Alfen (1950) and Hoyle (1955).
According to them the sun was originally surrounded by a disk of rotating gas. Later many rotating concentric whorls were formed by break up of the disc and by gravitation and rotation particles of each whorl collected and condensed to form a planet. Our earth was formed about 4.6 billion years ago as a red hot gaseous cloud of free atoms with temperature of 5000o to 6000oC.
Structure of present earth : The earth is orange like in shape. Its pole to pole diameter is 12640 km and equatorial diameter is 12783 km. It is about 15 crore km away from sun and about 484000 km away from moon. Due to tremendous temperature gases existed in atomic form but gradually they cooled down in hundreds and millions of years into molten core. According to density other elements got stratified. Earth contains the central solid core, the middle mantle and shell and outer crust. The earth rotates at its polar axis in one day and it rotates around the sun in days or one year.
Origin of Life
Life is the part and parcel of the universe and both are very intimately associated with each other. We know that “Life is the most unique, complex organisation of molecules, expressing itself through chemical reactions which lead to growth, development, responsiveness, adaptation and reproduction” that matter has achieved in our universe. Origin of life is a unique event in the history of universe.
(i) Ancient theories of origin of life : Various theories have been put forward to explain the phenomenon of origin of life. A few of them were only speculations while others were based on scientific grounds. These theories are –
(a) Theory of special creation : According to a Spanish Priest Father Suarez (1548 – 1617 B.C.), the whole universe was created in six days by the God. First day Earth and heaven, second day sky, third day dry land and vegetation, fourth day Sun, Moon and other planets, fifth day fishes and birds, and sixth day human beings other animals were created by God. This theory was based on some supernatural power.
(b) Theory of spontaneous generation or Abiogenesis : This theory postulates that life originated from non-living matter spontaneously from time to time. This theory was supported by Plato, Aristotle, Anaximander, John Ray, Needham, Von Helmont, etc., upto the end of seventeen century. Huxley (1870) criticised this theory and propounded the theory “life originated from preexisting life only.”
q Abiogenesis means origin of life from non-living organisms.
(c) Biogenesis : Scientists like Redi (1668) Spallanzani (1767), Louis Pasteur (1866–1862) provided experimental support for the Biogenesis concept of Huxley.
Francesco Redi (1668) showed that maggots could not be created from meat. Actually, the smell of meat attracts flies which lay eggs on the flesh. These eggs hatched into flies.
Spallanzani (1767) showed that even primitive, unicellular organisms cannot arise from non-living matter.
Louis Pasteur (1860-62) obtained air samples in the flasks of broth (yeast and sugar solution) whose drawn-out necks were sealed cooling these contained a partial vaccum. Where a sample was required, the flask was opened. Air was drawn in and the flask was resealed. Flasks were incubated. These flasks which were opened in the streets became turbid while those exposed to dust-free air rarely contained bacteria.
Louis Pasteur also, used swan-necked flasks whose long, curved necks permitted exchange of air between outside and inside of the flask, but dust and bacteria were trapped along the wall of the neck. On tilting the flask, the bacteria got washed down into the broth, so that the latter became cloudy due to bacterial growth.
(d) Cosmozoic or Extraterrestrial or Interplanetary or Panspermiatic Theory : Richeter (1865), Preyer (1880), Arrhenius (1908), Hoyle (1950) and Bondi (1952) believed in eternity of life. According to Arrhenius life was transferred from “cosmozoa” (life of outer space) to different planets small units called ‘spores’. The spores were covered by a thick protective covering. When the spores got favourable conditions and temperature, the spore coat was dissolved and gave birth to initial living organisms. This theory does not explain as to how the life originated in space and how the life originated in spores remain impenetrable by ultraviolet and gama rays.
(e) Theory of Catastropism or Theory of sudden creation from inorganic material :Cuvier (1769-1832) believed in catastrophism. According to him, the catastrophy destroys the whole life on earth, and after that, new life originates called it as Mechanistic theory.
(ii) Modern Theory / Oparin Haldane Theory / Chemical Theory / Naturalistic Theory / Materialistic Theory : Haldane, a British scientist, stated that in the early atmosphere of gas mixture probably carbon dioxide, ammonia and water vapours were predominantly present. When ultraviolet rays reacted on them, organic molecules were formed. Gradually, quantity of these oceans which later gave rise to amino acids, proteins, carbohydrates, nucleic acids, etc.
Oparin’s Modern Theory : Oparin (1924) proposed that “life could have originated from non-living organic molecules.” He believed in Biochemical origin of life. Haldane (1929) also stated similar views. Oparin greatly expended his ideas and presented them as a book “The origin of life” in 1936.
According to this theory, the Earth originated about 4,500 million years ago. When the earth was cooling down, it had a reduced atmosphere. In this primitive atmosphere nitrogen, hydrogen, ammonia, methane, carbon mono-oxide and water were present. Energy was available in the form of electric discharges by lightening and ultraviolet rays. As soon as the earth crust was formed, it was very much folded. Torrential rains poured over the earth for centuries and were deposited in deep places.
The atmospheric compounds, inorganic salts and minerals also came in deep places oceans, these molecules gave rise to a variety of compounds and finally to the self-duplicating molecules. Ultimately these molecules were enclosed in membranes derived from lipids and proteins, along with water and chemical compounds, giving rise to cell like units. Again random combinations may have led to the formation of chlorophyll– containing organisms which could produce their own food (autotrophs) by a process called photosynthesis. These organisms had a better chance to live because they synthesise starch from carbon dioxide and water in presence of sunlight. Starch could be used as further source of energy. During photosynthesis, oxygen was produced. The oxygen was used by other organisms for respiration. Also oxygen, when acted upon by ultraviolet rays, formed ozone layer through which ultraviolet rays cannot pass. This layer is formed about 25 km. from earth’s surface. After the formation of ozone layer, organisms could come to the surface of the ocean and could survive even on land, if thrown out of oceans. The Oparins’s and Haldane’s theory of origin of life is most accepted these days as it is supported by Miller’s experiment duly supported by David Buhal, Melvin Kelvin’s experiment etc.
q O2 is absent in the primordial atmospheres at the time of origin of life.
Miller’s Experiment: An American scientist (Biologist) Stanley Miller (1953) performed an experiment under support Oparin’s theory of origin of life. He believed that basic compounds which are essential for life can be synthesised in the laboratory by creation in the laboratory, on a small scale, the conditions which must have existed at the time of origin of life on earth.
Miller took a flask and filled it with methane, ammonia and hydrogen in proportion of 2:1:2 respectively at 0°C. This proportion of gases probably existed in the environment at time of origin of life. This flask was connected with a smaller flask, that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of tungsten were fitted. Then a current of 60,000 volts was passes, through gases containing bigger flask for seven days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this red substance was analyzed, it was found to contain amino acids, Glycine and nitrogenous bases which are found in the nucleus of a cell.
q An experiment to prove that organic compounds were the basis of life, was performed by miller.
From the above theory we conclude that life first originated in water. Therefore, water still continues to be an essential constituent of life.
The entire process of the origin of life, as proposed by Oparin, can be summarised as under :
(a) The Chemical Evolution
(1) Step 1 : Formation of simple molecules : The reactions between simple atoms like carbon, hydrogen oxygen and nitrogen in the primitive atmosphere led to the formation of simple compounds like water, ammonia and methane. But since the earth was very hot, all these substances remained in the form of vapours in the atmosphere. Gradually, as the earth started cooling down, the dense clouds began raining on the earth. But the liquid was still very hot. Therefore, as the liquid water touched the earth’s surface, it again got vaporized to be returned to the atmosphere. This process continued for millions of years. As a result of these heavy downpours, the earth’s surface got filled with water to form rivers and oceans. Ammonia and methane got dissolved in the oceanic water. The mineral elements, which were dissolved in rivers, were also carried into the oceans when rivers joined it.
The scientists have found that simple unicellular organisms (resembling modern cyanobacteria) were present on this earth about 3600 million years ago. It is believed, therefore, that life must have originated on this earth about 4600 to 3600 million years ago.
It must be clear that the earth’s atmosphere at that time was quite different from as it exists today. The earth’s atmosphere at that time was reducing, not oxidizing (as it is today). The primitive atmosphere of earth had hydrogen, nitrogen, water vapours, carbon dioxide, methane and ammonia abundance. Oxygen was not available in free state in sufficient quantities.
(2) Step 2 : Formation of simple organic compounds : Continuous rains provided opportunities for different types of molecules to collide with each other and react. Methane is an active compound, and it reacted with other compounds to form organic compounds like ethane, butane, propane, ethyl alcohol. From such organic compounds which were formed in the ocean and which played a role in the origin of life include –
(i) Sugars, glycerol andfatty acids : These were formed by the combination of carbon, hydrogen and oxygen.
(ii) Amino acids : These were formed by the combination of carbon, hydrogen, oxygen and nitrogen.
(iii) Pyrimidines and Purines : These were formed by the combination of carbon, hydrogen and nitrogen.
These compounds were formed at the time when sunlight could not reach earth because of dense clouds in the sky. Under such circumstances, the energy required for the synthesis of above–mentioned chemical substances must have been obtained from the cosmic rays and lightening in the sky. Haldane proposed that these simple organic compounds gradually accumulated in the water bodies and finally a ‘hot thin soup’ or ‘prebiotic soup’ or broth was formed. This set the stage for the chemical reactions.
(3) Step 3 : Formation of complex organic compounds : The simple organic compounds combined in different ratios to form complex organic compounds like polysaccharides, fats and proteins.
(i) Simple sugars combined in different ratios to form polysaccharides like starch, cellulose, glycogen etc. The formation of such compounds had been very important for the origin of life because cell walls are made up of cellulose and energy–giving molecules are stored in the form of starch and glycogen.
(ii) The reaction between glycerol and fatty acids yielded fats.
(iii) Different types of combinations between a variety of amino acids yielded different types of proteins. The formation of protein was a very important step in the origin of life because proteins are not only structural components of cell organelles, but many proteins, called enzymes, work as catalysts for biochemical reactions.
In fact, the present day organisms synthesise their complex molecules from simple organic substances with the help of enzymes only. However, enzymes themselves are proteins. Therefore, first of all protein must have been synthesized without the help of any enzyme.
(4) Step 4 : Formation of nucleic acids and nucleoproteins : The reaction between methane, ammonia and water resulted in the formation of purines and pyrimidines. Some of the purines and pyrimidines combined with sugar and phosphorus to form nucleotides. Many molecules of nucleotides combined to form nucleic acids–DNA and RNA. The formation of nucleic acid in the oceanic water was a big steps in the direction of origin of life. Nucleic acids combined with the proteins to form nucleoproteins. Some of the nucleoproteins developed the capability to synthesise molecules similar to themselves, from organic and inorganic substance present in the ocean. In other words, the capability to reproduce had evolved. As a result of continuos reproduction, the number of nucleoproteins went on increasing. Since, organic substance were required for this, the organic substances started being depleted resulting in competition between the nucleoproteins. Physical and chemical changes sometimes led to the changes in the competition of nucleoproteins, and new types of nucleoproteins came into existence by mutations. Those new nucleoproteins which were successful in the competition, increased in number.
(b) Organic Evolution (Biological Evolution)
(1) Step 5 : Formation of Coacervates : Oparin believed that the formation of protein was a very important step in towards the origin of life. The Zwitterionic nature of the protein molecules enabled these colloidal structures to maintain their identify inspite of being surrounded by water molecules–forming a type of emulsion. The coalescence of these colloidal structures led to the formation of structures called coacervates. These coacervates had the ability to exchange substances with the surrounding water and accumulating required substances within them.
r Under certain conditions scientists have obtained cell like structures. These are known as coacervates.
Sydney F. Fox’s experiment : Sydney F. Fox of Florida University, obtained some complex molecules by heating upto 90°C a dry mixture of many amino acids found in living organisms. The molecules so obtained very much resembled the proteins. He heated these molecules in water and allowed the mixture to cool down. In the fluid so obtained, he could observe minute structures resembling the cells. He called them microspheres. The microspheres are surrounded by membranes, and these also reproduce vegetatively just like yeast. Surprisingly the biochemical processes like breakdown of glucose also occur their. However, electron microscopic examination of these does not reveal any cellular structure. On keeping in distilled water, these become turgid, but these get shrunk if immersed in salt solution. Oparin’s coacervates and Fox’s microspheres are infact, similar structures or Protocells or Protobionts or Eobionts.
(2) Step 6 : Formation of Primitive living system : A primitive cell membrane was formed by the arrangement of lipid molecules between the surface of coacervates and external watery medium. This provided stability to the coacervates. It is believed that a primitive cell was formed when–
(A) Nucleic acids having the property of self-duplication entered the coacervates.
(B) Rearrangement of molecules occurred inside the coacervate surrounded by lipid molecules.
(3) Step 7 : Formation of first cell : Evidences available so far indicate that the cells of the earliest organisms did not contain either nucleus or cell organelles. The molecules of nucleic acid were surrounded by a colloidal mixture (may be called protoplasm) of proteins and organic compounds. This, in turn, was surrounded by a thin protein-lipid membrane. Water and soluble substances would pass through this membrane. Some proteins achieved the ablity to act as enzyme. Such cells which lacked nuclei were called prokaryotic cells. In 1966, some fossils have been discovered from 300 million years old rocks. These fossils are of prokaryotic organisms. Earliest organism is chemoheterotrops.
(4) Step 8 : Origin of autotrophism : In the primitive organisms, the process of metabolism began because all the substances required for reduction were available in water. Of course, oxygen was not available which was required for oxidation of substances to yield energy. Thus the first living organisms were anaerobes and heterotrophs. The primitive cells respired anaerobically i.e., these used to obtain energy by fermenting the organic compounds obtained form the water, with the help of enzymes due to fast nutrition, growth and multiplication, their number in the ocean increased greatly. As a result, scarcity of organic substances developed in the ocean. A struggle started between the cells for obtaining nutrition.
At such a time, some of these organisms developed the capability of synthesizing organic substances. Such organisms began synthesizing energy– giving substances (carbohydrates) from simple inorganic substances abundantly available in the environment. In this way, evolution of autotrophs from heterotrophs took place.
This was the beginning of autotrophic nutrition. However, it was quite different from the photosynthesis which is carried out by green plants, because it utilized energy obtained by anaerobic respiration (not solar energy). Therefore, such type of nutrition is also called chemoautotrophic nutrition. Such type of nutrition is observed even today in the sulphur bacteria.
At the same time, from different chemicals present in the oceanic water evolved porphyrins which where like modern chlorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2 O instead of H2S for photosynthesis. Thus they performed anoxygenic photosynthesis.
Till then, oxygen was not freely available in the atmosphere. However, gradually molecular changes in the bacteriocholorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2O instead of H2S for photosynthesis. Thus they performed oxygenic photosynthesis using water as hydrogen donar.
In this way, the prokaryotic cells which were chemoautotrophs, became photo autotrophic. These cells resembled modern cyanobacteria. In 1968, the forms of such types of cells have been recovered from 320 million years old rocks. These have been given the name Archaeospheroides barbertonensis. Due to the absence of well-defined nuclei in them, these have been included under the kingdom ‘Monera’. Thus, release of O2 in the atmosphere and its free avilability was the result of photosynthesis. This was a revolutionary change which greatly affected the course of organic evolution.
(5) Step 9 : Origin of Eukaryotic cells : As a result of photosynthesis, oxygen was released in the atmosphere which started reacting with methane and ammonia in the atmosphere. Its reaction with methane yielded CO2 and H2O. On the other hand, reaction between oxygen and ammonia resulted in the formation of CO2 and nitrogen. In the course of these changes, Ozone (O3) gas was formed from oxygen; the ozone spread in the form of an envelope surrounding the earth, the distance between the ozone layer and the earth’s surface being approximately 15 miles. Thus free oxygen changed the reducing atmosphere into oxidizing atmosphere. As free oxygen became available on the earth, gradual changes took place in cell structure also. Membrane bound organelles i.e., mitochondria, chloroplasts, golgi bodies, lysosomes evolved. Thus, prokaryotic cells. Most of the organisms on the earth today are eukaryotic.Gradual changes in the earth’s atmosphere led to gradual changes in the eukaryotic cells also. Instead of living separately, the cells started living together in the form of colonies. Simultaneously, multinucleation of multicellular structures forming tissues. Different types of tissue combined to form special organs. From the organs, organ systems and ultimately complex bodies of organisms were formed.
q Organic evolution would have not been taken place if individuals in a population did not show genetic variation.
q Synthetic theory is the most accepted theory of organic evolution.
q The greatest evolutionary change enabling the land vertebrates to be completely free from water, was the development of shelled eggs and internal fertilization.
q The material for organic evolution is mutation.
Viruses
The term ‘virus’ has been derived from Latin, which means poison or venom or viscous fluid. They remain inactive outside a living host but become active inside the host and multiply in it. They represent a transitional form of life between non–living and living world.
Luria (1953) defined virus as “Sub-microscopic entities capable of being introduced into specific living cells and reproducing inside such cells only. “Single virus is called ‘Virion’, most of the plant virus are RNA virus. Most of the animal virus are DNA virus.
(i) Important discovery of virus
Carolous causius(1576) recorded first viral disease in tulips.
Mayer (1886) found a disease in tobacco caused by virus and called it tobacco mosaic disease.
Ivanowski (1892), a Russian Botanist, discovered the infectious nature of the viruses. He was the person, who discovered the virus.
Beijerinck (1898) first used the word Virus.
Twort (1915) and D. Herelle (1917) discovered bacteriophages, a kind of virus which infected bacteria and destroyed them.
M. Stanley (1935) first time isolated tobacco mosaic virus (TMV) in crystalline form and showed that crystals were made up of proteins. Nobel prize was awarded to him for this work.
(ii) Nature of viruses : Viruses are regarded as intermediate between non-living entities and living organisms. It is very difficult to as certain whether they are living or non-living. Some characters of viruses suggest their non-living nature where as many other characters suggest their living nature. The two views are listed below –
Viruses are non-living : The following characters state that they are non-living.
(a) Viruses have no complete cellular structure. They are not surrounded by cell membrane or cell wall.
(b) They do not show cellular metabolism and lack respiration.
(c) They possess high specific gravity unlike living organisms.
(d) Viruses are active only when they are inside the living host cells. Out side the host, they are good as chemical substances. Thus, they do not have their independent existance.
(e) The viruses can be precipitated just like chemical substances.
(f) Postulates of Robert Koch are not true for the viruses. Virus cannot grow in “invitro” condition in lab.
Viruses are living organisms : The following characters state that they are living organisms –
(a) They have definite shape and morphology like that of a living organism.
(b) They possess genetic material (DNA or RNA), which determine their structure and development. Genetic material passes from generation to generation in usual manner.
(c) All viruses are intracellular obligate parasite and attack specific hosts. The bacteriophages recognise the real bacterial surface.
(d) They show property of mutation.
(e) They show irritability and respond to environmental conditions such as heat, ultraviolet rays, humidity, drought, alcohol, etc.
(f) They can grow inside the host and multiply enormously showing one of the most important property of living organisms.
(iii) Chemical composition : Chemically viruses are nucleoproteins. They are made up of central core of nucleic acid. Nucleic acid is only one, either DNA or RNA. This nucleic acid (DNA or RNA) represents the genetic characters of virus. TMV has RNA (like most plant viruses have) 10% RNA and 90% protein is present in influenza virus and PSTV (Potato Spindle Tuber Viroid) also has RNA but it does not have capsid (protein coat). Plant viruses contain RNA but in cauliflower mosaic virus contain DNA. Bacteriophages contain DNA and almost half animal viruses contain RNA and half contain DNA. But it is called that often animal viruses contain DNA. Cancer causing viruses reovirus contain both RNA and DNA, Only some enzymes are detected in viruses such as – Lysozyme in bacteriophages, transcriptase in vaccinia virus, reverse transcriptase and DNA or RNA polymerase in retroviruses.
Table : 1-6
DNA Viruses
Strands
RNA Viruses
Strands
Adenoviruses
DNA (2)
Avian leukemia virus
RNA (1)
Bacteriophage f X 174
DNA (1)
Bacterial virus F2
RNA (1)
Bacteriophage M13
DNA (1)
Bacteriophage MS–2
RNA (1)
Coliphage lambda (l)
DNA (2)
Coliphage R17
RNA (1)
Coliphage T2, T4, T6
DNA (2)
Influenza virus
RNA (1)
Coliphage T3, T7
DNA (2)
Poliomylitis virus
RNA (1)
Pox virus
DNA (2)
Tobacco mosaic virus (TMV)
RNA (1)
Herpes viruses
DNA (2)
Reovirus
RNA (2)
Popilloma virus
DNA (2)
Rice dwarf virus
RNA (2)
Polyoma virus SV 40
DNA (2)
Wound Tumour virus
RNA (2)
(iv) Shape : There is variation in shapes of viruses. On the basis of shape viruses have been placed in the following categories.
(c) Polyhedral virions, e.g.Turnip yellow mosaic, Tobacco ring spot virus.
(d) Tadpole like – Bacteriophages.
(e) Spherical – Influenza virus.
(v) Size : Viruses have a long range of size. They range from 10 mm to more than 300 mm in size. The virus of foot and mouth disease (FMD) of animals is smaller than the largest protein molecule. Largest virus is smallpox virus – variola (250 mm).
(vi) General structure of virus
Structurally viruses are made up of envelope, capsid, nucleoid and occasionally one or two enzymes.
Envelope : Some viruses possess an outer thin loose covering, called envelope. It is composed of proteins (from virus), lipids and carbohydrates (both from host). The smaller subunits of envelope are called peplomers The viruses, which do not possess envelope, are called naked.
Capsid : It is the protein coat that surrounds the central portion of nucleoid and enzymes (if present). The capsid consists of a specific number and arrangement of small sub-units called capsomeres. These sub-units possess antigenic properties.
Nucleoid : The nucleic acid present in the virus is called nucleoid. It is the infective part of virus which utilizes the metabolic machinery of the host cell for synthesis and assembly of viral components. The genetic material of viruses are of four types :
(a) Double stranded DNA (ds DNA)
(b) Single stranded DNA (ss DNA)
(c) Double stranded RNA (ds RNA)
(d) Single stranded RNA (ss RNA)
(vii) Life cycle : The word reproduction is not appropriate in case of viruses because they have no cellular components or cell organelles. They do not reproduce themselves but divide by a special mechanism as follows.
Attachment : The bacteriophage gets attached to bacterial cell wall with the help of caudal fibres.
Penetration : Bacteriophage dissolves the bacterial wall by an enzyme Lysozyme and makes a pore in cell wall. Through this pore DNA molecule enters in the cell after contraction of head protein, entire protein coat remains outside.
Latent period : Phage DNA controls hosts cellular machinery. Instead of formation of bacterial protein, phage protein formation begins. Cellular DNA and RNA is broken down and from this cellular DNA, phage DNA is formed. Now protein covers the DNA fragments to form a kid virus.
Maturation : This young virion is changed into an adult virus hence this process is called maturation.
Release : The viruses are mature, cell wall of bacterial cell is weakened by enzyme lysozyme. The release of viruses takes place by bursting of host cell and these are again ready for next infection or attack on other bacteria.
(viii) Economic importance of viruses
Uses of viruses
(a) Specific viral strains are cultured and attenuated to be used as vaccines against specific diseases.
(b) The addition of cyanophages LPP-1 and SM-1 are useful in controlling water blooms.
(c) Bacteriophage was used by Hershey and Chase to prove that DNA is the chemical basis of heredity.
(d) Bacteriophages are of interest to geneticists because these bring about transduction.
(e) Water of river Ganga is believed to have phages which destroy bacteria. That is why its water does not get spoiled.
Tobacco mosaic virus (TMV) : It was discovered by the Russian worker D. Ivanowski. Franklin etal (1957) described the ultrastructure of (TMV) – It is a rod–shaped virus having a central core of RNA surrounded by protein coat (capsid) to form the nucleocapsid. The nucleocapsid may be naked or may be surrounded by a loose membranous envelope. The protein coat (capsid) consists of 2130 identical subunits (capsomeres). The protein is 94% and RNA is only 6%. In the entire length a single RNA molecule runs in the form of spiral coils.
Bacteriophage : The viruses which attack bacteria are called bacteriophages. In outline they look like tadpole or sperm. The body can be divided into a hexagonal head neck and a tail. The hexagonal head has a central core of DNA, which is surrounded by protein coat. The DNA is double helix. The cylindrical tail is hollow and is entirely made up of proteins. At the end of this, there are six long threads called tail fibres or caudal fibres. These fibres help the virus while attaching to bacteria. Bacteriophage contain lysozyme enzyme.
Cyanophages : Generally some of the viruses are found which attack on blue green algae. Sofferman and Morris (1963) reported 11 filamentous forms of blue green algae (Lyngbya, plactonema and phormidium, hence called LPP-1) which were attacked by viruses. These viruses are usually called cyanophages. Cyanophages contain DNA as their genetic material. These viruses resemble with bacteriophages in morphology and behaviour.
Mycophages : Some fungi such as, Mushrooms, Penicillium, etc have also been found to be infected by viruses. These are isometric in shape and contain double stranded RNA.
Phycophages : These are virus which attack on Algae.
Viroids : Diener and Raymer (1967) discovered very simple smallest infectious agents called Viroids. Viroids consist of RNA only and capsid is lacking. Viroids contain only very low mol. weight. Diener and Raymer reported that causal agent of potato spindle tuber disease was a free RNA and no viral nucleoprotein particles were present in the infected tissue. T.O. Diener (1971) termed it viroid. Viroids are single–stranded, covalently closed circular as well as linear RNA molecules. Transmission is mechanical. The symptoms on host plants are almost similar to those of viruses. Viroids cause persistent infections. A number of other diseases caused by viroids are – Cadang Cadang of coconut, Cucumber pale fruit, Chrysanthemum stunt, Avacado sunblotch, etc.
Prions : Prusiner (1982) discovered it as a human disease causal agents. Stanley B. Prusiner discovered infectious agents which were prions. Prions are proteinaceous particles thought to cause a number of diseases including the slow virus diseases, therefore also called as slow viruses. They are made of proteins molecules only. Genetic material (DNA and RNA) is absent in prions. Kuru, a disease of central nervous system found in few canniblastic tribes of New Guinea is caused by prions. Other such disease is Creutzfeld–Jacob disease of humans and animals, similar to scrapie, gerstmann – strassler – scheinker syndrome. These all are diseases of central nervous system.
Interferons :G.M. Findleyand McCallum(1937) reported a phenomenon called viral interference in which the cell infected with one type of virus becomes resistant to super infection by other viruses. Alliac Issacs and Lindeman (1957) gave the term interferons to the chemical substances responsible for viral interference.
(i) Interferons are produced by cells in mammals, rodents, birds, etc., and provide resistance against viruses.
(ii) Interferons are protein molecules or polypeptides of low molecular weight which prevent viral multiplication.
Table : 1-7 Families of animal viruses, grouped by type of nucleic acid
Family
Virion Structure
Diameter (nm)
Examples/ Diseases
dsDNA
Papova virus
Naked polyhedral
40–57
Papilloma (human warts, cervical cancer); polyoma (tumors in certain animals).
Adeno virus
Naked polyhedral
70 – 80
Viruses that cause respiratory disease; some that cause tumors in certain animals.
Herpes virus
Enveloped polyhedral
150–250
Herpes simplex I (cold sores); herpes simplex II (genital); varicella zoster (chicken pox, shingles); Epstein–Barr virus (infectious mononucleosis, Burkitt’s lymphoma).
Pox virus
Enveloped complex
200–350
Variola (smallpox); vaccinia; cowpox.
ss DNA
Parvo-virus
Naked polyhedral
18–26
Most depended on co infection with adenoviruses for growth
Rubella virus; yellow fever virus; encephalitis virus (transmitted by insects).
Retrovirus
Enveloped polyhedral; two copies of genome per virion.
100–120
RNA tumor viruses (solid tumors and leukemia); AIDS
Table : 1-9 ss RNA that is a template for mRNA (– strand RNA)
Rhabdovirus
Enveloped helical
70–180
Rabies
Paramyxovirus
Enveloped helical
150–300
Measles, mumps
Orthomyxovirus
Enveloped helical; RNA in eight segments.
80–200
Influenza viruses
ds RNA
Reovirus
Naked polyhedral; RNA in ten segments.
60–80
Diarrhoea viruses
*ds = double– stranded; ss = single–stranded.
Table : 1-10 Important plant diseases caused by viruses
S.No.
Disease
Causal organism
(1)
Abutilon mosaic
Abutilon mosaic virus
(2)
Bunchy top of banana
Banana bunchy top virus
(3)
Cucumber mosaic
Cucumber mosaic virus
(4)
Little leaf of brinjal
Brinjal little leaf virus
(5)
Little leaf of cotton
Cotton little leaf virus
(6)
Papaya mosaic
Papaya mosaic virus
(7)
Potato leaf roll
Potato leaf roll virus
(8)
Potato mild mosaic
Potato virus X
(9)
Potato rugose mosaic
Potato virus X and Y
(10)
Stunt of S. C.
Ratoon stunt virus
(11)
Rosette of groundnut
Groundnut mosaic virus
(12)
Sugarcane mosaic
Sugarcane virus I
(13)
Tobacco mosaic
Tobacco mosaic virus
(14)
Tomato leaf curl
Tomato curl virus
(15)
Tristeza of citrus
Citrus Tristeza virus
Table : 1-11 Important human diseases caused by viruses
S.No.
Disease
Host
Causal organism
(1)
Encephalitis
Man
Encephalitis virus
(2)
Infectious hepatitis
Man
Hepatitis virus
(3)
Herpetic Keratitis
Man
Herpes virus
(4)
Influenza
Man
Influenza virus–a
(5)
Measles
Man
Measles virus
(6)
Viral bronchitis
Man
Parainfluenza virus
(7)
Poliomyelitis
Man (children)
Polio virus
(8)
Small Pox
Man
Pox virus
(9)
Common cold
Man
Rhino virus
(10)
Yellow fever
Man
Yellow fever virus
Evidence of Organic Evolution
The following are the evidences in favour of Organic Evolution :
(i) Evidences fromClassification : All the known living animals and plants have been classified into various species, genera, families, order, classes, phyla and kingdoms. The classification of a particular animal is attempted only after its extensive study. It is seen that every living being is related with other living being. The relations may be very close or may be quite apart. On their relationship, they are put under various orders, classes, phyla etc. On the superficial examination one can hardly believe that they are interrelated. But after their careful study they can be arranged in definite order, Protozoa (acellular) being at the base while Chordata at the top. No doubt, there is some sort of gap between chordates and non-chordates today but who known that this gap may be filled some day by some further discoveries. Moreover, the present-day types represent only the terminal twigs of a vast phylogenetic tree and for establishing relationship we should focus our attention on the main trunk. The animals can be classified as described above in a systematic order.
(ii) Evidences fromComparative Morphology and Anatomy : In all the living animals, the basic substance of life is Protoplasm. If the species had been created separately, then there should be no relationship in the various organs and systems of animals. But on the contrary, we see that large number of animals although unlike in appearance show most of the systems and organs made on the same plan. The resemblance are very close in the members of the same group. For example –
(a) Analogy and Homology : While examining the various structures in the bodies of different animals, one may come across certain organs of same origin but of different functions. For example, the forelimbs of salamander, crocodile, bird, bat, whale, and man, all have the same origin and essential structures but different functions to perform. Such structures are called homologous. On the other hand, the wings of insect, pterodactyle, bird and bat perform the same function, though they have different origin and entirely different structures. Such structures are termed analogous.
The homologous structures give us the answer that how the two different types of animals or organs have the same origin and functions to perform; this shows that they have changed themselves according to their different needs.
q Analogous organs are those, which are functionally similar.
q Homologous organs explains divergent evolution.
q Similarity developed in distantly related groups as an adaptation to the same function is called convergent evolution.
q The wings of an insect and a bat exhibit analogy.
(b) Vestigeal organs : There are present in the body of animals certain structures which have no function and are very much reduced. These structures are quite developed and functional in allied animals. Such useless organs are termed as vestigeal and are of frequent occurrence. In human body alone, there are as many as ninety such organs. For example, vermiform appendix is vestigeal in man but functional in rodents, horse and other herbivorous animals.
q Hind limbs is a vestigeal organ of python.
q Muscles of ear pinna is vestigial organ of man.
q Wisdom teeth is vestigeal organ of man.
(c) Atavism : Atavism or reversion is the sudden reappearance of some ancestral features which was either vestegial or altogether absent in parents. Such as Large canines, thick body hairs, short temporary tails, ability to move pinnae.
(d) Connecting links :Intermediate or intergrading forms between two groups of organisms :
Table : 1-12
Organism
Connecting link between
Viruses
Living and nonliving
Euglena (Protozoa)
Plants and animals
Proterospongia (Protozoa)
Protozoa and Porifera
Peripatus (Arthropoda)
Annelida and Arthropoda
Neopilina (Mollusca)
Annelida and Mollusca
Balanoglossus (Chordata)
Nonchordata and Chordata
Dipnoi (Lungfish)
Pisces and Amphibia
Archaeopteryx (Aves)
Reptiles and Birds
Prototheria (Mammalia)
Reptiles and Mammals
Missing links : These are the fossil forms transitional between two present day groups (taxa) of organisms.
E.g., Stegocephalous – between fishes and amphibians
Archaeopteryx – between reptiles and birds
Archaeornis – between reptiles and birds
Inostracevia – between reptiles and mammals
Manchurosaurus – between reptiles and mammals
(iii) Evidences from Physiology/Biochemistry : Various types of chemical tests exhibit many basic similarities in physiological and chemical properties that show a physiological relationship among animals.
Take the example of thyroxin hormones, which are similar in all vertebrates. The thyroid of human beings can be very easily replaced by the thyroid of cattle without any ill effects and so on.
Evidences from Serology : This is a method by which the reactions of blood serum is observed. From the blood are also extracted the crystals of Oxyhaemoglobin. The structure differs in different vertebrates, but in a definite order. The reaction is nearly identical in man and anthropoid monkeys, but slightly less identical with other mammals.
(iv) Evidence from Embryology : Van Baer (Father of modern embryology) put forward :
Germ layer theory : Various body structures arises from the same germ layers in different species of animals.
Baer’s Law : It revealed the fact about the sequence of development of structures in organisms (i.e., less general features ® more general features ® more special features).
Muller and Haeckel in year 1864 reinterpreted the Baer’s law in light of evolutionary theory as Biogenetic law. According to this law ‘structure of ancient origin develop earlier than structure of newer origin’ or ‘development of structures in an organism follow the same sequence as they evolved in is ancestors’ or Ontogeny (life history of an individual) repeats phylogeny (evolutionary history of race).
Table 1-13
Era
Age million years
Period
Epoch
Dominated Fauna
Cenozoic (Age of Modern life)
0.1
2.5
Quaternary
Recent
(Holocene)
Modern man, mammals, birds, fishes, insects.
Pleistocene
Extinction of great mammals. Primitive man common. Evolution of human society & culture.
7
Tertiary
Pliocene
Evolution of primitive man–like forms from man like apes. Formation of modern mammals.
Miocene
Mammals at peak. Evolution of man–like apes.
38
54
65
Oligocene
Rise of monkey, apes and monocots
Eocene
Diversification of placental mammals birds, origin of horse
Palaeocene
Origin of primates, placental mammals, angiosperm forests
Mesozoic (Era of Medieval life)
135
Cretaceous
Extinction of dinosaurs and toothed birds. Rise of first modern birds.
195
Jurassic (Age of Giant Reptiles)
Origin of advanced lizards, crocodiles, alligators, marsupials toothed birds. Dinosaurs became large. Reptiles dominant.
240
Triassic
Origin of dinosaurs & primitive mammals (egg–laying mammals). Extinction of primitive amphibians.
Paleozoic (Era of Ancient life)
285
Permian
Extinction of many marine invertebrates like trilobites. Rise of modern insects. Evolution of mammals like reptiles.
375
Carboniferous
(Age of Amphibians)
Origin of reptiles and winged insects.
420
Devonian
(Age of fishes)
Origin of first land vertebrates (amphibians) Fishes abundant.
450
Silurian
Origin of jawed fishes and wingless insects.
520
Ordovician
(Age of invertebrates)
Origin of chordates with first jaw– less fishes (origin of vertebrates). Invertebrates abundant.
570
Cambrian
All invertebrate phyla established. Trilobites (swimming crustaceans which do not exist today) dominant
Pre-Cambriam
2300
3600
3800
Proterozoic
(Era of early life)
Origin of marine metazoans including sponges, cnidarians, annelids, molluscs and arthropods.
Scanty fossils.
Origin of prokaryotes (Monera) and Eukaryotes (Protista).
4200
Archaezoic
Origin of life.
4200
–4600
Azoic
No life?
(v) Evidences from Palaeontology: The study of fossils and their interpretation forms one of the great evidences of evolution. An Italian scientist, Leonardo da Vinci, was the first person to recognize their importance and said they were either remains of organisms of their impressions on some sort of clay or rock.
A number of fossils have been discovered from time to time. Some of these fossils are very prefect in their state of preservation. For example, the fossils Archaeopteryx shows the characters of reptile on the one hand, and the characters of birds on the other hand, meaning that the birds have evolved from reptiles, the Archaeopteryx being a connecting link.
In the same way all the evolutionary stages of horse, elephant, camel and man etc., can be constructed. The earliest horse was known as Eohippus. It was eleven inches in height and made its appearance in Eocene time from some unknown five-toed ancestor. The Eohippus, after various evolutionary stages, transformed itself into the present-day horse Equus, which shown many dissimilarities from its great grand, grand ancestor. The various stages in its evolutionary life had been preserved as fossils. The same is the case with camel, elephant and man, etc. So this science of paleontology helps in a great deal in understanding the process of evolution.
q Wallace gave a theory very similar to that of Darwin.
q In Cenozoic era mammals and birds were evolved.
q There was no life in Azoic era.
q Fossils are remains of organisms present in the rocks.
q Ruling reptiles were dominant during Mesozoic era.
Fossils :
(a) Direct evidences of organic evolution are provided by fossils (L. fossil, dug up).
(b) The science of discovering and studying fossil record is called Palaeontology (Gr. palaeo, ancient; logy, study of).
(c) Fossils are remnants, models and impressions of extinct organisms.
(d) Fossils are found preserved in earth’s sedimentary rocks.
(e) Fossil may be an entire organism buried in sediment or snow, small part of ancient organism or impression of ancient leaf or stem.
(f) Fossilization occurs where organisms are buried and preserved by natural processes.
Age of fossils
(a) The age of the fossils or rocks can be determined by ‘Clock of the rock method’ or ‘Radioactive clock’.
(b) ‘Clock of the rock’ method is based on conversion of unstable radioactive nuclei into stable nuclei over a fixed period.
(c) Radioactive clock method was introduced by Boltwood in 1907.
(d) The common radioactive elements which lose their radioactivity and change into their non-radioactive isotopes at a fixed rate are :
Potassium 40 ® Argon 40
Carbon 14 ® Nitrogen 14
Uranium 238 ® Lead 207
Rubidium 87 ® Strontium 87
Thorium 232 ® Lead 206
Living fossils : Living fossils are the organisms which underwent little change during long geological periods e.g.,
Peripatus – Primitive arthropod
Limulus (king crab) – Arthropod
Neopilina – Mollusca
Lingula – Mollusca
Latimaria – Coelocanth fish
Sphenodon (fuatra) – Reptelia
Didephis – Opossum
Plastasterias – Echinodermata
Echidna and platypus – Mammalia
(vi) Evidences from geographic distribution : If the study of horizontal distribution of animals on the face of this earth is made, it would be seen that animals are not evenly distributed. Two identical places with the same climate and vegetation may not have same sort of animal fauna Alfred Russel Wallace (1823-1913) divided the whole world into six major biogeographical regions or realms. Nearctic – Northern America, Palaearctic –N. Africa, N. Asia, Europe, Neotropical – Central and S. America, Oriental –Asia (South of himalayas i.e., India, Ceylon, Malayasia, Indonesia, Philippines), Ethiopian – S. Africa, Australian –Australia, New Zealand. For example, Elephants and Rhinoceros occur in India and Africa only and not in Brazil, although the climatic conditions are alike. Again, members widely separated areas. For example, lung fishes are found in South America, Australia, Africa and nowhere else.
The explanation of this uneven distribution is quite obvious, and can be explained on the basis of organic evolution. A particular species, after arising from one place, migrates to other far off places. While doing so, it has to come across various climatic changes for which it tries to modify it self. In this way, the new species are added. After its dispersal, if some barriers arise, the species becomes confined to that particular region and is isolated from the parent species. This provides a very interesting theory on the mechanism of organic evolution.
Charles Darwin during his voyage around the world studied the fauna and flora of Galapagos islands (off the west coast of south America). Galapagos islands are called ‘a living laboratory of evolution’. Darwin studies 20 related varieties of the bird belonging to family Geospizidae different in shape and size of beak. These birds are now called ‘Darwin’s finches’.
(vii) Evidences from Genetics : Johan Gregor Mendel in 1866 published his work on experimental breeding. He bred two individuals differing in certain well-defined characters, and observed the ratio in which various contrasting parental characters appeared in successive generations. Since then, selective breeding has started in the case of domestic animals and plants to obtain the evidences of organic evolution. These furnish the direct evidences of evolution.
Conclusion : According to the above description, we have seen certain evidences in favour of organic evolution. Although the evidences are indirect, merely interpretations based on certain phenomenon, certain organs, systems and other facts but they throw enough light to show us that the present day forms of life have originated from simpler forms in millions and millions of years. The process was continuous gradual, and accompanied by trial and error.
Theories of Organic Evolution
(i) Lamarckism : Lamarck (1744 –1829) was one of the most brilliant stars on the horizon of the history of evolution. Though he was a man of great intellect yet he had to cut a sorry figure because of poverty, and secondly, as he could not get the approval of the famous and influential Cuvier. His work was recognized only few years after his death.
He was the first naturalist to put forward a general theory of evolution in his famous book. Philosophic Zoologique published in 1809. His evolutionary theory may be summarised in the form of following laws:
(a) The internal forces of life tend to increase the size of an organism : Lamarck believed that there is some kind of internal force which is constantly working in living beings. Its tries to make the animal bigger in size.
(b) The necessity in animals to produce new structures : According to this law, each organ and structure is the product of some continuous necessity in the animals, to develop it. He believed that it was not due to the direct influence of the environment, but acted through the nervous system, the process being very slow. The plants, he said, were directly influenced by their environmental conditions.
(c) The effect of use and disuse : Lamarck thought that the continuous use of a particular organ or structure tends to increase its size and its development. On the other hand, disuse results in disappearance of that structure, the process being slow and gradual. He supported this by a number of well-known examples. The long neck of giraffe was the result of continuous stretching to obtain food from the trees (long neck was acquired due to excessive use). The wings of certain birds Ratitae were not used for long, as there were no formidable enemies and food was available in abundance. So they did not fly and consequently lost the power of flight. Their wings became rudimentary (flightless condition acquired due to the disuse of wings).
(d) Inheritance of acquired characters : Lamarcks stated that all the characters, which are acquired in one’s own lifetime are inherited by offsprings.
Criticism : The greatest draw-back in the Lamarck’s work was that it was too theoretical and there were no proofs to support it. His ideas were theoretically sound but practically they had no standing. He met a severe criticism from various workers –
(1) The most serious blow came from Weisman who put his theory of continuity of germplasm which states that the inheritance is the sole concern of germ cells. Characters introduced in the germ cells will only be inherited and not those which are present in somatic cells.
(2) If the acquired characters were to be inherited, as Lamarck said, the world would have been full of cripples, blinds and deformed persons, as most of these characters are acquired.
(3) Some workers have practically proved that mutilations are not inherited by offspring’s even if practiced for generations.
Lamarck’s second and third laws show much truth in them; but they are not the sole cause of evolution. The laws of heredity of Weisman are practically opposed to the fourth law of Lamarck. Nevertheless, there came a number of workers who supported Lamarck and modified his laws. They were known as Neo-Lamarckian and the names of a few of them are : Cope, Haeckel, German savant, Gadow and Spencer.
(ii) Germplasm theory : Germplasm theory was given by Weisman. Later on, Weisman also admitted that the germplasm may become modified to a certain extent by some environmental factors.
q Key point to Lamarck’s view about organic evolution is that every offspring inherits characters acquired by the parental generation.
q Law of “Inheritance of acquired characters” is presented by Lamarck.
q Lamarck’s theory was most severely criticized by cuvier who greeted it by calling it “Nouvelle Folie.”
Table : 1-14 Difference between Lamarckism and Neo–Lamarckism
Lamarckism
Neo–Lamarckism
It is original theory by Lamarck.
It is a modification of the original theory of Lamarck in order to make it more suitable to modern knowledge.
The theory lays stress on internal force, appetency and use and disuse of organs.
Neo–Lamarckism does not give any importance to these factors.
It believes that changes in environment brings about a conscious reaction in animals.
The theory stresses on the direct effect of changed environment on the organisms.
According to Lamarckism the acquired characters passes on to the next generation.
Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.
(iii) Darwinism : Charles Robert Darwin was undoubtedly the first naturalist who put the idea of organic evolution on sound footing. His statements and theories were based upon practical experiences and large number of proofs which he collected directly from the nature. He devoted his whole life for the purpose of finding out proofs in support of the theory of organic evolution.
Table : 1-15 The natural selection theory may be summarized by a chart devised by wallace
Facts
Inference
A
1. Enoromous fertility
2. Limited food and space
Struggle for existance
B
1. Struggle for existance
2. Variations
Survival of fittest or natural selection
C
1. Survival of fittest
2. Continuous environmental changes
Origin of new species
q Charles Darwin wrote Orgin of species.
q Darwin was appointed up on a world survey ship of British government H.M.S. Beagle.
His main ideas about the evolution are given below –
(a) Over-production of offsprings : The power of reproduction is enormous in the living beings. The single Paramecium (Protozoa) divided about 600 times in a years. If all the progeny survive their total amount would exceed that of earth in a few months. Again if all the eggs of a lobster were to produce young ones, in about ten years time the sea would be full of lobsters and there will be hardly any space for other animals. Darwin gave one more interesting example of elephants. An elephant lives for about a hundred years and starts reproducing at the age of thirty. If each female produce six young ones, in 750 years, about 1,90,00,000 would be alive.
(b) Limited supply of food and shelter : The amount of food and shelter is limited in a particular area. It is sufficient only for a definite number of individuals.
(c) Struggle for existence : It is a common experience, that even with the enormous rate of reproduction, the number of species mostly remains stationary. The nature has provided a number of checks over their population. There is limited food, breeding places, shelter, presence of predators and parasites, etc.
There is always going on a struggle for existence among various agencies. The struggle for existence may be–
Inter-specific : When two different groups of animal species are opposed to each other, i.e., lion and deer or birds and insects.
Intra- specific : When there is struggle between the members of the same species, as all of them have same wants and requirements.
Environmental : It is the struggle between the animals and their environment (i.e., climate, vegetation, open spaces, jungles and water, etc.,) A change in climate may affect adversely, resulting in the extinction of some and the survival of others.
(d) Universal occurrence of variations : The “departures form the original pattern” and changes in animals were termed as variations. Darwin believed that continuous and useful variations constitute the raw material of evolution.
(e) Survival of the fittest or Natural selection : In struggle for existance, only those organisms survive which possess the most useful variations. This has been called ‘Natural selection’ by Darwin and ‘Survival of fittest’ by Spencer.
(f) Inheritance : The useful variations are inherited by the progeny.
(g) Origin of new species : Favourable variations accumulate over generations to ultimately form a complete new species.
q To explain inheritance of characters from one generation to another Darwin proposed ‘Theory of Pangenesis’. According to this theory each somatic cell produce pangene. All the pangenes from body cells accumulate in gametes and transfer characters to next generation.
Most of the biologists agree with the Darwin’s theory as the best explanation of organic evolution. But there are a number of objections to this theory.
q By performing the replica plating experiment, Lederberg supported the “Natural Selection Theory”.
Objections
(1) Darwin’s theory does not explain that the effects of ‘use and disuse’ of organs are inherited.
(2) He considered the minute fluctuating variations as the cause of natural selection, but most of the variations are non-heritable.
(3) He did not distinguish between germinal and somatic variations. His theory of Pangenesis has no basis at all.
(4) He believed that variations occur in all directions haphazardly. But now it is established that the variations occur only on definite lines of change.
(5) Darwin called mutations as ‘SPORTS’ but being unaware about genetics he couldn’t explain these.
(6) Darwin’s theory explains survival of the fittest but not about arrival of the fittest.
(7) It does not explain how natural selection could make use of certain adaptive characters in their initial stages, i.e., what would be the use of electric organs, electric fishes, until they have enough of power to produce a shock.
(8) It does not explain the over-specialised and vestigeal organs. Overspecialization of certain characters proved harmful such as Antlers of Irish elk, teeth of Swedolon, heavy armour of dionosaure.
(9) According to him, only useful characters are inheritable but on the contrary certain useless and non-adaptive characters are also passed on.
(10) Geologists and astronomers think that the time required producing organic world is much more than the actual age of the earth today.
In the light of these criticism and objections various workers after him modified his theory. Most of the work was done after the rediscovery of Mendel’s work in 1900. This modified theory of Darwin is known as Neo- Darwinism.
Examples of Natural selection
(1) Industrial melanism : Industrial melanism is a phenomenon where the moths living in the industrial areas, develop black colour (melanin pigments) to match the body to soot-covered background, on the bark of trees.
The industrial melanism is observed and worked out by a number of evolutionists like Fisher, Ford and Kettlewell.
Industrial melanism was observed in a peppered moth Biston betularia living, Manchester, in an industrial city of Great Britain and it is the example of ‘evolution taking place before the eyes’.
The change of the lighter coloured variety of peppered moth, Biston betularia (typica), to its darker variety (carbonaria) is due to mutation of a single Mendelian gene for survival in smoke-laden industrial environment.
The peppered moths exist in two forms, namely melanic forms and non-melanic forms. The melanic forms are black in colour because they contain melanin pigments. The melanic forms are also called carbonaria. The non-melanic forms are light coloured. The light colour is due to the absence of melanin pigments and are called Biston betularia typica.
(2) Resistance to DDT : The resistance is a character controlled by genes. The resistant flies arise as a result of the application of DDT. They reproduce more and more resistant flies. Thus the resistant populations are evolved as a selective advantage against insecticides.
(3) Resistance of bacterium to drugs : L.L. Cavalli and G.A. Maccacro (1952) experimentally proved that the colon bacteria Escherichia coli develop resistance to the antibiotic chloramphenicol 250 times as great as that tolerated by normal bacteria by exposing the bacteria to increased concentration of the drug.
Table : 1-16 Difference between Darwinism and Neo–Darwinism
Darwinism (Natural Selection)
Neo–Darwinism
It is the original theory given by Charles Darwin (1859) to explain the origin of new species.
Neo–Darwin is a modification of the original theory of Darwin to remove its short–comings in light of genetic researches.
According to this theory accumulation of continuous variations causes changes in individuals to form new species.
Instead of continuos variations, mutations and genetic variations are believed to help form new species.
It believes in the selection of individuals on the basis of accumulation of variation.
Variations accumulate in the gene pool and not in the individuals.
Darwinism does not believe in isolation.
Neo–Darwinism incorporates reproductive isolation as an essential component of evolution.
It can explain the origin of new characters.
The theory can explain the occurrence of unchanged forms over millions of years.
Darwinism cannot explain the persistence of certain forms in the unchanged condition.
Normally only those modification are transferred to next generation which influence germ cells or where somatic cells give rise to germ cells.
(iv) Mutation theory
(a) Hugo de Vries (1901), a Dutch Botanist, pioneered the theory of mutations to explain the mechanism of evolution.
(b) The plant on which de Vries had experimented was Oenothera lamarckiana (Evening primrose).
(c) The mutation observed by de Vries in Oenothera essentially was chromosomal number variant.
(d) Mutations are discontinuous variations, called ‘sports’ by Darwin and ‘saltatory variations’ by Bateson.
(e) Mutations are generally harmful and recessive.
(f) Role of mutations in evolution is genetic variations.
(g) Mutations are due to changes in chromosomes, genes or DNA.
(h) To be a successful event for evolution, a mutation must occur in germplasm DNA.
(i) Mutations are the changes which may or may not be inherited.
(j) Germinal mutation is a change that is inherited.
(k) Hereditary variations in plants have been produced by use of X-rays.
(l) Frequency of a mutated gene in a population is expected to increase if that gene is selected by nature.
(m) The possibilities of hereditary and evolutionary changes are greater in species that reproduce by sexual means.
(n) Organic evolution would not have taken place if individuals in a population did not show gene variations.
(v) Variations : Dissimilarities between members of the same species are called variations. Tendency to differ helps organisms in their adaptations for different environmental conditions. Heritable variations are responsible for changes in a species to form new species. Variations are thus important in evolution.
Variations are progressive factors in evolution. Members of the same species exhibit variations by structural, physiological or psychological dissimilarities.
Types of variations : Variations are classified in three sets.
(1) Germinal and somatic variations : Germinal variations arise in germplasm of the organism. They occur in the gene pattern and are inheritable. These variation reach the zygote through gametes and hence are inherited from generation to generation. For example colour of eyes or hair occur since birth whereas characters of height and body built develop later in life. Germinal variations are also called blastogenic.
Somatic variations are produced due to environmental factors. They develop in somatic cells or somatoplasm and are also called acquired variations. These are non-heritable. Darkness of skin due to working in the sun, development of intelligence by better education, achievements of a musician, muscular body of an athlete, bored nose and pinna in ladies, are some examples of acquired variations. Somatic or acquired variations are not important in evolution.
(2) Continuous and discontinuous variations : Continuous variations are small and graded variations which are found in the members of same species. Darwin called them as fluctuating variations and realised their importance in evolution. Darwin considered all fluctuating variations inheritable from one generation to other to form new species. They were considered important factor for natural selection. For example children of the same class and age show continuous variations in their height and intelligence but they will be much different from children of other classes.
Discontinuous variations arise suddenly and they are distinctly visible in the group. These ungraded variations deviate so much from the average character that they are seen in new form among members of the same species. There are no grades or intermediate stages in such variations. Darwin called these variations as sports while Hugo de Vries called these as mutations. These discontinuous variations are stable and inheritable. Polydactyly or more than five digits in hands or feet in man, occurrence of four horns instead of two in goat etc. are mutations.
(3) Determinate and Indeterminate variations : Determinate variations arise in a definite direction and time and are due to adaptations. These are found continuously and progressively in organic evolution and are affected by strong gene combination from generation to generation. Leaf eating moth Diabrotica soror is a good example of this variation. In this moth many different colours are found other than main colour.
Indeterminate variations do not arise under any special condition and can arise suddenly in any direction. These can develop to any extent.
Causes of variations
(a) Environmental conditions
(b) Inherent tendency to vary
(c) Dual parentage
(d) Nuclear reorganization
(e) Change in the gene pattern
(vi) Synthetic theory
(a) Dobzhansky (1937) in his book ‘Genetics and Origin of Species’ provided the initial basis of synthetic theory.
(b) ‘Modern synthetic theory of evolution’ was designated by Huxley in 1942.
(c) Some of the important workers who have contributed to the modern synthetic theory are : Th. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins.
(d) According to synthetic theory there are five basic factors involved in the process of organic evolution. These are :
(1) Gene mutations
(2) Changes in chromosome structure and number
(3) Genetic recombinations
(4) Natural selection and
(5) Reproductive isolation.
(e) While the first three factors are responsible for providing genetic variability, the last two are responsible for giving direction to the evolutionary processes.
(f) Besides the five factors outlined above, there are two accessory processes, namely migration of individuals from one population to another and hybridization between races, species and even related genera, which contribute to the evolution.
(g) The most accepted and recent theory of organic evolution is the synthetic theory.
(vii) Hardy-weinberg equilibrium
(a) Mutations introduce new genes into a species resulting a change in gene frequencies.
(b) G.H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, in 1908 established a simple mathematical relationship to the study of gene frequencies.
(c) If certain conditions existed, gene frequencies would remain constant.
(d) The conditions necessary for gene frequencies to remain constant are :
(1) Mating must be completely random.
(2) Mutations must not occur.
(3) Migrations of individual organisms into and out of the population must not occur.
(4) The population must be very large.
(5) All genes must have an equal chance of being passed to the next generation.
(e) According to Hardy-Weinberg concept, the gene frequencies will remain constant if all above five conditions are met.
(f) The distribution of genotypes could be described by the relationship where represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous recessive genotype and a2 represents the frequency of the homozygous recessive genotype.
(g) Constant gene frequencies over several generations indicate that natural selection and evolution are not taking place.
(h) Changing gene frequencies would indicate that evolution is in progress.
Human Evolution
During the course of evolution different animal species evolved special organs for a successful life like wings in insects, birds and bats for flying, claws for holding, burrowing and climbing in rats and squirrels and fins or paddles in aquatic life for swimming. Similarly, most significant event in human evolution has been evolution of brain which enabled him to become most superior member of animal kingdom. The large and more complex brain evolved greater ability of thinking, logical power and capacity of taking decisions depending on the situation.
Present human species is named as Homo sapiens sapiens (Sapient = wise). T. H. Huxley (1863) in his book ‘Man’s Place in Nature’ made first attempt to explain scientific grounds of upbringing of man. Charles Darwin (1871) in his book ‘The Descent of Man’ gave his ideas about ancestry of man.
(i) Systematic position of man in animal kingdom
Phylum :Chordata
Subphylum :Vertebrata
Class :Mammalia
Order :Primates
Suborder :Anthropoidea
Superfamily :Hominoidea
Family :Hominidae
Genus :Homo
Species :sapiens
(ii) Place of descent of man : Available fossils give the evidence that most of the fossils of prehuman ancestors including monkeys and apes have been recovered from Africa, Asia and Europe. However, descent of man is supposed to have occurred in Asia due to following reasons – (i) Civilization of Asia is oldest. (ii) Asia is the land of origin of all domesticated animals and crop plants. (iii) Migration of many animal species has taken place in Asia. (iv) Fossils of many ancestors of man have been found in Java and China which are part of Asia. (v) Climate of Asia was favourable at that time for the evolution of man.
(iii) Time of descent of man : Although time of descent of man is a controversial subject but fossil evidence indicates that time of evolution of man is from Miocene epoch to the beginning of Pliocene epoch of Tertiary period in Coenozoic era. It can be estimated that ancestral man evolved from man like apes about 1 crore 32 lakh years ago.
(iv) Evolutionary characteristics of man : The modern man possesses following special features, which have been acquired during the course of evolution–
(a) Bipedal locomotion.
(b) Large brain and cranial cavity.
(c) Grasping hands and feet.
(d) Erect posture.
(e) Stereoscopic (binocular) vision.
(f) Sensitivity
(g) Social organisation.
(h) Expression by speech.
Morphological changes that have occurred in man during the process of evolution are
(a) Increase in brain size and intelligence.
(b) Attainment of erect posture.
(c) Flattening of face.
(d) Shortening of body hair and reduction in their number.
(e) Elevation and narrowing of nose.
(f) Increase in height.
(g) Reduction of brow ridges.
(h) Rounding and enlargement of cranium.
(i) The bowl like form of pelvic girdle and broad ilia to support the viscera.
(j) Formation of chin.
The vertebrates of class mammalia as well as other primates have common ancestory with monkeys and apes. Mammals evolved from primitive reptiles in early Jurassic period (about 210 million years ago).
Humans belong to family hominidae in which Homo sapiens is the only living species. The evolutionary history of man has been built up on the basis of study of fossils and molecular homology.
The earliest hominid stock included fossils of Ramapithecus and Sivapithecus unearthed from Africa and Asia. Several species belonging to genus Homo can be recognised from fossil record. Human evolution took place in Africa and Asia.
A common ancestory for great apes and man has been deduced on the basis of similarities in DNA content, chromosome number and banding pattern of chromosomes.
(v) Early human ancestors : The fossils Ramapithecus and Sivapithecus which lived in Africa and Asia (about 1015 million years ago) are believed to be the forerunners of Hominids. These were first man–like primates. The first fossil of Ramapithecus was a fragment of upper jaw recovered from the Shivalik Hills of India. Ramapithecus and Sivapithecus must have a sort face, small brain case, thickly enameled large teeth and they must have been used to walk on their knuckles.
(a) Australopithecus(the first man–ape) : Its fossils were described by Raymond Adart in 1925 from South Africa. These were intermediate between Ramapithecines and genus Homo.
Australopithecines are considered to be ancestral to all hominids of genus Homo.
Australopithecines must have been small statured averaging about four feet. They walked nearly or completely straight. The vertebral column had a distinct lumbar curve with pelvis broad and basin–like. The teeth were larger than those of modern man. though jaws and teeth were larger than those of modern man. Their face was prognathous and a chin was absent. The bulge of occipital region was small. Eyebrow ridges projected over the eyes.
Their brain capacity ranged from 450–600 mli.e., slightly larger than that of modern adult Chimpanzee. Thus, Australopithecine’s represented man with an ape–brain.
q Ramapithacus has been known from shivalik hills in India.
q Australopithecus stood erect.
q G. E. Lewis discovered fossils of Ramapithacus.
q Australopithacus have 500 c.c. cranial capacity.
(b) Homo erectus (The forerunner of Modern Humans) : In the Middle Pleistocene period, Australopithecines were succeeded by large brained form which were described under the name Pithecanthropus or Java man. Its first fossils were obtained by Dubois (1891). These were named Pithecanthropus erectus (erect ape–man). Similar fossils were found in a cave near Peking, China, and were named Sinanthropus pekinenis.
Mayer (1950) has replaced these names by Homo erectus–
(1) Java Man (Homo erectus erectus = Pithecanthropus erectus) : Its fossils occurred in the Pleistocene deposits about 500,000 years ago. Its cranial cavity was about 940 c.c; (intermediate between that of Australopithecus (600–700 c.c.) and modern man (1400–1600 c.c.) It was more than five feet tall with skeleton much like ours. Its forehead was low and slanting. The face was prognathous, and jaws were massive with huge teeth. The chin was absent and bony eye. He might have learnt the use and construction of tools and knew how to lit fire.
q Fire was first used for protection and cooking by Java man.
(2) Peking man (Homo erectus pekinensis = Pithecanthropus pekinensis–Sinanthropus pekinensis) : These perhaps lived 500,000–2,00,000 years ago. It was very similar to Java man with heavy bony eyebrow ridges, low slanting forehead and chinless face. However, their cranial cavity was much larger as compared to Java man ranging from 850–1200 ml. and averaging 1075 c.c.
q The skill of pithecanthropus was found in Java.
q The fossils of sinanthropus pekinensis have been discovered in pleistocene epoch.
(3) Homo sapiens (Late Pleistocene Man) : Homo erectus were succeeded by early Homo sapiens, which were described under different names Homo neanderthalensis, Homo heildelbergensis, etc. But, since they are grouped under Homo sapiens.
The fossils of primitive man were found in Europe, Asia and Africa. These are Heildelberg man, Neanderthal man, Solo man and Rhodesian man.
Heidelberg man : Their jaw is large and heavy and lacks a chin. Teeth are like those of modern man. Heidelberg is regarded as an ancestor to Neanderthal man and contemporary to Homo erectus.
Neanderthal man : Their fossils were found in the Neanderthal valley in Germany. Previously, it was named as H. sapiens neanderthalensis. These arose some 1,50,000 years ago and flourished in Europe, Asia and North Africa. These were similar to us below the neck, and were heavily built with outwardly curved thigh bones.
The skull bones were thick, forehead was low and slanting and the eyebrow ridges were heavy. The jaw was deep with no chin. The cranial capacity was about 1450 c.c. (almost equal to the modern man). But its lower and posterior portions were larger than the upper and anterior parts. It was quite intelligent to use and construct tools. It buried its dead and could perform ceremonies as well as constructed hut–like dwelling structures.
q Neanderthal man lives in cave.
Solo man (Homo solonensis) : Fossils were discovered from the banks of solo river. They had heavy eyebrow ridges but forehead was receding type. Brain capacity was 1300 cc.
Rhodesian man (Homo Rhodesiensis): Fossils of Rhodensian men were found in Rhodesia in the large limestone cave. Their skull had a cranial cavity about 1300 c.c. with receding forehead and ridge was protruded out.
Homo sapiens fossils
Cro–Magnon Man : These lived during last 30,000 years or more in Europe. These succeeded Neanderthals and became extinct about 10,000 years ago in the last glacial period.
These were about 180 cm. in height with a large skull, broad face, rounded forehead, narrow nose and a prominent chin. They lacked eyebrow ridges. The cranial cavity was about 1660 c.c. These were cave dwelling and hunters. They made tools from stones and ornaments from ivory.
q Cro–Magnon had perfectly orthognathus face.
q Cro–Magnon is the most recent ancestor of “Homo sapiens”.
q Cro–Magnon man was expert in making tools, weapons, paintings etc.
q Cranial capacity of modern man is 1350–5000 cm3.
Modern Man (Homo sapiens–sapiens) : After last glacial period i,e., about 10,000 years ago, Homo sapiens–sapiens appeared and began to spread all over the globe. He learned to cultivate plants and domesticate animals of economic importance. These were the first settlers who started living a settled life.
? Father of Virology W.M. Stanley (American Microbiologist).
? Edward Jenner (1796) developed the first successful vaccine against viral disease small pox.
? D’ Herelle (1917) coined the term “bacteriophage” for bacterial virus.
? Caulimo virus (cauliflower mosaic virus) are double stranded DNA virus
? Lindemann (1957) did the first successful vaccination against Polio.
? Single virus observed under electron microscope, outside host is called “Virion”.
? The first virus to be cultured in human cells was Polio virus.
? Most of the phase are DNA virus.
? Retroviruses and reverse transcription were reported by Temin and Baltimore.
? Viruses can pass through bacteria proof filters. These are the intermediate connection between living and non living.
? Some animal viruses covered by a lipo-proteineous envelope. It also contains carbohydrate (found in influenza virus).
? Viruses have host specificity. A specific virus infects only a particular host.
? Virus are lack protoplasm.
? In the world which do not have cell are virus, viroids and prions.
? Substance which can inactivate to viral activities are known as antiviral agents or virucide.
? The synthesis of viral proteins takes place on host ribosomes.
? Viruses lack pigments metabolic activity, they made up by RNA only movement and sex organs, but some enzyms are found in them.
? AIDS is caused by HIV. It infects T–lymphocytes. HIV virus remains dormant for about 8 years. Infected person does not suffer a symptoms during this period. AIDS day is 1st December.
? Size of virus is 20nm. – 300 nm. Largest virus is – vaccinia or cow pox-virus (500nm). Smallest virus is – Alfa-alfa virus (17 nm).
? Pox virus is also known as vip virus.
? Five genes are present in a simplest virus.
? Hugo de Vries (1901) proposed Mutation theory of Evolution according to which new species are formed form the pre–existing species in a single generation by sudden and inheritable changes called mutations. The mutated individuals are called mutants. His theory was based on the analysis of his observation on the experiments of evening primrose (Oenothera lamarckiana).
Huge de Vries worked on Oenothera lamarckiana.
? Neo–Darwinism gains strength from a number of evidences like industrial melanism, origin of DDT–resistant mosquitoes, geographical distribution of sickle–cell anaemia etc.
? Artificial selection is the phenomenon in which man interbreeds the genetically different individuals and selects the genetically improved domesticated animals and plants.
? J. Lederberg and E. Lederberg provided experimental evidence for ‘selection’ in bacteria. By using replica plating technique, they demonstrated the process of ‘selection’ of antibiotic–resistant strains of bacteria.
? Speciation is the formation of new species. It may occur by gradual drifting apart of two or more species which later on cease to interbreed. Species formed may be allopatric or sympatric. Rapid speciation occurs by polyploidy.
(a) Aseptate and multinucleate (b) Septate and multinucleate (c) Aseptate and uninucleate (d) Septate and uninucleate Answer. (b) The mycelium of Aspergillus consist of branch septate hyphae. Cells of hyphae are multinucleate. (185)
(a) Parenchyma (b) Chlorenchyma (c) Plectenchyma (d) Collenchyma Answer. (c) Plectenchyma (Pseudoparenchyma) is nothing but the interwoven mass of fungal mycelium which in cross-section look like parenchyma. (108)
(a) Colletotrichum falcatum (b) Phytophthora infestans (c) Potato mosiac virus (d) Alternaria solani Answer. (b) Late blight of potato is a seed born disease which is caused by Phytophthora infestans. The disease is characterised by brownish to blackish dead areas …
