5 Editorial7 Father of Various Branches of Biology7 Important Branches of Biology8 Origin and Evolution of Life : Modern

Concept on the Origin of Life12 Cell : The Basic Unit of Life14 Biological Molecules of the Cells18 Classification of Plants29 Important Terms and Parts of Angio-

spermic Plant30 Modifications of Stem31 Modifications of Roots31 Ecology : Organism and Environment35 Air Pollution37 Genetics38 Monohybrid Cross41 Twins of Human41 Plant Nutrition41 Mineral Requirement of Plants43 Photosynthesis44 Human Nutrition45 Vitamins and Minerals

47 Digestion in Human48 Respiration49 Blood Circulatory System in Humans50 Human Blood51 Blood Clotting51 Lymphatic System52 Excretory System52 Kidney53 Endocrine System53 Pituitary Gland54 Thyroid Gland54 Adrenal Gland

55 Thymus Gland55 Pineal Body55 Pancreas55 Intestinal Mucosa55 Testis55 Ovary56 Fruit56 Placenta56 Phytohormones57 Scientists and their Contributions58 True and False Fishes59 True and False Worms60 Zoo and Museums60 Famous Research Institutes in India60 Some Important Facts About Human

Body61 Classification of Viruses on the Basis of

Genetic Material63 Important Canals and Ducts64 Some Interesting Plants and Animals65 Important Resin Yielding Plants65 Beverages65 Important Commercial Woods66 Some Important Fumitories and

Masticatories66 Spices and Condiments66 Gestation Period of Different Animals67 Important Diseases, Causal Organisms,

Symptoms and Control Measures ofRice, Wheat and Pulses

72 Descriptive Type Exam-OrientedQuestions-Answers

86 Agriculture92 Objective Questions

P.D./Gen. Sc.-2 / 3

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50.00

P.D. / Gen. Sc.-2 / 5

Make Loneliness a Boon of LifeMake Loneliness a Boon of LifeMake Loneliness a Boon of Life To be successful in life one has to go alone and not

wait for outer help. Working with full devotion and

concentration does not let a person feel lonely. God

is always with him as service of the society is synony-

mous with God.

Gurudeva Rabindra andMahatma Gandhi used to say ‘Goalone’ ‘,d`¥k p¥ks js’ Like this thereis another important adage that youare least alone while all alone.

When we are all alone we canbetter concentrate on any subject andthink over it seriously and we can doour work with full capacity. But is itpossible that a social being can be allalone ?

‘,d`¥k p¥ks js’ ‘go alone’ meansthat we should proceed on our patheven when we are all alone, thosewho desire to accompany us willcome along by themselves, but thisline has really a deeper meaning.

If we look at the deeper meaningof these two lines then we see thatthey mean almost the same thing. Thefamous dramatist Ibsen has used theword self-dependent for ‘all alone’.He has written that “he is the mostpowerful man in the world who liveslike a self dependent person.” Fromour point of view Ibsen has givenaltogether a new meaning to thisword ‘alone’ and opens up the newdimensions of our working system.Ibsen’s aim is quite obvious that oneshould try to live alone and thinkalone and that will make you live likean independent person.

Generally loneliness has twoforms—physical and mental. Physic-ally when we are alone, we are in aposition to do our work with fullcapacity and without any distur-bance. When mentally alone, we arein a position to fully concentration onthe subject, give it our seriousthought and then take independentand intelligent decision. In short,in both the situations we are able touse our capacity and capability to thefull extent. Pointing to this very con-dition the famous French EmperorNapolean had said, “They proceedfast, who move alone.” The English

Poet Wordsworth has written that“Expressing our innerself is the boonof loneliness.” As soon as yourposition of loneliness is disturbed orsome second person steps in yourthinking, your thinking becomesaffected by the thoughts of others oras aptly put by some wiseman, yourthinking becomes like a second-hand.Thought or a thought taken on loan.This type of thinking creates layer ofignorance and the person slowlydestroys his freedom of thought. Wecan be mentally alone even if we arenot physically in this position. Weshould practise to concentrate on ourinnerself so that we may able to doour work with our full capacity andconcentration. Napolean used to readbooks while riding horse and he usedto read the book with so muchconcentration that after reading it heused to throw the book and say that ithad become his. We can often meetsuch persons as can continue theirwork with concentration, unmindfulof the disturbances and noises madeall around. As a matter of fact suchpersons are so much devoted to theirsubject that they seem to have lost allrelationship and contact with theouter world. Their mind is theirwork, their goal or their inspirer andtheir companion. The result is thatthey do not feel alone even when noone is with them. Some poet haswritten that “One who has taken tothe service of God is not alonebecause God is with him”—

πeq[ eÍ jke uke] jke lsok gkƒ eÍrw vds`¥k ugÓ I;kjs] jke rsjs lkƒ eÍ\∏

The poet’s meaning is obvious.God’s name is the inspirer of serviceof the society and one who he devo-tes to the social service can neverfeel alone. His outer conduct andinner thinking do not let him feel likea lonely person. If we are fully devo-ted to our work and do it with full

capacity then there is no reason whywe feel like a lonely person.

When we are not able to do ourwork alone we take advice and helpfrom others. This type of help cannotbe of permanent advantage to us.This is just like the use of somemedicine, which helps us only to feelhealthy for a short period. How longa person can feel healthy on the helpof some medicine ? How long can aperson live on oxygen ? We shoulddevelop such power that can be ourpermanent possession. This possess-ion cannot be imposed from outsidebut it can be the result of innerdevelopment, this cannot be had fromthe crowd but it should come frominside only. Can a person who moveson clutches never be a healthy andindependent person ? Self depen-dence is our inner instinct. Ibsen hascalled this ‘loneliness’ when we don’taccept as a matter of fact, deny thepresence of God or we rebel againstthe fundamental law of nature.

Gautam Buddha had said that itis a sign of deep ignorance to thinkthat some outer power can help us orgive us salvation. Our true teacherand guide can be that person onlywho preaches us to be self dependentand go alone. Keeping this thing inview such things have been said,“Thou has to travel alone.” Let uskeep ourselves free from outer effectsand not let it get polluted. Being freefrom outer effects and outer pressu-res we can keep our mind pure andfree from all troubles. Any pollutionwill disturb loneliness of the person.There is purity in loneliness andcomplete purity is called completeloneliness.

Outer knowledge and means ofaction are not altogether useless.Saints have said that outer meansshould be used with discrimination.The discriminatory mind has beencompared with that wise goose whichseparates milk from water.

Mind is really very fluctuatingand it is very difficult to keep it undercontrol. Eternal vigilance is the priceof keeping the mind under control. Itis said one should try to make a boonof life.

●●●

P.D. / Gen. Sc.-2 / 7

Biology is a broad field ofknowledge concerned with the

study of all aspects of living orga-nisms which can be approached bythe methods of natural science.Because the field is so broad inscope, it has been divided into twomain areas—botany and zoology—each with a large number of specia-lized branches. No one worker canknow the entire field of biologythoroughly. Specialized subdivisionsof biology pertaining to both plantsand animals include taxonomy, mor-phology, cell biology, physiology,genetics, cytogenetics, evolution andecology. The term biology embracesthose principles of widest applicationon to the origin, growth and develop-ment, structure, function, evolutionand distribution of plants and animals.

For the most part no biologicaldiscipline is exclusive of the others.Each level of biological organizationhas an impact on or is influenced bythose above and below it, and, conse-quently, areas of biological studyfrequently overlap.

Father of VariousBranches of Biology

1. Father of Biology —Aristotle

2. Father of Zoology —Aristotle

3. Father of Botany —Theophrastus

4. Father of Endochrinology—Thomas Addison

5. Father of Modern Botany—Linnaeus

6. Father of Bacteriology—Leeuwenhoek

7. Father of Immunology—Edward Jenner

8. Father of Evolutionary Ideas

—Empedocles

9. Father of Genetics —G. J. Mendel

10. Father of Modern Genetics—T. H. Morgan

11. Father of Microbiology—Louis Pasteur

12. Father of Cytology —Robert Hooke

13. Father of Mycology —Micheli

14. Father of Plant Physiology—Stephan Hales

15. Father of Indian Paleobotany—Birbal Sahani

16. Father of Indian Ecology—R. Mishra

17. Father of Eugenics—Francis Galton18. Father of Mutation

—Hugo De Vries19. Father of Blood Circulation

—William Harvey20. Father of Blood Groups

—Karl Landsteiner21. Father of Taxonomy

—Carolus Linnaeus22. Father of Radiation Genetics

—H. J. Muller23. Father of Medicine —Hippocrates24. Father of Palaentology

—Leonardo da Vinci25. Father of Microscopy

—Marcello Malpighi26. Father of Genetic Engineering

—Paul Berg27. Father of Indian Phycology

—O. P. Iyengar28. Father of Gene Therapy —Anderson29. Father of Polygenic Inheritance

—Kolreuter30. Father of Palynology —Erdtman31. Father of Bio-diversity

—E. O. Wilson32. Father of Agronomy

—Peter De-cresenji33. Father of Indian Forestry

—Sir Dietrich Brandis (1824–1907)

Important Branches ofBiology

01. Agronomy—Management offarms and cultivation of fieldcrops.

02. Agrostology—Study of grasses.03. Anthology—Study of flowers.04. Anatomy—Study of internal

structures.05. Bacteriology—Study of bacteria.06. Biophysics—Physical aspects

of living systems.

07. Biochemistry—Study of chemi-cals constituting the body andchemical reactions.

08. Biotechnology—Utilization ofliving organisms or of substancesobtained from them in industrialprocesses.

09. Cell Biology—Structure, func-tion, reproduction and life-history of cells.

10. Ctetology—Study of acquiredcharacters of organisms.

11. Craniology—Study of skulls.12. Cytology—Detailed structure of

cells.13. Cytogenetics—Cytological basis

of inheritance.14. Cardiology—Study of heart and

its functioning.15. Dendrology—Study of shrubs

and trees.16. Ecology—Study of reciprocal

relationships between organismsand their environment.

17. Endocrinology—Study of duct-less glands and their hormones.

18. Ethology—Study of behaviourof animals.

19. Entomology—Study of life-history of insects.

20. Eugenics—Study of sciencedealing with factors whichimprove successive generationsof human race.

21. Euthenics—The science thatdeals with the improvement ofthe future of humanity by chang-ing the environment.

22. Euphenics—Study of productionof a satisfactory phenotype bymeans other than eugenics.

23. Enzymology—Study of enzymesand their activities.

24. Embryology—Study of develop-ment of embryo and its growthand repair.

25. Floriculture—Cultivation offlower yielding plants.

26. Genetics—Study of heredity andvariations.

27. Genetic Engineering—Geneticmanipulation to produce anorganism with a new combina-tion of properties.

28. Haematology—Study of bloodand disease caused by it.

29. Helminthology—Study of para-sitic worms.

P.D. / Gen. Sc.-2 / 8

30. Herpatology—Study of reptiles.31. Immunology—Study of natural

or acquired resistance to specificdiseases.

32. Ichthyology—Study of fish andits culture.

33. Karyology—Study of nucleus.34. Kinesiology—Study of muscle

movements.35. Limnology—Study of snails.36. Morphology—Study of form

and structure.37. Myology—Study of muscles.38. Microbiology—Study of micro

organisms like bacteria andviruses.

39. Molecular Biology—Study ofchemicals of living at molecularlevel.

40. Mammalogy—Study of mam-mals.

41. Malacology—Study of molluscsand their shells.

42. Neonatalogy—Study of newborns upto the age of twomonths.

43. Neurology—Study of neuronsand nerve rings.

44. Nosology—Classification ofdiseases.

45. Osteology—Study of skeletalsystem.

46. Odontology—Study of teeth.47. Organology—Study of different

organs.48. Ontogeny—Study of various

life-cycles.49. Ornithology—Study of birds.50. Ophiology—Study of snakes.51. Olericulture—Study of cultiva-

tion of vegetables.52. Oncology—Study of tumors.53. Physiology—Study of process

and function associated with life.54. Phylogeny—Evolutionary rela-

tionship among different groupsof organisms.

55. Palaeontology—Study of fossils.56. Parasitology—Study of para-

sites.57. Protozeology—Study of proto-

zoa.58. Pedology—Study of different

types of soil.59. Pomology—Study of fruit.60. Psychobiology—Study of be-

havioural aspects of animals.

61. Phytogeography—Study ofplant distribution on the earth.

62. Paleobotany—Study of distribu-tion and characteristics of fossils.

63. Radiology—Study of effects ofradiation on living being.

64. Sarcology—Study of muscles.65. Syndesmology—Study of bone-

joints and ligaments.66. Serology—Study of blood-

serum.67. Silviculture—Study of cultiva-

tion of timber-yielding plants.68. Tectology—Study of structural

organization of body.69. Trophology—Study of nutrition

science.70. Toxicology—Study of poisons,

including their nature and detec-tion and method of treatment.

71. Taxonomy—Study of nomencla-ture and classification of organ-isms.

72. Urology—Study of urine and itsdiseases.

73. Virology—Study of viruses.74. Zoophytology—Study of drift-

ing microorganisms such as,diatoms.

Origin and Evolution ofLife : Modern Concepton The Origin of Life

Russian biochemist A. I. Oparin(1968) stated that life is a specialform of matter in motion, arising inan orderly manner at a certain stagein the evolution of matter.

Most of the biologists of todaybelieve that life might have originatedon earth sometime in the remote pastfrom non-living substances. Modernbiologists do not believe in discon-tinuous origin of life. They are ofview that life was an eventualoutcome of long process of chemicalevolution which took place duringthe early stages of the earth’s history.The modern concept or thenaturalistic theory suggests that therewas no trace of life on this planetabout 2 billion years ago. The earthoriginated in about 3·5 to 5 billionyears ago.

Urey (1952) postulated that theearth cloud originally containedchiefly hydrogen, methane, inertgases and a large number of ironcompounds.

A. I. Oparin putforth the conceptthat living matter might have origi-nated from the non-living or inor-ganic matter. The same view wasalso expressed by J B.S. Haldane.Oparin published a fascinating bookthe Origin of Life in 1936, in whichhe stated that in the first place carbonmust have been present in theatmosphere of primitive earth in theform of hydrocarbons. Hydrocarbonsmight have united with other com-pounds to produce a variety of organiccompounds, such as alcohols, alde-hydes, oxalic acids and so on andsome of carboxylic acids could reactwith ammonia to form amino acids,the basic molecules of life.

An English biochemist, Dr.Haldane, wrote that the first formedsubstances must have been stored inthe primitive oceans before life origi-nated in them. He called such com-pound laden oceans ‘‘a hot dilutesoup”. He also suggested that onexposure to ultraviolet radiations theinorganic compounds should havebeen converted into organic com-pounds such as aminoacids and poly-peptides (proteins).

Stanley Miller (1953) performedan experiment in which he exposed amixture of water vapour (H2O),methane (CH4), ammonia (NH3) andhydrogen (H2) to continuous electricspark for a week and at the end of theexperiment he found sugars, alde-hydes, amino acids (glycine, alanine)and a number of more complexorganic compounds. This experimentprovided clue that such complexorganic compounds were formed inthe atmosphere as a whole in theremote past during continuous rainsand lightning. Cosmic radiationsmight have made the course ofreactions more easy by supplyingtremendous energy required in theprocess.

Before the first true cell arose,there would have been a protocell, astructure that has lipid-protein mem-brane and carries on energy meta-bolism. A true cell is a membrane-bounded structure that can carry onprotein synthesis. According toRNA-first hypothesis, RNA wouldhave been the first to evolve and thefirst true cell would have had RNAgenes. These genes would havedirected and enzymatically carried outprotein synthesis. Once the protocellwas capable of reproduction, itbecame a true cell and biologicalevolution began.

P.D. / Gen. Sc.-2 / 9

Theories of EvolutionOrganic evolution refers to the

continuous process by which livingbeings have come to their presentforms and functions.

1. Theory of eternity of presentconditions—This theory suggests thatliving forms have existed withoutundergoing any transformation andwill presumably continue to existunchanged in future also.

2. Theory of special creation—This theory is based on the descrip-tions of the Bible. It states that theworld was made by God in six naturaldays. On the first day, the basic mate-rials were made out of nothing. Onthe second day and third day, theancestors of all the living plantssuddenly came into existence. On thefifth day, the ancestors of all theliving animals were created andfinally on the six day God createdhuman.

3. Cosmozoic Theory—Accord-ing to this theory, the living material(protoplasm) might have been carriedaccidentally to the earth’s surface forother planets. But this idea was dis-carded because it did not explain as tohow the ultimate source of proto-plasm evolved.

4. Theory of Catastrophismand Spontaneous Creation—Empe-docles proposed the theory ofspontaneous creation or abiogenesiswhich states that life has originatedrepeatedly from non-living materialsspontaneously. Prominent supportersof spontaneous origin of life wereAristotle, Von Helmont, WilliamHarvey and John Needham.

The theory of castastrophism waspropounded by B. G. Cuvier in viewof the discovery of fossils of certainanimals whose existence antedatedsome of the living types.

5. Theory of Organic Evolu-tion—Charles Darwin (1809–1882)was beyond doubt the foremost figurein the history of organic evolution.The sources of evidence supportingthe doctrine of evolution have comefrom different branches of biology.These are outlined as under—● Evidences from palaeontology.● Evidences from taxonomy.● Evidences from comparative

morphology.● Evidences from biogeography.● Evidences from genetics and

cytology.● Evidences from embryology.

Theories Explaining theCauses of Evolution

1. Lamarckism—Jean BaptisteDe Lamarck (1744–1829) formulatedthe first theory of evolution. Hedevised “the theory of inheritanceof acquired characters”, whichexplains the following points—

(i) Living organisms and theircomponent pasts tend to increase insize continuously.

(ii) Production of a new organresults from a new need.

(iii) If an organ is in continuoususe, it will tend to develop inprogressive way, but those organswhich are in little or no use will tendto degenerate.

(iv) Inheritance of acquired char-acters—Modifications produced dur-ing the life of the individuals wouldeventually find their way into hered-itary material and would then betransferred to offspring.

Objections to Lamarckism—(i) Many organisms do not show

any tendency towards gigantism andin many cases size reduction has beena prominent feature of evolution.

(ii) The idea that organs incontinuous use will tend to develop ina progressive way and those in littleuse will tend to degenerate, appearsmeaningless in practice.

(iii) The idea that the acquiredcharacters are inherited has not beendemonstrated experimentally.

Neo-Lamarckism—A few biol-ogists accepted the Lamarck’s theoryof acquired characters of inheritencein modified form. The prominentamong them were Cope, Giard,Packerd Spencer and McBridge.According to Neo-Lamarckism, theadaptation is the universal feature oflivings. It arises as a result of inter-action between the structure, functionand environment. The changed envi-ronmental conditions can alter habitand structure of the organisms. So theorganisms acquire new adaptation inresponse to new environmental condi-tions and consequently variationsamong plants and animals may result.These variations become establishedgradually in the heredity of race. Thisis the modified version of New-Lamarckism because it does notfollow the general tendency in evolu-tion and stress mainly on the directaction of environment on organicstructure.

2. Darwinism—Charles RobertDarwin (1809–1822) during his

voyage period observed and collectedenormous amount of materials and hewas impressed with the strikingadaptations, found in the animals andplants he collected and with theprogressive changes in the differentspecies of given germs.

Darwin and Alfred RusselWalkice collectively proposed“Theory of Evolution”. The conceptsof this theory are given as below :

(i) Overproduction free fromenvironmental pressure—Everyspecies tends to multiply in geometricprogression. Thus the number of off-spring produced by plants andanimals is always greater than thatwhich could actually survive innature.

(ii) Struggle for existence—Ifthere is overproduction of species,these must start a struggle for exis-tence among members of the samespecies (intraspecific competition) oramong members of different species(interspecific competition).

(iii) Survival of the fittest—Inthe struggle for existence, therefore,individuals having several favourablevariations will have better chance ofsurvival and less fit species will bedestroyed by the physical or bioticfactors. This is known as ‘naturalselection’.

Thus, according to Darwin andWallace, environment is the principalcause of natural selection.

In order to explain the inheritanceof characters, Darwin, proposed TheHypothesis of Pangenesis, whichassumed that all the organs andperhaps all the cells in the body of anindividual produced minute particlesor pangeues which carried informa-tion about them.

Objections to Darwinism—(i) Natural selection is regarded

as a cause of the evolution of newspecies but it is not the sole cause.Because the variations of small mag-nitudes were not taken into consid-eration in the explanation of naturalselection.

(ii) It is not always the case thatuseful variations are selected.

(iii) Darwin’s theory did notexplain as to how variations arose inthe individuals.

Neo-Darwinism—Neo-Darwin-ism is the modification of Darwinismin the light of cytogenetics. Huxley,Dobjhansky, Haeckel and Weis-mann are the chief supporters of thistheory. Darwin’s theory did not

P.D. / Gen. Sc.-2 / 10

explain the origin of variations andcould not distinguish somatic (non-heritable) from germinal (heritable)variations.

Weismann and his supportersdiscarded Darwin’s theory except itsprincipal element of natural selection.Weismann proposed the ‘germplasmtheory’, which made clear cut distinc-tion between somatic and germinalvariations. Darwin was of the opinionthat the mutation resulted mainly bya single force, i.e., natural selection.According the Neo-Darwinism,adaptations resulted from multipleforces and the natural selection is oneof them.

This theory gives better mecha-nism of evolution and states that—● the sources of variations are

mutations and gene recombina-tions through several reproduc-tions.

● chromosomal organization andvariation affect genetic linkageand produce variations in genepool of cross-fertilizing popula-tions.Neo-Darwinism has received

great support from recent studies ofpopulation genetics.

Mutation Theory of Hugo DeVries—Hugo De Vries (1884–1935)propounded the mutation theory in1901 to explain the origin of newspecies. According to this theory, newspecies arise not only by gradualaccumulation of small variations inthe organisms over a long time butalso by appearance of some perma-nent changes, i.e., mutations. In thecourse of his studies on the eveningprimerose (Oenothera lamarckiana),De Vries marked the sudden appear-ance of new characters, which wereheritable. He called such suddenheritable changes mutations. Thenew individuals differed in flowersize, shape, arrangement of buds andsize of seeds, etc. He believed thatsuch differences were due to muta-tions and they provided sufficientmaterials for evolution.

De Vries concluded his theory asbelow—● Mutation occurs in nearly all

directions and may involve anycharacter.

● New elementary species appearsuddenly and attain full con-stancy atonce.

● The same new species are pro-duced in large number of individ-uals. This increases the chanceof selection by nature.

Objections to De Vries Muta-tion Theory—● ‘Mutation theory’ did not give

any proper direction to the evolu-tion unless natural selectionoperated on them.

● If mutations are only accidentsthen mutant animals may not benecessarily adapted to their envi-ronments.

● Genetic studies have clearlyshown that small variationsproduced mutations and generecombinations provide the ‘rawmaterials’ on which natural selec-tion operates.Biogenesis—Theory of sponta-

neous generation was disapprovedthrough the finding that life comesfrom pre-existing life (omne vivum evivo). Francesco Redi placed thor-oughly cooked meat in three jars—(i) uncovered, (ii) covered with perch-ment, and (iii) covered with muslin.Maggots developed only in theuncovered jar. No maggots devel-oped in jar covered by perchment.Flies visited the third, jar and laideggs on the muslin. Egg fallen in thejar produced maggots. Spallanzaniproved his theory of biogenesis byboiling nutrition broth in glass flasks,sealed the flasks and kept them forfew days. The broth remained clearindefinitely with no signs of anyliving things. Louis Pasteur tookbroth in flasks having straight andswan (bent S-shaped) necks, boiledand allowed the broth to cool. Nogerms developed in the broth ofswan-necked flasks though it wasconnected with the atmospherethrough curved necks of the flasks.The dirt particles or any other wastescould not reach the broth becausethey got trapped in the bend of theneck. When the swan necks werebroken, broth developed colonies ofmicroorganisms showing that thesame have come from the air. Thesame happened in the straight neckedflasks.

In a series of experiments basedupon those of Spallanzani, Pasteurdemonstrated the theory of biogenesisand finally disproved the theory ofspontaneous generation.

Modern Synthetic Theories ofEvolution—The modern theory ofevolution is a synthesis of Mendel’sand Darwin’s observations. Mende-lian population in this theory isdescribed as object of evolution. Theheritable variations which appearamong the individuals of such apopulation either by sexual recombi-

nations or by mutations are describedas the raw materials of evolutionaryprocess, and natural selection actingon the heritable variations of popula-tion have been described as mecha-nism of evolution. The principlesinvolved in the evolution of newspecies, according to Modern Syn-thetic theory, may be categorized intoisolation and hybridization.

Isolation—For bringing newspecies into existence, populationshould necessarily be segregated intosmaller units. This segregation ofpopulation into small units is referredto as isolation. Mutations also arisein the isolated species. Isolation maybe brought about by geographicalbarriers such as high mountains orlarge bodies of waters over whichplants and animals cannot pass (geo-graphical isolation) or by factorsrestricting or eliminating interbreed-ing or subsequent reproduction of thehybrids between them (reproductiveisolation).

Hybridization—The specieshybridization is believed to be impor-tant in the production of new formsof plants and animals, although therole which it plays is generallybelieved to be less important thanisolation. Hybridization is possiblebetween two different sexually repro-ducing species. The combinations ofgenes from different individualsincrease the number of variations uponwhich the natural selection operates.Sexual reproduction does not inducestruggle for existence or survival ofthe fittest in the population. Orga-nisms usually struggle rather veryrarely. Moreover, natural selectiondoes not eliminate the unfit indivi-duals from population.

Evolution at Molecular Level—Gene mutation although often ex-pressed in visible characteristics orphysiological activity of the orga-nism, are ultimately caused byrearrangement, addition or deletionof specific nitrogenous bases in thedeoxyribose nucleic acid (DNA).Changes in the order of bases resultin the modifications in the polypep-tide chain produced under the controlof gene and these modifications willresult in production of modifiedenzyme or protein. The modifiedenzyme may ultimately express itselfby affecting a change in the physio-logy of development of the organism.Since a change in one of the nitro-genous bases of DNA may theoreti-

Pratiyogita Darpan Extra Issue Series-6General Science (Vol-2) (Biology)

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