THEORIES OF ORGANIC EVOLUTION NEO-DARWINISM.pdf

THEORIES OF ORGANIC EVOLUTION NEO-DARWINISM.pdf

1. SCHOOL OF STUDIES IN ZOOLOGY
JIWAJI UNIVERSITY, GWALIOR. (M.P.)
Paper 401: Animal Taxonomy and Evolution
Topic:Theories of Organic Evolution (Neo-Darwinism)
Dr. Hemant Samadhiya

2. Contents
•Introduction
•Lamarckismtheory
•Darwin'stheory
•Mutationtheory
•Neo-Darwinism
•Conclusion
•References.

3. INTRODUCTION
ThetermorganicevolutionwasproposedbyCharlesDarwinand
signifiesas“Descentwithmodification”whichstatesthat,thepresent
complexplantsandanimalshaveevolvedfromearliersimplerformsof
lifebygradualchangesoveralongperiodoftime.
Accordingtoorganicevolution,unicellularorganismsevolvedfrom
simpletomulticellularcomplexforms.
Hence,theevolutionissimplyprocessofcontinuouschange.
ThevarioustheoriesoforganicevolutionareLamarckismtheory,
Darwinismtheory,Mutationtheory,Neo-Darwinismtheory.

4. MUTATION THEORY
•AccordingtotheHugoDeVaries“Evolutionisajerkyand
discontinuousprocesswhichiscausedbythesuddendiscontinuous
inheritablegeneticvariationsormutationsthatattainfullconstancyat
onceandgiverisetonewelementaryspecies”.
•Hencemutationsaretherawmaterialsforevolutionandtheymaybe
useful,uselessorharmful.
•Natureselectsonlyusefulvariations.
•Themainpostulatesofthistheoryare:

5. •Rawmaterial:Mutationsaretherawmaterialforevolutionand
evolutioncan’toccurwithoutmutation.
•Suddenvariations:Mutationsappearallofsudden.
•Intermediateforms:Mutationsdon’tfluctuatearoundthenormalsites
thespecies.
•Expression:Mutationsappearfullconstancyatonceandexpresstheir
effectimmediately.
•Naturalselection:Beneficialmutationsareselectedbynaturebutnot
harmfulmutations.
•Dis-contioniousevolution:Evolutionisnotacontinuousprocessbutit
isjerkyanddiscontinuousprocess.

6. NATURAL SECLECTION THEORY
ItwasgivenbyDarwin,accordingtothistheory,“Organicevolution
occursthroughnaturalselectionandaccumulationofsmall
variationswhichprovidestructuralandfunctionalsuperiorityover
othersintheirsurvivalanddifferentialreproduction.
Themainpostulatesofthistheoryareas:
Rapidmultiplication:Organismshavehighbioticorreproductive
potential.
•Limitedfoodandspace:Everyecosystemhasalimitedcarrying
capacityforitsdifferentmembersofbioticcommunity.

7. Struggleforexistence:Everyorganismhastostruggleforfood,
reproductionandspaceforsurvival.
Variations:Variationsoccurinallindividualse.g.size,shape,structure
etc.Onlyusefulvariationsareselected.
Naturalselection:Itiscommonlycalledas“survivaloffittest”.
Eachgenerationproducesalargenumberofoffspring's,however,only
fewsurviveswhileothersareeliminated.
Inheritanceofusefulvariations:Darwinbelievedthattheselected
individualspasstheirusefulvariationstotheiroffspring's,sothatthey
arebornfittothechangedenvironment.

8. NEO-DARWANISM
Itistheimprovementoforiginaltheoryof“naturalselection”inorder
toremoveitsdefectsandincorporatelatestinformation.
Itbelievesthatonlygeneticvariationsareinheritable.
Naturalselectionoperatesthroughdifferentialreproduction.
Themainpostulatesofthistheoryare:
Geneticvariability:Variationsaretherawmaterialsforevolution.
•Theunitsofbothheredityandmutationsaregeneswhicharelocatedin
linearmanneronthechromosome.
•Thevarioussourcesofthegeneticvariabilityingenepoolare
chromosomalaberrations,genemutations,geneticdrift,migration,
hybridizationandrandommatting.

9. •Natural selection: Natural selection of Neo-Darwinism differs from
that of Darwinism that “It does not operate through the survival of
fittest or Natural Selection” but it operates through the differential
reproduction and comparative reproductive success.
•Differentialreproductionmeans“Thosemembersofthepopulationare
bestadoptedtotheenvironmentthatproducemoreoffspring'sthan
thosewhicharelessadoptedandthegenesofsuchindividualswill
remainpredominantinthegenepoolandpasstothenextgeneration”.
•Reproductiveisolation:Itismustsoastoallowtheaccumulationof
variationsleadingtotheformationofnewspeciesbypreventing
hybridizationandithelpsinevolutionarydivergencei.e.,polytypic
species.

10. LAMARKASIM THEORY
Itisalsoknownas,theoryofinheritanceofacquiredcharacters,
givenbyLamarck.
Itisdefinedas,thechanges(variations)developedinthebodyofan
organismfromnormalcharacters,inresponsetochangesin
environment,orinthefunctioning(useanddisuse)oforgans,in
theirownlifetime.
Themainpostulatesofthistheoryare:
INTERNALVITALFORCE:
•Everyorganismhasaninternalvitalforcethatcausesthemtoincrease
insizeandundergoelaborationofitstraits.

11. Perfectingprinciple:
•Itisalsocalledasfitnessofdesign.
•Everyorganismhasatendencytoperfectlyadoptthemselvestothe
environment.
Changeinenvironment:
•Achangeinenvironmentsetsinanewchainofadaptationsintheorganisms.
Newneeds:
•Itisalsocalledas“DoctrineofDesires”or“Appetency”.
•Newneedsordesiressetinanewmovementthatculminatesintheformation
ofneworgan.
•Italsoproducesachainofmodificationsduetohabitchange,modificationsof
existingorgans.

12. Useanddisuseoforgans:
•Growth,development,formandfunctioningofanorgandependsupon
itsuseanddisuse.
•Continueduseoforgansmakesitlarger,moreelaborate,strongand
efficientandviceversa.
Inheritanceofacquiredcharacters:
•Newtraitsareacquiredbyorganismsduringtheirlifetimedueto
internalvitalforce,newneeds,useanddisuseoforgans,andarepassed
tothenextgeneration.
Isolation:
•Newtraitsdevelopduetoisolation.

13. CONCLUSION
•Organicevolutionsimplymeansdescentwithmodification,which
encompassesevolutionarychangeatgenetic,orginasimalorpopulation
level.
•Lamarckbelievesthatevolutionoccursthroughtheinternalvitalforce
anduseanddisuseoforgans.
•Darwin'stheorybelievedthatevolutionoccursthroughnaturalselection
orsurvivaloffittestandinheritanceofusefulvariations.
•Hugodevariesbelievedthat,evolutionoccursthroughusefulmutations
butnotharmfulmutations.
•Neo-Darwinismbelievesthatevolutionoccursthroughdifferential
reproductionandcomparativereproductivesuccess.