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1. EVIDENCES FROM COMPARATIVE CYTOLOGY Anothertype of evidence thatindicatesthatall formsof life are
relatedcomesfromthe cellularlevel.The veryfactthat the cell isthe unitof structure forall livingorganisms(except
viruses) isthoughttoreflectthe basicrelationshipamonglivingforms.Thisrelationshipisevenfurtheremphasized
by the fact that ithas beenpossibleforbiologiststoconstructa picture of the “generalized”cell fromwhichall other
typescan be inferred.Moreover,all cellsthathave beenexaminedthus farhave a DNA-RNA- proteininformation
and communicationsystem.All formscontainmembranesthatare made upof double-layeredlipoproteins.All cells
(exceptfewbacteria) utilize the glycolyticpathway.Mostbacterial forms,andall uni - and-multicellularorganisms,
have a Krebscycle and an electrontransportsystem.All are basedonATPas an energydonor.Certainlythese
factors provide anoverwhelmingdemonstrationof the interrelatednessof biological forms.EVIDENCESFROM
GENETICS Genetics,the science of heredity,dealswiththe variabilityof plantsandanimals.Hereditaryvariations
provide the rawmaterial of evolution.There are mainlytwosourcesof hereditaryvariationsnamelyrecombination
and mutation.While recombinationsafterhybridization yieldnew combinations,mutationswill create newgenetic
material whichneverexistedearlier.Forthe pastseveral thousandyearsmanhasbeenselectingandbreeding(i.e.,
hybridizing) animalsandplantsforhisownuses,anda greatmany varieties,adaptedfordifferentpurposes,have
beenestablished.These resultsof artificial selectionprovide strikingmodelsof whatmaybe accomplishedby
natural selection.Allof ourbreedsof dogshave descendedfromone orperhapsa very
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fewspeciesof wilddogorwolf,yettheyvaryso muchin colour,size andbodyproportionsthatif theyoccurredin
the wildtheywouldundoubtedlybe consideredseparatespecies.Theyare all interfertileandare knowntocome
fromcommon ancestors,sotheyare regardedasvarietiesof asingle species.A comparable range of varietieshas
beenproducedbyartificial selectionincats,chickens,sheep,cattle andhorses.Plantbreedershave establishedby
selectivebreedingatremendousvarietyof plants.Fromthe cliff cabbage,whichstill growswildinEurope,have
come cultivatedcabbage,cauliflower,Kohlrabi,Brussels,sprouts,brocoliandkale.Further,cytogeneticistshave
beenable totrace the ancestryof certainmodernplants bya combinationof cytologictechniquesinwhichthe
morphologyof the chromosomesiscomparedandbybreedingtechniqueswhichcompare the kindsof genesand
theirorderinparticularchromosomesina seriesof plants.Inthisway,the presentcultivated tobaccoplant,
Nicotianatabacum,wasshownto have arisenfromtwospeciesof wildtobacco,andcorn wastraced to teosinte,a
grass-like plantwhichgrowswildinthe AndesandMexico.Moreover,the cytologicdetailsof the structure of the
giantchromosomesof the salivaryglandsof fruitflieshave beenof prime importanceinunrevealingthe
evolutionaryhistoryof manyspeciesof Drosophila.Mutationshave alsoplayedaverysignificantrole inevolution.
Differentkindsof mutations,namelygrossmutation,due tovariationinchromosomenumber(polyploidyand
aneuploidy),andpointmutations(see chapterof mutation) introduce differentkindsof variationsinplantsand
animalsandconsequentlyresultinspeciation.Plantbreedershave employedinducedpolyploidymethodsin
producingnumerouseconomicallyimportantvarietiesof plants.Geneticistshave producedmanynew strainsof
microorganisms,plantsandanimals(Drosophila) byartificiallyinducingpointmutationsinthem.Thus,the artificial
selectionmethodsdue torecombinationorpolyploidyandinducedmethodsof mutationshave suggestedthe
fundamental processeswhichmaybe involvedinorganicevolution.