1. The document discusses different levels of evolution including microevolution (changes within populations), macroevolution (changes between species and higher taxa), and megaevolution (really large-scale evolution like changes between phyla). 2. It describes the hypothesis of punctuated equilibria proposed by Gould and Eldredge, which argues that most evolutionary changes occur rapidly during speciation events separated by long periods of stasis, rather than through gradual accumulation of changes. 3. Examples of macroevolutionary trends and principles are discussed, including adaptive radiation and the evolution of the horse family from small browsing ancestors to modern grazing horses.

1. MICROEVOLUTION,MACROEVOLUTION,MEGAEVOLUTION ANDHYPOTHESISOF PUNCTUATED
EQUILIBRIA
Goldschmidt(1940) dividedevolutionintomicroevolution (thatof subspecies) andmacroevolution
(thatof speciesandgeneraandperhapsalsoof highercategories). Simpson(1953) hasproposedthe
additional term“megaevolution”forreallylarge-scale evolution,suchasthat of families,orders,classes
and phyla.Evolutionatthese levelsreceivesmuchattentionfromstudentsof the fossilrecord.B.
Rensch(1959) has modifiedthe termsmicroevolutionintoinfra-specificevolutionandmacroevolution
intotrans-specificevolution.Calow(1983) has regardedmacroevolutionandcladogenesisassynonyms.
For several years(1976 to 1982) StephenJayGouldandNilesEldredge of the AmericanMuseumof
Natural Historyhave questionedthe conventionalview thatevolutionarychangesinthe distantpastare
principallythe outcome of the gradual accumulationof slightinheritedvariations.Theyadvocatedthat
mostevolutionarychangeshave consistedof rapidburstsof speciationalternatingwithlongperiodsin
whichthe individualspeciesremainvirtuallyunmodified.GouldandEldredge maintainthatmost
lineagesdisplaysuchlimitedmorphological changesforlongintervalsof geologictime astoremainin
stasis,or in“equilibria”.Conspicuousorprominentevolutionarychangesare concentratedinthose brief
periods(“punctuations”) whenthe lineagesactuallysplitorbranch.Thisiscalledhypothesisof
punctuatedequilibria.Further,branchingfromasingle lineage(orasingle line of descent)during
macroevolution,will ultimatelyproduce aclusterof lineagesknownasclade.Thus,branching
sometimesiscalledcladogenesis.Whenreferringtoa clade or to a group of relatedclads,itiscommon
to use the adjective
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EVOLUTION (EvolutionaryBiology)104
phylogenetic(nottobe confusedwiththe similarwordphyletic). Phylogeneticpertainstoa portionof
phylogenyconsistingof more thanone lineage,whereasphyleticreferstoasingle lineage.Moreover,a
lineage reconstructedfromfossildatamayexhibitsufficientevolutionarychange thata taxonomist
deemsitappropriate todivide itintotwointergradingspecies.Suchspeciesare knownas
chronospecies,successional species,palaeospeciesorevolutionaryspecies(seeStanley,1979).
1. Microevolution
Evolutionarychangesinpopulationsare ordinarilyvisualizedasgradual,builtuponmanysmall genetic
variationsthatarise andare passedon fromgenerationtogeneration.The shiftinggene frequenciesin
local populationsmaybe thoughtof as microevolution.The progressive replacementof light-coloured
mothsby dark (melanic) mothsinindustrialregionsinEngland(i.e.,industrial melanism) exemplifiesthe
microevolution.Mostpopulationgeneticists endorsethe view thatthe same microevolutionary
processeshave beeninvolvedinthe majortransformationsof organismsoverlongerspanof geologic
time (macroevolution).The traditional outlookisthatsmall variations graduallyaccumulate inevolving
lineagesoverperiodsof millionsof years.
2. Macroevolution
The most significantfeature of macroevolutionisaprogressive,sustainedtendencyforcertain
characters to developalonganevolutionaryline.Trends(ordirectionsof evolution) of thissortare
numerousinthe fossil record.Long-termprogressingtrendsrarelyappearinonlyone structure,but

2. almostalwaysinvolveacomplex of differentfeatures.Infact,trendsare producedbythe drivingforce
of natural selectionoperatingwithinthe limitsof aparticularadaptive zone orsubzone.Evolutionisnot
random,althoughcertainelementsinthe processare random, andtrendsleadingtogreaterefficiency
are to be expected.Evolutionarytrendsare generallyadaptive movementsalongone pathway,butthey
are neverexclusivelysequential andalwaysinvolvedivergentandrepeatedtakingonof one or the
othercharacters importantinthe trend.The populationsundergoingchange are constantly
experimentingwithinthe adaptivepathway,andparallel probingsof anew subzone byrelatedbut
differentlinesare the rule.The classical example of evolutionarytrendsinmacroevolutionisprovided
by the horse family,Equidae.Evolutionof horses.Inreconstructingthe phylogenyof ananimal group,it
has beenthe standardpractice to showthe emergence fromacentral,generalizedstockof alarge
numberof divergentbranchesorlineages.Notall branchespersist;indeedthe generalrule isthatall but
a fewperish.The disappearance of manybranchesinthe distantpastmightleadthe observertodayto
the mistakenimpressionthatthe evolutionof aparticulargroupwas notat all elaboratelyforked.Thus,
the evolutionof horsescouldbe erroneouslydepictedasanundeviatingstraight-line progressionfrom
the small,terrier(small dogof variousbreeds)sizedHyracotherium(Eohippus) tothe large modern
horse,Equus.Onthe contrary,a wealthof evidence hasrevealedconvincinglyapatternof many
divergentlineages(Fig. 8.12).Figure 8.12 demonstratesverynicelythe gradual change froma dog-like,
browsingcreature withpaddedfeettoa horse withgrazinghabitsandhoofedspringingfeet.Infact,the
appearance of the onetoedconditionwasalandmark inhorse history. The fleet-footed,one-toed
Pliohippusemergedduringthe Pliocene epochfromthe slow-footed,three-toedMerychippus.This
majoreventisusuallyinterpretedasagradual change withina single lineage.Inotherwords,adirect
ancestral-descendantrelationshipbetweenMerychippusandPleiohippushasbeenwidelyaccepted.
The paleontologistSimpson(1951) departedfromthe view of slow andsteadychange whenhe
suggestedthatsucha major transitionmighthave involvedthe accumulationof small geneticchangesin
relativelyrapidsuccession.Thisacceleratedpace of phyleticgradualismwascalled“quantumevolution”
by Simpson.Proponentsof hypothesisof punctuatedequilibriaexplainedthe suddenappearance of
Pliohippusinaquite contrastbut convincing way.Thus,one mightargue thatMerychippusexistedfora
longperiodwithlittle ornoevolutionarychange (stasis),andeventuallysufferedextinction.Pliohippus
isnot a lineardescendantof Merychippus,butadramaticallynew formthathadearlierarisenasa small
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EVOLUTION ABOVESPECIESLEVEL 105
Fig.8.12. Phylogenetictree of horsesthroughtime,withitsmanydivergentbranches.All branchesdied
out,save one whicheventuallyculminatinginthe moderngroupof horses,Equus.The history of horses
datesback to earlyCenozoictimes,some 60millionyearsago.The Cenozoiceraisdividedintoperiods,
whichinturn are dividedintoepochs(afterSavage,1969).
SouthAmericaNorthAmericaOldWorldRecentPleistocenePiloceneMiocene Oligocene Eocene Equus
Equus toes 1–1 springingStylohippariontoes3– 3 springingNannippusNeohipparionCalippus
PliohippusHippidionGrazersHipparionMegahippusHypohippusAnchitheriumArchaeohippus
MerychippusParahippusMiohippusBrowsersMesohippusEpihippus
Orohippus
toes3 – 3 withpads

3. To Rhinoceroses
Hyracotherium
toes4 – 3 withpads
peripheral populationof the parentspecies.Itmaybe noticed(Fig.8.12) that not all offshootsof
Merychippusevolvedthe progressivesingle-toedcondition.Severallinesof Pliocene horses,suchas
HipparionandNannippus,retainedthe conservative three-toedpattern.TheseconservativePliocene
horseswere evolutionaryblindalleys,andmaywell have represented“punctuations”thatfailed.
Hipparion
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EVOLUTION (EvolutionaryBiology)106
General principlesof macroevolution.The followingcommonfeatureshave beenderivedfromthe
phylogenyof horsesregardingmacroevolution:1. Macroevolutionfollowsthe acquisitionof new
general adaptationorentrance intoa new adaptive zone.Forexample,Darwin’sfinchesradiatedafter
an apparentlygeneralizedfinchancestorarrivedtooccupythe previouslyunoccupiedGalapagosIslands.
Reptilesradiatedaftercompletelyterrestrial developmentwasestablished asa general adaptation.2.
Macroevolutionalwaysinvolvesevolutionarydivergence.Macroevolutionisnotlinearbutradiating.
Usuallyradiationfollowsgeneral adaptationandthe invasionof anew adaptive zone throughspecial
adaptationindifferent divergentdescendantlines.3.Adaptive radiationormacroevolutiontendsto
produce evolutionarylinesthatconverge inspecial adaptationwithotherdistantlyrelatedgroups
differingintheirmatrix of general adaptation.Forexample,ichthyosaursshow amarkedevolutionary
convergence withanumberof fishgroupsintheirbodyform, mannerof locomotion,foodhabits
(fishes) andfree-swimmingpelagiclife.Theyare convergentinspecialadaptationsbutstill share agroup
of general adaptationswithall otherreptiles.4.Macroevolutionproducesgroupsof parallelspecial
adaptationsamongdivergentbutrelatedstocks.All of themhave acommongeneral adaptation.Among
reptiles,groupaftergrouprepresentativesof everyorderexceptthe cotylosaurshad invadedaquatic
adaptive zonesandbecome completelyaquatic.Amongthese,some groupsflourishandlaterbecome
extinct.These groupswere replacedbyotherparallelgroupswhichinvadedthe same zone and
underwentdifferentiation.The phytosaurs(Triassic) of the orderThecodontiaandthe crocodiles
(Triassictorecent) of the orderCrocodiliainvadedthe aquaticadaptive zone.Butlaterphytosaurs
became extinctandthenthe crocodilesreplacedthem.These provide anexample of parallelismand
ecologicreplacement.Thus,wherethere are repeatedevolutionaryexperimentswiththe same broad
groupof zones,some groupsarise,flourishandbecome extincttobe replacedbyparallel groupsthat
invade the same zone andundergodifferentiationintheirturn.5.Macroevolutionultimatelyleadsto
extinction.Somegroupsacquiredspecial adaptationsforanarrow adaptive subzonesanddue tothese
specializedadaptationstheycouldnotmove intonew majorzones.Because all adaptive zonesfinally
change and disappear,soall groupsrestrictedtoa narrow zone alsodisappeared.Hence,the groups
whichcouldchange the adaptive zonesbyacquiringgeneral adaptationsurvivedandothersbecame
extinct.
3. MegaevolutionMegaevolutionorthe originof new biological organizational plansisrare andshares
featurescommontomicroevolutionandmacroevolution.Onlyafew majorgeneral typeshave

4. developedduringthe historyof life,butalmostall of thempersistwithoutextinction,althoughafew
have perishedandothersare relict.All of the phylaandmostclassesof microorganisms,plantsand
animalsrepresentmarvellouscomplex andcoordinatedgroupsof general adaptations,eachforminga
basic,distinctive biological organization,unique anddominantinabroadrange of characteristic
adaptive zones.About200 of these majorbiological organizationplansare knowntohave developedin
last3 billionyears.The originof these systemsisthe mostsignificantof all evolutionaryevents,yetthe
processesleadingtothese eventsremainthe leaststudiedof biological problems.Megaevolutionhas
the followingcharacteristics:1. The breakthroughalwaysfollowsevolutionaryexperimentationand
explorationbydivergentlinesof the ancestral stock,until one of themcrossesthe ecologicbarrierinto
the newzones.2. The breakthroughandshiftare alwaysrapid;otherwise theyfail because of the
extreme negative selectioninunstable ecologiczones.3.The new zone isalwaysecologicallyaccessible,
isdevoidof competitionandrequiresanew general adaptivetype foritsinvasion.4.Adaptive radiation
alwaysfollowsthe initial shift.
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EVOLUTION ABOVESPECIESLEVEL 107
The ProcessinvolvedinMacroevolutionandMegaevolution
The major feature of organicevolutionisdivergence guidedbythe mouldingforce of natural selection.
Evolutionatthe populational level isdrivenbythe elemental forcesof mutation,selectionanddrift.
Basicallypopulationevolutionmaybe sequential,butitisalwaysdivergentinthe end.Speciation,
macroevolutionandmegaevolutionrepresentstatesorlevelsinacontinuumof evolution;all are driven
by the elementalforcesbutare subjecttoincreasinglycomplicatedeffectsfromlessunderstoodforces
as well.Selectionbecomesof greater significance above the microevolutionarylevel.Of the greatest
importance inspeciationisthe isolation.Selectionmayactto produce divergenceinthisprocess
wheneverfragmentsof anoriginallyinterbreedinggene pool becomespatiallyorreproductively
isolated.New speciesoriginatedfromoldonesthroughthe originof reproductive isolationand
independentmicroevolution.Thus,inadditiontoelementaryforces,isolation,microevolutionand
macroevolution,megaevolutioninvolvesthe followingprocesses(Fig.8.13) : (1) The possessionof new
general adaptationoroccupancyof a new adaptive zone.(2) The breakthroughintonew zonesor
subzoneswithinthe new adaptive zone bydevelopmentof special adaptation.(3) The lossof
evolutionaryflexibility,andchannelizationintogreaterandgreaterspecializationwithinsubzones.(4)
The ecological reinvasionof azone or subzone whenitbecomespartiallyunoccupiedbecause itsoriginal
occupiersare nowspeciallyadapted(ecologicreplacement).(5) The irreversibilityof evolution.Since
each stepisdependentuponthe previousprogressive changes,once agroupis on an adaptive road,itis
usuallytrappedinanadaptive zone andcannotreverse evolutionagainstthe directionof selection.4.
Doctrine of PunctuatedEquilibriaA single lineof descentorlineagemaypersistforlongreachesof
geologictime.Assmall changesaccumulate overperiodsof millionsof yearswithinone lineage,the
descendantpopulationsmayeventuallybe recognizedasa species distinctfromthe antecedent
population.The persistent
accumulationof small changeswithinalineage hasbeentermedphyleticgradualism,andthe
transformationof a lineage overtime hasbeentermedasanagenesis.How organismscome toterms
withand keepintune withtheirenvironment,iscalledanagenesis(see Calow,1983). Asdepictedin
Figure 8.14 A,a newspecies(labelled“B”) arisesfromthe slow andsteadytransformationof alarge

5. antecedentpopulation(“A”).If onlytransformationoccurred,thenlifewouldcease assingle lineages
became extinct.Hence,anewspecies(“C”) canalsoarise bythe splittingorbranchingof a lineage.As
Interbreedingblursthe distinctionbetweenspecies.(a) The myrtle warblerand(b) Audubon'swarbler
were formerlythoughttobe twoseparate speciesbutare now consideredtobe merelylocal varietiesof
one widespreadspecies.(a) (b)
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EVOLUTION (EvolutionaryBiology)108
alreadydescribed,the splittingof one phyleticlineage intotwoormore lines istermedcladogenesis
(viz.,the evolutionaryprocessof genesisof variety).Buthere againthe splittingisbelievedas
proceedingslowlyandgradually,withthe twobranchinglineagesprogressivelydiverging,without
significantreductioninpopulation size.Thus,palaeontologiststendtoview lineagesplittingintermsof
gradual morphological divergence.GouldandEldredge maintainthatphyleticgradualismistooslowto
produce the majoreventsof evolution.Accordingtotheirtheoryof punctuatedequilibria,the
prominentepisodesof evolutioninthe historyof lifeare associatedwiththe splittingof lineages,but
the splittingisnotseenasslowand steadycladogenesis.The new speciesarisesthroughrapidevolution
whena small local population becomesisolatedatthe marginof the geographicrange of the parent
species(Fig.8.14 B).Indeed,the successful branchingof asmall isolatedpopulationfromthe periphery
of the parental range virtuallyassuresthe rapidoriginof anew species.Ingeologictime,the branching
issudden—thousandsof yearsorless,comparedtothe millionsof yearsof longevityof the species
itself.The punctuational change doesnotcause anyunconventional evolutionaryphenomenon.The
suddenappearance of a speciesis fullyconsistentwithErnstMayr’srapidselectioninperipheral
isolates.The morphological differentiationof geographical isolatesoccursearlyandrapidly,especially
whenthe populationissmall andadaptingtothe new abnormal environmentatthe peripheryof the
range.Once establishedasanewspecies,the assumptionisthatlittle evolutionarychange will occurin
that speciesovergeologictime.A well-studiedexample of punctuational modeof evolutionisprovided
by PeterG.Williamsin1981. He studiedthe fossilizedfreshwatersnails(molluscs) inthe Lake Turkana’s
basinregionof Africa(NorthernKenya).The thick,undisturbedfossil bedscontainmillionsof preserved
shellsrepresentingatleast19 speciesof snails.Several lineagesof Cenozoicsnailswere exceptionally
stable for3–5 millionyears.Whenmorphologicalchangesinshell shapedidoccur,theywere
concentratedinbrief periodsof 5,000 — 50,000 years.These rapidlyevolvednew populationsthen
persistedvirtuallyunmodifieduntiltheybecame extinct.Some authorshave convincinglycommented
that so-calledsuddenchangeswithin50,000 yearsmightappear to be instantaneousto a
palaeontologistbutwouldbe almostaninfinitytoanexperimental geneticist.Infact,tomany
geneticists,aninterval of 50,000 years(or 20 generationsinsnails) wouldbe sufficienttime forthe
morphological changesinthe snail populations,toaccumulate graduallyratherthandramatically.Fig.
8.13.The processof evolutionabove the specieslevel — adiagrammaticrepresentationof
macroevolutionandmegaevolution(afterSavage,1969). extinction,irreversibilitydiversification
environmental opportunitymacroevolution(special adaptation) megaevolutionaryshift(new general
adaptation) evolutionaryexperimentationtime of breakthroughoriginalstockzone A numerous
subzonesadaptive gridZone Btime trend
Contents

6. EVOLUTION ABOVESPECIESLEVEL 109
WhetherHumanEvolutionisGradual or Punctuated?
On the basisof phylecticgradualismmode of evolution, ithasbeenstandardpractice toput Homo
sapiensatthe endof a continuousseriestracingbackthroughat leasttwootherspecies(Fig.8.15).
Accordingto certainmodernauthorssuchas Stanley(1979), Calow (1983) andVolpe (1985) varioustaxa
involved inhumanphylogenyare the following:1. Ramapithecus.ThisgroupalsoincludesSivapithecus.
These ancestral forms(apes) livedapproximately8–14 My (=millionyears) agoandcontaineda
flattenedface withashortmuzzle.Firstfossil (afragileupper jaw) of RamapithecuswasrecoveredbyG.
Edward Lewis(1930) inShivalikHillsof NorthernIndia.Anotherfossilof thisgroupwasrecoveredfrom
Lake VictoriainAfricaby Leakyin1955. The fossil evidence clearlyindicatedthatsuchgeneraas
Ramapithecus(fromRama= Hindugod + pithekos= Greekforape) and Sivapithecus(fromSiva=Hindu
God) whichlivedinAfricaandEurasia were the forerunnersof Hominids.2.Australopithecusafarensis.
(L. austral = southern).The fossil of thisfemaleape-man(calledLucy) wasrecoveredfromAfarlocalityin
Hadar, EthiopiabyDonaldJohanson(1973). Thisfossil wasprobablythe oldestamongall the primitive
man described.The afarensisremainsdatedbetween3and4 millionyears(My) ago.3.
Australopithecus.This grouplived1.5—5My ago. Theyhad a flattenedface.The orientationof the skull
relative tothe spinal cord,andthe bonesof the hind-limbsrelativetothe pelvissuggestanuprightgait.
There are followingtwogroupsof these ape-men:(i) Robustaustralopithecines(Australopithecus
robustus) whichhada heavyskull anddentitionsuggestingavegetationdiet.The fossilremainsof this
groupwas recoveredbyRobertBroomand RaymondA.Dart (1938) fromcavesin Sterkfontein,
Kromdraai and SwartkransinSouthAfrica.(ii) Gracile australopithecines(Australopithecusafricanus)
whichhas a more delicate skeleton(jaw waslightandslender)andsupplementedtheirdietwithanimal
food(i.e.,theyhadsmallermolarsthanthe A.robustus).Firstfossilremainsof A.africanuswere
obtainedbyR.A.Dart in1924 froma
SpeciesA SpeciesB
Time
SpeciesCSpeciesB
SpeciesB
SpeciesA
B PunctuatedequilibriaA Phyleticgradualism
SpeciesA
Fig.8.14.A–Accordingto phyleticgradualism, speciesA graduallytransformsinto speciesB,andmay
splitintoa slowlyevolvingspeciesC;B– Accordingto theoryof punctuatedequilibriaasmall isolate
fromthe parental population(speciesA) evolvesrapidlyintoanew entity(speciesB).SpeciesA
undergoesalongperiodof stasisduringwhichlittle ornomorphological change takesplace.SpeciesB
doesundergominorstructural modificationwithtime (afterVolpe,1985).
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EVOLUTION (EvolutionaryBiology)110

7. limestonequarrynearTaungvillage inTransvaal regionof SouthAfrica.LouisLeakyandhiswife Mary
Leaky(1959) uncoveredbonyfragmentsof arobustaustralopithecine withenormousteethfrom
Olduvai Gorge inTanzania.It wascalledNutcrackerMan or Zinjanthropusandwasabout1.8 million
yearsold.ZinjanthropusisconsideredaneastAfricanvarietyof A.robustusandhasbeenassigneda
statusof separate species—Australopithecusboisei.4.Homohabilis(“handyman”).The fossil remains
of thishominidwere recoveredbyLeakyin1964 fromrocks of Olduvai Gorge inEast Africa.Thisgroup
livedabout1.5— 2 millionyearsago.Theyhadan erectgait,relativelylarge brainsandwere tool-users.
Theyfabricatedcrude toolsoutof pebblesandusedthemtogathera varietyof plantmaterials(nuts,
seeds,fruits,tubersandroots) andsmall animals(lizardsandrodents).Cooperative behaviourevolved
inthemand promotedthe sharingof the foodfor the firsttime.5. Homo erectus(“uprightwalking
man”).Thisgroup lived1millionyearsago.Theyhada gait evenmore like oursandlargerbrainsthan
the habilines.H.erectususedtoolsof bone andstone andusedfire.Fossil remainsof H.erectuswere
discoveredatTrinil,Java,in1894 by Eugene Dubois.Theyhave beencalledJavamenandhad a cranial
capacityof 775 to 1000 cubiccentimetres(orcc).Anothertype of fossilsof H.erectus,calledpeking
fossils(i.e.,Pekingmen),were discoveredbyDavidsonBlackin1920s. Pekingmenhave the cranial
capacityof 900 to 1200 cc. 6. Homosapiens(“thinkingman”).The grade of sapienscontainsatleasttwo
contrastinglydifferentanatomical types — the bulkilybuiltandheavilymuscledNeanderthal andslim-
bodiedCro-Magnons.The skeletonof Neanderthalman(Homosapiensneanderthalensis) wasfirst
unearthedin1856 in a lime stone cave inthe Neanderravine nearDusseldorf,Germany.The
Neanderthalswere1.5lakhyearsold cave dwellers,short(about5 feet) butpowerfullybuilt,with
prominentfacial browridges.Theyhadlarge brainswithanaver
Fig.8.15.Phylogenyof human.AustralopthecusafarensisfollowedbyHomohabilisandHomoerectus
are believedtohave existedsuccessivelyingeological timeandtoforma lineage leadingtoHomo
sapiens.Inthe distantpast,the Homo assemblage co-existedwiththe Australopithecusgroups.This
phylogenyisbasedonthe viewsbyDonaldJohanson(1979) (afterVolpe,1985).
Homo sapiens
0
1
2
3
H. erectusH. habilisAustralopithecusafarensis
A. africanus
A. robustus
Millionsof yearsago
(A) A fossil skullof Homoerectusshowthatthe human braingraduallygrew larger,beginningabout2.5
millionyearsago.Atthe same time the ridgesabove the eyesbecame smaller,andthe face andjaw
beganto jutout less.Increasingintelligenceandthe abilitytomake bettertoolsapparentlywent

8. together.(B) The earlieststone tools,made inAfricaabout2.5 millionyearsago,were simplerthanthe
handaxe shownhere.
(A)
(B)
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EVOLUTION ABOVESPECIESLEVEL 111
Millionsof yearsago
Australopithecusrobustus
Homo sapiens
Homo erectus
Homo habilis
Australopithecusafricanus
Australopithecusafarensis
* suddenemergence of newspecies
0
1
2
3
4
age capacityof 1,450 cc as opposedto1,350 cc inmodernhumans.Neanderthalsmade complex stone
tools,were accomplishedhuntersof large game,usedanimal hidesasclothingandwithstoodthe rigors
of the bittercoldclimate of the lastglaciation.There isevidence thatthe Neanderthalsburiedtheir
deadwithvariousritual objects.The fossil remains of Cro-Magnons(Homosapienssapiens),a
representative of ourspecies,Homosapiens,wereunearthedfromaFrenchrock-shelterinthe village of
Les Ezgies.Theywere about35,000 yearsold.The CroMagnonpeople occurredinEurope andleft
behindveryelaborate cave paintingsshowingattainmentof aformof culture notunlike ourown.We
do notknowabout the birthplace of modernHomosapiens.Modernhumansmayhave beencradledin
Asiaor Africa.In the pastthree decades,Africahasbecome increasingly the focusforinvestigationsinto
humanoriginsanddifferentiation.Several sitesinAfricahave yieldedfossilsthatreveal differentphases
of the humanstoryfrom the earlieststages.The differencesof opinionthatexistconcerningthe
phylogenyof the Hominidaeare vividlyrevealedbythe ever-changingreconstructionsof the humanand
evolutionarytree,almostonayearlybasis.One of the mostwidelyheldviewsisthatagradual
ancestral-descendantrelationshipexistedforeachsuccessivehominidgroup.Thus,the
australopithecinesshadedfaintlyintoHomohabilis,whointurngradedslowlyintoHomosapiens.The

9. theoryof punctuatedequilibriaclaims(see Stanley,1979) that the evolutionarychangesinhominids
occurredin rapidbursts,concentratedinspeciationevents(Fig.8.16).Such
punctuatedepisodeswere separatedbylongperiodsof stasisinhominidlineageduringwhichlittleor
no morphological change tookplace.Suchlongperiodsof stasisoccurredwithinboththe
australopithecine andhominidgroupsandthisisratherstartlingconsideringthe highnetrate of
evolutionwhichischaracteristicof humanevolution.Figure8.16 showsthatthispatternof evolution
appears“rectangular”ratherthan tree-like.
The skull of Neanderthalssuggestsabrainat leastas large as that of modernhumans.Theywere very
strongand stockyand seemto have beenwell suitedtothe Ice Age climate.
0
1
2
3
4
Fig.8.16. The rectangularpatternof hominidphylogenyasexpressedbythe controversialtheoryof
punctuatedequilibria(afterVolpe,1985).
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EVOLUTION (EvolutionaryBiology)112
SIMPSON’SHOPEFULMONSTER There are geneticchanges,knownashomeoticmutations,whichare
responsible forsuchdramaticdevelopmental effectsasthe transformationof one majororganinto
another.Inthe fruitfly,homeoticmutationstransformanantenna intoaleg,or a balancerintoa wing,
or an eye intoan antenna.Althoughfascinatingfordevelopmental studies,mostevolutionistswouldnot
evenconsiderthatthe originof a newspeciescouldeverinvolvesuchdrasticmajormorphological
changesproducedbysingle homeoticgene changesormacromutation.However,in1940s, the
prominentgeneticistRichardGoldschmidtassertedthathomeoticmutantsdoillustrate apossiblemode
of originof novel typesof bodystructure,and,hence,couldbe responsible forthe rapidemergence of a
newspeciesinthe distantpast.He acknowledgedthatthe vastmajorityof macromutationwouldhave
disastrousconsequences(“monsters”).Anoccasional macromutation,however,mightconceivable
immediatelyadaptthe organismto a new wayof life,a“hopeful monster”inGoldschmidt’s
terminology.Goldschmidtwasconvincedthatthe gradual accumulationof small mutationswastooslow
a processto account for broadmacroevolutionarytrends.He favouredthe ideaof adramatic,abrupt
transformationof a populationbyafavourable,instantaneousmacromutation,whichcould take the
formof a radical rearrangementof geneticmaterial (viz.,agrosschromosomal change).The conceptof
punctuatedequilibriadoesnotrequire orimplychance favourablemacromutations.Rather,inthe
processof allopatricspeciationinperipheralisolates,the incipientspeciesdeveloptheirdistinctive
featuresrapidly.However,the significance of one aspectof Goldschmidt’sviewscannotbe denied:
structural chromosomal rearrangementsmayplayaprimaryrole inthe initiationorachievementof
reproductive isolationbetweentwodivergingpopulations.ORTHOGENESISANDORTHOSELECTION The

10. observedtendencyof apart or organto change progressivelyinsize iscalledorthogenesisorevolution
ina straightline (Moody,1970). The theoryof orthogenesiswhichwasoriginallyproposedbyEimer
(1897), laysstressuponthe fact shownbymany examplesof evolutionarytrendswhichare gradually
directedandproceedingalongastraightline course.Some of the well knownexamplesof orthogenesis
are the following:1. The steadyincrease inthe lengthof horninrhinoceros-like animal.2.Fossil ‘line’
leadingfromHyractotherium(orEohippus) toEquus,showingtrends inchangesin(a) increase inbody
size;(b) reductionof lateral digitsand(c) specializationof teeth.Recently,however,manydivergent
lineageshave beensuggestedforevolutionof horses(see Volpe,1985).3. Paludina(snail) fossils
showinggraded changesof shape of shell.4.Megaloceros(fossilIrishelk)whichhadgiganticantlers.5.
Human line of evolutionshowingthree straighttrends,viz.,the increase insize of craniumandbrainand
the perfectionof bipedallocomotion(see Nayar,1985). The selectionforlargeness(increase insize of
body) musthave beenproduceddue tothe advantage of becomingstrongerandresistingpredators
(e.g.,elephants,ungulatesanddinosaurs).Someof the progressiveseriesthatare observedare
explainable as the resultof differentialgrowthrates(allometry).Otherprogressiveseriesare explicable
as the resultof the operationof natural selectiononorganismslivinginastable environmentoran
environmentthatischangingwitha constanttrend(e.g.,becomingincreasinglydry).Undersuch
conditionsnatural selectionpromotesmore andmore perfectadaptationtothatenvironmentandthe
resultingchangesmaytake the formof a progressive series.Natural selectionoperatinginthismanner
issometimescalledorthoselection(Simpson,1953).Nayar (1985) has differentiatedthe twoterms—
orthogenesisandorthoselection—asfollows:Thus,accordingtoorthoselectionthe change isdependent
uponthe constancy of the favouringenvironmentalopportunity.However,orthogenesismayinvolve
straightline progresswhichmayevengobeyondevenafterthe selectionhaspasseditsuseful stage.
REVISION QUESTIONS
1. What is adaptive radiation?Explainthisphenomenonbycitingexample of mesozoicreptiles.2.
Describe the evolutionof horse.3.Give an accountof phylogenyof man.4. Write shortnoteson the
following:(1) Macroevolution;(2) Punctuatedequilibria;(3) Orthogenesis.
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