1) Macroevolution describes large-scale evolutionary events such as speciation, extinction, and major trends seen in the fossil record over long periods of time. 2) Stephen Jay Gould recognized three tiers of evolutionary timescales: population genetics (tens to thousands of years), rates of speciation and extinction (millions of years), and major extinction events (tens to hundreds of millions of years). 3) Differential rates of speciation and extinction among lineages over geological time, known as species selection, represents an expansion of Darwin's theory of natural selection and helps explain macroevolutionary patterns seen in the fossil record.

1. MACROEVOLUTION:MAJOR EVOLUTIONARY EVENTS
Macroevolutiondescribeslarge-scale eventsinorganicevolution.Speciationlinksmacroevolutionand
microevolution.Majortrendsinthe fossil recordare clearlywithinthe realmof macroevolution.Patternsand
processesof macroevolutionarychange emerge fromthose of microevolution,buttheyacquire some degreeof
autonomyindoingso.The emergence of newadaptationsandspecies,andthe varyingratesof speciationand
extinctionobservedinthe fossil recordgobeyondthe fluctuationsof allelicfrequencieswithinpopulations.
StephenJayGouldrecognizedthree different“tiers”of time atwhichwe observe distinctevolutionaryprocesses.
The first tierconstitutesthe timescaleof populationgeneticprocesses,fromtenstothousandsof years.The second
tiercoversmillionsof years,the scale onwhichratesof speciationandextinctionare measuredandcompared
amongdifferentgroupsof organisms.
Punctuatedequilibriumisatheoryof the secondtier,explainingthe occurrence of speciationandmorphological
change and theirassociationovermillionsof years.
The third tiercoverstensto hundredsof millionsof years,andismarkedbyoccurrence of episodicmassextinctions.
In the fossil recordof marine organisms,massextinctionsrecurat intervalsof approximately26millionyears.Fiveof
these massextinctionshave beenparticularlydisastrousThe studyof long-termchangesinanimal diversityfocuses
on the third-tiertimescale .
Speciationand Extinction Through Geological Time
Evolutionarychange atthe secondtierprovidesanew perspectiveonDarwin’stheoryof natural selection.Although
a speciesmaypersistformanymillionsof years,itultimatelyhastwopossible evolutionaryfates:itmaygive rise to
newspeciesorbecome extinct withoutleavingdescendants.
Ratesof speciationandextinctionvaryamonglineages,andlineagesthathave the highestspeciationratesand
lowestextinctionratesproduce the greatestnumberof livingspecies.The characteristicsof a speciesmaymake i t
more or lesslikelythanotherstoundergospeciationorextinctionevents.Because manycharacteristicsare passed
fromancestral to descendantspecies(analogoustoheredityatthe organismal level),lineageswhose characteristics
increase the probabilityof speciationandconferresistance toextinctionshoulddominate the livingworld.This
species-level processthatproducesdifferential ratesof speciationandextinctionamonglineagesisanalogousin
manyways to natural selection.
It representsanexpansionof Darwin’stheoryof natural selection.Thisexpansionisparticularlyimportantfor
macroevolutionif one acceptsthe theoryof punctuatedequilibrium,whichstatesthatthe evolutionarilyimportant
variationoccursprimarilyamongratherthan withinspecies. Speciesselectionencompassesthe differential survival
and multiplicationof speciesthroughgeological time basedonvariationamonglineages,especiallyinemergent,
species-level properties.These species-level propertiesinclude matingrituals,social structuring,migrationpatterns,
geographicdistribution,andall otherpropertiesthatemerge atthe specieslevel.
Descendantspeciesusuallyresembletheirancestorsinthese properties.Forexample,a“harem”systemof mating
inwhicha single male andseveral femalescomposeabreedingunitcharacterizessome mammalianlineagesbutnot
others.We expectspeciationratestobe enhancedbysocial systemsthatpromote foundingof new populationsby
small numbersof individuals.Certainsocialsystemsmayincreasethe likelihoodthata specieswill survive
environmental challengesthroughcooperativeaction.Suchpropertieswouldbe favoredbyspeciesselectionover
geological time.
Differential speciationandextinctionamonglineagesalsocanbe causedby variationinorganismal-level properties
(suchas specializedversusgeneralizedfeeding) ratherthanspecieslevel properties.Organismsthatspecialize in
eatinga restrictedrange of foods,forexample,maybe subjectedmore readilythangeneralizedfeedersto
geographicisolationamongpopulations,because areaswhere theirpreferredfoodisscarce orabsentwill function
as geographicbarrierstodispersal.Suchgeographicisolationcouldgeneratemore frequentopportunitiesfor
speciationtooccur throughoutgeological time.
The fossil recordsof twomajor groupsof Africanantelopessuggestthisresult.A groupof specializedgrazersthat
containsblesboks,hartebeests,andwildebeestsshowshighspeciationandextinctionrates.Since the late Miocene,

2. 33 extinctand7 livingspeciesare known,representingatleast18 eventsof branchingspeciationand12 terminal
extinctions.
In contrast,a group of generalistgrazersandbrowsersthatcontainsimpalasshowsneitherbranchingspeciationnor
terminal extinctionduringthissame intervalof time.Interestingly,althoughthese twolineagesdiffergreatlyin
speciationrates,extinctionrates,andspeciesdiversity,theydonotdiffersignificantlyintotal numberof individual
animalsalive today.
PaleontologistElisabethVrba,usesthe term effectmacroevolution todescribe differentialspeciationandextinction
ratesamong lineagescausedbyorganismal-level properties.She reservesthe termspeciesselectionforcaseswhere
species-level emergentpropertiesare of primaryimportance.Some otherevolutionarypaleontologistsconsider
effectmacroevolutionasubsetof speciesselectionbecause fitnessdifferencesoccuramongdifferentspecies
lineagesratherthanamongvaryingorganismswithinspecies.
Mass Extinctions Whenwe studyevolutionarychange onanevenlargertimesscale,we observe
episodiceventsinwhichlarge numbersof taxagoextinctsimultaneously.These eventsare calledmassextinctions.
The most cataclysmicof these extinctionepisodeshappenedabout225 millionyearsago,whenatleasthalf of the
familiesof shallow-watermarine invertebrates,andfully90% of marine invertebrate speciesdisappearedwithina
fewmillionyears.Thiseventwasthe Permian extinction. The Cretaceousextinction,whichoccurredabout65 million
yearsago, markedthe endof dinosaurs,aswell asnumerousmarine invertebratesandmanysmall reptiliantaxa.
Causesof mass extinctions and evolutionarytimingof massextinctions atintervalsof approximately26millionyears
are difficulttoexplain.Some people have proposedbiological explanationsforthese episodicmassextinctionsand
othersconsidermanymassextinctionsartifactsof ourstatistical andtaxonomicanalyses.
WalterAlvarezproposedthatthe earthwasperiodicallybombardedbyasteroids, causingthesemassextinctions.
The drastic effectsof suchbombardmentof aplanetwere observedinJuly1994 whenfragmentsof Comet
ShoemakerLevy9bombardedJupiter.The firstfragmenttohitJupiterwasestimatedtohave the force of 10 million
hydrogenbombs.Twentyadditional fragmentshitJupiterwithinthe followingweek,one of whichwas25 times
more powerful thanthe firstfragment.Thisbombardmentwasthe mostviolenteventinthe recordedhistoryof the
solarsystem.
A similarbombardmentonearthwouldsend debrisintothe atmosphere,blockingsunlightandcausingdrastic
changesof climate.Temperature changeswouldchallenge ecological tolerancesof manyspecies. Alvarez’s
hypothesisisbeingtestedinseveral ways,includingasearchfor impactcraters leftbyasteroidsandforaltered
mineral contentof rockstrata where massextinctionsoccurred.Atypical concentrationsof the rare-earthelement
iridiuminstrataat the Cretaceous-Tertiaryboundaryimplythatthiselemententeredthe earth’satmosphere
throughasteroidbombardment. Sometimes,lineagesfavoredbyspeciesselectionare unusuallysusceptible tomass
extinction.
Climaticchangesproducedbythe hypothesizedasteroidbombardmentscouldproduce selectivechallengesvery
differentfromthose encounteredatothertimesinthe earth’shistory.Selective discriminationof particular
biological traitsbyeventsof massextinctionistermedcatastrophicspeciesselection.
For example,mammalssurvivedthe end-Cretaceousmassextinctionthatdestroyedthe dinosaursandother
prominentvertebrateandinvertebrate groups.Followingthisevent,mammalswereable touse environmental
resourcesthatpreviously hadbeendeniedthem, leadingtotheiradaptive radiation. Natural selection,species
selection,andcatastrophicspeciesselectioninteracttoproduce the macro-evolutionarytrendsseeninthe fossil
record.Studiesof these interactingcausal processes have made modernevolutionarypaleontologyanactive and
excitingfield.