Biology 1001A Lecture Notes - Lecture 22: Denisovan, Sister Group, Gene Flow

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BIO1001ALecture22HumanEvolution
Reinforcement:selectionfavoursevolutionofprezygoticisolation
● Ifpopulationscomebackintosecondarycontact,and
ifpostzygoticisolationhasalready
occurred…
● Thenindividualswhoavoidhybridizinghaveaselectiveadvantage
○ Favoursfastevolutionofprezygotic
isolation
● Prezygoticisolationlesscostlythanproducingalowfitnesshybrid
○ Femaleswilldobetteriftheymateassortativelywithintheirownpopulation
● Secondarycontactcanacceleratespeciation,orinduce‘speciescollapse’…just
dependsonthetiming
○ Relativelylatecontact(afterpostzygoticisolation):acceleratesspeciationthrough
prezygoticisolation
○ Contactearlyindivergence(speciationprocess):speciescollapse→geneflow
(interbreeding)resumesbetweentwospecies
Humans,Neanderthals,Denisovans
● Over100000yearsago,Homosapiens
sharedtheworldwithotherhominids
○ Hominids:morecloselyrelatedtohumansthanchimps
○ Branchedoffmorerecentlythanhumansorchimps
● Speciesofhominids:
○ Neanderthals
■ buriedtheirdead,hadaparticularlanguage
■ 40000yearsago,Neanderthalswentextinct
○ Denisovans
■ Sistergrouptotheneanderthals
■ Discoveredin2008throughagirl’sfingerbone,andatooth
■ FoundinacaveinSiberia
● Inthesamecave:
○ Denisovanevidencewasfound
○ Neanderthalfossilsfound
○ Humanmadetoolsfound
■ Arguably3species(orsubspecies)ofhominidsoccupiedthesamecave
Differentspeciesofhominid?
● Basedongeneticevidence,ourancestorsinterbred(hybridized)withotherspeciesof
hominid
○ Thereisactuallyafourthhominin(asidefromHumans,Neanderthals,and
Denisovans)
○ Interbreedingstillconsideredtoberare
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○ MoregeneflowbetweenNeanderthalsandHumans(moderate)than.
DenisovansandHumans(alittle)
● Whethertheyaredistinctspeciesorsubspeciesdependsonwhichspeciesconceptwe
use
○ Morphological:Neanderthalscanbeidentifiedfromnonneanderthals
○ Phylogenetic:Eachstillconsideredtobedistinct
○ Biological:Sometimesinterbredundernaturalconditions,butnotthatmuch
■ Wedoknowsomehybridssurvivedsincewehavesomeoftheirgenesin
ourgenomenow
■ However,wedon’tknownormalfitnessofhybrids
Arehumansstillevolving?
Noconsensusamongevolution’sAlisters
● Darwin:Believedthathumanswouldstillevolveandadapt
○ “BelievingasIdothatmaninthedistantfuturewillbeafarmoreperfectcreature
thanhenowis…”
○ Selection/evolutionstillhappening,peoplewillcontinuetoadapt
● StephenJayGould:Nochange,noevolution
○ “There’sbeennobiologicalchangeinhumansin40,000years…everythingwe
callcultureandcivilizationwe’vebuiltwiththesamebodyandbrain.”
○ Morphologically,ourskeleton/brainsizehasn’tchanged
● SteveJones:Thereisnomorehumanevolution
○ (Adaptive)“Humanevolutionisover”
Dotheseevolutionarymechanismsstillapplytohumans?Strongerorweakernow?
● Selection
○ Dependsonwhereyouareintheworld
■ IntheWesternworld,99%ofbabiessurvivetoadulthood,thanksto
developmentsinmodernmedicine(antibiotics/antiviraldrugs)
■ Inotherpartsoftheworld,alargeproportionofchildrendiebeforethe
ageof5
○ Selectionmaybestrongerinsomepartsoftheworldthanothers
● GeneticDrift
Weaker
now
○ Thebiggestcontributortogeneticdriftispopulationsize
■ Populationisbiggernow,sogeneticdriftisweaker
● Mutation
○ Theenvironmentcaninfluencemutationrate
○ Themajorsourceofmutationisoldmenhavingchildren
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