genetic drift genetic drift known drift wright effect biologist change frequency gene variant population random sampling organisms offspring sample parents chance role determining given individual survives population frequency fraction copies gene share particular form genetic drift cause gene variants disappear completely reduce genetic variation copies effect genetic drift larger copies effect smaller early 20th century vigorous debates occurred relative importance natural selection versus neutral processes including genetic drift explained natural selection held view genetic drift plays minor role evolution remained dominant view decades population debate neutral theory molecular evolution claims instances genetic change spreads population necessarily changes caused genetic drift acting neutral currently scientific debate evolution caused natural selection genetic drift analogy process genetic drift illustrated represent organisms population consider starting population half red half blue colours correspond different gene population new generation organisms reproduce random represent reproduction randomly select marble original deposit new marble colour offspring new selected marble remains original repeat process new second second contains second generation offspring consisting colours second contains exactly red blue random shift occurred frequencies repeat process number times randomly generation form numbers red blue picked generation red blue analogous genetic drift change population frequency resulting random variation distribution generation possible generation particular colour chosen meaning offspring example red selected representing new generation contains blue offspring happens red lost permanently population remaining blue fixed future generations entirely blue small populations occur generations probability frequency mechanisms genetic drift illustrated simplified example consider large colony bacteria isolated drop solution bacteria genetically identical single gene labeled neutral meaning affect bacteria ability survive reproduce bacteria colony equally likely survive reproduce suppose half bacteria half frequency drop solution food sustain bacteria bacteria die survive sixteen possible combinations bacteria original solution equally likely survive solution survivors random sample original colony probability survivors given probability particular combination occurs solution formula original population size large sampling effectively happens replacement words sixteen possible combinations equally likely occur probability counting combinations number following table shown table total number possible combinations equal number probability total number possible alternative combinations probability unequal number original colony began equal number chances number remaining population members equal case genetic drift occurred population frequencies changed random sampling example population contracted random survivors phenomenon known population number copies survive given column table calculated directly distribution success probability probability given present probability copies combination given formula number surviving bacteria mathematical models genetic drift mathematical models genetic drift designed branching processes equation describing changes frequency population wright fisher model think gene populations consisting individuals copies gene individual copies different frequency frequency wright fisher model named assumes generations overlap example annual plants exactly generation year copy gene found new generation drawn independently random copies gene old generation formula probability obtaining copies frequency generation formula symbol signifies function expression formulated formula model model assumes overlapping generations time step individual chosen reproduce individual chosen die number copies given stay means transition matrix means mathematical solutions easier model wright fisher model hand computer usually easier perform wright fisher model fewer time steps need calculated model takes generation effective population size wright fisher model takes practice model wright fisher model similar results genetic drift runs twice fast model models drift number offspring greater given distribution assumed wright fisher model given overall speed genetic drift effective population size genetic drift powerful force compared selection higher moments offspring number distribution exceed distribution force genetic drift substantially weakened random effects sampling error random changes frequencies caused effects sampling error example random changes selection pressure important alternative source important genetic drift genetic draft genetic draft effect selection linked mathematical properties genetic draft different genetic drift direction random change frequency generations drift hardy principle states sufficiently large populations frequencies remain constant generation disturbed migration genetic selection finite populations new gained random sampling passed generation sampling cause existing disappear random sampling remove replace random declines increases frequency influence expected generation genetic drift drives population genetic time reaches frequency said fixed population reaches frequency lost smaller populations achieve faster limit infinite population achieved fixed genetic drift comes halt frequency change new introduced population gene flow genetic drift random process acts eliminate genetic variation time rate frequency change drift assuming genetic drift evolutionary force acting generations replicated populations starting frequencies frequency populations formula time loss assuming genetic drift evolutionary force acting given time probability eventually fixed population simply frequency population time example frequency frequency given unlimited time probability ultimately fixed population probability fixed expected number generations occur proportional population size predicted occur rapidly smaller populations normally effective population size smaller total population determine effective population takes account factors level stage population smallest fact neutral genes genetically linked selection effective population size gene population forward looking formula expected time neutral fixed genetic drift according wright fisher model formula number generations effective population size initial frequency given result number generations expected pass occurs given population given size frequency expected time neutral lost genetic drift calculated formula appears population large initial frequency simplified formula average number generations expected neutral formula average number generations expected loss neutral time loss drift apply present population subject natural selection lost gained drift influence time loss prone loss begins fixed population lost rate expected time generations loss population given formula formula equal constant represents waiting time destined loss loss occurring relatively rapidly genetic drift taking time second represents time needed loss accumulation cases time dominated term affected effective population size genetic drift versus natural selection law large numbers population large effect genetic drift reproductive population small effects sampling error alter frequencies significantly genetic drift considered mechanism evolutionary change primarily small isolated populations processes affect evolution genetic drift operates randomly natural selection functions non randomly natural selection direction guiding evolution adaptations current environment genetic drift direction guided mathematics chance result drift acts frequencies population regard effects contrast selection favors spread effects increase survival reproduction carriers frequencies cause unfavorable traits ignores neutral natural populations genetic drift natural selection act isolation forces play migration magnitude drift frequencies generation larger absolute number copies small small populations magnitude drift large selection selection divided effective population size mathematics genetic drift depend effective population size clear related actual number individuals population genetic genes selection reduce effective population size experienced neutral higher rate decreases local effect effective population size effect visible molecular data local rate genetic diversity negative gene density diversity dna regions associated genes selection sampling error known genetic draft order distinguish genetic drift frequency small drift selection large populations example usually eliminated quickly large populations new vulnerable loss genetic drift neutral frequency reaches certain threshold genetic drift effect general global solutions challenges evolve smaller effective population size local solutions evolve separately challenge population population population contracts significantly smaller size short period time random environmental event true population odds survival member population purely random improved particular inherent genetic advantage result radical changes frequencies completely independent selection impact population sustained caused time event natural interesting example causing unusual genetic distribution relatively high proportion individuals total rod cell color increases increases damage process known depression worst selected leading loss genetically linked process background selection harmful selection enhanced consequence genetic leads loss genetic diversity addition sustained reduction population size increases likelihood fluctuations drift generations come population genetic variation greatly reduced beneficial adaptations permanently eliminated loss variation leaves surviving population vulnerable new selection pressures disease climate change shift available food source adapting response environmental changes requires sufficient genetic variation population natural selection place known cases population recent past prior arrival europeans north american habitat millions greater prairie chickens illinois numbers million birds birds 1990s declines population resulted hunting habitat destruction random consequence loss species genetic diversity dna analysis comparing birds mid century birds 1990s documents steep decline genetic variation decades currently greater prairie chicken experiencing low reproductive success genetic drift lead genetic loss increases fitness seen hunting caused severe population northern elephant seal 19th century resulting decline genetic variation comparing southern elephant seal aggressively hunted founder effect founder effect special case population occurring small group population splinters original population forms new random sample formed new colony expected original population respects possible number genes original population larger number gene copies founders making complete representation impossible newly formed colony small founders strongly affect population genetic far future documented example found migration pennsylvania members new colony shared van syndrome members colony descendants tend religious remain relatively result generations syndrome prevalent general population difference gene frequencies original population colony trigger groups significantly course generations difference genetic distance increases separated populations distinct genetically genetic drift natural selection gene flow contribute divergence potential relatively rapid changes colony gene frequency led scientists consider founder effect extension genetic drift significant driving force evolution new species attach significance random drift small newly isolated populations shifting balance theory following wright created models decline genetic variation small population size following founder effect critically important new species develop support view today hypothesis tested repeatedly experimental research results best founder effect investigated ussr soviet scientists history concept concept genetic drift introduced founders field population use term drift time sense directed process change natural selection random drift means sampling error came known wright effect entirely comfortable given wright referred changes frequency steady drift selection random drift sampling error drift came adopted technical term sense exclusively today usually defined narrowly terms sampling error narrow definition universal wright wrote restriction random drift drift component effects accidents sampling tends lead confusion considered process random genetic drift means sampling error equivalent means later work shown distinct early days modern evolutionary synthesis scientists beginning blend new science population theory natural selection working new framework wright focused effects small relatively isolated populations introduced concept landscape phenomena cross breeding genetic drift small populations push away peaks turn allow natural selection push new peaks wright thought smaller populations suited natural selection sufficiently intense create new interaction systems random drift intense cause random genes wright views role genetic drift evolutionary scheme controversial beginning influential critics colleague fisher conceded genetic drift played role evolution insignificant fisher accused misunderstanding wright views criticisms fisher argue wright rejected selection entirely fisher viewing process evolution long steady progression way explain increasing complexity simpler forms debates continued lean wright model evolution selection drift play important role debate neutral theory molecular evolution claims genetic changes caused genetic drift acting neutral role genetic drift means sampling error evolution criticized argue selection linked sites important force
Genetic drift