Genetic DriftA common conception about evolution is that the features of anA common conception about evolution is that the features of an organism have evolved due to random (undirected) change. Some processes in biology are random –like mutation. pgyNatural selection is not random. Natural selection is the nonrandom preservation of random variation.Genetic drift is a random process that can be important in the evolution of some populations.One of the requirements for the maintenance of allele frequencies in populations is a very large population size. G ti d ift i th f fi it l ti iGenetic drift is the consequence of finite population size.Alleles that do not affect fitness fluctuatefitness fluctuate randomly in frequency. Random fluctuation eventually results in the loss of alleles (allelic extinction) from )populations. One allele becomes fixed – the only allele in the populationallele in the population.Sampling error (randomSampling error (random chance) results in some gene copies being lost dh iiand others continuing.Different populations will lose different alleles.The probability that a particular allele will be fixed in a population in the future equals the frequency of the allele in the population If there are two alleles in a populationAandawithpopulation. If there are two alleles in a population, Aand a, with frequencies p and q, the probability that the allele a will be lost at some point in the future is p and the probability that allele A will be lost is q.If a large number ofnumber of populations is considered, hdifieach drifting, the total heterozygosity yg yoverall will decrease.Starting with a population size of N with two alleles in equalof N with two alleles in equal frequencies p and q, the likely magnitude of divergence from the initial frequencies increases with time.After 2N generations, all allele frequencies are equally likelylikely.The average time to fixation of one of the alleles is 4N ooeo e eess Ngenerations.The effective population size is the number of individuals in the population that successfully pass genes tosuccessfully pass genes to the next generation. This is usually smaller than the actual number (censusactual number (census number) in the population.The smaller the effectiveThe smaller the effective population size, the faster a population will drift, and the ffhlllifaster one of the alleles in the population will become fixed.The effective populations size (Ne) is affected by biological parameters other than the number of individuals in the population.Variation in offspring b i di id lnumber among individuals can reduce Ne. If some individuals produce more poffspring than others their alleles, even those that have no effect on fitnesshave no effect on fitness, will be passed on at higher rates.A sex ratio other than 1:1 produces a similar reduction in NeNatural selection at one gene can produce differences in offspringNatural selection at one gene can produce differences in offspring number among individuals and reduce NeInbreeding within or between generations reduces the number of diff i f d h i ddifferent copies of a gene passed to the next generation and effectively reduces Nelii liiddFluctuations in population size reduce Ne. Temporary decreases in population size have greater effects than temporary increases in population size. ppA temporary reduction in lti i i lldpopulation size is called a bottleneck.Read: A Strong Bottleneck Reduced the Heterozygosity of Elephant SealsWh ll b fidiid lf ltiWhen a small number of individuals from a source population establish a new population genetic variation can be lost. The loss of genetic variation due to such an extreme bottleneck is called the founder effect.Founder effects may make formerly rare alleles common.Simulations of founder effects hllbsuggest that a small number founders and a small population growth rate (r) result in greater loss g()gof genetic diversity.Eventually mutation will restore genetic variation in a founding population.Studies of laboratory and natural populations confirm the theoretical expectations of genetic drift models.pg107 populations of fruit flies each started with 16 heterozygotes, 8 males and 8 females.8dlh8 randomly chosen males and females were used to start each generation.Genetic Drift and Natural Selection•Directional selection will cause the frequency of alleles thatDirectional selection will cause the frequency of alleles that confer high fitness to increase. • In small populations drift may increase or decrease allele f i i d d t f th fit th ffrequencies independent of the fitness they confer.• Very strong selective regimes may make drift negligible• Very small population sizes may make the effects of drift so large that selection is negligible.Dobzhansky and Pavlovsky’siStarted 10 replicates ofexperiment:• Used fruit flies with two different inversions P and A.Started 10 replicates of populations with 20 flies – the high drift treatment• Created heterozygotes for both inversions – PA hybrids•They knew that PA hybridsStarted 10 replicates of populations with 4000 flies –the low drift treatment.They knew that PA hybrids had higher fitness than homozygotesAdfit fAA•And, fitness of AA homozygotes was greater than PP homozygotes.• So, they expected the P allele to decrease in culturesThey found frequencies of the two types varied among cultures much more in the high drift treatment But, the average frequency of the P allele in the two treatments was nearly the same at the end of the experiment.Kimura showed mathematically that selection is more important if h l i ffi i ( ) i h 1/2Nif the selection coefficient (s) is greater than 1/2Ne. The probability of fixation of a favored allele due to naturalallele due to natural selection increases with increased fitness d t d ithadvantage and with increased population size.Wh h i f l i f l l i i bWhen techniques for analysis of molecular variation became available (protein electrophoresis and then later gene sequencing) the amount of genetic variation within and between populations was gppsurprising. It had been expected that there would be little variation within natural populations because natural selection would have caused the fixation of high fitness allelescaused the fixation of high fitness alleles.The neutral theory of molecular evolution holds that although some genetic variation is selectively advantageous orsome genetic variation is selectively