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NICHOLLS BIOL 370 - Genetic Drift

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1Genetic DriftA common conception about evolution is that the features of an organism have evolved due to random (undirected) changeorganism have evolved due to random (undirected) change. Some processes in biology are random – like mutation. Natural 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 li f lievolution of some populations.One of the requirements for the maintenance of allele frequencies in populations is a very large population size. Genetic drift is the consequence of finite population size.Alleles that do not affect fitness 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 population.Sampling error (random chance) results in some gene copies being lost and others continuing.2Different 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 population, A and a, with f i d th b bilit th t th ll lill b l t tfrequencies 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 of populations is id dconsidered, each drifting, the total heterozygosity overall will decrease.3Starting with a population size of 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 equallyallele frequencies are equally likely.The average time to fixation of one of the alleles is 4N generations.4The effective population size is the number of individuals in the population that successfully pass genes to the next generation This isthe next generation. This is usually smaller than the actual number (census number) in the population.The smaller the effective population size, the faster a l ti ill d ift d thpopulation will drift, and the faster 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 number among individuals can reduce Ne. If some individuals produce more offspring than others their alleles, even those that have no effect on fitness, will be passed on at higher trates.A sex ratio other than 1:1 produces a similar reduction in NeNatural selection at one gene can produce differences in offspring number among individuals and reduce Ne5Inbreeding within or between generations reduces the number of different copies of a gene passed to the next generation and effectively reduces NeFluctuations in population size reduce Ne. Temporary decreases in population size have greater effects than temporary increases in population size. A temporary reduction in population size is called a bottleneck.Read: A Strong Bottleneck Reduced the Heterozygosity of Elephant Seals6When 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 suggest that a small number founders and a small population growth rate (r) result in greater loss of genetic diversity.Eventually mutation will restore genetic variation in a founding population. pp7Studies of laboratory and natural populations confirm the theoretical expectations of genetic drift models.107 populations of fruit flies each started with 16 heterozygotes, 8 males and 8 females.8 randomly chosen males and females were used to start each generation.Genetic Drift and Natural Selection• Directional selection will cause the frequency of alleles that confer high fitness to increase. • In small populations drift may increase or decrease allele frequencies independent of the fitness they confer.V l i i k d if li ibl•Very strong selective regimes may make drift negligible• Very small population sizes may make the effects of drift so large that selection is negligible.8Dobzhansky and Pavlovsky’sexperiment:• Used fruit flies with two different inversions P and A.• Created heterozygotes for Started 10 replicates of populations with 20 flies – the high drift treatmentStarted 10 replicates of ygboth inversions – PA hybrids• They knew that PA hybrids had higher fitness than homozygotes• And, fitness of AA homozygotes was greater populations with 4000 flies –the low drift treatment.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.9Kimura showed mathematically that selection is more important if the selection coefficient (s) is greater than 1/2Ne. The probability of pyfixation of a favored allele due to natural selection increases with increased fitness advantage and with increased population size.When 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 surprising. It had been expected that there would be little variation pg pwithin natural populations because natural selection would have caused the fixation of high fitness alleles.The neutral theory of molecular evolution holds that although some genetic variation is selectively advantageous or disadvantageous and can result in natural selection, most genetic variation is effectively neutralvariation is effectively neutral.The neutral theory predicts that most of the variation between populations or species is selectively neutral.10Alleles of a gene can be selectively neutral due to • synonymous codon changes• nonsynonymous changes that have little functional effect•changes in noncoding regionschanges in noncoding regions The Molecular Clock - early in the history of the study of molecular evolution it was hypothesized that molecular evolution may be different than morphological evolution. Molecular evolution may


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