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lecture 4

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Biology 3466B
Yolanda Morbey

Hand out 4  Migration as a mechanism of evolution o Migration is the movement of alleles between populations. Migration means gene flow: ht e transfer of alleles from the gene pool of one population to the gen pool of another. Varies greatly among different species. o Life cycle with migration  Start with the allele frequency of all A1A1 (one allele is fixed)  Migrate A2A2 people so they join the breeding pool of adults  We can now predict the final allele frequencies  All A1A1 will make A1A1 gametes  All A2A2 will make A2A2 gametes  Therefore the allele frequencies have change which means evolution happened  Migration violates the HWE, excess in homo and deficit of hetero. Random mating will put this back into HWE o Migration in lake Erie  Genetic drift o Violation of HWE from violation of the assumption of infinite population size. o In small populations chance events produce outcomes that differ from theoretical expectations o Happens due to sampling error, en you don’t have a infinite sample size therefore HWE is not shown o Depends on population size, the smaller the population size the more deviation o Nothing more than cumulative effect of random events, genetic drift can not produce adaptations. But it can cause allele frequencies to change. o Selection is differential reproductive success that happens for a reason; genetic drift is differential reproduction success that just happens o The founder effect: when a new population is founded by a small number of individuals, it is likely that chance alone will cause the allele frequencies in the new population to be different from those in the source of the population  Silvereyes: have migrated 5 times in history to different islands. Took blood samples and tested the DNA of each sample from the different islands. Graphing showed that there was a decline in allelic diversity in each new population but each is so small it is undetectable. However when put together the small declines in diversity added up and evolution occurred due to genetic drift. o Graphs A B and C show 100 successive generations of genetic drift in different population sizes. Three patterns are evident  Because the allele frequency from one generation to the next are caused by random sampling error, every population follows a unique evolutionary path.  Genetic drift has more rapid effect on allele frequencies on small populations than large ones  Given enough time genetic drift can make the same difference in diversity in large populations than small ones  Fixation of alleles o As any allele moves between frequencies of 0 and 1 sooner or later it will reach a fixed rate. If it hits 0 it is lost forever. If it hits 1 then the allele is said to be fixed. o Assume the population size is N and the number of copies of a unique allele is 2N o The probability that a unique allele A will become fixed = ½ N o The probability that an allele with multiple copies will become fixed = x/2N o As the alleles drift to fixation or loss, the frequency of heterozygosity in the population declines o The frequency of heterozygotes is called heterozygosity. We like to be able to predict this decline. We do this by this relationship  H g+1= Hg[1-1/2N]  Where H g+1is the heterozygosity of the next generation and H gf the present.  Effective population size o Is the size of an ideal theoretical population that would lose heterozygosity at the same rate as an actual population of interest  N e (4N Nm/(f +N)m f
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