Lecture 5 - Evolution in finite populations. I.

Lecture 5 - Evolution in finite populations. I. - 1 Lecture...

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1 Lecture 5 - Introduction to Evolution in Finite Populations From last time. .. (redraw a cline, w/ p on the y, lat on the x) III. B. Migration and Selection What about selection ? We can imagine two patches, i and j , the A1 allele is favored in one and the A2 allele in the other, with a rate of migration m ij and m ji representing migration from patch i to patch j and vice versa. Genotypic fitnesses in patch i are w 11,i , w 12i , w 22i ; fitnesses in patch j are w 11,j , w 12,j , w 22,j . Population sizes are equal in the two patches. Set the model up as random mating followed by selection followed by migration (imagine selection acts mostly on juvenile survivorship, as for many fish). After selection but before migration, let the frequency of the A1 allele in population i be p i *, which follows from the general viability model for population i . So, the frequency of the A1 allele in the next generation in population i is: p i ' = (1-m ij )p i * + m ji p j * The equilibrium condition is (you guessed it!) marginal overdominance, i.e., the average fitness of the heterozygote is highest, even if it isn't the most fit in either population. The equilibrium frequency will also depend on the migration rates. Note that natural selection can also produce a cline in allele frequency. For example, the gene for alcohol dehydrogenase (Adh) exhibits a latitudinal cline in several species of Drosophila , on several continents. There are two major allozyme alleles, "fast" and "slow". One allele appears to be favored in warmer climates, the other in cooler climates. Other examples of latitudinal clines include growth rate in Atlantic Silversides (a fish), ovariole number in several species of Drosophila (on several continents) How would you determine if a cline was maintained by restricted migration (e.g., stepping stone) or by natural selection? Evolution in a finite population: Introduction to Random Genetic Drift I. A familiar example of genetic drift. There ~25,000 genes in the human genome, and since we are diploid, that means we have two copies of each. On average, there is a polymorphic nucleotide site about every 1000 bp (1 kb). Let's further say the average gene is 1 kb long. So by this crude reasoning, we are all heterozygous at every locus, on average.
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2 Now, suppose you are an only child. You got one copy of a gene from mom and the other copy from dad. Since you are their only child, that means that one allele from each gene in each parent's genome did not make it into the next generation (i.e., was "lost"). Clearly, natural selection did not favor anything like all 15,000 (times two) of the
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Lecture 5 - Evolution in finite populations. I. - 1 Lecture...

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