Lecture 6 - Evolution in finite populations. I.

Lecture 6 - Evolution in finite populations. I. - 1 Lecture...

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1 Lecture 6 - 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. 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 alleles that made it into your genome and disfavor the 15,000 (times two) that did not. Almost all of the alleles that made it into your genome made it AT RANDOM. THIS IS GENETIC DRIFT! Now suppose your mom and dad each had an identical twin and they married each other and had fourteen kids. The chances that any allele from your aunt and uncle's genome made it into the next generation are clearly much greater than in the case of your mom and dad's genomes because they had 14 times as many chances. Note that (1) genetic drift is more important ("stronger") in small populations than in large populations and (2) the cause of genetic drift is RANDOM variation in fitness. II. The Wright-Fisher Model of Genetic Drift Assumptions: o. One locus, two alleles (for now. ..) i. Diploid population of constant size N individuals = 2N copies of every gene ii. Discrete (non-overlapping) generations iii. Number of gametes is >> number of individuals (e.g., oysters) iv. Random mating v. No recombination, migration, or selection (for now. ..) SHOW FELSENSTEIN'S FIGURES HERE Binomial sampling Suppose a population of N diploid individuals is produced by a random sample of 2N gametes? Two alleles, A1, A2 w/ freqs
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Lecture 6 - Evolution in finite populations. I. - 1 Lecture...

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