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Lab 2 - Evolution - Student

# Lab 2 - Evolution - Student - BSC2011 Lab 2 Evolution and...

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1 Introduction Modern biologists define evolution as the change in allele frequency in a population over time. There are several mechanisms that can cause a population’s allele frequencies to change including migration, mutation, selection and genetic drift. If none of these mechanisms are operating, then allele frequencies will remain constant from one generation to the next (i.e. the allele frequencies are in equilibrium). This central evolutionary tenet is referred to as the Hardy- Weinberg Equilibrium Theorem. Hardy-Weinberg Equilibrium theory can be summarized with a simple equation that allows us to track both allele and genotype frequencies in a one gene, two allele model: 1 = p 2 + 2 pq + q 2 where p = frequency of the first allele, q = frequency of the second allele, p 2 = frequency of the p homozygote genotype, q 2 = frequency of the q homozygote genotype and 2 pq = frequency of the heterozygote genotype. For example, suppose a gene has two alleles, A and a. By sampling the population, we find the frequency of the alleles to be A = 0.2 and a = 0.8 (remember, p + q = 1). We can then predict the genotypic frequencies by assigning A and a to the variables p and q in the above equation, such that p 2 = (0.2) 2 = 0.04, q 2 = (0.8) 2 = 0.64 and 2pq = 2 (0.2 * 0.8) = 0.32 (see Figure 1). Thus, if 20% of the alleles in a population are A , then we expect only 4% of the population to be AA homozygous. After our calculations, if we find the AA genotype to differ from 4% (e.g. 15%), then something has occurred to

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