Mechanisms of Evolution  Lesson goals:  1. Define evolution in terms of genetics.  2. Using mathematics show how evolution cannot occur unless there are conditions that cause a change in allele frequencies. (Hardy- Weinberg principle).  3. Identify and describe the patterns that can be observed in evolution.

The Hardy-Weinberg principle  What is it?  Mathematical model that can be used to predict the frequencies of certain genotypes, if you know the frequency of other genotypes within a population.  Refresher question: what is a genotype? Give an example of a genotype.

The Hardy-Weinberg principle  H-W principle is given by:  in words  (frequency of AA) + (frequency of Aa) + (frequency of aa) = 100%  And (frequency of A) + (frequency of a) = 100%  In symbols:  p² + 2pq + q² = 1.0 and (where p² = AA, 2pq = Aa, q² = aa)  p + q = 1.0

Example:  There is a genetic condition controlled by two alleles (S and s), which follow the rule of simple dominance at a single locus. The condition affects only homozygous recessive individuals. (the heterozygous phenotype shows no symptoms). The population size we are studying is 10,000 individuals and there are 36 individuals affected by the condition. Based on this information, use the Hardy Weinberg equations to answer the following questions:  1 – Calculate: what are the frequencies of the S and s alleles?  2 – Calculate: what are the frequencies of the SS, Ss, and ss genotypes?  3 – Calculate: how many people, in total, is likely to be carrying the s allele, whether or not they are aware of it.

Task:  Suppose that, in one generation, the frequency of the A allele is 40% (p = 0.4) and the frequency of the a allele is 60% (q = 0.60).  If this population is in genetic equilibrium (i.e. no evolution is happening) calculate the chances of an individual in the next generation having genotype AA, genotype aa, genotype Aa.  Identify and list the conditions for a population to remain in genetic equilibrium (i.e. for the hardy-Weinberg predictions to be upheld). (reference p. 432 in textbook).

Task 2 – evolution as genetics change in populations  Define the following ways in which natural selection acts on a organism’s phenotype.  1. stabilizing selection.  2. directional selection.  3. disruptive selection.  Beside your descriptions represent each of the above with large neat labelled graphs showing the effect on phenotype. Provide an example for each mode of natural selection and describe the effect on phenotype ratios on the population you researched (you are not allowed to use the examples already listed in the textbook).