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: what percentage of 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).

Hardy – Weinberg activity.

Scenario 1 – testing equilibrium  You have 50 white rice and 50 black rice in a bowl (population is in equilibrium) – these represent alleles.  You decide which color is “p” and which is “q”  Two members from your group each blindly pick one allele from the bowl – the alleles picked out represent the allele of one of the offspring.  Record the offspring’s genotype.  Put the rice back into the bowl and repeat the experiment for 40 breedings.

Scenario 2 – advantage  Repeat experiment again but this time follow these rules:  1. every time the genotype qq is born – the offspring does not survive.  2. when this happens – DO NOT record the result, throw the rice back into the bowl and pick alleles again.  Keep repeating until you have recorded 40 live offspring.

Scenario 3 - Set the scene  Who remembers what this is? Talk about it.

Sickle cell anemia  Is a hereditary blood disorder. It is caused by an abnormality in the oxygen-carrying hemoglobin molecule in red blood cells – this turns them sickle shaped.  People get this disease by inheriting sickle cell genes from both of their parents – meaning they are homozygous for this trait.  People who have this disease have a big chance of dying before reproductive age.

Scenario 3 – the heterozygous advantage  In Africa two of the major causes of death are the genetic sickle cell disease and the infectious disease malaria.  If a person is born with sickle cell disease (qq) they may die early.  If a person is pp – they have a big chance of dying from malaria.  However – if a person is heterozygous, they do not die from sickle cell disease and due to the fact their red blood cells are poorly oxygenated the malaria parasite cannot survive well – so they do not get malaria.  It seems to be an advantage to be heterozygous

Scenario 3 – the heterozygous advantage  Repeat the experiment  Follow these rules:  qq offpring still die  pp offspring – toss a coin to see if they survive or not (50:50)  pq – survive regardless.  DO NOT record qq  DO NOT record pp when they die after the coin toss.  Keep picking until you reach 40 breedings.

Lab report Guide  Should contain  Appropriate Title.  Introduction: (State your aim, briefly explain how you set up your investigation i.e. how will you achieved your aim, briefly explain how you collected your data, explain the key terms in your investigation and show your understanding of the Hardy- Weinberg equation).  Materials, Procedure, Experimental design.  Data: need a table and graph showing the ratios of offspring genotype for each scenario. (tables & graphs should be labeled clearly and have a title which clearly describes what information is contained in each).

Analysis  First make a claim about the Hardy – Weinberg Equation. E.g. does it run true in nature? Why not? Use your data from our tests to help you explain.  For each scenario the population was in genetic equilibrium at the beginning of the test. (Describe the results from the simulations over the 40 generations)  1 – for each scenario: Was the population in genetic equilibrium after the test? Explain why - if you were or were not.  2 - In your own words explain if the sickle cell allele in the United States would show a similar distribution to the one in Africa. Why or why not? (scenario 3 question)