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. (HardyWeinberg 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
 p + q = 1.0
(where p² = AA, 2pq = Aa, q² = aa)
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 HardyWeinberg 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)