Download Mechanisms of Evolution - Mr. Doyle SUIS Science

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: 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).