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SBI3U: Biology, Grade 11, University Preparation
Unit 3: Evolution
Activity 3: Mechanisms for Evolution
Overview | Expectations | Content | Assignment
Overview
In this activity you will learn about adaptation and how it is vital to natural selection. You will also
learn how random changes can occur in a population, but how natural selection is not random,
instead selecting for survival of the fittest. At the end of this activity you will complete a lab where you
will observe selection on a micro level.
SBI3U: Biology, Grade 11, University Preparation
Unit 3: Evolution
Activity 3: Mechanisms for Evolution
Overview | Expectations | Content | Assignment
Content
In the previous activity you learned about how the organisms that are most fit for a particular
environmental change have a better chance to survive long enough to produce offspring. But, how is
it that one individual’s fitness can translate into a permanent genetic change that is passed on from
generation to generation?
Did you know?
Sickle cell anaemia is caused by a mutation in a gene that makes
haemoglobin. The allele that causes the disease occurs most frequently in
parts of the world where malaria is common. In these areas, sickle cell
anaemia carriers are naturally selected because carrying the trait gives
them some immunity to malaria.
Although sickle cell anaemia greatly curtails life span, individuals who carry
the trait have some immunity to malaria and on average live longer with the
trait than without. This is an example of natural selection.
Variation
Recall from genetics that alleles are alternate forms of the same gene. Alleles can be created by
random mutations in the DNA. If the mutated alleles are harmful to the organism, usually the
organism will die. However organisms that have the beneficial mutation may be naturally selected.
A variety of molluscs
Here is one species of mollusc,
Donax variabilis. Within this
species there is variability in the
phenotype of the shells. This
variability likely originated from point
mutations in the DNA of these
molluscs. If you were to take the
frequencies of how many times
these phenotypes appear in the
population of molluscs and graph
them, you would probably obtain a
curve that shows a normal
distribution like the one on the right
A
normal distribution curve showing the
percentage of variation/deviation under the
curve. This graph shows continuous
variation within a population.
Adaptation
An adaptation is a genetic variation that allows for a population to be favoured by natural selection.
The ability of an organism to change its phenotype in response to different environmental conditions
is called phenotypic plasticity. The phenotypic plasticity seen in the Donax in the illustration above
allows them to live in a variety of conditions. Changes in the conditions may cause one type of shell
colouration to be favoured over another. Thus the shape of the normal distribution curve might shift
to the right or left, or even split in the middle.
Hardy-Weinburg Principle
After a dialogue with Reginald Punnett, two mathematicians, Godfrey Hardy and Wilhelm Weinberg,
came up with a solution to identify the frequency of each allele in a population. They showed that
there was a mathematical relationship between two alleles in a population and that the allele
frequencies would not change from generation to generation unless there are disruptive influences.
This relationship is known as the Hardy-Weinburg Principle and holds true if:

populations are large;

there are equal opportunities for mating;
no migration occurs;
no mutations occur;
no natural selection, but equal opportunity for all.



Under these conditions a given gene pool remains the same from generation to generation. This
does not allow for much evolutionary change.
Let’s look at conditions where the Hardy-Weinburg Principle does not apply:
1.
Genetic Drift
When populations are small and some individuals leave the population, this reduces the
frequency of certain alleles in the population. For instance, in the island scenario from the
previous activity, what if two of the eleven individuals can overcome an infection (they carry the
alleles to do this) that strikes all the people. However they decide to build a raft and leave the
island. This would affect the frequency of this particular allele in this small population. There is
a chance that no one would survive, and those two individuals would not pass on these alleles
to the next generation. This is an example of genetic drift. In severe situations, genetic drift can
give way to the bottleneck effect and founder effect.
Learn more on Bottleneck Effect, Founder Effect and Genetic Drift
Bottleneck Effect
Founder Effect
Genetic Drift
Portfolio:
Notes help prepare you for tests. Keep all of your notes in your portfolio. Be prepared to submit
your portfolio notes and organizers to your teacher for assessment at various times during the
course.
Use your portfolio to make detailed notes on Bottleneck Effect, Founder Effect and Genetic Drift.
2.
Gene Flow/Migration
When organisms migrate from one population to another, this can alter the frequency of alleles
in both populations. This is termed gene flow. Gene flow can also occur when individuals from
two populations mate without moving. When this occurs, gene flow can make populations more
similar.
3.
Mutation
Mutations allow for new alleles to be formed. Mutations can be beneficial and allow organisms
to adapt to change. They can be neutral or they can be harmful and not allow the organism to
be more fit, but rather adversely affect their chances of survival. You have learned of several
types of mutations such as point mutations, translocations, inversions, deletions, and
duplications. Those that affect evolution are gene duplication, pseudogenes, and polyploidy.
Learn more on the mutations that affect evolution.
Pseudogenes
Polyploidy
Evolution by Polyploidy
4.
Natural Selection
Natural selection is a mechanism of evolution. Before natural selection can occur, there needs
to be variation in a population’s gene pool. Variation means that there are multiple alleles in a
population. Some changes in the environment will result in individuals with alleles that code for
traits that suit the new environment and therefore be more fit to survive. The survivors will pass
on their genes to the next generation. When members of a population separate from the original
population, they face new environmental challenges and new traits may be advantageous. The
genes coding for these traits will be passed on to the next generation. Populations of individuals
can become more varied from each other since the gene flow between the populations is not
possible. Increased differences can ultimately lead to speciation (the formation of two
genetically unique species) and these two populations can no longer mate due to genetic
differences.
Extinction
In Activity 2 you learned about extinction. You also researched some of the factors that cause
extinction such as loss of habitat, loss of food source, and environmental change. Unfortunately,
these species will be lost from the planet forever. In terms of evolution, however, extinction is
not necessarily considered negative. Can you think why this is?
Learn more on Natural Selection and Extinction
Natural Selection
Extinction
Portfolio:
Notes help prepare you for tests. Keep all of your notes in your portfolio. Be prepared to submit your
portfolio notes and organizers to your teacher for assessment at various times during the course.
Use your portfolio to make detailed notes on Gene duplication, Polyploidy, Evolution by Polyploidy
and Extinction.
Try this simulation lab:
http://download.elearningontario.ca/repository/1235730000/SBI3UPU03/SBI3UPU03A03/mme/natura
lsel/preloader.html