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SBI3U: Biology, Grade 11, University Preparation
Unit 3: Evolution
Activity 4: Being Selective
Overview | Expectations | Content | Assignment
Overview
Evolution can be defined as the change in inherited traits of a population over time. Evolution is a
concept that involves many different mechanisms, all acting independently and randomly over
millions of years. In this activity you will look in more detail at the selective forces that act in
evolution. An assessment gauges your understanding of the content. You will have the opportunity
to take a quiz to assess your own understanding of evolution. You will also be able to demonstrate
your understanding of the forces acting on evolution through a class discussion.
SBI3U: Biology, Grade 11, University Preparation
Unit 3: Evolution
Activity 4: Being Selective
Overview | Expectations | Content | Assignment
Content
Now that you understand how natural selection and genetic variation can cause changes in a
population’s genetic makeup, you will look at selective forces that favour some phenotypes over
others in the population.
Sexual Selection
Sexual selection favours the selection of any trait that affects
mating success. This is especially the case when male and female
members of the species show clear differences in their
phenotypes. The picture to the right shows the phenotypic
differences between the male and female peacock. The male’s
plumage is a trait that increases his chances of mating.
The difference in appearance of the
male and female (as seen in the lion
and lioness) of the same species is
called sexual dimorphism.
Artificial Selection
This type of selection is directed by humans and these
organisms are intentionally bred for specific traits. Artificial
selection has been practiced for years by breeders, and
farmers. For example, the picture to the right shows a
purebred Arabian horse. These horses have been bred for
centuries for their stamina and intelligence.
Artificial selection has occurred in many other organisms
as well – including bacteria and plants. Some scientists
argue that genetically modifying foods and animals using
genetic engineering and recombinant DNA is a more
modern application of artificial selection.
Modes of Selection
Stabilizing Selection
This type of selection favours the most common phenotypes in
the population. For example, most hummingbirds have an
average bill. If their bill is too long or too short, it is not
advantageous to draw nectar from particular flowers. Extremes
in bill length are not selected for.
Directional
Selection
Disruptive
Selection
Here the
environment
favours individuals
with more extreme
phenotypes of a
particular trait.
With the
hummingbird, the
long billed bird
may be selected.
Here the
environment
favours both
opposite extremes
of a trait. In the
case of the
hummingbird, both
long and short
billed birds are
selected for.
Types of Selection
Resources
Find out more on forces of selection
1.
2.
3.
4.
5.
Sexual Selection
Artificial Selection
Stabilizing Selection
Directional Selection
Disruptive Selection
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 sexual, stabilizing, directional, disruptive and artificial
selection.
Next
Content
Let’s recap what you have learned about how evolution works.
Evolution takes millions of years. It can be defined as the change in inherited traits of a population
over time. Genetic variation makes evolution possible. Selective forces act on a population to cause
genes to be expressed that help the population contend with the selective pressure. Evolution also
relies on mechanisms that allow for change to happen:

mutation

gene flow/migration
genetic drift
natural selection
sexual selection
artificial selection (more recent)




These selective forces can allow for populations to become more skewed (directional selection), more
polarized (disruptive selection), and more average (stabilizing selection) in their phenotypic
expression.
Learn more on evolutionary mechanisms
Mechanisms of change
Mechanisms of evolution
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 mechanisms of evolution.
Review
Take some time to review all your notes and assignments from the previous activities.
Check Your Understanding
1.
Which of the following is considered a vestigial organ?
a.
b.
c.
d.
appendix
pelvis
rectum
large intestine
Answer
2.
Which type of selection is occurring when the average phenotype is selected for and extreme
phenotypes are selected against?
a.
b.
c.
d.
disruptive
stabilizing
sexual
directional
Answer
3.
Which of the following is a type of genetic drift?
a.
b.
c.
d.
directional selection
stabilizing selection
mutation
bottle neck
Answer
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