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Types of Natural Selection
Sickle-Cell Anemia
• Mutation + genetic variation + natural selection =
evolution of a population
• Sickle- cell allele resulted from a single base mutation
in the DNA coding for hemoglobin
• Heterozygous individuals, Ss, are resistant to malaria
 better chance of survival
• Sub-Sahara Africa- if you carry the allele, S, than you
will survive to reproduce, and pass on the allele to the
next generation
• Therefore, an increase in the frequency of the allele
Directional Selection
• Occurs when selection favours an increase or
decrease in the value of a trait from the current
population average.
– Favours individuals with a more extreme
variation of a trait
• Results in a shift away the average condition
• Read Hummingbird example on the bottom of pg.
Directional Selection Example
-The soil in the original
environment was a medium
- After a volcano the soil
becomes darker
-Mouse population evolved
to blend in with the darker
-Therefore, environment
favoured individuals with
more extreme variations of
the trait
Stabilizing Selection
• Selection against individuals exhibiting traits that
deviate from the current population
The average phenotype within a population is favoured by
the environment
• Read the human birth weight example on the
bottom of page 327.
Stabilizing Selection Example
• The soil remains
medium brown
• Medium brown mice
survive from predation
• Medium brown mice
pass of their allele for
medium brown fur to
their offspring
• Light/ dark brown mice
are eaten more ->
those alleles are not
passed on
Disruptive Selection
• Selection that favours two or more variations
of a trait that differ from the current
population average.
– Favours opposite extremes of a trait over
individuals with intermediate variations
Disruptive Selection Example
• Mouse move to a new
environment with light
brown rock and dark
brown volcanic rock
• Light brown and dark
brown mice blend in with
their environment, but
medium brown mice do
• Medium brown mice are
eaten by predators
• Light/ dark brown mice
pass on their genes to
future generations
Hardy- Weinberg Equilibrium
• If allele frequencies change over time, the
population is NOT in Hardy-Weinberg
• Therefore, a ‘disturbing factor’ is acting on the
Natural Selection
Small population size
Immigration/ Emigration
Sexual selection
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