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Chapter 21

BIOA01H3 Chapter Notes - Chapter 21: Heterozygote Advantage, Allele Frequency, Balancing Selection

Biological Sciences
Course Code
Mark Fitzpatrick

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Sharan Chinniah BIOA01
Chapter 21.4:
Natural Selection: The differential survival and reproduction of individuals due to differences in
Deleterious mutations are wiped from a population through natural selection; beneficial
mutations result in adaptation to the environment overtime.
Almost all currently living organisms are extremely adapted to living in their
Positive selection is the favour of an advantageous allele even under initial
unfavourable situations.
Negative selection is the decrease in frequency of deleterious alleles.
o When a b is introduced as a heterozygote it has no deleterious effect. However,
as the frequency of the b increases it will eventually result in deleterious bb. As
this point natural selection will reduce the frequency of the bb.
Balancing selection acts to maintain 2 or more alleles in a population.
o When an organism’s fitness is optimal as a heterozygote, both alleles are
maintained (heterozygote advantage).
o When a species live in 2 environmental conditions; as a result, favourable alleles
will survive from both sides.
Charles Darwin:
Species are not unchanging organisms, they adapt and change according to their
Natural Selection is the mechanism for which organisms have become to adapted to
their environments.
In 1858, Darwin was motivated to publish his findings when Alfred Wallace, living in
Asia, had developed the same theory of evolution through natural selection.
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