BIOA01H3 Chapter Notes - Chapter 21: Heterozygote Advantage, Allele Frequency, Balancing Selection
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Sharan Chinniah BIOA01
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
• 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.
• 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
• 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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