Sickle-cell anemia is the best
documented example of natural selection
in humans. In this short video, I'll
discuss natural selection and it's
relationship to sickle cell anemia and
Malaria. All of our human genes have
different alleles. Most humans are
homozygous to the hemoglobin A allele (HbA),
therefore producing normal hemoglobin.
Hemoglobin is a protein in our blood
that transports oxygen throughout our
body. However, there is a rare mutation
to the hemoglobin allele called the
hemoglobin S allele (HbS). Which causes an
abnormal form of hemoglobin, and as I
will soon explain
for some people this has major effects
in transporting oxygen across
the body. Sickle cell trait happens when
a person is heterozygous to the
hemoglobin allele, meaning this person
has one hemoglobin A allele and one
hemoglobin S allele.
Although this causes some of the
hemoglobin to be abnormal, there is still
enough normal hemoglobin in the body for
the person to function normally.
Sickle cell anemia happens when a person
is homozygous to the hemoglobin S allele,
meaning both of his alleles are
hemoglobin S. Sickle-cell anemia is rare
and very serious. The abnormal hemoglobin S
reduces the ability of red blood cells
to transport oxygen across the body. When
people with sickle cell anemia increase
their demand for oxygen, for example by
exercising, the red blood cells will
collapse forming a sickle shape. This red
blood cells can't carry the right amounts
of oxygen throughout our body.
These red blood cells will also clump
together and block capillaries,
restraining good blood flow across our
body and to major important organs.
Life expectancy in the U.S. for people
with sickle cell anemia is less than 45
years. And this is with treatment.
The hemoglobin S mutation occurs in all
populations, however it is especially
present in central and western Africa,
where it can appear as often as 20% of
the time.
Given how bad this allele is, how can it
be that is allele survived natural
selection over centuries? The answer to
this is one word, Malaria. Malaria is a
disease caused by a parasite. This
parasite is transmitted by the bite of
an infected mosquito. After the bite, the
parasites invade the blood cells
where they gain the oxygen they need in
order to reproduce. Malaria kills about
one to three million people each year
and in parts of Africa where Malaria is
always present, as many as 50 to 75
percent of 2-9 year-olds are infected. So
how are sickle cell anemia and Malaria
related? Well, people with sickle cell
trait have a greater resistance to
Malaria than people with normal
hemoglobin. Why? Because the
cells that contain hemoglobin S are not good
for the parasite to reproduce. The
parasite is not attracted
to this allele, to this hemoglobin.
Therefore, this provides protection from
Malaria. Because of this, in regions where
Malaria is present, people with
sickle cell trait live longer, therefore,
producing more offspring than people
with only normal hemoglobin who are more
prone of dying from Malaria. This is the
perfect example of natural selection
because malaria acts as a
selective agent that favors the
hemoglobin S allele. This allele will be
maintained in the population
and therefore, there will always be
people with sickle-cell anemia.
