So, these are all cases of directional selection where eventually you go to one
form, eventually you go to monomorphism.
Here's a different type of selection, this is the case of heterozygote advantage,
this is often called overdominance.
In that particular case, the most fit genotype is the heterozygote or the Aa.
So this one is more fit than both kinds of homozygous.
The two homozygotes may differ in fitness from each other, but
the highest fitness is associated with the heterozygote, okay?
In this case, unlike with directional selection,
one allele does not replace the other.
This does not lead to monomorphism over time.
A classic example of this, that people always site is that of
sickle cell anemia and malaria resistance, so let me introduce this to you briefly.
Malaria is a big threat for a large part of the developing world.
In fact, in places like sub-Saharan Africa,
your odds of death by malaria are around 4%, that's pretty high overall.
Now, malaria is transmitted by mosquito bites,
which are giving this Plasmodium protozoa, which is what actually causes malaria.
Now, how does this relate to sickle cell anemia?
Well, sickle cell anemia is not a great thing, either.
This is a recessive genetic disease, so only aa individuals are fully afflicted.
In this particular case, the sickle cells die faster than normal red blood cells.
They deliver less oxygen to cells.
So people who have this disease are tired all the time, they have chronic pain.
They have, quote unquote,
episodes where they sometimes have to go to the hospital associated with this.
So this is clearly not a good thing.
Interestingly the heterozygote, so the big A little As,
these people are often called having the sickle cell trait.
They are usually okay, sometimes they can have some sickling in their cell
when they're exercise very hard and have some type of intense physical exertion.
But overall they're good.
But interestingly, these individuals who are heterozygous for
sickle cell are actually more resistant to malaria than
individuals that are heterozygous for the non sickle allele.
Now, the thought a long time ago with this was that invasion, growth and development
of Plasmodium was actually somehow reduced in these heterozygous blood cells.
But more recently, I cite two studies down here.
You're welcome to look them up if you'd like.
One thought is the heterozygote is more tolerant to sickle cell symptoms, but
actually still retains a connection.
Whereas another study showed that the infected Heterozygous cells,
are more likely to be eliminated by the spleen.