This is the Human Geographic Diversity lecture in
the last lecture we discussed the genetic component of diversity, where it comes from, and how it is introduced into a
population. Now that we have a better understanding of genetics and [the] ways in [which] variation
can be introduced into the genes, we need to move on to discuss how we understand population history
from genetics.
In this lecture we are going to discuss variation within and between
populations. If you remember in the first lecture we discussed [Lewington]
1972 research
where he showed that more genetic variation exists with within a population
than between two populations. In this lecture, we're going to actually discuss
how this is measured. We will also discuss what evolutionary forces can affect
population variation, and how each force can either increase or decrease
variation in a population or both.
When we begin assessing variation within and between
populations we are doing what's called population genetics; this is an entire field of study.
And I've defined it here on the slide:
It's the quantitative study of how genes are distributed within and across populations.
But before we really get into some other definitions. I think it would be [useful] to give you an example of what
population genetics studies do and the sort of questions that these type of studies are essentially trying to answer.
Something I discuss in my introductory course
is sickle cell anemia and this is something that we're going to kind of
keep coming back to throughout the semester for various examples. A lot of people assume
that this is a disease that is racial; however, this is another topic all together, and we're going to discuss this later in the semester.
But sickle cell anemia is an example
that I typically give in my introductory course for what is called
Heterozygote Advantage.
This is a genetic phenomenon where one genetic type, in this case the heterozygous form, is being favored in a
population. And I think I briefly mentioned this in the last lecture.
But sickle-cell anemia is a deficiency of the red blood cells, so typically in a normal
Individual that [is] not affected by this disease their red blood cells are going to be normal cells,
so they're going to have a really nice,
round shape
to them. And you [can] actually see a drawing of what we would consider to be normal
red blood cells on the Powerpoint slide. This is important because
red blood cells are the components, the things, that carry oxygen throughout your body.
So having them be nice, plump, round,
healthy cells will
allow them to be able to do this efficiently; to be able to carry oxygen throughout your body. If
a person [is] affected [by] Sickle cell anemia,
their red blood cells have a sickle shape. So you can see in the Powerpoint slide a picture here of
a slide of red blood cells where most of them are
sickled, and only a few of them actually have a normal shape to them.
For these individuals that are affected it's often deadly,
because obviously their body is having issues getting oxygen.
But their body is not able to function
properly because they're not effectively carrying oxygen throughout the body.
So if a person is
normal for the trait we could classify them as a homozygous-dominant individual.
So let's just say we'll use "A's" for this particular example.
Homozygous-dominant capital "a" capital "a." If they have sickle cell anemia then they [are] homozygous-
recessive for the trait, so this would be a lowercase "a" and a lowercase "a" . If they are heterozygous for the trait,
they would be considered what we call a carrier of sickle cell anemia.
Carriers therefore have some cells that are normal and
then they have some cells that are sickle, so they kind of have a little bit [of] both.
But what's really interesting about this disease is that research [has] shown
that there's a
geographical pattern for its occurrence.
The presence of heterozygous individuals is very prevalent in areas throughout Africa.
There's a reason [for] this and we're going to discuss it in more detail later in the semester.
But let's relate this example to population genetics; that's really the only reason I'm bringing this example up right now.
First of all this type of knowledge, knowing that this particular form of genetic loci
occurs more heavily in a certain geographical area, is due to population genetic research.
But you can also use this type of example to understand the research that is conducted; for instance,
okay, so,
African populations carry a higher frequency of this allele,
so some questions
you would want to ask. Ss this due to chance? Is this due to cultural practices? Not only that, but,
what has played a role in creating an
increased resistance to this allele and other populations?
Why is it that the heterozygous form is being favored over the homozygous-dominant form?
Since obviously it would be more advantageous to have all
normal cells.
So these are the types of questions that population genetics is designed to answer.
The entire purpose of population genetics is to better understand the variation of alleles within a population,
but to also understand why and how have these alleles changed over time and [how] they
responded to
forces; evolutionary forces. So
we're going to look briefly at how population genetic studies are actually conducted.
First I want to [start] with just some simple definitions.
I keep using the term population so we need to actually define what I mean by population. A
population consists of reproductive age
individuals that may contribute offspring to the next generation.
Defining which individuals belong to a population can be quite [tricky].
But if a population is isolated then it's very clear obviously who their members are.
However, once you begin distributing populations continuously,
across the landscape, then your boundaries of one population versus another can get kind of blurred.
When assessing or measuring how much genetic variation exists within a population or when comparing the
variation between two populations,
geneticists use probability theory.
Probability theory is a way of measuring the frequency with which events occur or
stating the likelihood that they will occur and
relating the probability of specific events, so when assessing
probability there are two rules that are used.
These rules allow us to see how individual probabilities can be combined.
So let's start with understanding the "or" rule. This rule is used when you are trying to assess the
probability that one or
the other
will occur in one event, or the other will occur. For example,
it would be used to ask the question. What is the probability that a
Gh-
Heterozygote will produce a "g" gamete, or an "h" gamete?
Keep in mind that this rule does [not] allow for the to vent [two] events to happen together.
The "and" rule is used to find the probability
that two independent events will occur.
So an example would be to assess the probability that a gh-
Heterozygote will produce two gametes a "g" and then another "g."
So when geneticists use these rules of probability they assess population variation
through the Hardy-Weinberg equilibrium.
This is a mathematical construct that allows
geneticists to predict the structure of a population over time. If
certain criteria are met, in other words, it
provides a baseline
expectation for a population to not change; to not undergo evolution.
So I've gone back and forth about how detailed I want to get in terms of making you all
calculate this equation, and I've decided
that for this class I
expect that most of you have already done Hardy-Weinberg
calculations in the past, so I'm not going to get into the calculations.
But I am more concerned about you knowing
how the Hardy-Weinberg is
used as a mathematical equation. How is it used to assess variation in a population?
So as I said, the Hardy-Weinberg equilibrium is just a
baseline expectation of what we would expect the genotype frequencies to be if a
population was in equilibrium.
So to do this using your known allele frequencies for the population,
you would then calculate the frequencies that you would quote "expect" using Hardy-Weinberg.
So again if you haven't done this in a previous class
this is basically a lot of mathematical calculations that you would use to determine these numbers.
So the expected
frequencies are the numbers that you would expect if the population is not undergoing
evolution. So let me say that again, the expected
Hardy-Weinberg
frequencies are the allele frequency calculations that you would expect if the population is not
undergoing evolution.
So on this slide, I put an example of some frequencies
for a population when the three genotypes for a trait are assessed.
"MM", "MN", and "NN." As you can see the expected frequency for the "MM"
genotype is point two five and the actual frequency for the genotype in
the population is point two four. So expected frequencies or what you would expect the
population to...
the frequencies that you would expect the population to have if they are in equilibrium;
if they are not undergoing evolution. The observed or actual frequencies is what you are actually
observing in the population.
So as you can see point two five,
point two four. These two frequencies aren't really differing that much from each other. You can also
compare the other two genotypes and see that the expected and the observed
frequencies
do not differ greatly from each other. So what this means is that for this particular trait ,that is being observed,
the population is not undergoing
evolution. That's because the actual frequencies that you're observing are fairly close to what [you] would expect the
population to be if they are not undergoing evolution. So if these numbers were drastically different from one another,
let's say that
"MM" expected frequency was  point 75 and our observed frequency was point
25, then this would
indicate that this particular genotype is not in equilibrium and
that
something is affecting that genotype; some type of evolutionary force is causing that genotype to undergo evolution.
So what does this all mean? Why is the Hardy-Weinberg important? Why is it used to assess variation?
Essentially, when we say that a population is in equilibrium,
we are saying that some trait that describes that population is remaining
unchanged through time.
If the population is deviating from Hardy-Weinberg
this is what's interesting, this is what tells us because these are the traits that suggest
that some type of evolutionary forces are at work in the population.
These forces are those that we have already discussed and include on
mutation, selection, drift, and migration. I want to briefly
discuss how each of these can change a little frequencies. And again, this is a really simplified version of
population genetics and quantitative genetics, but I just kind of want to give you the basics of
of
assessing a
variation in a population so that we can move on to really getting into the details and
the more, I guess to me, interesting discussion of
human variation.
So mutation, as we previously discussed,
this is the only way that you can introduce
new alleles into the population.
The rate of this can be measured using Hardy-Weinberg
calculations, but for the most part
the effect of a mutation on allele frequencies is really small, so when compared to the other forces,
mutation is considered to be a weak force and that it doesn't change a allele frequencies at a
high rate, in other words, it really isn't a force that will pull a
population that far from Hardy-Weinberg equilibrium.
The only way it will have noticeable effects is if it occurs over a long period of time.
Mutation is also one that acts in conjunction with other forces,
meaning that the other forces of evolution operate with mutation to produce changes in allele frequencies.
Most mutations are considered to be deleterious, but there have been
advantageous ones that occur as well.
Selection is a force that must operate on genetic variation, so as I've previously mentioned,
mutation is the precursor for
evolution to occur through natural selection.
Also keep in mind that selection is occurring within a population
because there's some type of differential survivorship.
So if we are looking at a particular trait in a population, let's just use stature as an example,
then how the
population responds to selection is
determined by where the population is most fit or where it shows the greatest survivorship.
So there are two categories of a selection that population geneticists consider:
directional selection and balancing selection.
Directional selection occurs when one allele is being favored. So let's go back to our example using height as a trait.
This would [be] as the one extreme is being favored, maybe tall individuals,
balancing selection occurs
when two alleles are being favored in the population, so this would [be] as though the average height,
not short individuals, not really tall individuals, but just your average height is being favored.
The most common cause of this is heterozygote advantage and when this occurs an
individual that has two different copies of an  allele is more fit than
individuals with one copy of that allele. For example,
let's say we are referring to a allel "g"
individual with
heterozygous for this trait, capital "g" a lowercase "g", will have a higher fitness
than individuals with both the
homozygous form; whether that be dominant or recessive.
The heterozygote individuals are favored over
both homozygous forms of the tree.
Because the heterozygous individual is being favored [then] both alleles are going to be maintained in the population.
Two roles to remember about evolution through selection.
Evolution by natural selection can occur rapidly.
If the dominant allele is being selected against, while on the [other] hand it will occur
slowly if the selection is against the recessive allele.
So why is this?
Well, it helps me to remember this rule by thinking about the phenotypes.
Remember when viewing the phenotype the recessive alleles are essentially hidden in the heterozygote.
So, for example, let's just say we are talking about
selection against plant height, maybe we have a
field full of plants and some of them are really tall some of them are really short plants.
Let's argue that, or let's denote,
capital "T"
for tall plants and a lowercase "t" for short plants. If
There was a, I don't know, a hungry goat
that was only eating every tall plant in the population, in other words, selection; the goat, is
acting upon the tall plants, then in one generation
all tall plants could possibly disappear.
Selection, which is the goat here remember, is only seeing the phenotype.
The goat is only seeing tall plants and
therefore evolution in this population would be really fast.
If
selection was against the short plants, this would slow evolution down, because the goat would eat all of
the plants that are homozygous-
recessive, but the recessive allele wouldn't actually
disappear from that population
because it still exists in the heterozygous dominant plants; which are the tall ones. In fact, this allele can't be
completely
eliminated because of this. So keep in mind that
that recessive allele is being hidden in the
heterozygous form of the trait, but all you're seeing the phenotype is what's being seen by the selective pressure.
This moves us to discussing the effect of genetic drift.
Remember, genetic drift is a change in allele frequencies within small
populations that occurs across time.
This is due to chance event so it's a completely random force.
It can have significant impacts on [the] evolution of a population if the population is small.
If the population is large, you're not going to see this random genetic change in terms of evolution.
You just won't see it. Why is that?
Well, a good way to think about this is to imagine that you have a population that's
represented by three colors, so you can see here in the first picture on the left side [of] the slide you have red,
yellow, and blue
beads in this jar.
Let's just imagine that these colors represent all of the possible alleles in the population
so
lots of different variation even though
there's only three colors we're kind of looking at here.
Let's just pull out a small handful of those beads and then you can move to the second jar,
the second generation,
so now the frequency of these alleles are not equal to the original frequency that we had.
You can see there's
less reds, a lot more blues.
Another handful taken out of that jar and
You continue to minimize the frequency [of] the alleles in each generation.
So through time, this is randomly
shifting, there's no pattern there's no no
process to this, but it's randomly shifting the frequency of alleles and will produce
drastic evolutionary impacts on the population. So for example, you can see here in the last picture of
the jars that the population now only contains
blue and yellow beads...
and it's skewed toward a larger number [of] blue ones.
This is a completely random process,
but it can have a really drastic impact on the population.
It is common that when genetic drift occurs in a population, there tends to be a deficiency of
heterozygotes and an excess of
homozygotes.
This leads to an interesting interplay with other forces of evolution like
motation, since typically mutation will act to increase heterozygosity.
Lastly, migration is a force of evolution that affects population variation.
When the term migration is used though
it's essentially referring to gene flow so keep in mind that gene flow is considered [to] be the one of the four forces of evolution.
But when we're talking about population genetics, we typically use the term
migration for this, so this is a movement of alleles.
There is a genetic phenomenon that occurs with migration referred to as isolation by distance.
So when you think of two populations exchanging genes
typically one population will exchange genes with their nearest neighbors, so they're not going to go further than they have to
But the population that [is]...
so they're going to exchange genes with their nearest neighbor; the population that's living closest to them.
So this is going to lead to
populations
that are closer together in terms of geography. They are going to be more genetically similar to one another
than populations that are further apart.
So this is what is referred to as
isolation by distance, so migration actually acts to increase variation within a population.
Which makes sense if you think about its definition, essentially, the allele frequency of already-existing
alleles has the potential to increase,
or you could introduce new alleles into the population if there's some type of new
mutation being introduced
from the neighboring population; from the migrating population. I
want to finish this lecture with a discussion of the assigned article for the week and something you are
probably already
realizing is that the articles that are assigned each week in some way relate to the lecture.
But they are chosen also to move us forward to our discussion for the second portion of the course.
I'm not sure if you've realized but the second portion of the course is a little bit heavier in
lectures and heavier in material.
And a few more weeks longer than the first
portion of the course.
The assigned article for this week is by Jared Diamond, and if you haven't read it
please be sure to do so but I
wanted to briefly summarize this article and just kind of talk about a few points that
relate to what we have discussed in this lecture.
First of all, one of the main purposes of this article, and something you should be kind of thinking about
as you're reading, is that Diamond's trying to
look at answering the question of, how is it that traits become geographically patterned? And
Diamond focuses on looking at skin color kind of as the main example throughout the article
But the answer to this question can be applied to all sorts of different traits.
He mentioned some ways in the article that geographical
patterns can occur. First,
one is through natural selection, so this is a force that we already discussed
but variability is first produced in the population due to some selective force acting upon it,
and he [actually] provides an
example of the differences between
Northern Hares and Weasels and the Southern ones. The Northern species
produces white fur during the wintertime because selection is
providing them with a camouflage throughout the winter months but for the southern species
selection
maintains this brown colored fur year-round.
Traits can also vary
geographically due to sexual selection.
This is something that we
didn't discuss in this lecture, and so I just want to briefly mention it here.
Sexual selection is a lot like natural selection.
It's essentially the same thing, it can do the same things in terms of heterozygote advantage and stuff.
It can do those same things in a population. It's just how it comes about is different.
It's like natural selection except that traits are being selected due to
individuals of 1 sex being attracted to mates of the opposite sex
based upon a specific trait, so depending on what trait is being selected
this this can create differences between
populations.
So take note of the examples that Diamond provides for humans and he gets a number of them.
Examples for sexual selections and also examples for natural selection.
So I just wanted to point out that these are really good things to  focus on throughout the article
as they [hint] make for good test questions.
So that wraps up this lecture, and i've tried to keep this lecture as simple as possible.
But I do realize [that] most lectures on quantitative genetics is all math
and I just really want you to understand how genetics plays a role in producing
population variation.
And how is it that those various forces of evolution act to increase or do they decrease
population variation.
