Introduction to Genetics and Evolution is a college-level class being offered simultaneously to new students at Duke University. The course gives interested people a very basic overview of some principles behind these very fundamental areas of biology. We often hear about new "genome sequences," commercial kits that can tell you about your ancestry (including pre-human) from your DNA or disease predispositions, debates about the truth of evolution, why animals behave the way they do, and how people found "genetic evidence for natural selection." This course provides the basic biology you need to understand all of these issues better, tries to clarify some misconceptions, and tries to prepare students for future, more advanced coursework in Biology (and especially evolutionary genetics). No prior coursework is assumed.
Case Studies and Examples (G)
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Introduction to Genetics and Evolution
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From the lesson
Population Genetics II
This module extends the previous one to specifically examine the effects of natural selection and genetic drift on genetic variation in natural populations.
Meet the Instructors
Dr. Mohamed NoorEarl D. McLean Professor and Chair,
Biology
Hello, and welcome back to Introduction to Genetics and Evolution.
In the previous videos, we talked about the general principles of natural
selection as applied to single loci or as applied to phenotypes.
What I wanted to do in this video is to present to you some elegant
case studies and examples of natural selection.
Cases where looking at animals or
plants in the field, we tend to see these observations that
seem to be best explained by the action of natural selection.
So let's go ahead and get started.
I have three general categories of things that we'll be talking about.
First, we'll look at some case studies of mimicry.
Mimicry, as you may know from the term itself,
suggests imitating something else in appearance.
We'll look at variation in space and time within a species or
across species, and we'll look at the principle of sacrifice for family members.
Why does it happen?
Evolution by natural selection isn't always ruthless [LAUGH],
as some may have you think.
So, looking at mimicry to start with.
Mimicry, again,
it just means the organisms evolving to resemble one another.
Now when can this adaptive?
The simplest time this would be adaptive is
if the model has some sort of warning coloration.
Basically, if you want to look like something that is scary, that makes it so
other things are less likely to threaten you or to eat you or things like that.
So if what's referred to as the model, the thing that you're evolving to look like,
has a warning coloration and is actively avoided, then your appearing
like the model species will make it less likely that you will be harassed or eaten.
I think this is not a particularly good example here of a dog
trying to look like a skunk, and it's clearly not evolutionary, but nonetheless,
let's look at some real examples.
There's two general types of mimicry.
The first one we'll talk about is called Batesian mimicry.
This is the case where the mimic is not dangerous, but the model is dangerous.
In this case again, it's useful for the mimic to look like the model because
others will fear it or avoid it for whatever reason.
So we have here example of the non-venomous kingsnake,
which as you see, looks very, very similar to the venomous coral snake.
We have an example here of a fly that looks very much like a wasp.
And wasps, of course, can sting you, and that's very painful, and
therefore they're typically avoided.
This fly however has no stinger, but it looks so much like a wasp that it
tends to be avoided, and therefore its fitness is higher.
In both of these cases, they are more likely to eventually survive and
have more offspring because they are less likely to be harassed and/or eaten.
This is a really elegant picture.
This is a caterpillar that, as you see,
looks a lot like snake when it puts its rear up like that.
So this is Batesian mimicry, the case where the mimic is not dangerous, but
the model is.
Let me show you a different one.
This is Müllerian mimicry.
In this case, this is a lot of dangerous species, or multiple dangerous species,
evolving to look like one another.
In that sense, they derive an advantage from each other.
So what we have here,
these are all, the top row here is all from the species Heliconius melpomene.
This is a nice tropical butterfly.
The lower ones are all Heliconius erato.
What you see here is this melpomene looks a lot like this erato,
this melpomene looks like this erato, etc., etc., etc.
Now, importantly, why am I showing you all these different pictures?
In each of these cases, the vertical group here are from one local population.
So in one particular population, melpomene look like this, and
the erato look like this.
In another population, the melpomene look like this, and the erato look like this.
It's very dramatic, that within each of these local populations,
they've evolved to look like each other even though they are different from their
own species in other populations.
And again, in this case they derive the advantage from each other.
Because imagine if a bird eats a Heliconius erato and
it has a bad digestive reaction to it.
It's less likely to eat any more erato, but
it's also less likely to eat any melpomene.
They all derive this mutual advantage.
In the case of Batesian mimicry, the advantage is only to the mimic.
In this case, the advantage is for all the models to come together.
It's a really elegant case of natural selection, don't you think?
So moving on.
I wanna talk a little bit about variation across space and time.
Now there are a couple of things that are often referred to as ecogeographic rules.
So these ecogeographic rules are patterns of variation either within a species or
across a species group that tend to correlate with geography.
So for example, having a particular trait in cool environments, but
not having that trait in warm environments.
And when you see this pattern over and over again, either within species or
across species, you know within a particular species group,
that's indicative that there's some reason behind this.
That there's some adaptive value to having a particular trait in the cold
but not in the warm.
So one of these is referred to as Bergmann's rule.
This was named after the German biologist Bergmann in the 19th century, who observed
that animals in general tend to be bigger in cold environments or in high latitudes.
Now his classic work on this in house sparrows, but
it's been seen across and within various other species groups too.
Now why might this be?
Well, again, as you're larger, you're increasing your volume quite a bit,
but you're not increasing your surface area at the same rate.
So therefore you should be better able to retain heat.
Now this is just one possible explanation.
We're not positive that this is the cause for it, nor are we positive
in every individual case that this is a result of natural selection.
But we see this pattern over and over again, and
it's suggestive of natural selection, and it makes sense.
So we'll come back to this sort of principle a little bit later on in this
class as well.
So Bergmann's rule, again, is that animals tend to be larger in cold environments or
in high latitudes.
A similar rule is Allen's rule,
is that animals tend to have shorter appendages in cold environments.
Now why would you do that?
Why would you have shorter appendages in cold environments?
Again, think of it in the context of surface area, that if you have long,
lanky arms [LAUGH], then you might be losing more heat in a cold environment.
In contrast, if you have short,
little stubby arms, then you're not gonna be losing so much heat.
So one classic example of this that's been cited repeatedly
is in the context of the ears of the polar bear compared to the ears of other bears.
Again, shorter appendages in colder environments.
Now interestingly, we see this with respect to bills or
beaks in birds as well.
So beaks and bills, as you can imagine, do lose some heat, and this is a thermal
graph to show you that they actually do have heat within them that can be lost.
And this graph here on the right shows various different species groups.
These are the lines in the graphs, various different species groups, and
in each case you tend to see this correlation with latitude,
where at higher latitudes, the bill size tends to be smaller.
At lower latitudes, the bill size tends to be larger.
Again, consistent with what you would expect from evolution by
natural selection.
The last one that I'll mention here, though there are others as well,
is referred to as Gloger's rule,
that animals tend to be more heavily pigmented in high humidity.
Now, this is generally or typically true near the equator, and this has been
observed particularly in various bird species such as song sparrows.
In those cases, it's possibly related to bacterial activity,
though again we're not certain.
But again, this is another ecogeographic rule where we see it over and over again.
It's been suggested to be true potentially in humans as well, but again,
the evidence is not perfect in this regard so we don't know.
Nonetheless, all these ecogeographic rules are cases where it looks like
the action of natural selection has been present because we see these same patterns
over and over again with respect to geography.
And you wouldn't expect this by random chance.
Why would it always, or why would it typically be that the cold environments
have the larger animals, just by chance?
We don't think that's true.
Now I'm talking about this so far with respect to phenotypes,
but you see the same thing sometimes with respect to alleles,
in cases where we can look at the alleles directly.
Drosophila pseudoobscura has a set of alleles on its third chromosome, and
in this video, I'll focus on one that's referred to as arrowhead, or
in this case abbreviated AR.
This is a species that I actually conduct research on as well,
but we'll be talking about the classic work by Dobzhansky from the 1930s and
40s all the way through his passing in the 1970s.
One of the things that Theodosius Dobzhansky observed was a correlation with
altitude, not latitude this time but altitude, how high a population is, and
the allele frequencies of arrowhead.
So one of the places he studied was in California, and he found that at
low altitudes, like here in Jacksonville, arrowhead tended to be pretty rare.
But as you went higher and
higher in altitude, you tended to get more and more of this arrowhead allele.
Interestingly now, he did some laboratory experiments where he varied the humidity,
and in those cases again, as he changed the humidity to match what you
would observe in low altitude populations, arrowhead became rare.
When he changed it to match high altitude populations, it became more common.
And the same thing also happens with respect to latitude.
So this is quite striking,
where all these different sources of evidence are supporting the same idea that
there is some selective benefit to the arrowhead allele
at environments similar to what you would see in high altitude or in high latitude.
Now, this was true with respect to geography, but remember I said before,
this was space and time.
You see the same thing looking over time as well.
Remember now, Drosophila species are, they breed very quickly.
Drosophila pseudoobscura has its generation time in the lab of
three to four weeks, so you have many generations over the course of the year.
So there's lots of opportunity for
natural selection to change the abundance of alleles over seasons.
So he collected Drosophila pseudoobscura from this place called Mather,
CA, just outside of Yosemite National Park, in 1946 and 1947.
And what he found was early summer, he tended to find a lot of arrowhead.
As you got later into the summer,
as it was warmer, you tended to see a decrease in arrowhead.
So you see this a little bit in 1946, but you see it much more strikingly in 1947,
that arrowhead gets less abundant as it gets hotter each year.
Remember, this is a slightly high altitude too, so that's why June is actually still
a little bit on the cool side in that particular area.
So again, the abundance of the arrowhead allele decreases
from spring to fall each year, and then it comes back the following spring.
Very dramatic.
So this was again a case of variation across space and
time, consistent with evolution by natural selection.
The last principle I wanted to briefly touch on was this concept of sacrifice for
family members, or it's often referred to as kin selection.
So why does this happen?
Why do you sacrifice yourself to help your family members, or
why do some animals do this?
They'll often sacrifice to help others if they are relatives.
Now Darwin, as with most things, had some ideas on this.
And he pointed out in his Origin of Species book, selection may be applied to
the family as well as to the individual, and may thus gain the desired end.
It's important to remember that you're related to your family, so when you
help your family, you're actually helping the spread of your own alleles too.
So let's look at this with respect to some pictures and stuff.
So let's say this is you, okay?
This is you there, and let's say that generally speaking,
if you just were selfish and helped yourself, you would have two kids.
And let's say then on average, you pass half your information to each daughter,
and therefore your alleles live on.
Half goes to this daughter, half goes to that daughter.
On average, you've reproduced yourself.
There may be particular alleles that were lost, or some that you have that
are now in two copies in the population, but on average, it's passed on.
Passing on nothing is clearly bad,
if you were just to sacrifice yourself for nothing, clearly that's not advantageous.
So,let's look at this third option.
Let's say that now you have four sisters.
You share half your genetic information with each sister, assuming you're diploid,
which I assume you all are who are watching this.
So each of these has already half of your genetic material.
Now, you didn't make your sisters.
They were made by your parents, but let's say they're there.
Now, let's say each of your sisters has the potential to be selfish, and
they're going to have two kids each.
With each of these nieces, you share one quarter of your genetic information.
So, this one has one quarter of your genetic information,
this one has one-quarter of your genetic information, etc.
So let's assume that your help is critical
to the reproduction of your sisters and to the survival of your nieces.
And you have the choice now where you could either help these four sisters
produce their kids, or you could have your own kids.
Which one is actually better for you?
Well, counterintuitively, if you help four sisters and
die in the process, your alleles live on twice as well as if you had kids yourself.
Cuz in this case, you've passed on two halves, 2 x 1/2, so
you've passed on a total of one copy of you.
Here, you've passed on 8 x 1/4, or essentially two copies of yourself.
Now basically your alleles live on better because you helped your sisters
than if you were selfish and helped yourself.
Now there were some assumptions in this, but nonetheless we do see this concept
that sometimes it will be beneficial for you to sacrifice yourself, or for
an organism to sacrifice itself, to help relatives reproduce.
Now again, this comes back to Darwin's quote
here about selection may be applied to the whole family.
And the term for this is kin selection, and
it's the evolutionary strategy that favors the reproductive success of an organism's
relatives, even when at the cost of the organism's own reproduction.
There's a famous quote by JBS Haldane, an evolutionary biologist.
Now he was asked if he would give his life to save a drowning brother, and he said,
no, but I would save two brothers or eight cousins.
[LAUGH] Now of course it's a little bit silly, but you see where this concept is,
that if he saves two brothers and each of them has half his genetic information,
it's basically the same as saving himself.
You may not feel like that with respect to your brothers,
but that's at least what natural selection would favor.
Now, there are some possible examples of kin selection out there.
Now one of them is with respect to the Belding's ground squirrel alarm calls.
Now these squirrels will, when they see a threat, will stand up on a rock or
something and make a little chip sound.
And it's potentially dangerous to themselves that they're doing this,
because they're making themselves very conspicuous to whatever the predator is or
whatever the threat is that's imminent.
Now, it's thought that this may have evolved and spread to help relatives.
And the reason for that is that females do alarm calls more frequently when there
are relatives around, rather when there are no relatives around, and this is for
one particular alarm call.
So again, this is a possible example of kin selection.
So again, overall,
what I wanted to do in this video is just give you some case studies of natural
selection in areas where it's been applied to some interesting natural observations.
First with respect to mimicry, so the resemblance of something that is a threat.
Looking at variation across space and time, where often you see
these repeated observations suggestive of the action natural selection.
And of course sacrifice for family members or kin selection.
Well, I hope you enjoyed this.
Thanks so much.
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