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
