Greetings, Bio400 people. So, today we're going to talk about gene flow and
genetic drift in the context of the Galapagos. This is going to be
familiar territory for a few of you guys who have researched Galapagos. We're
going to look at it under the context of evolution today. So, for those of you guys
who are not necessarily familiar with where the Galapagos are, they're a couple
hundred miles off the coast of Ecuador in South America, and they're actually a
perfect laboratory for studying evolution for the reasons that we're
going to discuss in a couple of minutes. But, one of the things that I want you to
bear in mind is, if we look at this map of the Galapagos, there are many
different islands. None of them are actually inhabited by humans; there is a
military base on one of these islands, but aside from that there is really
nothing there they're all protected lands. But, if you look down here at this
key, you can see that there are actually lots of different kinds of environments
on a lot of these islands. And, some of them are mountainous, some of them are flat,
some of them are more heavily forested, and that's going to play into a lot of
the evolution that we see here. Before we can dive into evolution, we need to
review something very important from earlier this term. So, this should look
very familiar to you; this is the central dogma of biology where DNA codes for
mrna, which codes for trna, which codes for amino acids, and amino acids are folded
into proteins.
However, we're going to add something to it and we're going to remind ourselves that
the different proteins that you create are going to influence your some of your
traits physically, and also some of your behavior, so having different genes and
having different DNA can actually change the way that you conduct yourself and the
way that you behave. So, we are looking at the Galapagos today; we're going to see
how different forms of the DNA molecule can actually lead to differences in
traits in animals and also to different ways that they behave. So, an important
concept for us to understand as we talk about the Galapagos is going to be the idea
of the gene pool and the gene pool is just all of the different genes that are
present in a breeding population. And the idea of the gene pool inspired this kind
of funny thing called the "Darwin Award" and the Darwin Award is given annually to a
person or a group of people who do something so stupid that they
remove themselves from the gene pool. And usually that means they do something so
dumb that they actually died doing it, and because they've died doing whatever
the stupid thing is, they're no longer adding their genes for "dumbness" to the
general population so thus they've removed themselves from the gene pool.
So if you're hoping for a good laugh at the end of the school year if you need a
little release you can check out the darwin awards online they're actually
kind of funny.
So, one of my personal favorites actually involves a terrorist who was trying to
send a bomb through the mail but he didn't put enough postage on the
envelope and so it was sent back to him marked "return to sender" and then he
promptly opened it and then of course the bomb detonated and he died. So, as we
discussed in our last screencast, genetic drift is actually a really important
feature in evolution; it's one of the major drivers of evolution.
Essentially, it's just the change in allele frequency in the population
because of random chance.
So, if you had a new mutation that happened, if you had a new predator that
was introduced that looks for creatures that had certain traits, you had a
catastrophic event that happened, but all these things are going to be much more
heavily impacted if the population is really small. Gene flow is actually a
little bit different because it happens whether the population is big or small,
and it's essentially the movement of genes or alleles between populations.
Here, I have species A in one location and species A in another location. So, we
have population one on the left and population two on the right. So it would be
pretty normal for population one to interbreed with population two and then
for population two to interbreedwith population one. But where you get
problems is if you have a barrier of some kind that's preventing species A
from population one and species A of population two from ever meeting up. So, for instance, if
you had a really big snowy mountain in between the area with population one
and population two, you might not get nearly as much crossover between the two
populations. Even though populations one and population two start out as the same
species, you might have a mutation in population one that causes them to
behave or look a little bit different. So, let's just say that we had a couple of
mutations that
cause some of these red dots on the left to turn green instead, and if that
happens to be a really advantageous mutation, that maybe caused them to
blend in with the greenery a little bit better and thus be less likely to be
eaten by predators. That would actually become much more common and so you would
see multiple green ones popping up in the population and fewer red ones
because the red ones will be more easily seen.
So, we can see that the number of green organisms in population one is now
different than the number in population two, and so the green allele is now much
more frequent than it was originally. But now let's imagine that we had a
different kind of catastrophic event that passed over, so we had maybe a big
wind storm that blew some of species a over this mountain and into population
number two over here on the right. Now, depending on whether or not the green
mutation is also advantageous in this other area, you might also see a change
in the number of green organisms that you have. You might only start off with
one or two, but if it's also an advantageous mutation here you might see
more of the green ones breeding over here as well. But because this area originally
didn't have great gene flow because the mountain was in the way, we wouldn't see
the green organisms popping up in population number two unless you had
some kind of abnormal event. So now we're actually gonna look at a real world
example from the Galapagos. We have two types of iguanas in this general
area and they originally were the same specie,s and you can see that they
actually look really similar between the one on the left and the one on the right.
The one on the right that's green is actually a tree-dwelling iguana and the one
on the left is called a marine iguana and they came from the same species
originally but the marine iguana was all the way out of the Galapagos Islands. And
this iguana actually knows how to swim, and will actively swim around to hunt
food whereas the green tree dwelling iguana basically never comes down from
trees, it lives up there. And because there's a big ocean in between the Galapagos
Islands and the coast of Ecuador, you don't see marine iguanas swimming
all that distance
across back to Ecuador and so you wouldn't really have any gene flow
between the two populations. So, one of the things that makes the Galapagos such
an interesting environment for studying evolution, is that it actually has four
distinct types of ecosystems on the islands. So you can sort of get a sense
of that if you look at this map you can look at
the key here, but you can see there are lots of different types of elevations,
lots of different types of ecosystems across the different islands.
Environment number one is actually called the arid lowland; they're pretty
dry they're not very far above sea level and they have lots of things like small
shrubs and cacti. Environment number two is a subtropical forest; it has got lots of
trees, a little bit less dry but it's not very far above sea level. Then
environment number three is actually going to be at much higher elevation and
so it's going to be a little bit cooler and a little bit more moist, less dry, you can see that
it looks kind of like the subtropical forests, but its trees are very
closely packed together. Then lastly we have high elevated areas with lots of
ferns and these are also going to be a little bit cooler, a little bit less dry.
Even though all of these islands are very close together geographically, they
have a huge variety in terms of the conditions on each Island and the
conditions on each island are going to favor certain types of organisms and
certain adaptations more than others. And so, even if you have the same species on
each island, you might have slightly different adaptations. One of the most
famous examples of evolution in the Galapagos has to do with something
called Darwin's finches. When he was on his second expedition with the HMS
Beagle, he noticed that there were about 13 different kinds of this same bird and
they were all the same species but they had slightly differently shaped heads
differently shaped beaks and were different sizes ever so slightly. And so
he observed that all of these different birds were eating different things, and that their
beaks were very well adapted for eating certain things. Before I erase the lines
showing what they eat, take an educated guess as to what each of these different
types of birds might be eating based on the shape of their bill and where they
might be living. So now you can see that there are a few different kinds; some of
them eat cacti and some of them eat seeds, and some eat insects and some
of them eat fruit.
Birds that are eating really large seeds are going to have a very
large crushing bill and then something that's eating something pointy like cactus
fruit, they're going to need to have a thin bill in order to dig in between
the needles of the cactus, then something that's eating insects is going to have
something that's in between a crushing bill and a probing bill. So that's pretty
much an overview of evolution in the Galapagos, and I hope you bring your
questions to class.