Hello. It's Mr. Andersen and welcome to biology essentials video number
8. This is on reproductive isolation and speciation. In other words how we go from one species
to two. And we do that through reproductive isolation, or isolating them reproductively.
An example of this, a really good example of this is, this is Diane Dodd in the 1980s.
She took a group of fruit flies and she just fed them different things. So this group ate
just starch and these ones ate just maltose. And after eight generations, when they were
done, a group of individuals that originally would interbreed ignored each other. In other
words the group that had just eaten starch and the group that had eaten maltose,
even though they're in the same jar, they wouldn't interbreed. And so what she had done
is she'd created reproductive isolation. That's the first component you need to create brand
new species. It's weird. Imagine if we had a group of humans where some were eating hamburgers
and the other ones were all vegetarians, and eventually they just wouldn't interbreed anymore.
This is probably not going to happen but in fruit flies it did. And so this is what I
am going to talk about. We have to start with a group, one species, so this is a group of
individuals that can interbreed and produce fertile offspring. And we're eventually going
to end up with two species where that process is called speciation. So what's the first thing
we that have to do? We have to create a barrier. And so that barrier could be physical. So
it could be geographic barrier, in other words one population is isolated. And we could also
have changes just within that population. We'll talk about that in a second. But it
also, those could be pre and post-zygotic. And this word, zygote, means fertilized egg.
And so it could be something before the egg is fertilized or after it's fertilized. But
these barriers eventually create reproductive isolation. And what does that do? We add one
species that can't have gene flow. In other words you've eliminated gene flow. So the
genes aren't being mixed within that population. And that eventually can create species that
have the inability to breed. Sometimes that speciation happens really fast. An example
would be like in polyplodian implants, and sometimes it can happen over millions and
millions of years. But we know this. Once we have speciation, we've created one group
that can't interbreed with the other. And so let's talk about how that might actually
occur. First of all let me talk about geographic isolation. Geographic isolation is when there
is an isolation in the population due to where they exist. So an example I'll talk about
in a second. Well first of all let me define these up here. Mainly you hear these two terms,
allopatric and sympatric speciation. "Patric" means homeland. And so allopatric is when
you have two groups that have different lands or they live in different lands. Sympatric
is when they live in the same land. But we can kind of tweak that and I'll talk about
that in just a second. Example, meadowlarks. So we have meadowlarks in North America. But
during the last ice age as ice moved down through the middle of the continent it broke
those meadowlarks into two populations. We call that allopatric speciation. Now the ice
is melted, they're back again and they're not interbreeding, generally in that middle
hybrid area. And so that'd be a brand new species. Sympatric speciation occurs when
you have something just within that population. An example, in plants you can have a mistake
in the number of chromosomes that they have so they can not interbreed anymore. It's actually
really really common. I'll talk about that in just a second. That's sympatric or in the
same land. But we can also have a gradient - peripatric, parapatric. Let me give you
an example of that. When I was growing up I thought there was just 2 different types
of elephants. And there really are. There's the African elephant and the Indian elephant.
There's some huge differences phenotypically when you look at them. So this would be the
typical. It's a big male savannah African elephant. But what you may not know is that
there's a group of forest elephants, sometimes they're referred to the pygmy elephant that
live in a different area. And if we compare the DNA of these two the forest elephant,
some scientists consider it a subspecies and some might even say it is a separate species itself.
In other words, its DNA is 2/3 the difference between an African elephant and an Indian
elephant. And so they maybe well on their way to forming a brand new species. How did
they do that? It's probably one population or one group where they moved into a different
area. They're exploiting a different niche, they live in the forest. And so then there's
reproductive isolation within that. So where you live can create isolation. What you do
can also create isolation as well. And so these are all pre-zygotic barriers. And so
a zygote is an egg that's fertilized by a sperm. So a fertilized egg is referred to
as a zygote. And so these three types of isolation, temporal, mechanical and behavior are all
things that occur before the zygote is actually formed. So the first type of isolation is
called temporal. This right here is an American Toad and this is a Fowlers Toad. If you put
them in the lab and let them mix they'll interbreed. You can get them to produce fertile offspring
that will survive. Unfortunately, or that's just the way it is, in nature they may live
in the same area but the American Toads generally will breed in the springtime and the Fowlers
Toads will breed in the fall. And so that's a temporal. And the way I always remember
temporal is the word time, they breed at different times of the year. And so even though they
could produce fertile offspring, they don't, because of the timing. Example of mechanical
isolation, this is a study that was done in snail's in Japan. You can see species that
live right next to each other. So this one right here looks almost exactly like this
snail right here. You think same species. But if you look a little bit more carefully
you'll find that this one right here, it spirals in one direction, so we could call that left-handed
and this one is going to spiral in the other direction so we call that right-handed. And
so even though these are very similar, their DNA is almost identical and they are very
closely related, they don't interbreed because their sex parts can't get next to each other.
So that is mechanical isolation. You couldn't even transfer the sperm to the egg because
they are isolated mechanically. And lastly we could have behavioral isolation. So I talked
about these. These are two types of meadowlarks the Western and the Eastern Meadowlark. They
were separated during the last the ice age, where ice started to come down through the
middle of North America. So now we had the Western Meadowlark, which is our state bird
in Montana. Eastern Meadowlark. And so now that we've eliminated that isolation and they
live in this hybrid zone, they don't interbreed and the reason why is that they attract mates
through their songs and a lot of birds do that. And so the males are able to attract
a mate by singing a song. And the more songs that they can sing the more likely they are
to attract a mate. But during this period of time, the songs have separated and so now
we have a behavior that's different. So there's no sperm meeting egg. It is a pre-zygotic
barrier. Sometimes we'll actually have organisms living in the same area and the sperm and
the egg will get together but that zygote may die. And so in reefs what we will find
is that sperm is transferred from one coral to another. It'll fertilize the egg, making
a zygote, but that zygote immediately dies. And so that's an example of zygote mortality.
Sometimes you'll have different species living in the same area, so for example horses and
donkeys. You can actually fertilize the egg. You can create a brand new offspring. That's
called a mule, but it's sterile. It can't produce more offspring. And so these are all
post-zygotic barriers. They're in the same area, they are able to fertilize the egg but
the offspring are sterile. And so it is not able to move any farther than that. And so
what does that produce? Well that produce eventually a reduction in the gene flow. And
so if you ever have reproductive isolation, the genes can't flow from one area to another.
A great study was done on the Great China Wall. This wall was built, you have plants
on either side. But some plants are being impacted by that, just that production of the wall.
And so Ulmus pumila
is a type of plant that's grown on either side of the wall. But it is fertilized by
wind. In other words pollen must be transferred by the wind and that wall serves as a block
to that wind. And so what's happening is you're creating populations on either side that are
reproductively isolated. In other words, we're seeing a decrease in the DNA, decrease in
the genetic variability. Now there are other plants that live on either side of the wall
that aren't pollinated by wind. They're actually pollinated by insects. And insects have no
problem getting over the wall. And so we're seeing that there's actually genetic diversity
that's remaining there. And so reproductive isolation can essentially break your species
down into two different populations that can't interbreed. Eventually you can create brand
new species through that. Now the speciation rate is going to vary. In other words how
fast this occurs, it can happen very quickly or it can happen slowly over time. So polyploidy
is an example of very fast speciation. And so essentially what you have is a mistake
in the chromosome number. So we're going from a diploid organism to a tetraploid organism.
But it can even get crazier than that. Now what eventually happens, eventually this organism
can't interbreed with the normally diploid organism. And so you eventually have a brand
new species forming. Now we find in plants that is incredibly common. Something like
30% of brand-new fern species form through this mistake. And 15% of angiosperms, which
is all of the plants you're looking at came to be through polyploidy or a mistake in the
chromosomes. Wheat, for example, has been formed through multiple polyploid events.
It's rare in animals that you can have this. This is an example if the viscacha rat. Hopefully
I'm pronouncing that right. It was formed through polyploidy. In general if you have
any kind of mistake in the chromosome numbers in animals they die. And the reason why is
that you get a duplication of the sex chromosomes. And so what we think happened in this rat
is they actually shed that extra xy chromosome or those sex chromosomes and they're able
to reproduce as a tetraploid animal. Now if we put that aside, there's been a debate going
on over the actual rate of speciation. And so this is the phylogenetic tree that was
drawn by Darwin. The belief that through time, so if we put t in this direction, speciation
occurs gradually over time. Now there's been a tweak to that. It's just a different form
of gradualism called punctuated equilibrium. It's most famous proponent is this man Steven
J. Gould who is an incredible writer if you are interested in evolution you can read a
Panda's Thumb is a great place to start. But his idea is that it doesn't occur gradually
over time. It actually occurs very quickly. In other words there's some kind of a change
in the environment which forces speciation to occur. And that would account for why we
don't see a lot of these transitional fossils and also when we actually study evolution
in the lab, we're finding that it can occur very very quickly. And so that's just another
idea on how fast speciation can occur. And that's kind of up for debate now. But what
we do know about speciation is it starts with reproductive isolation. So I hope that's helpful.