Speciation


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.

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