Bozeman science: Speciation

hi 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 example of this a really good example of this is Diane Dodd in the 1980s she took a group of fruit flies and she just fed them different things so this group at eight 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 in the group that I eat maltose even though they are 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 um that's probably not going to happen but a fruit flies it did and so this is what I'm 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 or that process is called speciation so what's the first thing that we 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 group or one population is isolated we could also have changes just within that population will talk about that in a second but also those could be pre and post I gotik and this words I go means fertilized egg and so it could be something before the before the egg is fertilized or after its fertilized but these barriers eventually create reproductive isolation what does that do we had 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 polyploidy in 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 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 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 allopatric patrick means homeland and so allopatric is when you have two groups that are that have different 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 meadowlark so we had meadowlarks in North America but during the last ice age as ice move down through the middle of the continent it broke those meadowlarks into two populations we call that allopatric speciation now the ice has melted they're back again and they're not inter breeding generally in that middle hybrid area and so that beat brand new species sympatric speciation occurs when you have something just within that population example in in implants you can have a mistake in the number of chromosomes that they have so they can't interbreed anymore it's actually really really common I'll talk about that in just a second that's sympatric we're in the same land but we can also have a gradient para patrick para patrick let me give you an example of that when i was growing up i thought there were just two different types of elephants and there really are there's the African elephant and the Indian elephant there's some huge differences you know typically when you look at them so this would be the typical is a big male savanna African elephant but what you may not know is that there's a group of forest elephants sometimes they refer to as the pygmy elephants that live in a different area and if we compare the DNA of these to the forest elephant is some scientists consider the subspecies and some might even say it's a it's a separate species itself in other words its DNA is two-thirds the difference between an African elephant and an Indian elephant and so they may be 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 an effort [ __ ] 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 a egg that's fertilized by a sperm so a fertilized egg is referred to as as I go 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 Fowler's toad if you put them in the lab and let a mix they'll interbreed you can get them to produce fertile offspring that will 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 Fowler's toads will breed in the fall and so that's a temporal 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 on in snails 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 careful 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 spoil in the other direction so we called that right-handed and so even though these are very similar their DNA is almost identical and they're very closely related they don't interbreed because their sex parts can't get next to each other so that's mechanical isolation you couldn't even transfer the sperm to the egg because they're isolated mechanically and lastly we could have a behavioral isolation so I'd talked about these these are two types of meadowlarks the Western and the eastern meadowlarks they were separated during the last ice age where I started to come down through the middle of North America so we now we have 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 those songs have separated and so now we have a behavior that's different and so there's no sperm meeting egg it's 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'll 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 Zygo mortality sometimes you'll 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 so it can't produce more offspring and so these are all posts i gotta carriers there in the same area they are able to fertilize the egg but the offspring are sterile and so it's 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 great study was done on the the great china wall so that 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 almost Pamela is a type of plant that's grown on either type 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 our reproductive reproductively isolated in other words we're seeing a decrease in the DNA decreasing 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 that's there and so reproductive isolation can essentially break your species down into two different populations that can't interbreed eventually you can create that 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 brand new species for me now we find implants that's incredibly common something like thirty percent of brand new firm species form through this mistake and fifteen percent of angiosperms which is all the plants that you're looking at came to be through polyploidy or a mistake in the chromosomes wheat for example has been formed through multiple polyploid event it's rare in animals that you can have this this is an example of a the vision the Shaka rat hopefully I'm pronouncing that right was formed through polyploidy in general if you have any kind of mistake in the chromosome numbers and 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 X X Y chromosome are 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 the phylogenetic tree that was drawn by Darwin there it's the belief that through time so if we put tea 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 Stephen Jay Gould who is an incredible writer if you're interested in evolution you could read panda's thumb is a great place to start but his idea is that it doesn't occur gradual 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 how fast speciation can occur and that's kind of up for debate now but what do we knew know about speciation is it starts with reproductive isolation so I hope that's helpful