Learn about speciation, including allopatric and sympatric speciation and mechanisms of reproductive isolation. Created by Sal Khan.
Want to join the conversation?
- Since the beefalo (cattle and bison) can produce fertile offspring, are bison and cattle the same species?(3 votes)
- No, they aren't even in the same genus!
(However, it took decades of experiments with crossbreeding cattle and bison to come up with males that were fertile, so this wasn't something that was likely to happen in "the wild".)
Hybridization is known to happen between what are generally accepted to be different species — sometimes this leads to new species, other times some traits from one species become incorporated into another species. Hybridization is quite common in plants, but there are many examples in animals† as well — including most modern humans§.
Note that the definition of "species" is hotly debated among evolutionary biologists and none of the (numerous) proposed definitions seems to adequately cover all cases. (This is a lot like our attempts to define "life" — we can list characteristics, but they don't all apply in all situations.)
This website has an accessible discussion of some of the complications associated with trying to define species:
†Note: For other examples see this Wikipedia list:
At least one of the hybrids on that list is hypothetical rather than actual and there are many verified hybrids seen in the wild that are not included.
For example hybrids are somewhat common in the Wood warblers of the Americas:
§Note: Most modern humans (including everyone with European or Asian ancestry) contain some genetic material that came from Neanderthals (Homo neanderthalensis).(10 votes)
- in the sympatric speciation, you said that 3n is produced at the last, but it was also written that it is sterile and cannot reproduce, so how will that kind of speciation even continue to the next generation. I don't think that answers the question to how new species come to exist, as in this case, we got a sterile 3n cell which will not even pass its genes to the next generation. could someone please clarify as I might have misunderstood the concept?(4 votes)
- Though what Ivana said might also be true, I think Sal was trying to imply that as Biological Species Concept stated, even if both parents (diploid one and tetraploid one) may interbreed, since their offspring is infertile, each of the parents itself is already considered different species to one another. In other words, the tetraploid plant is the new species.(2 votes)
- Which form of speciation is usually faster: Allopatric speciation, or Sympatric speciation? In other words, which one creates now species faster?(4 votes)
- I think sympatric speciation is faster since it relies on polyploidy and can be done within one generation (cultivation).(2 votes)
- At8:05, Sal mentions tetraploid plants. Just out of curiosity, but what would happen if that was somehow naturally introduced into something prokaryotic, like a bacterial cell or a prokaryotic protist?(2 votes)
- Prokaryotes generally don't have sex the way eukaryotes do§ — as a consequence they usually only have one version of their genetic information. In other words, most prokaryotes are effectively "haploid".
When polyploidy does occur in prokaryotes this is usually just multiple copies of the same genetic information — i.e. they have copied their genome faster than they divided.
Does that help?
§Note: There are some fascinating exceptions that may help explain the origin of eukaryotic sexual reproduction. See for example:
- If a homosapien has more or less chromosomes than 23, and cannot reproduce with other homosapiens, then is there a new species of homosapiens? Also, some people cannot reproduce with most people so does that make them a new species?(2 votes)
- No, it is not a new species just because aneuploidy happened (syndromes). At the farthest, they will leave a few decades but not leaving fertile offspring.(2 votes)
- How did the grand canyon perfectly divide both spiecies? wouldn't some of the northern ones stay on the south side and some of the southern ones stay on the north side by chance?
It would be very hard for them to seperate perfectly...(1 vote)
- I think you might want to rewatch this video as you've missed a key point — there was only one species before the Grand Canyon formed!
Once the animals to the north and south of the Colorado River were isolated by the forming canyon, they stopped exchanging genes (breeding with each other). These separate populations then began to diverge as random genetic changes accumulated. This eventually resulted in so much divergence that they became separate species.(4 votes)
- What exactly is polyploidy?(2 votes)
- Polyploidy is when an organism has more than the usual number of chromosome sets.
Most eukaryotic organisms like us are normally diploid, meaning we have two homologous copies of each type of chromosome. This is because there is one copy in each of our parents' gametes that fuse together. Humans have 23 types of chromosomes, so normally a human egg cell has 23 chromosomes, a sperm cell also has 23, and they fuse to create offspring that has 46 chromosomes.
Sometimes, errors can happen in meiosis that result in two copies being included in a gamete. If an egg cell has two copies instead of one, and fuses with a normal sperm cell, the offspring will have three copies (so 69 chromosomes in our human example) of each chromosome. This condition is called triploidy. Tetraploidy can happen when two gametes with two copies each fuse, resulting in four copies.(2 votes)
- what are the advantages of genetic variation(2 votes)
- Maybe it would help you to think about this from the other direction — what do you think might be the disadvantages of a lack of genetic variation?
I'll give you one example by way of an analogy.
We are advised to not use the same password for multiple websites — this is because if someone steals the password for one account, they can then get into all of your accounts. Can you see how to apply this idea to individuals in a species?
Another advantage has to do with how a population deals with changes in the environment — what do you think is likely to happen if all the individuals are very similar?
Do these hints help?(2 votes)
- what if there would be an error in 4n plant cell's meisosis too and its egg/sperm become 4n ? Then what happens(2 votes)
- Well, for one thing individual cells don't have eggs or sperm, only the plant itself has it. And to answer your question assuming you mean that the plant's egg or sperm became 4n, either the sperm/egg simply wouldn't work with another plant's egg/sperm or it'll create a non-viable or infertile 6n hybrid. Or if it happens in multiple plants, it could create 8n plants.(2 votes)
- [Voiceover] In any discussion of biology or discussion of evolution, the idea of a species will come up over and over again. And we have a whole separate video on species. But the general idea, or the mainstream definition of a species is a group of organisms that can interbreed, interbreed, and produce fertile offspring, fertile, fertile offspring. So for example, in this picture right over here, you have a bunch of species of both modern elephants and previous, or now nonexistent species, that are related to modern elephants. But today on earth you have Asian elephants and you have African elephants, and they are each a species. An Asian elephant can interbreed and produce fertile offspring with another Asian elephant, and an African elephant can interbreed and produce fertile offspring with another African elephant, but they can't do it with each other. An Asian elephant and an African elephant cannot get together and interbreed to produce fertile offspring. We know, people have actually tried this. But the next question, or the most obvious question, and this is one of the central questions of evolution, is, well, how do you get these species? We see drawings like we have on the right, on the left here we have actually some of Darwin's original drawings showing this evolutionary tree, showing how over and over again we have this branching from a parent species into two, I guess you could say different child species. You see this here with the elephants. At some point, the Asian elephant and the African elephant shared a common ancestor, and it was also, based on this diagram, a common ancestor of the mammoth. And you go even further back, it's the common ancestor of this species that I'm not familiar with, the Anancus. And you can keep going back. But how does this tree branch? How do you actually get speciation? How does the variation within a population, within a species get so extreme and in some ways so separate from each other that they can no longer interbreed and produce fertile offspring? Well, there's a couple of ways to think about it. The most obvious way that you could imagine this happens, or maybe the most intuitive way that you could imagine this happening, is through geographic separation. And the technical term for speciation, which is the formation of new species, so speciation, actually, let me just write it this way, the technical term for speciation due to geographic separation is allopatric. Allopatric speciation. So speciation is just how, the formation of new species. And "allo" comes from the word "other," and "patric" comes from the root or the word "homeland." So it's really talking about other geographies, or other homelands, or geographic separation. And one commonly cited example here are the antelope squirrels. So if you go to the, if you were to go to the American Southwest a long time ago, before the Grand Canyon was a canyon, when the Colorado River was just kinda going through and wasn't a major barrier, there was a parent species, an ancestral species to both of these characters that lived on both sides of the river. And on different times of the year, it was able to get across the river, it's able to, the squirrels on the north and the squirrels on the south were able to interbreed and produce fertile offspring, so they were all one species. But over time, the Colorado River started to erode more and more soil and rock, and so this became what we now consider to be the Grand Canyon. And so over time, this became a very significant geographic barrier. No longer could, before they could travel across, but once it became the Grand Canyon, it became very difficult or impossible for them to travel across. And so now you have these two different populations. They have the same parent species, but they're now geographically isolated. And since the creation of the, or while we have the creation of the Grand Canyon, since it became very hard or impossible them to cross, you've now had enough, both genetic drift, also natural selection, these are the evolutionary processes that we've talked about, where the Harris' antelope squirrel, which lives on the south side and is right over here, it's this picture, and the white-tailed antelope squirrel, which lives on the north side. Even though they look quite similar, as you can see from these pictures, they have now diverged enough that they are different species, that they no longer will be able to interbreed and produce fertile offspring. So it's fairly intuitive how allopatric speciation can work. Geographic separation, no longer can interbreed, and over time their genes change through natural selection and genetic drift. But what about situations where they stay in the same place, where theoretically, they could get together, they could interact. Could you still have speciation? And the answer is yes. And that form of speciation, where you are still in the same geography, that is called sympatric speciation. Let me write that down. Sympatric speciation. Speciation. And examples of sympatric speciation are a little bit less obvious, or a little bit less intuitive, but there's an example that people believe is sympatric speciation happening before our eyes. So this species, the technical term Rhagoletis pomonella, I know I'm mispronouncing it right over here, this is native to North America, and before European settlers brought apples to North America, they hung out and they laid their eggs and their maggots were inside of, or they leveraged the hawthorn fruit right over here. So they would go to the hawthorn trees, and they would lay inside of the haw, they would use the hawthorn fruit for their, to lay their eggs, and for their young to kind of consume. But once the European settlers came and introduced apples into North America, a certain, I guess you could say a subgroup, of Rhagoletis pomonella started to leverage the apples, so started to lay their eggs and their eggs and their maggots would grow inside of the apples. And they've actually now diverged, not into fully different species now. In theory, they can still interbreed and produce fertile offspring. But they don't tend to do it any more. That it tends to be, even though they're in the same geography, and it's not hard to fly from the hawthorn tree to the apple tree, they don't tend to do it. And because of this behavioral divergence, that some decide to go to the apple, some decide to stay at the hawthorn, they actually are now developing different traits that are selected for depending on that, I guess you could say initial preference, or that initial bias for which fruit they want to use to lay their eggs in. So for example, the ones that are in the apple tree, they now have, their breeding cycle is more aligned with the growing season for apples, while the ones in the hawthorn tree, their breeding cycle is more aligned with what, for the growing cycle for the hawthorn. And so biologists believe that this is an example of sympatric speciation happening before our eyes, that if we were to wait a few hundred more years, possibly a thousand years or more, that this will diverge into two different species that will no longer be able to interbreed and produce fertile offspring. Another example of sympatric speciation, which is a little bit more wild in some ways, it's a little bit more out there, but this would be an example with plants. So as we learn in other Khan Academy videos, organisms like human beings, and in fact many sexually reproducing organisms, they're diploid organisms. They have two sets of chromosomes. For example, human beings have two sets of 23 chromosomes, for a total of 46 chromosomes, 23 from your mom, 23 from your dad. And so we are diploid organisms. And in general, there are errors that occur during reproduction and errors that occur during meiosis that can lead to polyploidy, where an organism can have more than two sets of, or can start, or a potential organism could have more than two sets of chromosomes. In the animal kingdom, that doesn't work out too well. Usually, that does not produce a viable embryo, a viable zygote. But in the plant kingdom, this is, it tolerates it a little bit more. So you could have a situation where you have a diploid plant, and through meiosis, through an error in meiosis, instead of producing haploid egg and sperms, it produces diploid egg and sperm, which then are able to get together to form a tetraploid plant, so a plant that has four sets of chromosomes instead of two sets of chromosomes. And then once that tetraploid plant exists, it might only be able to reproduce with other tetraploid plants versus the diploid plant. So you have the diploid plants here, when meiosis is working "properly," and I'll put that in quotes, 'cause maybe, you know, arguably this error is what helps for a speciation sometimes, especially in the plant kingdom, it is, it can produce this haploid egg or sperm. The tetroid plant would then, would, through its meiosis, its ploidy, I guess you could say halves, when it goes to the egg or the sperm, but you're now going to have a nonviable or infertile triploid plant, because the separation won't happen properly in meiosis for this egg, if it's even viable. So all of a sudden this tetraploid plant is now a, you've had speciation occur. It could be viewed as a new species. And you could think about things like this happening as a potential, and we don't understand all of it, we don't understand how all of the speciation that we now observe has actually occurred, but you can even imagine as this being a mechanism for why you have an increase in the number of chromosomes in certain species versus others. So hopefully this is starting to answer some questions, and hopefully even introduce more questions, 'cause this is a very exciting topic, about how we get species from, more species, how do we get the diversity from parent species or ancestral species.