If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:47
AP.BIO:
EVO‑3 (EU)
,
EVO‑3.D (LO)
,
EVO‑3.D.1 (EK)
,
EVO‑3.D.2 (EK)
,
EVO‑3.F (LO)
,
EVO‑3.F.1 (EK)
,
EVO‑3.F.2 (EK)

Video transcript

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 non-existent 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 can not 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 a one of the central questions of evolution is well how do you get the 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 evolutionarily 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 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 of the mammoths and you go even further back it's the common ancestor of this this species that I'm not familiar with the ananka's and you could 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 can imagine this happens or maybe the most intuitive way that you can imagine this happening is through gee 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 mo the technical term for speciation due to Geographic separation is allopatric allo patric speciation so speciation is just how the formation of new species and allo comes from the word other and Patrick 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 kind of going through and wasn't a major barrier there was a parent species and ancestral species to both of these characters that lived on both sides of the river and on different times of the year was able to get across the river it's able to the 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 the genetic drift also natural selection these are these evolutionary processes that we've talked about where the Harrises antelope squirrel which lives on the south side and is right over here it's this picture and the 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 in Purdue they will no longer be able to interbreed and produce fertile offspring so I it's fairly intuitive how allopatric speciation can work Geographic separation no longer can interbreed and over time they their 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 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 sim sim patrick 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 a technical term Rhaego lettuce hammam nala 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 in side of inside of the or they leveraged the Hawthorn fruit right over here so they would go to the Hawthorn trees and they would lay inside of the heart they would use a Hawthorn but fruit for their to lay their eggs into and for their young to kind of consume but once once the European settlers came and introduced apples into North America a certain I guess you say a subgroup of rageh lettuce Pam Enola started to leverage the apples so started to lay their eggs and their eggs and their maggots to grow inside inside of the apples and they've actually now diverged not into fully different species now in theory they can still enter breed and produce fertile offspring but they don't tend to do it anymore 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 decided to go to the Apple some decided to stay at the Hawthorn they actually are now developing different traits that they that are 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 they they're their breeding cycle is more aligned with the 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 these that this will diverge into two different species that will no longer be able to interpret 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 organisms they have two sets of chromosomes for example human beings have two sets of 23 chromosomes for 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 more than two sets of or or can start may 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 in sperms it produces diploid egg in sperm which then are able to get together to form a tetraploid plant so a plant that has four sets of or 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 because maybe in 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 tetrode plant will then with through its meiosis it's 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 non viable or infertile triploid plant because it's the separation won't happen properly in meiosis for this thing if it's even if it's even viable so all of a sudden this tetraploid plant can is is now 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 and we don't understand how all of the speciation that we now observe has actually occurred but you can even imagine as this this being a mechanism for why you have an increase in the number of chromosomes in in certain species versus others so hopefully this is starting to answer some questions and hopefully even introduce more questions 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
Biology is brought to you with support from the Amgen Foundation