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:12:39

Video transcript

in these pictures we might look at the fin of the shark and say they are very similar to the flippers of a dolphin for the Dolphins we don't call them as fins we call them as flippers they look very similar and they're both used for swimming so we could say hey maybe they are very related to each other and similarly if you look at the wings of the bat let me use a different color wings of a bat and the wings of say an eagle again both are used for flying they look kind of similar we might say they are related to each other but it turns out that in reality they are not as closely related to each other and in fact it turns out surprisingly it's these two structures are more closely related than these two or these two yeah this sounds very confusing I know and so in this video we'll explore why certain structures which might look similar and perform the same function need not necessarily mean that they are related to each other and on the other hand we will see that all those some other structures might look very different and perform different functions they can be related to each other to avoid confusions let's get rid of this picture and start fresh so if you take a look at the wings of a bat the flippers of a dolphin and our hands then although they look very different and they perform different functions you look carefully you look at their bone structures you find the bone structures are very similar let me show you if we zoom in a little bit let me zoom into the wings and the flippers and the hand and here are the bones look at them approximately this is how the bones would be look at them do you see similarities to make it more clear let me color them appropriately so if I color them now look at these bones carefully over here we see one born one born one one and this now is an elbow or elbow do you get two bones - bones - bones then we get the wrist bones wrist bones wrist bones then you get the hand bones over here five of them when they're only four here again five of them and then we have fingers finger bones and finger bones so can you see how similar structures they are the number of bones and the kind of bones are very very similar to each other of course yeah there are of course some differences and there are different species altogether so some differences are fine but because of so much similarity we can say that all these bones evolved from a common ancestor because of such similarity in the structure right so let me just write that down let me zoom out a little bit same thing I zoomed out a little bit and now we can say that all of these all these bones they have evolved from a common ancestor and sister so same structure from the common ancestor performing different functions okay let me give you a separate example an extreme example in plants of the same thing if you were to look at these three plants this is a pitcher plant which contains the pitcher and what it does is if insects fall into that it will close the lid it will not allow it to escape it will start digesting it and this is a Venus flytrap it's like a mouth like thing so if a fly comes in it basically closes the mouth and starts digesting it and this is our cactus which has a lot of spines or a thorn like structures they're called spines pointy spines now what's interesting to see over here is that if you look carefully research if you do a careful study on these structures then we can see that the picture of the pitcher plant the mouth of the Venus flytrap and the spines they are all actually highly modified leaves now I know it's hard to believe how a leaf can modify into these different different things but it turns out to be true we will trust our biologists okay one on this so again what does this mean since they are all modified from the leaf itself from the same leaf itself we can again say hey that means these structures also must have come from a common ancestor common ancestor so with these couple of examples of evolution what is the common underlying feature you can find out in these examples what is common in these evolutions can you think about it well in both these examples we see structures are evolved from the same ancestors but they end up performing different functions see over here they perform flying and swimming and our hands can do so many things like I don't know maybe scratching or climbing over here the these are used for catching insects here also they catch insects but the spines are used as a protection so different functions but same ancestry such structures which have different functions but same ancestor we call them homologous homologous structures or homologous function I just write homologous okay so what's important is that the word homo homo means same same same what same ancestors and that's important same ancestor but they can have different functions can have different different functions and I say the word they can have different functions is because they can also have same functions for example if you compare my our hands with the hands of a chimpanzee okay they're also homologous I am not drawn the bone structure but bone structures will be very similar and chimpanzees hands also perform the same function as our hands so homologous structures can perform same functions they can also perform different functions as we see over here what's important though is that they have the same ancestry or more same ancestry on the other hand if you compare the wings of a bat and an eager let's see what we get we've already seen the wings of the bat we already saw the bone structure of this now let's look at the wing of an eager if i zoom in and I look at the bone structure it looks somewhat like this so again we see one bone here then we have two bones over here you can see the wrist and a little bit of that as well and when it comes to fingers and hand that's where things get a little different but again the bone structures are pretty similar so we can say the four limbs four limbs means you know the front limbs in humans four limbs are just hands so the four limbs we can say still came from the same ancestor because of the same structure so we can still say the four limbs of the bat and the four limbs of the the Eagles they are homologous okay the structures are the same but let's look at the wings now that's important concentrate on the wing structures for the bat look at how the wings are formed the wings are actually skin this is the skin of the bat and and and the skin of the bat kind of makes a web board here right it's attached to the fingertips can you see that the skin is stretched over the fingertips and that's how they have evolved because of which they can fly but look at the wings of the eagle these are not skin these are feathers and the feathers are attached to the entire arm they're not web shaped like in the case of the bats they're not skin they're not connected to the fingers like at the bats so a completely different structure if you look at the wings isn't it so can we say that these wings might have evolved from the same ancestor I don't think so because they have such different structures we can now say that maybe this wing evolves separately from some ancestor I'm gonna call this as ancestor one may have evolved from some ancestor and this wing might have independently evolved from a different ancestor all together from a different ancestor altogether and so are the bat wings and eagle wings homologous the answer is no because they have such different structures they must have evolved from different ancestors so they're not homologous so if they are not homologous what do we call them and before I tell you the name of this let me tell you and let me show you another example if you look now at the flipper of a dolphin and compare that with the fin of a shark and if you look at the bones of the shark it might look somewhat like this this is an approximate drawing okay and if you're asking if you're if you're wondering why I haven't colored it mainly because there's nothing similar between them to color okay these are made of bones very similar to our hands these are not even bones these are not to be made of cartilages very different structure so because they have such different structure we can now again say hey they must have evolved separately from different ancestors so this must have evolve from a different ancestor compared to this one these are separate examples okay not related to each other so now again if you look at these two what common feature do you find in this kind of evolution in this we see that structures come from different ancestry but they perform the same functions such structures are called analogous structures and now legis structures are analogous organs okay what's important over here they have different ancestors different ancestors different ancestors but the important thing is that they have the same functions they carry out same functions have to carry out the same functions like over here both of them swim over here both of them are used in flying but underlying structures are super different therefore different ancestors so what is the big picture that I'm trying to paint over here well the main thing that I'm trying to tell over here is when we are studying evolutionary biology just because we see certain structures which look very different at first and perform different functions they can still have the underlying same structure and so they could still be related by a common ancestor homologous structures and on the other hand just because we find some structures in some species to be very similar it look very similar than the perform same function need not necessarily mean that they are related to each other it's totally possible that they evolved independently from separate ancestors all together and so these are called analogous structures and one last thing before we wind up is that if you look at the homologous structures you see it from one common ancestry from the same underlying structure we are getting different functions so diversification is happening over here right same structure is used for many different things so homologous structures give us something called divergent evolution alright same structure used for many different things many different functions divergent evolution on the other hand if you look at analogous structures their exact opposite two completely different structures from completely different ancestry have the same functions have evolved independently to give us the same functions so different structures are coming together sort of to give us the same function so it's like converging this evolution is converging right converging to give same function so we call this convergent evolution so analogous structures are our evidence for convergent evolution all right convergent evolution that's pretty much it