Transcript for:
Understanding Natural Selection and Evolution

alrighty boy it is finally time for the long-awaited ap bio unit 7 i know it's crazy and i know i say that every time i have a video that i haven't done for a while but this one's absolutely been long awaited dude you got to keep commenting you guys want you ap volume 7 and 8 you guys want like a b cam unit 8 9. don't worry i'm gonna get to it okay olympia season the first round at least is done so we're chilling hello everybody i'm kara and today we are going to be doing unit 7 natural selection and it will be fun this one's actually a really long unit so hopefully we get through it in some short amount of time but we shall see all right let's get the party started shall we okay so the big idea in this unit is really just evolution okay everything in this unit is about evolution and the one thing you have to understand about evolution is that it's a change right now in the context of biology obviously you're not talking about the change where it's like pokemons like converting into pikachu or something i don't know how how the evolution of pokemon works don't don't ask me but in biology it really does change in your characteristics right and obviously i think i know from mendelian inheritance like your traits are encoded by alleles so at the bare bones definition though like the most low level definition you can have evolution is that it's a change in allele frequency or a change in your genetic code okay hooray we have finally finished the unit hooray we are done let's go no i'm kidding okay we still got to talk about all the nonsense that caused the evolution but basically the idea is if an uh a trait becomes more common in a population right like let's say brown hair like brown hair gets all the girls right so like the brown haired guys are going to have more success and like brown hair will increase in the population right so that's an example of evolution because the allele frequency of brown hair is increasing so that brings us to the next point okay very nice segue natural selection so a lot of people think that like natural selection and evolution are the exact same thing they're not okay like natural selection is what causes evolution right evolution all evolution is a change in allele frequency okay natural selection is what actually causes it now basically the way i like to think about natural selection is like in terms of domestication right like if you're breeding dogs right and let's say you want dogs with white colored hair the beauty is basically going to have a ton of dogs and you're going to take the one with the most white hair right and eventually if you only let the ones with white hair breed you're only going to have dogs with white hair right so essentially darwin when he went on is like tripped all over the globe in the hms beagle he basically was like oh what happened to nature did it in exactly the same way and it turns out that it does it's crazy so essentially like let's look at bacteria i think that's my favorite example of it so we have a bunch of epic bacteria and they are all chilling doing totally fine stuff right and essentially when an antibiotic comes around right antibiotics are supposed to kill bacteria it goes in and slaps all these guys kill some of them epic very cool but let's just say let's just say that my boy bob over here was a special kind of bacteria okay he had a slight difference from all his other friends and he was slightly more resistant to this antibiotic than all the friends right so essentially all of his friends are going to die out right and now bob is going to reproduce right and now the bacteria is purely resistant to the antibiotic right it went from one single guy resistant to the antibody to everybody is resistant to the antibody right so this is clearly evolution right like the allele frequency of resistance is increasing now obviously it doesn't have to be this intense right it might just be like some people have a slightly higher chance of surviving right so basically the common example is mobs right so you have like the grey moss and you have white moths which i'll just do a very light gray because white on white is not going to be very cool and essentially the white moths were doing super super good at some point right because all the trees the bark was white okay so obviously if the bark is white you're more likely to survive if you have a wider color right so all the green moss kept dying and they did have babies so the proportion of white mods increase right so it makes sense so this is the color of the tree trunk the white mods were pretty good at hiding on that right but then this one factory went haywire right and it turned all the tree trunks great because of like ass or something i don't know the exact details of suit and all that good stuff and essentially now what do you think is gonna happen i wonder this is a very very complicated thing but basically all the white ones disappeared it's crazy and the reason is basically the exact same as the breeder right when the breeder chose only white dogs to reproduce then only white dogs are going to be left after like a couple of generations right so in this case nature in having this different colored tree trunks is selecting or giving these guys a higher chance of reproducing so essentially these guys are going to like produce less offspring so their numbers are going to go down and these guys are going to produce more offspring so their numbers are going to go up so that is the fundamental idea behind natural selection right like nature is choosing which animals are more likely to survive and those animals that are more likely to survive are going to increase in frequency and that is evolution now the one thing about natural selection is that it's completely random right you first need that one guy to be resistant to the antibiotic for it to spread to the whole population right like if all of the bacteria were not resistant to the antibody all of them would die out and boom there's no evolution happening so essentially natural selection requires variation in order to cause all the stuff right you need somebody to be different in order for him to like survive better than everybody else so that gives a very important point right natural selection needs variation if everybody had black hair or the brown hair guys would never like take over the world you see what i'm saying and basically the variation comes up in a bunch of ways right like obviously there's sexual reproduction right like your kids are similar to yourself but they're slightly different right so they might survive better or worse than their peers right it also just comes up for mutations right like your dna just might randomly change and by some like total random change it might actually be helpful and that like mutation might become more common because it so helps and then of course there's like horizontal gene transfer between like um bacteria and stuff that's not as important just know like sexual reproduction and mutation are the main ways that variation is produced all right so those are the basics of natural selection the main thing we got to remember is first that like natural selection means that some people survive better than others and are more likely to reproduce right survival of the fish second idea is that you pass on your genes to your kids right so if you have the genes that are making you more likely to survive your kids are also going to have those genes so that means that your genes if you're more likely to survive are going to spread throughout the whole population and then third natural selection requires variation right because if everybody's the same then like nobody is more likely to survive than everybody else okay so natural selection is only one of the things that cause evolution it's the main thing but it's not the only thing and of course bio has a bunch of fancy terms for the rest of them so let's get into the other stuff all right so the next thing we're going to talk about is genetic drift right and like whenever you hear this drift just think of random right like drift is random you're not actually controlling where you're going it's completely random so genetic drift just means random chance changes in the allele frequency right like let's just say that you have only two people in a population right only two people exist in the whole universe okay and one has like a dominant allele and a recessive allele and one of them has two dominant alleles right so essentially if you just draw the punnett square right there is a half chance your kid is hh and a half chance that they're double big h right so let's just say by chance right these two guys have like two kids that are both hh right just by chance like this is completely random just by chance the h allele is completely gone it used to have a 25 chance now it's just completely gone this is so sad what did the little easterly will do wrong why you gotta do this to him but basically the point is random changes can cause a lot of change in your allele frequency and that's what's called genetic drift now the only other thing you should keep in mind when you're thinking about genetic drift is that you need like very few people in order for a genetic drift to occur right like for example in our example we only have two people right like obviously chance is probably going to play a role in it so you need a small small population like basically the way i like to think of it is like let's say you're trying to figure out whether a dice is weighted right so you roll it once and you get a one you're not all of a sudden you're gonna be like oh shoot that dice is unfair i gotta 100 of the time right like just because you rolled it very few number of times there is a very very good possibility that like one roll like one number takes over the whole thing right but if you roll that dice like a million times and every single time it's a one you probably should have like looked at the dice and actually see that all the faces are one right but the point is as you have like average over a ton of stuff then there's very little likelihood that your allele frequency is going to change much due to random chance right but if you only do one rule there's a very good likelihood that it's going to change a lot so remember that genetic drift caught is caused by a small population side so there are basically two types of genetic drift you have to know right there is the founder effect and the bottleneck effect very cool i gotta say of all the biology things there are out there the genetic difference have the most descriptive names founder effect and bottleneck effect are exactly what they sound like all right founder effect just take a guess take a guess what it might be i have i like have no idea oh maybe it is when a couple guys found a new colony holy moly i i never would have thought of that what the heck that's crazy so essentially like let's say you have like a really really really big population right and and they're basically like all like relatively the same well let's just say there's two different types of people in this population right and they're roughly equal now let's just say by chance just by chance this like tiny subset of them get cut off from the rest of the population right like maybe like an earthquake happened and like you know the dinosaur movie where like the two have i don't even remember the name of that anymore what was it called you know i need to find this the little dinosaur movie what i'm gonna search no that's not what it is what uh little dinosaur animated series oh this one yes land before time yeah yeah that's what it was oh my god i watched this one so much holy moly okay i got distracted there but the point is this tiny little segment of the population got cut out so in the whole population there's 50 of the red guys right but now in a new population they got separated from everyone else they go and found a new population and in this new population they're 75 percent red people right we basically went from 50 of the people being red to 75 percent of them being read right so it's kind of crazy that's why we took a tiny little portion of the population we completely changed the leo frequency that is what the founder effect is when a couple guys found a new colony and just due to chance because there's such a small founding population it results in a very big change in allele frequency okay now the bottleneck effect is also pretty self-explanatory so essentially it's if you take a very big population right let's just copy this over actually i'm a lady who needs to draw another second time you know what i'm saying so basically the bottleneck effect is like the exact same thing as the founder effect except a little bit more gruesome basically all these guys would die right i shouldn't be laughing at that but you know what i'm saying you guys have not okay and essentially these are the only guys who are left and for the same reason at the founder effect the audio frequency changes a lot right so there you go the two type of genetic drift it's like a bottleneck right like you take a really big population you squeeze them through the bonnet and only a couple guys get pooped out you guys probably do not need the imagery but it helps me remember okay all right and then the next one is gene flow right this also could cause evolution if a bunch of people with a certain phenotype come into your population right like if you have like a bunch of only like non-red guys right and then you bring the red guys in from another population right like a couple guys move in obviously your allele frequency is going to change right gene flow pretty self-explanatory gene is flowing from one population to another all right and now for the infamous hardy-weinberg equilibrium i love this stuff oh my god there's no okay i'm kidding i hate this stuff but it's okay basically all bio students know this like equilibrium as the most math you're gonna have to do in a bio class that's great i love math i mean it's good for me i don't know about you guys but but it's math okay you had to know quite a bit of math to do this so basically the idea behind hardy-weinberg equilibrium is like allele frequency right like we keep talking about this is the leo frequency thing i've been pretty big about it right i've just said like it's how like frequent a trait is but what an allele frequency actually is is the number of a certain allele over the total number of alleles in the whole population so let's say you have a population of like i don't know let's say a hundred um h 100 h and a hundred like little h little h right then the way you find the allele frequencies is you find how many of each allele there are right so let's just say we're trying to find the dominant allele right these guys have two of the dominant allele right so there's 200 from the first guys then each of these guys have only one so there's plus 100 from these guys right and that these guys have no not in the big one right and then we divide by the total number so if you have 300 guys each of them have two how much is that that's right a total of 300. so your allele frequency is just 0.5 okay and then obviously if you have one allele at 0.5 the rest of them have to be the other allele so they should also be 1 minus 0.5 which is equal to 0.5 all right so that's basically the concept of allele frequency now let's just get into hardy weinberg so i think like teachers don't really like talk about this too much but the whole point of hardy weinberg is just probability right is basically if you know that your population is like point five of one allele and point five by the other how likely is it that you get like two of the same allele that is right the likelihood of that i mean you know i should probably change up the numbers so that it's only not the same thing and it's less confusing let's just say it's point six and point four right so h and this is little h then what's the probability of getting two inches right so do you think the rule of probability if you want to get the same thing like at the same time you basically gotta multiply it together right so the probability of getting your first h is 0.6 and the probability of getting your second h is 0.6 which basically means that 36 of the population should have the big h allele right should be like homozygous dominant then what about if you wanted two of little ages right then it's just 0.4 times 0.4 is equal to 0.16 gives you h right and then what about if you wanted to get big h to little h then essentially you can either get your like little age from your mom or you can get a friend pump right so essentially there's two ways to order like which side you get each of them from right you could either get a big age for your dad a little way for your mom or little age from your dad big a for your mom and then you have to multiply like the probability of each right so the probability your dad had it at 0.6 and the probability your mom has the other one is 0.4 and vice versa that's why you multiply by 2. and this is just going to be 0.48 now the one thing you should do whenever you want to like check if you're right just make sure that all of these things add up to one right and you want to make sure that these two things add up to one right all right so why don't we just do like a couple of typical problems right like this is obviously one example right you are given the allele frequency then you want to find the frequency of each genotype right but there's also you're given the genotype and you want to find each of the two um frequencies right all right so let's just say you're given a thousand dudes dude with two o's very cool and and let's just say we have like um what's a good allele let's say let's just do the hair thing again because i love hair like double h are like h codes for black hair and little h codes for brown hair so let's just say that like um of these guys only 40 have a brown hair and the rest of them the 960 other guys have black hair right now i know that this is not very consistent with the fact that brown hair people are objectively more attractive but let's just do this for the sake of argument right you know what i'm saying so eventually like whenever you approach hardy-weinberg problems just ignore the dominant allele right because the dominant allele like these guys could either be big h big h or big h little h you don't know anything they're like lame why would you even care about these guys they have multiple alleles what the heck amanda having multiple wheels that's crazy but these guys there's only one possible thing that the brown hair dudes could be and that's h8 right so what is the frequency of hh dudes that's 0.04 right 40 over 1000 and how did we find the percentage of guys who have little h little h we took the frequency of h and multiplied it by itself right because the probability you get an h the first time is the problem like the percent of h and the percent you get at the second time is the same thing right so essentially we could say q squared is equal to that right and then you just take the square root you get q is equal to 0.2 and you gotta know how to subtract from one so that means that p is equal to zero point eight very cool and if you just wanna check whether like you get like point nine six remaining okay so let's just do it out right so um the percent of big eight big h is the point eight 0.8 times 0.8 which is equal to 0.64 and then the percentage of these guys is of 2 times 0.8 times 0.2 is equal to 0.32 add these guys together and you get 0.96 as desired very cool so obviously that's like the practical application of hardy-weinberg but basically at a high level what hardy weinberg is saying is that like if you just think about it in terms of probability right the illegal frequency is going to stay constant and the frequency of each type of offspring is going to stay constant as well however hardy-weinberg does not occur that often in nature because it needs five very specific conditions okay and we already talked about a lot of these right essentially essentially you could just think of hardy weinberg as being purely like not due to chance right like everything is like determined by the laws of probability so as we talked about before that means you need big numbers if you have small numbers you have genetic drift and that is bad you also can't have mutations right because literally the definition of equilibrium is that nothing is changing right three you can't have natural selection right because if you had natural selection the mating would not be completely random right like some people are more likely to mate than others so you can't have natural selection sorry no mutation no but the exclamation point point yeah there we go and nope to natural selection and then the fourth thing is no gene flow right we basically don't want any like causes of evolution and of course random mating because you don't want like people to specifically choose like all the good looking guys that cause it like that obviously now there would be more than 40 brown haired people right thing i don't know why i'm suddenly addicted around here i promise i'm not like this in real life i think hopefully so obviously like this looks a bit intimidating but you don't have to actually remember it okay you just have to remember the big idea that hardy-weinberg is purely random there can't be any form of evolution like hardy-weinberg equilibrium everything stays the same everything follows the law the probability so that's all you gotta know these things are pretty clear from that so as long as you know that you should be good okay nice and now we are kind of done with the natural selection e part kind of this so now what we got to talk about is phylogeny and that's basically the idea of the study of species right and like the relationships between each other so before we talk about microevolution right like evolution in terms of like actual like individual organisms mating with each other right but now we want to talk about macro evolution which is basically like over time like over millions of years we went from dinosaurs to humans and clearly humans are superior oh i don't know about that dinosaur is pretty cool but yeah macroevolution is basically over a very very long period of time how like lineages of animals have changed so we kind of saw like from the idea of natural selection is that if you have one dude right like this one guy like let's just say one type of bacteria right and then eventually like let's just say that one part of the bacteria was subjected to the antibodies i mean not antibodies antibiotics and one was not then the the one that was subjected to the antibody is going to be completely different from the one that wasn't subjected to the antibodies right and eventually they'll split from their original species they'll be now two species so then the idea is that like there's a common ancestor that splits up due to natural selection and eventually you get a bunch of different species right like keep splitting keep splitting keep splitting and you get like a ton of cool very cool species so basically what phylogeny studies is what are these common ancestors who who did who did each of us evolve from right like for us we share a common ancestor a pretty like recent common ancestor with chimpanzees that is why we are very closely related though so the main concept of phylogeny that we are going to be talking about is speciation i wonder what that means yeah it means making new species and like obviously that is caused by natural selection but we still got to talk about what specifically causes speciation because it is a little bit more complicated than that so the first thing you have to know is what is the species a species is just defined like people who can like interbreed and have viable offspring right like the offspring could also reproduce and they could also live properly okay so this is called the biological species concept there are other species concepts but this is the most legit one just remember the biological species concept is in terms of whether they could breed with each other or not right like the like a very common example is like horse and donkey right horsepower donkey equals mule very quick math now the reason why horses and donkeys are separate animals even though they could have offspring together is because the mules actually can't have any kids so their often are not viable so technically horses and donkey are not the same species and basically there's two ways that speciation can occur there's allopatric right i think this is the first thing that i thought was very cool and vile i was like wow i love patrick speech jason is such a cool word dude i really do i believe i literally spewed allopatric speciation everywhere my protable username with allopatric speciation my like i don't know i just use the allopathic speciation everywhere it was a very cool word not gonna lie and then there is sim patrick which is not quite as cool but it is decent is these and i'll give it that but basically this just differs in weird species form right so allopatric means they are geographically split apart right the lan flame before time kind of thing but sympathy basically means they're still in the same place but they still form different species like alan patrick's speciation is pretty obvious how it like happens right like uh obviously like one population gets stuck over here one population gets stuck over here and maybe like the guys over here find a new food source because they're in a completely different place right and then they're gonna get naturally selected to uh get adaptations for that food store and then their friends back home are gonna like face completely different pressures and they're just gonna like have completely different adaptations and then eventually you have new species very cool like i think there's a ton of good examples like any island is a very good example of allopatric speciation right like manic asparagus lemurs which are found nowhere else in the world and lemurs are very cool you should switch them up but the point is like obviously lemurs are completely different from everywhere else because they got separated from everybody else like geographically and now they like evolved completely differently then australia bro why can i not keep my pencil in my hand today and as i was saying before i was so rudely interrupted by my pencil is that really i forgot no no so australia so basically australia is also a very good example too right because literally there's that's like the one place where there's only marsupials right like literally every single thing like all the carnivores the fossa is an example of a carnivore the like kangaroos the herbivore they're also marsupials everybody the marsupial just because they were in a completely different environment from everywhere else they didn't mix with all the eutherians or like the non-marsupial so when you think about allopatric fishing let's think about islands okay geographic split and then as you might imagine if aloe means like a part then sim like sim is like the same thing right so they're in the same place thin patrick now like sympathetic speciation like how it occurs and not very like fun and like honestly it doesn't really matter that much uh the one thing you could think about is like polyploidy i think that's the only example that the book gives but basically all that means that you have an error in meiosis you got like extra chromosomes and then you create a new species you don't have to actually worry about the exact specifics of that okay now another thing the cause of speciation those are the two types of like geographic things but there's actual things that cause the species to form right like the only way you could have species form is if they can't reproduce with each other so the like one criteria for species to be considered separate is reproductive isolation so if you haven't remember one thing about speciation is that they have to be reproductively isolated okay and that's like the one definition of species right so essentially reproductive isolation can occur in several ways right like either you could stop them from reproducing in the first place or once they reproduced you could just kill their kid right okay that was that was kind of cool but the idea is like they're if even if they do breed their kid they're not gonna be like viable so stopping them from being in the first place it's called poster right like zygote is your kid right so obviously if you stop them before it's pre-zygotic oh did i say it post i got it no it's pre-zygote i got it before the zygote and then if you do it after if your kid is like not viable then it's probiotic because the zygote was already formed okay so p diagonally there's like a bunch of different examples they don't really matter honestly speaking but like i'll just go through the uh five types so habitat right like they live in different habitats temporal they have like mate at different times right so if they made the different times they can't mate together because like obviously you can't like time travel you know what i'm saying then the mechanical which basically means that their parts don't fit together which is kind of nasty but the example of the book is is the snail right there their shells curl in different directions so they can't actually make then there is comedic isolation so basically even if like the gametes come together they won't actually fuse because there's some like different like physiological things like with coral they really just throw their sperm out there it's kind of crazy but literally like the sperm that they release could only fertilize like eggs from the same species so that's an example of comedic isolation and then of course there's behavioral right like um different animals have different courtship behavior than if you can't perform the courtship behavior none of the other species is going to choose you as a mate right so that's another example so then we like posteriority because obviously your kid is not viable so the one way that reduced hybrid viability right that's literally what i just said so that means that like you're like offspring is literally like you can't even live past a certain age it just dies off really quickly another one is reduced hybrid like fertility right so it can't have kids of its own and of course hybrid breakdown that doesn't mean that like eventually like even if the kids of the kids can't have kids eventually like the genetic like damage or like whatever caused the hybrid it just doesn't work out and eventually at some point it dies out hybrid breakdown okay very cool you don't have to know any of that john kidding you probably should like have an idea that exists but like you don't have to like memorize every single one of these and what they do just remember the high level pre-zygotic and post-zygotic and you should be chilling okay so essentially from what we've done already we've seen that like one species puts into two right so that basically gives us the idea of common ancestry so let's talk about that now so essentially scientists want to figure out which animals have are like more like have a common ancestor with which other animals right so like we obviously know that chimps and humans are pretty related because they have a very recent common ancestor right which basically means they have less time to have differences right because there used to be literally one species just like a couple years like that i don't know hundreds of thousands of years ago and then suddenly they diverged right so essentially relatedness is basically proportional to how recent your common ancestor is so the more recent your common ancestor the more related you are now now how do we actually figure out whether people are related right now you might just say like look and if they look the same right like humans have hands chimps have hands like obviously they should be similar they should be pretty related and yeah that's exactly what scientists do they know like fancy well they're fantasy like molecular stuff but at a high level they literally just look at them and see okay what features did they have in common and these features are called homologous features i wonder why homo saying homologous you know what i'm saying it makes sense and basically these are just the features that are the exact same do the common ancestry like the reason that whales flippers have the same bones as human hands is because we both like evolve from the same common ancestor and we have the same bones that's because we derived from the same common ancestor the reason why bats have the same finger like bones in their wings is because we have a common ancestor so the wings and the hands are homologous structures then of course there is vestigial structures right like like for humans an example is our tailbone which we do not use for anything other than getting roasted when we accidentally fall on our butt talks but let's not talk about that but the reason we have it is because our primate ancestors like monkeys they had tails right so essentially for us we didn't need it anymore so eventually it got reduced down to just your tailbone so basically we have it to do to common ancestry but if we don't use it anymore and then the last one that you have to know are analogous structures right so like in an analogy it's not the exact same thing like me analogizing genetic drift to uh rolling a dice they're not the same thing right but they both get across the same point so in the same idea these structures are not like they don't come from the same idea like they don't come from the same thing but they both serve the same purpose like take for example bird wings and insect wings right like obviously insects do not have bones if you thought they had bone that would be very sad but like birds do have bones in their way so even though both the wings are used to fly they're not like from the same idea they're not like the same thing exactly so essentially um uh same function and these are caused by the same like selection pressure right like both of them needed a fly so they both found different ways to fly but not common ancestor not not due to common ancestry all right so just know these then you should be good i think homologous and analogous are pretty easy to remember then the sigils that's the one that's left over which is just like what our teal bone is wait this is so big easy okay well that's that you guys have got this okay and then other than anatomy you can also find it out using molecular body right like as i said like if two things have a more recent common ancestor their genetic codes are going to be more similar right because they have less time to diverge right another way you could determine the difference is biogeography right like like obviously if a island separates before animals have a chance to um differentiate like all the marsupials on australia are likely more related to each other than they are to all the other animals in other places of the world and then of course you could look at fossils right like um basically in your fossils like they're they're in these layers called strata like strata literally means layers and you can see how old this fossil is based on where it is because it gets buried over time so obviously older fossils are going to be more buried but basically you can see as you go from bottom to top the soul gradation and like how how species have changed over time and that would give you a certain like idea of who's related to who right okay and now the last thing we got to talk about phylogenetics is phylogenetic trees okay and how to read them so i mean it's pretty easy i mean i i don't know there's not too much to say about it but let's just say we have like this very very very very fancy to fill up five final genetic tree and immediately ask you like let's say a b c d e f g h okay and ask you who's more related e and h or a and e and basically you could look at common answers you just go up here you go up here and you go there and you can see that both of these meet first time like as you go to the top they meet for the first time here right so this is their most recent common ancestor okay let's look at a and h right all righty so their common ancestor is all the way up here which means that their common ancestor is longer away or sorry a and e write that same thing but it's farther like it's farther in the past so essentially e and h are more related than e and a even though like e and a are pretty close together in terms of alphabet but like just just here like you can see that their common ancestor is more recent i guess the other thing to mention about phylogenetic trees is that like essentially if something evolves here right then all of these guys should probably have that trait right like let's just say that this is this these whole guys are all like mammals right then this should be live birth right so all the guys over here should not give birth to live yeah all the guys here should probably give birth to live yeah now obviously it's possible that somebody might have evolved to not do live birth anymore but the idea that most of the guys in this area should have live birth and then basically like if something evolved here all of these guys should have it right and something evolved here all of these guys should have it because this line is a common ancestor of both of these guys very cool all right we're almost at the end this is such a long unit but we're getting there okay mass extinctions okay these ones are pretty self-explanatory you really don't even need to know that much about them honestly speaking but really just a mass extinction like a ton of the species on earth get destroyed so the three like there's five biggest ones like five like actual mass extinction that ever occurred but there's like three main ones right there's permian triassic so this is basically right before the dinosaurs came then there is a kt like cretaceous tertiary i don't know why why this followed with a cave but it seems cool and that's basically when all the dinosaurs died out when a meteor struck and it like caused like a bunch of debris and they couldn't get sunlight for their plants and they all died out so that is what caused dinosaurs to die and i think those are actually the two main ones but like there's obviously like one that's supposedly happening right now right okay honestly not much else to say about it basically just when some really bad disturbance happens right like i think the permeate triassic was caused by like oxygenation no no one one of the mass extinction was caused by oxygenation and a bunch of people who can't survive with oxygen like started dying out then obviously credentials tertiary was caused by a big meteor like that that's as big of a disturbance as you get and that right now it's human right we're like completely changing the environment okay and now for the final section of this unit we have the origin of life so let us start talking about history because i know all the guys who like science also like history that is how it works it's totally not inversely proportional but we're going to talk about the origin of the earth so let me take you back 4.6 billion years ago to the start of the earth well i mean the book says 4.5 i don't know it's about 4.6 or 4.5 i don't know the exact date earth was formed then essentially like earth was like really really hot when it first formed right like literally molten and it keeps getting bombarded by meteors so eventually at like 3.9 billion years ago or something it started cooling down enough for like water to start actually condensing on the surface so you can actually have oceans and then around 3.5 billion years ago like some point between like oceans forming and 3.5 billion years ago life formed and the reason we know this is because we found some epic fossils called stromatolites which are basically like these like layered rocks they look pretty cool they're like this and then there's like a ton of layers in them and each of the layers are like a bunch of bacteria so essentially stromatolites are just bacteria fossilized and how did we get from oceanforming to like stamatolytes basically if people think that the first life to form was self-replicating rna and essentially the reason why that is it's called rna it's like much more versatile than dna it could literally it literally acts like an enzyme sometimes it's called a ribozyme but basically that's called the rna world hypothesis where rna is the first um first y form but then how did rna form in the first place what the heck that's crazy well basically what scientists think they don't actually know but like what they think is that it went from inorganic substances they eventually got like they reacted together to form building blocks of bigger molecules right like amino acids like singular nucleic acids like monosaccharides that stuff and then eventually those building blocks came together to form polymers like rna and then of course like organic stuff might have also come from meteorites that's also an option because even the meteorites that hit earth today they also come with like a couple organic compounds all right so you don't need to know too much of this right but like the one thing you got to know about the origin of the one thing is the miller yuri experiment it has a very catchy name so i don't think you're gonna forget it anytime soon okay miller yuri that that's the coolest name you could ever ask for but basically all they did is they just took like a bunch of gas they just like slapped in a bunch of gas into a container they sparked it up very cool and they turned inorganic into organic just using like a couple gases and an electric spark so essentially this suggested that like organic compounds could have actually formed from the inorganic ones to produce lime so likely to all you got to know like basically they found that this experiment was not actually accurate to earth's initial atmosphere but like since then they've done a bunch more experiments and they've shown that it's still possible even in earth's initial um like atmosphere you still could produce organic substances from inorganic substances now now obviously they're like the most dumbed down explanation of military if you want to search it up go ahead but the main thing yet i like note is that they just mixed a bunch of gases they sparked it and it produced organic molecules from inorganic ones all right we finally made it this is crazy i hope you guys enjoyed thank you guys so much for watching as always if you enjoyed the video leave a like and subscribe for more but like another pretty big unit uh like if i i might have like explained stuff badly or something but let me know in the comments how i did and on that that's all i got for you guys today so thank you guys for watching again and see you guys next time