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Main content
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Video transcript

the whole process of natural selection is to some degree dependent on the idea of variation that within any population of a species you have some genetic variation so for example you know let's say I have a bunch of well this is the circle species and you know one guy is that color and then I got a bunch more maybe some are that color that's the same color that one and that one in that one and for whatever reason it's um sometimes there are no environmental factors that will predispose one of these guys to be able to survive and reproduce over the other but every now and then there might be some environmental factor and it makes may be all of a sudden this guy is more fit to reproduce and so for whatever reason this guy is able to reproduce more frequently and these guys less frequently and some of them got get killed or whatever eaten by birds or whatever or they're just not able to reproduce for whatever reason and then maybe these guys are something in between and so over time the the frequency of the different traits you see in this population will change and if they are drastic enough maybe these guys start becoming dominant and start not liking these guys because they're so different or whatever else we could see a lot of different reasons these could this could eventually turn into a different species now the obvious question is is what leads to this variation right you know in a population sorry in a population what leads to this you know in fact even in our population what leads to one person having you know dirty blonde hair one person having brown hair one person having black hair and you know we have the spectrum of skin complexions and and and Heights is pretty much infinite what causes that and then one thing that I kind of point to and we talked about this a little bit in the DNA video is this notion of mutations you know DNA we learned is just a sequence of these bases so adenine guanine let's say got some thymine going I have some more adenine some cytosine and that these code if you have enough of these in a row maybe if a few hundred or a few thousands of these these code for proteins or they code for things that control other proteins but maybe you have a change in one of them maybe this cytosine for whatever becomes a guanine randomly or maybe these get deleted and that would change the DNA now but you can imagine I mean if if I went to someone's computer code and just randomly started changing letters and randomly started inserting letters without really knowing what I'm doing most of the time I'm going to break the computer program most of the time the great majority of the time this is going to go nowhere go nowhere it'll either do nothing for example if I go into someone's computer program and if I just add a couple of spaces or something that might not change the computer program but if I start getting rid of semicolons and start changing numbers and all that it'll probably make the computer program break so either do nothing or it'll actually kill the organisms most of the time mutations mutation sometimes they might make the actual cell kind of go run amok and will do a whole maybe series of videos on cancer and and that itself obviously would hurt the organism as well as a whole although if it occurs after the organism is reproduced it might not be something that selects against the organism but anyway it wouldn't it also wouldn't be passed on but anyway I won't go to detail to that but this the whole point is that mutations don't seem to be a satisfying source of variation they could be a source or kind of contribute on the margin but there must be something more profound than mutations that's creating the diversity even within or maybe I should call the variation even within a population and the answer here is really it's kind of right in front of us it really addresses kind of one of the most fundamental things about biology and it's so fundamental that a lot of people never even question why it is the way it is and that is sexual reproduction sexual reproduction and when I mean sexual reproduction it's this notion that you have and pretty much if you look at all cells that have all organisms that have that have nucleuses and we call those eukaryotes maybe I'll do a whole video on eukaryotes versus prokaryotes but it's the notion that if you look universally all the way from plants not universally but if you look at you cells that have nucleuses they almost universally have this phenomenon that you have males you have males and you have females in some organisms an organism can be both a male and a female but the common idea here is is that that both that all organisms kind of produce versions of their genetic material that mix with other organisms version of their genetic material if mutations were the only source of variation then you know I could just butt off other cells you know maybe other cells would just butt off for me and then you know randomly one Sal might be a little bit different and whatever else but that would as we already talked about most of the time we would have very little change very little variation and whatever variation does occur because of any kind of noise being introduced into this into this kind of a budding process where I just replicate myself identically most of the times it'll be negative most of the times it'll break the organism now when you have sexual reproduction what happens well you keep mixing and matching every possible combination of DNA in in in a in a kind of an in a species pool of DNA so let me let me make this a little bit more concrete for you so let me erase this horrible drawing I just did so we all have we all let me stick to humans because that's what we are we have we have 23 pairs of chromosomes and each of those in each pair we have one chromosome from our mother and one chromosome from our father so let me draw that so I'll drew my father's chromosomes in blue so I have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 and I'm running out of space let me do more here 16 17 18 19 20 21 22 and then I'll throw another one here that looks a little bit different I'll throw one here that looks like a Y and we'll talk more about the X's in the Y chromosomes and I have 23 chromosomes from my mother and not to be stereotypical but maybe I'll do that in a and a more feminine color let's see so I have 23 chromosomes from my mother one two I just have to go 1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 so what's going on here I have 23 from my mother I have 23 from my father 23 from my from my father now each of these chromosomes and I and I made them right next to each other so 4 he's in on one pair of these so let's say we look at chromosome number let's call it this chromosome number 3 so let me zoom in on chromosome number 3 I have one from my mother right here and remember actually maybe I'll do it this way remember chromosome is just it's just a big if you take the DNA the DNA it just keeps wrapping around and actually wraps around all these proteins and it creates the structure but it's just a big you just see it like that you're like oh maybe the DNA you know no but this has this could have millions of base pairs so you know maybe it'll look something like that it's a densely wrapped version of well it's a it's a long string of DNA and when it's normally drawn like this which is not always the way it is and we'll talk more about like that they draw this densely packed like that so let's say that's for my mother and that's from my father now these are both we call them you know I'll call them they're the same let's call this chromosome 3 they're both chromosome 3 and what the idea is here is that I'm getting different traits from my father and from my mother for example and I'm doing a gross oversimplification here but this is really to just give you the idea of what's going on this this this chromosome 3 maybe it contains the straight for hair color and maybe my father my father had had and I'll use my actual example my father had very straight hair so you know let's say he had some place on this on this chromosome there is a gene for hair straightness let's say it's a little thing right there and you remember that gene could be thousands of base pairs but let's say this is hair straightness so my father's version of that gene or the he had the allele for straightness and remember an allele is just a version of a gene allele so I'll call it the allele straight for straight hair for a straight hair now this other chromosome that my mother gave me this had this essentially and there are exceptions but for the most part it codes for the same genes and that's why I put them next to each other so this will also have the gene for hair straightness or curliness but my mom does happen to actually have curly hair so she has the gene right there for curly hair so she has the the version of the gene here is let's see allele curly the gene just says look this is the gene for whether or not your hair is curly each version of the gene is called an allele allele curly now when I got both of these in my body or in my cells and these this is an every cell of my body every cell of my body except for and we'll talk a little in a few seconds about my germ cells but every cells other than the ones that I used for reproduction have this complete set of chromosomes in it which I find amazing but only certain chromosomes are extremes will be completely useless in my fingernails because all of a sudden the straight and the curly don't matter that much and I'm simplifying maybe they will on some other dimension but let's say for simplicity they won't matter in certain places so certain genes are expressed in certain parts of the body but every every one of your body cells and we call those somatic cells and we'll separate those from the sex cells or the or the or the or the or the germ cells that we'll talk about later so this is my body cells so bodies so this is the great majority of your cells and this is opposed to your germ cells and the germ cells I'll just write it here just so you get it clear and for male that's the sperm cells and for a female that's the egg cells or the ova ova so but most of my cells have a complete a collection of these what I want to give you the idea is is that for every for every trait I essentially have two versions one from my mother and one from my father now these right here are called homologous chromosomes homologous homologous what that means is you know every time you see this this the the prefix you know a cha malaga sore you know if you see like a homo sapien or i'm even the word homosexual or homogeneous it means same right and you see that all of the time so what they what homologous means that they're almost the same they they are coding they're coding for the same for the most part the same set of genes but they're not identical they're not identical they actually that they might code for slightly different versions of the same gene so depending on what versions i get you know my what is actually expressed for me so my genotype let me introduce another word and i'm overwhelming you with words here so my genotype is exactly what g what alleles i have what versions of the gene so i got like you know the fifth version of the Curley allele there could be multiple versions of the of the Curley allele in our in our gene pool and maybe I got some version of the straight allele that is my genotype my phenotype my phenotype is what my hair really looks like so for example two people could have different genotypes with us the same but they might code for hair that looks pretty much the same so it might have a very very similar phenotype so one phenotype can be represented by multiple genotype so that's just one thing to to think about and we'll talk a lot about that in the future but I just want to introduce you that into that there now my whole I entered this whole discussion because I wanted to talk about variation so how does variation happen well what's going to happen when I so first of all well let me put it this way what's going to happen when I reproduce and and and I have I have a son well my son my contribution to my son is going to be a random collection of half these jeans I'm going to contribute either one for each homologous pair I'm either going to contribute the one that I got from my mother or the one that I got from my father right so let's say that the the sperm cell that went on to to fertilize my wife's egg it just happened to have let's say it happened to have that one that one well I could just pick one from each of these 23 sets and you could say well how many combinations are there well for every set there's two I can pick one of two different I could pick one of the two homologous chromosomes and I'm going to do that 23 times 2 times 2 times 2 this is 2 to the 23rd so there's 22 to the 23 different versions that I can contribute to my to my to any son or daughter that I might have and we'll talk about how that happens when we talk about meiosis or mitosis that when I generate my sperm cells sperm cells are essentially takes one that you have you have instead of having 23 pairs of chromosomes in sperm you only have 23 chromosomes so for example I'll take one from each of those and through the process of meiosis which will go into I'll generate a bunch of sperm cells and each sperm cell each sperm cell will have one from each of these pairs one version from each of those pairs so if maybe for chromosome this chromosome I get it from my dad I get you know from the next chromosome I get it from my mom then I get a couple then I donate a couple more from I shouldn't draw them next to each other I donated a couple more from my mom then for the chromosome number five it comes from my dad and so on and so forth but there's two to the 23rd combinations here because there are 23 pairs that I'm collecting from now my wife's egg is going to have the same situation they're two to the 23 different combinations of DNA that she can contribute just based on which which of the homologous pair she will contribute so if so the the possible combinations that just one couple can produce and this is you know I'm using my my life as an example but you could use this this applies to everything this applies to every species that experiences sexual reproduction is that so if I can give to 220 to the 23rd combinations of DNA and my wife can give to to the 23 combinations of DNA then we can produce 2 to the 46th combinations now just to give an idea of how large of a number this is this is 12,000 roughly 12,000 times the number of human beings on the planet today so there's a huge amount of variation that even one couple can produce and and if you thought that even that isn't enough it turns out that amongst these homologous pairs you and we'll talk about when this happens in meiosis you can actually have DNA recombination and all that means is is that when these homologous pairs during meiosis line up near each other you can have this thing called crossover where all of this DNA here crosses over and over and touches over here and all of this DNA crosses over and touches over there so all of this goes there and all of this goes there and what you end up with after the crossover is that one DNA the one that came from my mom or that I thought came from my mom now has a chunk that clump came from my dad and the chunk that came from my dad now has a chunk that came from my mom now let me do it in that right color it came from my mom like that and so that even increases the amount of variety even more so you can almost now instead of talking about the different chromosomes that you're contributing where the chromosomes are each of these collections of DNA you're now talking about you can almost go to the different combinations at the gene level and now you can think about an almost infinite form of area of variation and you can think about all of the variations that might emerge when you start mixing and matching different versions of the same gene in a population and you don't just look at one gene I mean the reality is is that genes by themselves very seldom code for a specific you can very seldom look for one gene and say oh that is brown hair or look for one gene and say oh that's intelligence or that is how likable someone is it's usually a whole set of genes interacting an incredibly complicated you know hair might be coded for by this whole set of genes on multiple chromosomes and this might be coded for a whole set of genes on multiple chromosomes and so then you can start thinking about all of the different combinations and then all of a sudden maybe some combination that never existed before all of a sudden emerges and that's very successful but I'll leave you to think about because maybe that combination might be passed on or it may not be passed on because of this recombination but we'll talk more about that in the future but I wanted to introduce this idea of sexual reproduction to you because this really is the main source of variation variation within a population and I mean in me it's kind of a philosophical idea because we we almost take the idea of having males and females for granted because it's this it's this universal idea but I did a little reading on it it turns out that this actually only emerged about 1.4 billion years ago that before that this is almost a useful trait because once you introduce this level of variation the natural selection can start you can kind of say that when you have this more powerful form of variation than just pure mutations and maybe you might have some primitive form of crossover before but when now that you have the sexual reproduction and you have this variation natural selection can occur in a more efficient way so that species that were able to reproduce and essentially recombine their DNA and mix-and-match it in this way we're able to produce more variety and we're able to essentially be selected for the environment in a more efficient way so they started to essentially outnumber the ones that couldn't so became a kind of a very universal trait but you know you could have imagined a world and there are science fiction books written about this where you have three genders where you have you know gender one two three you could have ten genders and it just happens to be that in on earth this this notion of having two genders turned out to be a very efficient and stable way of introducing variation into a population so hopefully you found that interesting in the next video I'll go more into the detail of how exactly meiosis and mitosis works
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