1 00:00:00,000 --> 00:00:00,440 2 00:00:00,440 --> 00:00:03,569 The whole process of natural selection is to some degree 3 00:00:03,569 --> 00:00:06,689 dependent on the idea of variation, that within any 4 00:00:06,690 --> 00:00:11,449 population of a species, you have some genetic variation. 5 00:00:11,449 --> 00:00:15,629 So, for example, let's say I have a bunch of-- well, this 6 00:00:15,630 --> 00:00:19,500 is a circle species, and one guy is that color, and then I 7 00:00:19,500 --> 00:00:22,960 got a bunch more, maybe some are that color-- oh, that's 8 00:00:22,960 --> 00:00:26,080 the same color-- that one, and that one, and that one. 9 00:00:26,079 --> 00:00:28,629 And for whatever reason, sometimes there are no 10 00:00:28,629 --> 00:00:32,030 environmental factors that will predispose one of these 11 00:00:32,030 --> 00:00:34,800 guys to be able to survive and reproduce over the other, but 12 00:00:34,799 --> 00:00:36,529 every now and then, there might be some environmental 13 00:00:36,530 --> 00:00:40,620 factor, and it makes maybe, all of a sudden, this guy more 14 00:00:40,619 --> 00:00:42,089 fit to reproduce. 15 00:00:42,090 --> 00:00:45,170 And so for whatever reason, this guy is able to reproduce 16 00:00:45,170 --> 00:00:49,820 more frequently and these guys less frequently. 17 00:00:49,820 --> 00:00:53,140 And some of them get killed, or whatever, or eaten by 18 00:00:53,140 --> 00:00:56,460 birds, or whatever, or they're just not able to reproduce for 19 00:00:56,460 --> 00:00:59,070 whatever reason, and then maybe these guys are something 20 00:00:59,070 --> 00:00:59,859 in between. 21 00:00:59,859 --> 00:01:03,960 So over time, the frequency of the different traits you see 22 00:01:03,960 --> 00:01:05,400 in this population will change. 23 00:01:05,400 --> 00:01:07,710 And if they are drastic enough, maybe these guys start 24 00:01:07,709 --> 00:01:10,909 becoming dominant and start not liking these guys, because 25 00:01:10,909 --> 00:01:12,329 they're so different or whatever else. 26 00:01:12,329 --> 00:01:14,109 We could see a lot of different reasons. 27 00:01:14,109 --> 00:01:16,459 This could eventually turn into a different species. 28 00:01:16,459 --> 00:01:21,759 Now, the obvious question is what leads to this variation? 29 00:01:21,760 --> 00:01:27,520 In a population what leads to this-- in fact, even in our 30 00:01:27,519 --> 00:01:30,709 population, what leads to one person having dirty blonde 31 00:01:30,709 --> 00:01:33,489 hair, one person having brown hair, one person having black 32 00:01:33,489 --> 00:01:38,629 hair, and we have the spectrum of skin complexions and 33 00:01:38,629 --> 00:01:40,589 heights is pretty much infinite. 34 00:01:40,590 --> 00:01:41,900 What causes that? 35 00:01:41,900 --> 00:01:44,930 And then one thing that I kind of point to, we talked about 36 00:01:44,930 --> 00:01:47,500 this a little bit in the DNA video, is 37 00:01:47,500 --> 00:01:49,439 this notion of mutations. 38 00:01:49,439 --> 00:01:52,310 DNA, we learned, is just a sequence of these bases. 39 00:01:52,310 --> 00:01:56,430 So adenine, guanine, let's say I've got some thymine going. 40 00:01:56,430 --> 00:01:58,990 I have some more adenine, some cytosine. 41 00:01:58,989 --> 00:02:01,780 And that these code, if you have enough of these in a row, 42 00:02:01,780 --> 00:02:04,030 maybe you have a few hundred or a few thousand of these, 43 00:02:04,030 --> 00:02:06,719 these code for proteins or they code for things that 44 00:02:06,719 --> 00:02:10,020 control other proteins, but maybe you have a 45 00:02:10,020 --> 00:02:11,770 change in one of them. 46 00:02:11,770 --> 00:02:14,330 Maybe this cytosine for whatever reason becomes a 47 00:02:14,330 --> 00:02:18,270 guanine randomly, or maybe these got deleted, and that 48 00:02:18,270 --> 00:02:19,820 would change the DNA. 49 00:02:19,819 --> 00:02:23,150 But you could imagine, if I went to someone's computer 50 00:02:23,150 --> 00:02:26,219 code and just randomly started changing letters and randomly 51 00:02:26,219 --> 00:02:29,479 started inserting letters without really knowing what 52 00:02:29,479 --> 00:02:31,789 I'm doing, most of the time, I'm going to break the 53 00:02:31,789 --> 00:02:33,060 computer program. 54 00:02:33,060 --> 00:02:35,800 Most of the time, the great majority of the time, this is 55 00:02:35,800 --> 00:02:37,050 going to go nowhere. 56 00:02:37,050 --> 00:02:39,150 57 00:02:39,150 --> 00:02:41,390 It'll either do nothing, for example, if I go into 58 00:02:41,389 --> 00:02:43,239 someone's computer program and if I just add a couple of 59 00:02:43,240 --> 00:02:45,189 spaces or something, that might not change the computer 60 00:02:45,189 --> 00:02:47,199 program, but if I start getting rid of semicolons and 61 00:02:47,199 --> 00:02:49,609 start changing numbers and all that, it'll probably make the 62 00:02:49,610 --> 00:02:50,840 computer program break. 63 00:02:50,840 --> 00:02:53,050 So it'll either do nothing or it'll actually kill the 64 00:02:53,050 --> 00:02:55,600 organisms most of the time. 65 00:02:55,599 --> 00:02:59,729 Mutations: sometimes, they might make the actual cell 66 00:02:59,729 --> 00:03:03,060 kind of run amok, and we'll do a whole maybe series of videos 67 00:03:03,060 --> 00:03:06,920 on cancer, and that itself obviously would hurt the 68 00:03:06,919 --> 00:03:09,839 organism as well as a whole, although if it occurs after 69 00:03:09,840 --> 00:03:12,500 the organism has reproduced, it might not be something that 70 00:03:12,500 --> 00:03:14,750 selects against the organism and it also 71 00:03:14,750 --> 00:03:15,930 wouldn't be passed on. 72 00:03:15,930 --> 00:03:17,790 But anyway, I won't go detailed into that. 73 00:03:17,789 --> 00:03:21,469 But the whole point is that mutations don't seem to be a 74 00:03:21,469 --> 00:03:23,469 satisfying source of variation. 75 00:03:23,469 --> 00:03:26,349 They could be a source or kind of contribute on the margin, 76 00:03:26,349 --> 00:03:29,909 but there must be something more profound than mutations 77 00:03:29,909 --> 00:03:32,969 that's creating the diversity even within, or maybe I should 78 00:03:32,969 --> 00:03:36,280 call it the variation, even within a population. 79 00:03:36,280 --> 00:03:38,800 And the answer here is really it's kind of 80 00:03:38,800 --> 00:03:39,710 right in front of us. 81 00:03:39,710 --> 00:03:43,300 It really addresses kind of one of the most fundamental 82 00:03:43,300 --> 00:03:46,260 things about biology, and it's so fundamental that a lot of 83 00:03:46,259 --> 00:03:49,789 people never even question why it is the way it is. 84 00:03:49,789 --> 00:03:51,909 And that is sexual reproduction. 85 00:03:51,909 --> 00:03:59,460 86 00:03:59,460 --> 00:04:02,670 And when I mean sexual reproduction, it's this notion 87 00:04:02,669 --> 00:04:07,869 that you have, and pretty much if you look at all organisms 88 00:04:07,870 --> 00:04:10,680 that have nucleuses-- and we call those eukaroytes. 89 00:04:10,680 --> 00:04:14,780 Maybe I'll do a whole video on eukaryotes versus prokaryotes, 90 00:04:14,780 --> 00:04:18,050 but it's the notion that if you look universally all the 91 00:04:18,050 --> 00:04:20,189 way from plants-- not universally, but if you look 92 00:04:20,189 --> 00:04:23,639 at cells that have nucleuses, they almost universally have 93 00:04:23,639 --> 00:04:29,639 this phenomenon that you have males and you have females. 94 00:04:29,639 --> 00:04:33,509 In some organisms, an organism can be both a male and a 95 00:04:33,509 --> 00:04:39,920 female, but the common idea here is that all organisms 96 00:04:39,920 --> 00:04:42,530 kind of produce versions of their genetic material that 97 00:04:42,529 --> 00:04:44,959 mix with other organisms' version of 98 00:04:44,959 --> 00:04:46,139 their genetic material. 99 00:04:46,139 --> 00:04:49,409 If mutations were the only source of variation, then I 100 00:04:49,410 --> 00:04:53,900 could just bud off other Sals. 101 00:04:53,899 --> 00:04:58,639 Maybe just other Sals would just bud off from me, and then 102 00:04:58,639 --> 00:05:01,550 randomly one Sal might be a little bit different and 103 00:05:01,550 --> 00:05:02,319 whatever else. 104 00:05:02,319 --> 00:05:04,629 But that would, as we already talked about, most of the 105 00:05:04,629 --> 00:05:06,730 time, we would have very little change, very little 106 00:05:06,730 --> 00:05:09,500 variation, and whatever variation does occur because 107 00:05:09,500 --> 00:05:14,160 of any kind of noise being introduced into this kind of 108 00:05:14,160 --> 00:05:17,650 budding process where I just replicate myself identically, 109 00:05:17,649 --> 00:05:19,099 most of the time it'll be negative. 110 00:05:19,100 --> 00:05:21,790 Most of the time, it'll break the organism. 111 00:05:21,790 --> 00:05:24,660 Now, when you have sexual reproduction, what happens? 112 00:05:24,660 --> 00:05:27,380 Well, you keep mixing and matching every possible 113 00:05:27,379 --> 00:05:35,750 combination of DNA in a kind of species pool of DNA. 114 00:05:35,750 --> 00:05:39,509 So let me make this a little bit more concrete for you. 115 00:05:39,509 --> 00:05:42,779 So let me erase this horrible drawing I just did. 116 00:05:42,779 --> 00:05:47,299 So we all have-- let me stick to humans because 117 00:05:47,300 --> 00:05:48,550 that's what we are. 118 00:05:48,550 --> 00:05:50,879 119 00:05:50,879 --> 00:05:56,699 We have 23 pairs of chromosomes, and in each pair, 120 00:05:56,699 --> 00:06:00,209 we have one chromosome from our mother and one chromosome 121 00:06:00,209 --> 00:06:01,549 from our father. 122 00:06:01,550 --> 00:06:02,470 So let me draw that. 123 00:06:02,470 --> 00:06:04,890 So I'll draw my father's chromosomes in blue. 124 00:06:04,889 --> 00:06:15,269 So I have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15-- 125 00:06:15,269 --> 00:06:17,279 I'm running out of space. 126 00:06:17,279 --> 00:06:25,409 Let me do more here-- 16, 17, 18, 19, 20, 21, 22, and then 127 00:06:25,410 --> 00:06:26,770 I'll throw another one here that looks 128 00:06:26,769 --> 00:06:27,969 a little bit different. 129 00:06:27,970 --> 00:06:30,660 I'll throw one here that looks like a Y, and we'll talk more 130 00:06:30,660 --> 00:06:32,340 about the X's and the Y chromosomes. 131 00:06:32,339 --> 00:06:35,869 Then I have 23 chromosomes from my mother. 132 00:06:35,870 --> 00:06:39,470 And not to be stereotypical, but maybe I'll do that in a 133 00:06:39,470 --> 00:06:41,220 more feminine color. 134 00:06:41,220 --> 00:06:44,360 Let's see, so I have 23 chromosomes from my mother. 135 00:06:44,360 --> 00:06:56,480 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 136 00:06:56,480 --> 00:07:02,740 18, 19, 20, 21, 22, 23. 137 00:07:02,740 --> 00:07:03,730 So what's going on here? 138 00:07:03,730 --> 00:07:07,870 I have 23 from my mother. 139 00:07:07,870 --> 00:07:14,620 I have 23 from my father. 140 00:07:14,620 --> 00:07:18,069 Now, each of these chromosomes, and I made them 141 00:07:18,069 --> 00:07:19,319 right next to each other. 142 00:07:19,319 --> 00:07:24,870 So let me zoom in on one pair of these. 143 00:07:24,870 --> 00:07:28,389 So let's say we look at chromosome number-- I'll just 144 00:07:28,389 --> 00:07:31,339 call this chromosome number 3. 145 00:07:31,339 --> 00:07:33,149 So let me zoom in on chromosome number 3. 146 00:07:33,149 --> 00:07:36,329 I have one from my mother right here. 147 00:07:36,329 --> 00:07:37,649 Actually, maybe I'll do it this way. 148 00:07:37,649 --> 00:07:43,229 Remember, chromosome is just a big-- if you take the DNA, the 149 00:07:43,230 --> 00:07:45,930 DNA just keeps wrapping around, and it actually wraps 150 00:07:45,930 --> 00:07:48,100 around all these proteins, and it creates this structure, but 151 00:07:48,100 --> 00:07:50,430 it's just a big-- when you see it like that, you're like, oh, 152 00:07:50,430 --> 00:07:53,860 maybe the DNA-- no, but this could have millions of base 153 00:07:53,860 --> 00:07:56,009 pair, so maybe it'll look something like that. 154 00:07:56,009 --> 00:08:01,099 It's a densely wrapped version of-- well, it's a long string 155 00:08:01,100 --> 00:08:04,230 of DNA, and when it's normally drawn like this, which is not 156 00:08:04,230 --> 00:08:06,640 always the way it is, and we'll talk more about that, 157 00:08:06,639 --> 00:08:08,240 they draw it as densely packed like that. 158 00:08:08,240 --> 00:08:10,170 So let's say that's from my mother and 159 00:08:10,170 --> 00:08:11,990 that's from my father. 160 00:08:11,990 --> 00:08:21,120 Now, let's call this chromosome 3. 161 00:08:21,120 --> 00:08:22,680 They're both chromosome 3. 162 00:08:22,680 --> 00:08:26,269 And what the idea here is that I'm getting different traits 163 00:08:26,269 --> 00:08:28,409 from my father and from my mother. 164 00:08:28,410 --> 00:08:30,990 And I'm doing a gross oversimplification here, but 165 00:08:30,990 --> 00:08:32,620 this is really just to give you the idea of 166 00:08:32,620 --> 00:08:35,070 what's going on. 167 00:08:35,070 --> 00:08:37,820 This chromosome 3, maybe it contains this 168 00:08:37,820 --> 00:08:39,460 trait for hair color. 169 00:08:39,460 --> 00:08:45,930 And maybe my father had-- and I'll use my actual example. 170 00:08:45,929 --> 00:08:48,469 My father had very straight hair. 171 00:08:48,470 --> 00:08:53,529 So someplace on this chromosome, there's a gene for 172 00:08:53,529 --> 00:08:54,470 hair straightness. 173 00:08:54,470 --> 00:08:56,210 Let's say it's a little thing right there. 174 00:08:56,210 --> 00:08:58,610 And remember, that gene could be thousands of base pairs, 175 00:08:58,610 --> 00:09:00,539 but let's say this is hair straightness. 176 00:09:00,539 --> 00:09:05,360 So my father's version of that gene, he had the allele for 177 00:09:05,360 --> 00:09:05,800 straightness. 178 00:09:05,799 --> 00:09:10,229 And remember, an allele is just a version of a gene, so 179 00:09:10,230 --> 00:09:13,800 I'll call it the allele straight for straight hair. 180 00:09:13,799 --> 00:09:16,579 181 00:09:16,580 --> 00:09:21,770 Now, this other chromosome that my mother gave me, this 182 00:09:21,769 --> 00:09:24,149 essentially, and there are exceptions, but for the most 183 00:09:24,149 --> 00:09:26,750 part, it codes for the same genes, and that's why I put 184 00:09:26,750 --> 00:09:27,860 them next to each other. 185 00:09:27,860 --> 00:09:31,210 So this will also have the gene for hair straightness or 186 00:09:31,210 --> 00:09:33,519 curlyness, but my mom does happen to 187 00:09:33,519 --> 00:09:35,600 actually have curly hair. 188 00:09:35,600 --> 00:09:40,250 So she has the gene right there for curly hair. 189 00:09:40,250 --> 00:09:43,700 The version of the gene here is allele curly. 190 00:09:43,700 --> 00:09:46,610 The gene just says, look, this is the gene for whether or not 191 00:09:46,610 --> 00:09:47,600 your hair is curly. 192 00:09:47,600 --> 00:09:50,500 Each version of the gene is called an allele. 193 00:09:50,500 --> 00:09:52,639 Allele curly. 194 00:09:52,639 --> 00:09:56,689 Now, when I got both of these in my body or in my cells, and 195 00:09:56,690 --> 00:10:00,040 this is in every cell of my body, every cell of my body 196 00:10:00,039 --> 00:10:04,079 except for, and we'll talk in a few seconds about my germ 197 00:10:04,080 --> 00:10:07,330 cells, but every cells other than the ones that I use for 198 00:10:07,330 --> 00:10:12,060 reproduction have this complete set of chromosomes in 199 00:10:12,059 --> 00:10:13,329 it, which I find amazing. 200 00:10:13,330 --> 00:10:17,690 But only certain chromosomes-- for example, these genes will 201 00:10:17,690 --> 00:10:20,360 be completely useless in my fingernails, because all of a 202 00:10:20,360 --> 00:10:22,759 sudden, the straight and the curly don't matter that much. 203 00:10:22,759 --> 00:10:23,879 And I'm simplifying. 204 00:10:23,879 --> 00:10:25,980 Maybe they will on some other dimension. 205 00:10:25,980 --> 00:10:27,700 But let's say for simplicity, they won't 206 00:10:27,700 --> 00:10:28,850 matter in certain places. 207 00:10:28,850 --> 00:10:32,240 So certain genes are expressed in certain parts of the body, 208 00:10:32,240 --> 00:10:35,600 but every one of your body cells, and we call those 209 00:10:35,600 --> 00:10:38,980 somatic cells, and we'll separate those from the sex 210 00:10:38,980 --> 00:10:45,050 sells or the germs that we'll talk about later. 211 00:10:45,049 --> 00:10:48,059 So this is my body cells. 212 00:10:48,059 --> 00:10:50,219 So this is the great majority of your cells, and this is 213 00:10:50,220 --> 00:10:52,029 opposed to your germ cells. 214 00:10:52,029 --> 00:10:54,079 And the germ cells-- I'll just write it here, just so you get 215 00:10:54,080 --> 00:10:58,950 a clear-- for a male, that's the sperm cells, and for 216 00:10:58,950 --> 00:11:01,210 female that's the egg cells, or the ova. 217 00:11:01,210 --> 00:11:04,550 218 00:11:04,549 --> 00:11:09,309 But most of my cells have a complete collection of these, 219 00:11:09,309 --> 00:11:13,369 and what I want to give you the idea is that for every 220 00:11:13,370 --> 00:11:16,200 trait, I essentially have two versions: one from my mother 221 00:11:16,200 --> 00:11:17,629 and one from my father. 222 00:11:17,629 --> 00:11:21,064 Now, these right here are called homologous chromosomes. 223 00:11:21,065 --> 00:11:25,780 224 00:11:25,779 --> 00:11:32,839 What that means is every time you see this prefix homologous 225 00:11:32,840 --> 00:11:35,850 or if you see like Homo sapiens or even the word 226 00:11:35,850 --> 00:11:41,110 homosexual or homogeneous, it means same, right? 227 00:11:41,110 --> 00:11:42,639 You see that all the time. 228 00:11:42,639 --> 00:11:46,990 So homologous means that they're almost the same. 229 00:11:46,990 --> 00:11:52,230 They're coding for the most part the same set of genes, 230 00:11:52,230 --> 00:11:55,100 but they're not identical. 231 00:11:55,100 --> 00:11:58,950 They actually might code for slightly different versions of 232 00:11:58,950 --> 00:11:59,660 the same gene. 233 00:11:59,659 --> 00:12:02,919 So depending on what versions I get, what is actually 234 00:12:02,919 --> 00:12:06,120 expressed for me, so my genotype-- let me introduce 235 00:12:06,120 --> 00:12:08,940 another word, and I'm overwhelming 236 00:12:08,940 --> 00:12:10,490 you with words here. 237 00:12:10,490 --> 00:12:17,629 So my genotype is exactly what alleles I have, what versions 238 00:12:17,629 --> 00:12:18,169 of the gene. 239 00:12:18,169 --> 00:12:21,209 So I got like the fifth version of the curly allele. 240 00:12:21,210 --> 00:12:23,850 There could be multiple versions of the curly allele 241 00:12:23,850 --> 00:12:25,490 in our gene pool. 242 00:12:25,490 --> 00:12:27,820 And maybe I got some version of the straight allele. 243 00:12:27,820 --> 00:12:29,980 That is my genotype. 244 00:12:29,980 --> 00:12:36,170 My phenotype is what my hair really looks like. 245 00:12:36,169 --> 00:12:39,719 So, for example, two people could have different genotypes 246 00:12:39,720 --> 00:12:42,879 with the same-- they might code for hair that looks 247 00:12:42,879 --> 00:12:44,559 pretty much the same, so it might have a 248 00:12:44,559 --> 00:12:47,869 very similar phenotype. 249 00:12:47,870 --> 00:12:48,980 So one phenotype can be 250 00:12:48,980 --> 00:12:50,639 represented by multiple genotypes. 251 00:12:50,639 --> 00:12:53,689 So that's just one thing to think about, and we'll talk a 252 00:12:53,690 --> 00:12:55,780 lot about that in the future, but I just wanted to introduce 253 00:12:55,779 --> 00:12:57,379 you to that there. 254 00:12:57,379 --> 00:13:01,200 Now, I entered this whole discussion because I wanted to 255 00:13:01,200 --> 00:13:02,430 talk about variation. 256 00:13:02,429 --> 00:13:04,569 So how does variation happen? 257 00:13:04,570 --> 00:13:09,129 Well, what's going to happen when I-- well, let 258 00:13:09,129 --> 00:13:09,639 me put it this way. 259 00:13:09,639 --> 00:13:11,289 What's going to happen when I reproduce? 260 00:13:11,289 --> 00:13:13,990 And I have. I have a son. 261 00:13:13,990 --> 00:13:18,159 Well, my contribution to my son is going to be a random 262 00:13:18,159 --> 00:13:22,529 collection of half of these genes. 263 00:13:22,529 --> 00:13:25,829 For each homologous pair, I'm either going to contribute the 264 00:13:25,830 --> 00:13:28,740 one that I got from my mother or the one that I got from my 265 00:13:28,740 --> 00:13:30,250 father, right? 266 00:13:30,250 --> 00:13:37,360 So let's say that the sperm cell that went on to fertilize 267 00:13:37,360 --> 00:13:43,440 my wife's egg, let's say it happened to have that one, 268 00:13:43,440 --> 00:13:45,400 that one, or I could just pick one from 269 00:13:45,399 --> 00:13:46,829 each of these 23 sets. 270 00:13:46,830 --> 00:13:49,020 And you say, well, how many combinations are there? 271 00:13:49,019 --> 00:13:56,240 Well, for every set, I could pick one of the two homologous 272 00:13:56,240 --> 00:13:59,549 chromosomes, and I'm going to do that 23 times. 273 00:13:59,549 --> 00:14:02,359 2 times 2 times 2, so that's 2 to the twenty third. 274 00:14:02,360 --> 00:14:09,220 So 2 to the 23 different versions that I can contribute 275 00:14:09,220 --> 00:14:12,800 to any son or daughter that I might have. We'll talk about 276 00:14:12,799 --> 00:14:16,309 how that happens when we talk about meiosis or mitosis, that 277 00:14:16,309 --> 00:14:20,659 when I generate my sperm cells, sperm cells essentially 278 00:14:20,659 --> 00:14:26,589 takes one-- instead of having 23 pairs of chromosomes in 279 00:14:26,590 --> 00:14:29,540 sperm, you only have 23 chromosomes. 280 00:14:29,539 --> 00:14:32,759 So, for example, I'll take one from each of those, and 281 00:14:32,759 --> 00:14:35,029 through the process of meiosis, which we'll go into, 282 00:14:35,029 --> 00:14:37,199 I'll generate a bunch of sperm cells. 283 00:14:37,200 --> 00:14:44,740 And each sperm cell will have one from each of these pairs, 284 00:14:44,740 --> 00:14:46,440 one version from each of those pairs. 285 00:14:46,440 --> 00:14:51,980 So maybe for this chromosome I get it from my dad, from the 286 00:14:51,980 --> 00:14:54,370 next chromosome, I get it from my mom. 287 00:14:54,370 --> 00:14:56,840 Then I donate a couple more from-- I should've drawn them 288 00:14:56,840 --> 00:14:57,820 next to each other. 289 00:14:57,820 --> 00:14:59,580 I donate a couple more from my mom. 290 00:14:59,580 --> 00:15:03,540 Then for chromosome number 5, it comes from my dad, and so 291 00:15:03,539 --> 00:15:04,199 on and so forth. 292 00:15:04,200 --> 00:15:06,200 But there's 2 to the twenty-third combinations 293 00:15:06,200 --> 00:15:08,240 here, because there are 23 pairs that 294 00:15:08,240 --> 00:15:09,289 I'm collecting from. 295 00:15:09,289 --> 00:15:13,529 Now, my wife's egg is going to have the same situation. 296 00:15:13,529 --> 00:15:17,600 There are 2 to the 23 different combinations of DNA 297 00:15:17,600 --> 00:15:20,730 that she can contribute just based on which of the 298 00:15:20,730 --> 00:15:24,590 homologous pairs she will contribute. 299 00:15:24,590 --> 00:15:28,389 So the possible combinations that just one couple can 300 00:15:28,389 --> 00:15:34,990 produce, and I'm using my life as an example, but this 301 00:15:34,990 --> 00:15:35,980 applies to everything. 302 00:15:35,980 --> 00:15:39,430 This applies to every species that experiences sexual 303 00:15:39,429 --> 00:15:41,189 reproduction. 304 00:15:41,190 --> 00:15:46,280 So if I can give 2 to twenty-third combinations of 305 00:15:46,279 --> 00:15:50,620 DNA and my wife can give 2 to the 23 combinations of DNA, 306 00:15:50,620 --> 00:15:55,240 then we can produce 2 to the forty-sixth combinations. 307 00:15:55,240 --> 00:15:58,220 Now, just to give an idea of how large of a number this is, 308 00:15:58,220 --> 00:16:02,560 this is roughly 12,000 times the number of human beings on 309 00:16:02,559 --> 00:16:03,489 the planet today. 310 00:16:03,490 --> 00:16:07,210 So there's a huge amount of variation that even one couple 311 00:16:07,210 --> 00:16:08,269 can produce. 312 00:16:08,269 --> 00:16:12,259 And if you thought that even that isn't enough, it turns 313 00:16:12,259 --> 00:16:15,559 out that amongst these homologous pairs, and we'll 314 00:16:15,559 --> 00:16:18,509 talk about when this happens in meiosis, you can actually 315 00:16:18,509 --> 00:16:20,629 have DNA recombination. 316 00:16:20,629 --> 00:16:24,450 And all that means is when these homologous pairs during 317 00:16:24,450 --> 00:16:27,730 meiosis line up near each other, you can have this thing 318 00:16:27,730 --> 00:16:32,340 called crossover, where all of this DNA here crosses over and 319 00:16:32,340 --> 00:16:34,430 touches over here, and all this DNA crosses over and 320 00:16:34,429 --> 00:16:35,199 touches over there. 321 00:16:35,200 --> 00:16:38,550 So all of this goes there and all of this goes there. 322 00:16:38,549 --> 00:16:44,009 What you end up with after the crossover is that one DNA, the 323 00:16:44,009 --> 00:16:46,980 one that came from my mom, or that I thought came from my 324 00:16:46,980 --> 00:16:49,980 mom, now has a chunk that came from my dad, and the chunk 325 00:16:49,980 --> 00:16:55,149 that came from my dad, now has a chunk that came from my mom. 326 00:16:55,149 --> 00:16:56,699 Let me do that in the right color. 327 00:16:56,700 --> 00:16:59,110 It came from my mom like that. 328 00:16:59,110 --> 00:17:01,980 And so that even increases the amount of variety even more. 329 00:17:01,980 --> 00:17:04,328 So you can almost now, instead of talking about the different 330 00:17:04,328 --> 00:17:06,139 chromosomes that you're contributing where the 331 00:17:06,140 --> 00:17:09,359 chromosomes are each of these collections of DNA, you're now 332 00:17:09,358 --> 00:17:11,578 talking about-- you can almost go to the different 333 00:17:11,578 --> 00:17:13,700 combinations at the gene level, and now you can think 334 00:17:13,700 --> 00:17:17,410 about it in almost infinite form of variation. 335 00:17:17,410 --> 00:17:20,910 You can think about all of the variation that might emerge 336 00:17:20,910 --> 00:17:23,720 when you start mixing and matching different versions of 337 00:17:23,720 --> 00:17:25,559 the same gene in a population. 338 00:17:25,559 --> 00:17:27,149 And you don't just look at one gene. 339 00:17:27,150 --> 00:17:31,350 I mean, the reality is that genes by themselves very 340 00:17:31,349 --> 00:17:35,289 seldom code for a specific-- you can very seldom look for 341 00:17:35,289 --> 00:17:38,329 one gene and say, oh, that is brown hair, or look for one 342 00:17:38,329 --> 00:17:41,329 gene and say, oh, that's intelligence, or that is how 343 00:17:41,329 --> 00:17:42,549 likable someone is. 344 00:17:42,549 --> 00:17:45,019 It's usually a whole set of genes interacting in an 345 00:17:45,019 --> 00:17:46,839 incredibly complicated way. 346 00:17:46,839 --> 00:17:50,429 You know, hair might be coded for by this whole set of genes 347 00:17:50,430 --> 00:17:52,720 on multiple chromosomes and this might be coded for a 348 00:17:52,720 --> 00:17:54,880 whole set of genes on multiple chromosomes. 349 00:17:54,880 --> 00:17:56,630 And so then you can start thinking about all of the 350 00:17:56,630 --> 00:17:57,690 different combinations. 351 00:17:57,690 --> 00:17:59,500 And then all of a sudden, maybe some combination that 352 00:17:59,500 --> 00:18:02,190 never existed before all of a sudden emerges, and that's 353 00:18:02,190 --> 00:18:03,450 very successful. 354 00:18:03,450 --> 00:18:06,019 But I'll leave you to think about it because maybe that 355 00:18:06,019 --> 00:18:08,769 combination might be passed on, or it may not be passed on 356 00:18:08,769 --> 00:18:10,579 because of this recombination. 357 00:18:10,579 --> 00:18:12,480 But we'll talk more about that in the future. 358 00:18:12,480 --> 00:18:15,720 But I wanted to introduce this idea of sexual reproduction to 359 00:18:15,720 --> 00:18:21,690 you, because this really is the main source of variation 360 00:18:21,690 --> 00:18:24,080 within a population. 361 00:18:24,079 --> 00:18:26,730 To me, it's kind of a philosophical idea, because we 362 00:18:26,730 --> 00:18:31,579 almost take the idea of having males and females for granted 363 00:18:31,579 --> 00:18:34,079 because it's this universal idea. 364 00:18:34,079 --> 00:18:36,399 But I did a little reading on it, and it turns out that this 365 00:18:36,400 --> 00:18:40,310 actually only emerged about 1.4 billion years ago, that 366 00:18:40,309 --> 00:18:43,819 this is almost a useful trait, because once you introduce 367 00:18:43,819 --> 00:18:48,359 this level of variation, the natural selection can start-- 368 00:18:48,359 --> 00:18:50,299 you can kind of say that when you have this more powerful 369 00:18:50,299 --> 00:18:53,909 form of variation than just pure mutations, and maybe you 370 00:18:53,910 --> 00:18:56,540 might have some primitive form of crossover before, but now 371 00:18:56,539 --> 00:18:59,220 that you have this sexual reproduction and you have this 372 00:18:59,220 --> 00:19:01,980 variation, natural selection can occur in a 373 00:19:01,980 --> 00:19:03,319 more efficient way. 374 00:19:03,319 --> 00:19:06,509 So that species that were able to reproduce and essentially 375 00:19:06,509 --> 00:19:09,650 recombine their DNA and mix and match it in this way were 376 00:19:09,650 --> 00:19:13,060 able to produce more variety and were able to essentially 377 00:19:13,059 --> 00:19:15,119 be selected for their environment in a more 378 00:19:15,119 --> 00:19:17,739 efficient way so they started to essentially outnumber the 379 00:19:17,740 --> 00:19:20,049 ones that couldn't, so it became a kind of very 380 00:19:20,049 --> 00:19:21,720 universal trait. 381 00:19:21,720 --> 00:19:24,759 But you could have imagined a world, and there are science 382 00:19:24,759 --> 00:19:27,180 fiction books written about this, where you have three 383 00:19:27,180 --> 00:19:30,160 genders, where you have gender one, two, three. 384 00:19:30,160 --> 00:19:31,410 You could have 10 genders. 385 00:19:31,410 --> 00:19:36,040 It just happens to be that on Earth, this notion of having 386 00:19:36,039 --> 00:19:39,750 two genders turned out to be a very efficient and stable way 387 00:19:39,750 --> 00:19:42,640 of introducing variation into a population. 388 00:19:42,640 --> 00:19:44,070 So, hopefully, you found that interesting. 389 00:19:44,069 --> 00:19:46,139 In the next video, I'll go more into the detail of how 390 00:19:46,140 --> 00:19:49,790 exactly meiosis and mitosis works. 391 00:19:49,789 --> 00:19:50,843