Evolution and natural selection
Variation in a Species How variation can be introduced into a species.
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- 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, let's say I have a bunch of-- well, this
- is a circle species, and one guy is that color, and then I
- got a bunch more, maybe some are that color-- oh, that's
- the same color-- that one, and that one, and that one.
- And for whatever reason, 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 maybe, all of a sudden, this guy 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 get killed, or whatever, or 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.
- So over time, 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.
- This could eventually turn into a different species.
- Now, the obvious question is what leads to this variation?
- In a population what leads to this-- in fact, even in our
- population, what leads to one person having dirty blonde
- hair, one person having brown hair, one person having black
- hair, and we have the spectrum of skin complexions and
- heights is pretty much infinite.
- What causes that?
- And then one thing that I kind of point to, we talked about
- this a little bit in the DNA video, is
- this notion of mutations.
- DNA, we learned, is just a sequence of these bases.
- So adenine, guanine, let's say I've 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 you have a few hundred or a few thousand 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 reason becomes a
- guanine randomly, or maybe these got deleted, and that
- would change the DNA.
- But you could imagine, 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.
- 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 it'll either do nothing or it'll actually kill the
- organisms most of the time.
- Mutations: sometimes, they might make the actual cell
- kind of run amok, and we'll do a whole maybe series of videos
- on cancer, and that itself obviously would hurt the
- organism as well as a whole, although if it occurs after
- the organism has reproduced, it might not be something that
- selects against the organism and it also
- wouldn't be passed on.
- But anyway, I won't go detailed into that.
- But 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 it 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.
- And when I mean sexual reproduction, it's this notion
- that you have, and pretty much if you look at all organisms
- that have nucleuses-- and we call those eukaroytes.
- 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 cells that have nucleuses, they almost universally have
- this phenomenon that 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 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 I
- could just bud off other Sals.
- Maybe just other Sals would just bud off from me, and then
- 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 kind of
- budding process where I just replicate myself identically,
- most of the time it'll be negative.
- Most of the time, 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 a kind of species pool of DNA.
- So 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-- let me stick to humans because
- that's what we are.
- We have 23 pairs of chromosomes, and in each pair,
- we have one chromosome from our mother and one chromosome
- from our father.
- So let me draw that.
- So I'll draw my father's chromosomes in blue.
- So I have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15--
- 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 and the Y chromosomes.
- Then I have 23 chromosomes from my mother.
- And not to be stereotypical, but maybe I'll do that in a
- more feminine color.
- Let's see, so I have 23 chromosomes from my mother.
- 1, 2, 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.
- Now, each of these chromosomes, and I made them
- right next to each other.
- So let me zoom in on one pair of these.
- So let's say we look at chromosome number-- I'll just
- call this chromosome number 3.
- So let me zoom in on chromosome number 3.
- I have one from my mother right here.
- Actually, maybe I'll do it this way.
- Remember, chromosome is just a big-- if you take the DNA, the
- DNA just keeps wrapping around, and it actually wraps
- around all these proteins, and it creates this structure, but
- it's just a big-- when you see it like that, you're like, oh,
- maybe the DNA-- no, but this could have millions of base
- pair, so maybe it'll look something like that.
- It's a densely wrapped version of-- well, 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 that,
- they draw it as densely packed like that.
- So let's say that's from my mother and
- that's from my father.
- Now, let's call this chromosome 3.
- They're both chromosome 3.
- And what the idea here is that I'm getting different traits
- from my father and from my mother.
- And I'm doing a gross oversimplification here, but
- this is really just to give you the idea of
- what's going on.
- This chromosome 3, maybe it contains this
- trait for hair color.
- And maybe my father had-- and I'll use my actual example.
- My father had very straight hair.
- So someplace on this chromosome, there's a gene for
- hair straightness.
- Let's say it's a little thing right there.
- And 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, he had the allele for
- straightness.
- And remember, an allele is just a version of a gene, so
- I'll call it the allele straight for straight hair.
- Now, this other chromosome that my mother gave me, 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
- curlyness, but my mom does happen to
- actually have curly hair.
- So she has the gene right there for curly hair.
- The version of the gene here is 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
- this is in every cell of my body, every cell of my body
- except for, and we'll talk in a few seconds about my germ
- cells, but every cells other than the ones that I use for
- reproduction have this complete set of chromosomes in
- it, which I find amazing.
- But only certain chromosomes-- for example, these genes 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 one of your body cells, and we call those
- somatic cells, and we'll separate those from the sex
- sells or the germs that we'll talk about later.
- So this is my body cells.
- 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
- a clear-- for a male, that's the sperm cells, and for
- female that's the egg cells, or the ova.
- But most of my cells have a complete collection of these,
- and what I want to give you the idea is that 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.
- What that means is every time you see this prefix homologous
- or if you see like Homo sapiens or even the word
- homosexual or homogeneous, it means same, right?
- You see that all the time.
- So homologous means that they're almost the same.
- They're coding for the most part the same set of genes,
- but they're not identical.
- They actually might code for slightly different versions of
- the same gene.
- So depending on what versions I get, 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 alleles I have, what versions
- of the gene.
- So I got like the fifth version of the curly allele.
- There could be multiple versions of the curly allele
- in our gene pool.
- And maybe I got some version of the straight allele.
- That is my genotype.
- My phenotype is what my hair really looks like.
- So, for example, two people could have different genotypes
- with the same-- they might code for hair that looks
- pretty much the same, so it might have a
- very similar phenotype.
- So one phenotype can be
- represented by multiple genotypes.
- So that's just one thing to think about, and we'll talk a
- lot about that in the future, but I just wanted to introduce
- you to that there.
- Now, 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-- well, let
- me put it this way.
- What's going to happen when I reproduce?
- And I have. I have a son.
- Well, my contribution to my son is going to be a random
- collection of half of these genes.
- 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 sperm cell that went on to fertilize
- my wife's egg, let's say it happened to have that one,
- that one, or I could just pick one from
- each of these 23 sets.
- And you say, well, how many combinations are there?
- Well, for every set, I could pick one of the two homologous
- chromosomes, and I'm going to do that 23 times.
- 2 times 2 times 2, so that's 2 to the twenty third.
- So 2 to the 23 different versions that I can contribute
- to any son or daughter that I might have. We'll talk about
- how that happens when we talk about meiosis or mitosis, that
- when I generate my sperm cells, sperm cells essentially
- takes one-- 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 we'll go into,
- I'll generate a bunch of sperm cells.
- And each sperm cell will have one from each of these pairs,
- one version from each of those pairs.
- So maybe for this chromosome I get it from my dad, from the
- next chromosome, I get it from my mom.
- Then I donate a couple more from-- I should've drawn them
- next to each other.
- I donate a couple more from my mom.
- Then for chromosome number 5, it comes from my dad, and so
- on and so forth.
- But there's 2 to the twenty-third combinations
- here, because there are 23 pairs that
- I'm collecting from.
- Now, my wife's egg is going to have the same situation.
- There are 2 to the 23 different combinations of DNA
- that she can contribute just based on which of the
- homologous pairs she will contribute.
- So the possible combinations that just one couple can
- produce, and I'm using my life as an example, but this
- applies to everything.
- This applies to every species that experiences sexual
- reproduction.
- So if I can give 2 to twenty-third combinations of
- DNA and my wife can give 2 to the 23 combinations of DNA,
- then we can produce 2 to the forty-sixth combinations.
- Now, just to give an idea of how large of a number this is,
- this is 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 if you thought that even that isn't enough, it turns
- out that amongst these homologous pairs, and we'll
- talk about when this happens in meiosis, you can actually
- have DNA recombination.
- And all that means is 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
- touches over here, and all this DNA crosses over and
- touches over there.
- So all of this goes there and all of this goes there.
- 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 came from my dad, and the chunk
- that came from my dad, now has a chunk that came from my mom.
- Let me do that in the 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 it in almost infinite form of variation.
- You can think about all of the variation 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 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 in an
- incredibly complicated way.
- 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 it 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
- within a population.
- To me, it's kind of a philosophical idea, because we
- almost take the idea of having males and females for granted
- because it's this universal idea.
- But I did a little reading on it, and it turns out that this
- actually only emerged about 1.4 billion years ago, 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 now
- that you have this 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 were
- able to produce more variety and were able to essentially
- be selected for their environment in a more
- efficient way so they started to essentially outnumber the
- ones that couldn't, so it became a kind of very
- universal trait.
- But you could have imagined a world, and there are science
- fiction books written about this, where you have three
- genders, where you have gender one, two, three.
- You could have 10 genders.
- It just happens to be that on Earth, 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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