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AP®︎/College Biology
Variation in a species
Natural selection relies on genetic variation within a species. Mutations alone aren't sufficient for creating diversity; sexual reproduction plays a crucial role as well. By combining genetic material from two parents, offspring inherit a unique mix of traits, resulting in vast potential for variation. This process allows for more efficient adaptation and survival, making sexual reproduction a widespread biological strategy. Created by Sal Khan.
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- At14:05
With so many possible combinations of chromosomes, it seems unlikely that siblings would look similar. How does this happen?(14 votes)- Each of those genes govern one small characteristic (that's similar to either from your mom or your dad). On average, you'll be getting about half from each. So let's use a hypothetical, really simplified example: there's 4 genes that control hair for curliness, color, texture, and density. Your mom's hair is very curly, brown, smooth, and thick. Your dad's hair is not curly, blonde, sandy, and thin. Your hair could be very curly, blonde smooth, and thin. Your sister's hair could be very curly, blonde, sandy, and thick. That sounds pretty similar.
Secondly, dominant traits also come into play. Some visible physical traits are much more common even if you're genetically different on the inside.
And most importantly, you guys are both taking genes from the same two pools. There are countless more variants for curliness, hair colors, textures, and thickness and all the other genes that you won't have any chance of inheriting if your parents don't have them. So compared to all the other people outside the family, siblings usually look pretty similar. I mean, from an elephant's perspective, all humans probably look exactly alike. ;)(16 votes)
- Haha. I don't know if anyone else noticed. But this video seems like a review. Middle school review. If you think you knew some of these things just to be curiouss, vote this up. If no, then comment. I am very curious.(17 votes)
- Was there sexual reproduction (or some other version of DNA transfer and recombination) before the advent of male & female 1.4 billion years ago?(7 votes)
- Nobody knows for sure, though modern Bacteria can engage in horizontal gene transfer via conjugation.(15 votes)
- I noticed a correction asin the video. The correction says 'Sal says "gender" but means "sex"' 9:32
What is the difference between gender and sex?(5 votes)- In this context, "sex" refers to the physical traits about being male or female or otherwise -- such things as genetic distinctions, differences in reproductive organs, and sexual dimorphism (in species where that occurs).
"Gender" refers to the psychological, social, and/or culture aspects about sexual identity and roles.(12 votes)
- I have another question. He said there are 3 billion codes for the DNA. This is very complex. But if humans could interpret them with some high technology computers and change these DNA codes using some biogenetic stuff or chemical combinations even before the baby is born, is it possible to bring out desired results? And if during a test-tube baby, the sperm is genetically changed to have some other characters instead of what it has, and them the ova is fertilized under observation where necessary changes are brought somehow, then can the baby have that particular character? For eg., if the doc makes changes in the DNA structure of the sperm and inserts the gene for high IQ removing the original gene of low IQ, then is it possible that after fertilization if the sperm's genes remain dominant in the IQ subject, that the baby born will have high IQ? If this is possible, we can have a a new studying stream called baby engineering!!(6 votes)
- Yes this is possible and true. Most store bought fish are actually clones of the desired breed of fish, size, and health.(2 votes)
- How did variations originally come about: i.e. how could humans develop black hair if thousands of years ago lets say all humans had brown hair. How could a gene for black hair develop?(6 votes)
- Well, mutations for black hair could arise randomly in a population- it could be passed down if this mutation happens to be dominant.(5 votes)
- Do we have the same sets of chromosomes in every cell? I mean, just one cell can't identify whether you have curly or straight hair, can it? In each cell, if there are 23 chromosome pairs, are they the same in every cell or does every cell have different pairs that "code" something else in the body (because 23 pairs of chromosomes can't really make you up, even if the same type of pairs are in every cell)? I'm a bit confused, and would like some help.
Thanks(4 votes)- Yes, we have the same chromosomes in every cell with the exception of our reproductive cells. In most cells in the human body there are 23 chromosome pairs, and in reproductive cells there are 23 single chromosomes. Each cell in your body has the genetic information for the makeup of your entire body. So even the cells at the end of your big toe have the encoded information for the curly, or straight hair on the top of your head. So yes, 23 pairs of chromosomes really can contain the information for the makeup of your entire body. In fact there is a significant amount information encoded in your DNA that is essentially switched off, because it is no longer needed. So those 23 chromosomes not only include the information for your entire body in each and every cell, they contain some information about your ancestry as well.(7 votes)
- If sexual reproduction can only use current and existing genes from both parents, how do we account for "new" traits, and not just novel gene combinations?(4 votes)
- New genetic traits occurring that are not the result of novel gene combinations happen by mutations. These mutations happen as the gametes are forming. This is not all that unusual, most gametes have a few mutations. Most of the time the mutation has little or no effect on the phenotype. When there is an effect that matters, it is usually harmful. But, once in a while, the mutation is beneficial.
There are many kinds of mutations. Let me (greatly over-simplifying) mention one class of mutation that is more likely (not always of course) to lead to a beneficial change in traits. This is gene duplication.
In gene duplication the offspring gets an extra copy of a gene, perhaps even on a completely different chromosome. Sometimes this is catastrophic, but a lot of times it can enhance an existing trait or simply just provide a backup copy of an existing gene. Gene duplication is a very common mutation and there are many genes we have extra copies of.
Now, if we have a spare copy of a gene, then some of the time that allows a major mutation to occur in one of the copies without causing a catastrophic problem. Now, in particular, if one of the copies undergoes what is called a frameshift mutation, then that gene will now code for a completely different protein. This might do very little, might be harmful, or might actually cause a new trait to appear rather suddenly. There are documented cases of this kind of mutation.(6 votes)
- Ok, so hypothetical question. Say you and your spouse have the maximum number of combinations of kids. If you have one more kid, then it would be a repeat of a combination. Does that mean this kid would be exactly the same as the other kid who had the same combination? Or would they still be different?(3 votes)
- Even if two of my however many quadrillion children happened to be genetically identical at fertilization, they would not physically identical. They wouldn't even be genetically identical for long after fertilization. This question could also be asked about identical twins. Since one zygote divides into two embryos in the case of identical twins, shouldn't they be, well, identical? But during embryonic development, when the cells are dividing and their DNA is almost constantly being replicated, hundreds of mutations will occur and accumulate. Also, environmental differences will cause changes in each individual's epigenetics - changes in gene expression not controlled by mutations or other changes in the DNA itself. The mutations and epigenetic changes will result in two different individuals. They will only be slightly different physically, but they will still be different.(5 votes)
- If my cells contain chromosomes that contain the information passed down from both of my parents, is it possible for me to pass down a characteristic that I did not inherent? For example, I inherited black hair from my mother but my father has blonde hair. Since my chromosomes contain that information, can my offspring inherit the blonde hair trait?(6 votes)
- Yes this is possible because some alleles are dominant and some are recessive. Due to this you can carry an allele without expressing it but you are able to give the particular allele to the next generation(6 votes)
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, 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.