Hello, biology people, this screencast going to be about the costs and benefits of sexual
reproduction. We're going to discuss what it is, why it works, and also why we do it.
However, before we can talk about sexual reproduction we need to talk about this
term; evolution. Evolution is the change in gene distribution and frequency over
time. When we talk about "over time" we're talking over many, many generations.
Ultimately, this is going to lead to an increase in diversity at every level of
biological organization; the more diversity you have the better chance
your offspring have of surviving particularly in an environment that
happens to be constantly changing. We will come back to this in a few minutes, but
keep this in mind as we start talking about sexual reproduction. Let's begin by
reviewing some differences between mitosis and meiosis. We're going to start
over here on the left with mitosis. Mitosis occurs in almost all of the
cells in your body with the exception of sex cells. In mitosis, one cell produces two
exact copies of itself; essentially these two are clones.
Inside the nucleus of a human body cell, you have 46 chromosomes. These 46
chromosomes are divided into twenty three different pairs. Because we have 23
pairs of cells, we call this a diploid cell, which is also indicated by the
symbol 2n. In mitosis, one diploid cell is going to duplicate itself to
form two diploid cells. Remember that it duplicates its genetic material to avoid
losing some of it when it divides in half. Meisosis also starts with the
diploid cell; 23 pairs of chromosomes. However, after meiosis occurs, we now have
four daughter cells each of which are called haploid meaning they have 23
single chromosomes and we use the symbol n or 1n to indicate they are haploid.
Remember also that these cells are all going to be genetically diverse from one
another.
Let's zoom in on this set of cells a little bit more closely. So let's imagine
that we have two haploid cells; we have a sperm cell and an egg cell.
Ignore the fact that the sperm cell and egg cell here appear to be the same size
generally speaking the egg cell is significantly bigger but also less mobile. Each of
these cells are haploid, meaning that they have 23 chromosomes. I drew all of them
in blue except for this red one in the bottom corner right down here, and the
reason that this is red is because this is going to be the unusual chromosome.
In an egg cell, this is always what we call an X chromosome, but in the sperm cell it can
be either an X or Y depending on which variety of the 23rd chromosome this
particular sperm cell has. It will determine whether or not the resulting
zygote is going to be male or female. If the sperm cell has an X chromosome as
the 23rd chromosome, the resulting zygote will be female. If it's a Y chromosome, however,
the resulting zygote will be male. Let's now imagine
fertilization has occurred, and the sperm and egg have united to form one zygote.
This zygote is going to be dipoid; it has one set of chromosomes from the mother
and one from the father, meaning that you now have TWO of each variety of
chromosome. This zygote would ultimately be male because it has an X and Y
chromosome for the 23rd pair. Each of the varieties of chromosome will line up with
the corresponding chromosome from the other contributing parent. These are
called homologous pairs. Each of these chromosomes is going to contribute one
set of traits from the mother and one set of traits from the father. This is
why you don't ever wind up looking or behaving exactly like one of your
parents. Because each chromosome is going to bring a different version of the same
kind of characteristic. These light blue chromosomes for instance
might encode how tall you end up being, but you're probably not going to wind up
being exactly the same height as either of your parents because we're going to be
combining the different height genes from your mother and your father.
Remember also that before you combine your chromosomes with another
individual, each of your chromosomes in your sex cells are going to be slightly
different from one another, so every cell that you produce is going to have a
slightly different version of each of these chromosomes. So now we have to ask
ourselves what all of these details have to do with asexual versus sexual
reproduction, and we also want to remind ourselves that about 90% of all
eukaryotic organisms are going to undergo sexual reproduction at some
point during their reproductive cycle. So, there must be some benefits to doing
reproduction in this way. When an individual cell or an organism
reproduces asexually, all of its offspring are going to be clones of the
parent organism or the parent cell. Since all of the organisms are going to have
exactly the same genetic code, this works really well in a static environment
where things aren't changing very often. If the environment isn't going to change
and your genetic code is working really well for you, there's no reason to expand
extra energy to change your genetic code if things are already working out really
well for you. However, if your environment is changing pretty frequently, you run
the risk of having your exact clone offspring dying with the environment
changed to something that's less suitable. Another major perk of reproducing a
asexually is that the energy expenditure is extremely low.
You don't need to produce specialized gamete cells like eggs and sperm, you
don't need to be pregnant, and you don't need to take care of your offspring
after they've been created.
Finally, you also don't need to find a mate if you don't reproduce with another
organism. This is beneficial in a number of different ways because you don't need
to develop attractive characteristics and you also don't need to go find a
mate, which is very helpful in low population areas where organisms are
really far apart. However, not reproducing with a mate can also be a disadvantage
because you have pretty much any organism that is healthy enough to
reproduce reproducing, not just the absolute strongest best organisms. And so,
overall your population is going to stay a lot more varied in terms of how fit
the organisms actually are. The biggest advantage of reproducing using sexual
reproduction is that it introduces variability into the population before
you even combine your gametes with that of another organism. Your gametes are
going to have different genes than you yourself have because of the way that
meiosis works. Between the process of crossing over and homologous
recombination, all of your gametes are going to have slightly different genetic
codes than you have, even though they came from you.
Combining with another organism also leads to still more variation leading
into a greater variety and greater diversity of organisms in a population.
Diversity is not always going to be good, you're going to have some combinations of
genes that make the organisms less healthy and less fit, but you also have a
chance of producing better organisms over time. This is a huge advantage in an
environment that
happens to be pretty variable. For instance if a pathogen came along that
was really really dangerous to a certain kind of organism, you have a
chance to get around that pathogen if you have a slightly different genetic
code. But if all of your organisms are clones, unfortunately if one of them is
vulnerable, every single one is vulnerable. Because there are so many
more changes happening from generation to generation with sexual reproduction,
the rate of evolution actually increases. Remember that evolution is "blind" and it
occurs without a goal in mind, so some changes that occur might actually be
disadvantageous while some other changes might actually be advantageous. This is
going to result in a much more rapid accumulation of genes that increase the
"fitness" of an organism. For the purposes of evolution, we define the word "fitness"
as the ability to do two things. Number one is to survive and number two is to
reproduce. If you think about it, an organism is not useful at all of it
doesn't survive long enough in order to pass along its genes. In addition, when
you reproduce using sexual reproduction, you tend to get better quality offspring
that are more well-equipped in order to survive. However, this is definitely a
trade off because organisms that reproduce sexually tend to have fewer
offspring and it also takes much longer. In addition to the time demands of finding
a mate and maintaining the characteristics that are necessary to
attract the mate, it also tends to require a much greater investment in
terms of time and energy even after the gametes have been combined. The more
complex an organism is, the more likely it is that they will spend a long time
taking care of their offspring after they've reproduced. If you're going
to spend a lot of time taking care of your offspring, you can only have a small
number because each one of them places great demands on your time. However, if
you're going to produce clones of yourself
generally speaking those offspring don't need to spend a lot of time growing up
and needing care. OK, that
pretty much wraps up the sexual reproduction vs sexual reproduction chat
and I will see you all in class. Don't forget to subscribe. Bye!