Sex.
If you’re a human, this word can mean a whole lotta things.
When it comes to life on Earth, however, sex is one of many processes that lead to more
life on Earth.
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A more widely applicable term is “reproduction”, and sexual reproduction is just one version.
Reproduction can be broadly defined as the process of creating new organisms from already
existing ones.
We usually call the new organisms offspring, and the already existing ones parents.
First of all, let’s answer the why.
Why do organisms reproduce?
Well, this one is pretty straight-forward, but still worth mentioning.
Organisms reproduce because there’s this crazy, ingrained need for life on Earth to
make more life.
Weird, right?
Let’s get specific about this making more life idea.
It’s really about passing on DNA, because that’s the only thing that really goes directly
from parent to offspring during reproduction.
There are two main types of reproduction: sexual reproduction, and asexual reproduction.
The difference between these two is how many individuals contribute DNA to the offspring.
With asexual reproduction, an individual produces offspring without another individual of that species.
There’s only one set of DNA involved, so all of the offspring are technically clones
of the parent.
With sexual reproduction, the genetic material of two individuals combines to make offspring
that have a unique set of DNA.
So, like, why actually have sexual reproduction?
Why doesn’t everything just make clones of itself?
For sexual reproduction to have come along, and stayed, it must have been advantageous
in some way.
It turns out, the biggest advantage of sex is genetic diversity.
Genetic diversity is lower for species that produce asexually since offspring are clones
of the parent.
These species typically have a ton of offspring and have to rely on mutation and sheer numbers
to provide genetic diversity.
Mixing and matching DNA by having sex, however, is a straightforward way to increase genetic diversity.
This type of diversity is the driving force of evolution- having variety means that if
an environment changes, there will be some individuals in the species that will thrive
instead of die.
And that is a major advantage.
Alright, so what are the drawbacks?
Well, the physical act of sexual reproduction requires energy.
Not only that, but you have to find a mate.
And once you do, the distraction of actually reproducing can make the individuals vulnerable
to things like predation.
Risky, but hey- it’s worth it for that lovely, adaptive genetic diversity.
Some species can actually reproduce asexually and sexually, and the times when they chose
to do either method reveal the tradeoffs between the two.
When environmental conditions are optimal- like having abundant food, good shelter, lack
of diseases, optimal climate and pH- species like aphids and some starfish will use asexual
reproduction to make a lot of offspring really quickly that will all be adapted to that environment.
Can you imagine that?
Being like, “hey everyone, we’re sorta killin it right now so we should probably
all clone ourselves and keep up the party!”
Now that we’ve looked at the pros and cons of sexual and asexual reproduction, we should
check out the different ways that species do their thang.
If you’ve never really thought about sex outside of the human world, then hold on tight:
life on Earth can get really wild when it makes more of itself.
Let’s explore some of these methods:
We’ll start with asexual reproduction- that’s the one that only requires a single organism.
One way that organisms reproduce asexually is by using a process called “fission”.
We’re going nuclear!
In a type of fission called “binary fission”, a parent cell literally splits into two new,
identical cells.
For single-celled organisms, the splitting of one cell is the splitting of the whole organism!
This is the way that most bacteria reproduce, as well as some single-celled non-bacteria
like amoebas.
“Budding” is another type of asexual reproduction, and it happens when an outgrowth of an individual
separates completely and becomes its own individual.
This happens a lot in corals and hydras.
With hydras, the little outgrowth, or bud, actually fully matures before breaking off
from the main body.
So what about a version of fission where there’s a big difference in size of the resulting offspring?
This is called “fragmentation”, and you can think of sea stars as an example.
If a sea star has an arm break off, that arm can actually regenerate into a new sea star.
Talk about a rebound!
The last type of asexual reproduction we’ll discuss is called “parthenogenesis”.
This is when a sex cell, called an egg, grows into a completely mature adult without being fertilized.
No big deal, just, y’know, did it on my own.
Bees use parthenogenesis to their benefit.
This process allows the creation of male drone bees and female worker bees.
When an egg is actually fertilized, this egg will usually become the queen.
There are vertebrate species that reproduce through parthenogenesis as well.
These include some reptiles, amphibians, and fish.
This leaves us with sexual reproduction, which, I gotta be honest, is kind of boring once
you’ve gone on a fission and parthenogenesis trip.
With sexual reproduction, two reproductive cells from different individuals combine to
make a unique third individual.
But wait - there’s more!
There’s actually a version of sexual reproduction that includes only one individual.
This version is called “hermaphroditism”, and it happens when individuals have both
versions of reproductive parts.
That means they can have sex with other individuals, or can self-fertilize.
Basically, in situations where they can’t move much, these individuals can have sex
with themselves.
Way to go, snails!
Most of the examples we’ve used here were for animals.
But what about the other kingdoms of life?
Bacteria will undergo binary fission and budding, which are both types of asexual reproduction.
But check this- in some cases, bacteria will actually reproduce sexually, and exchange
genetic information.
This genetic recombination can happen by “conjugation”, which is when two bacteria join up through
a special tube called a “pilus” or by “transformation”, which is when a bacteria
picks up and incorporates DNA from the remnants of dead bacteria in the environment.
There’s even a special process called “transduction”, which involves a special type of virus called
a “bacteriophage”.
In transduction, the DNA from one bacteria is taken over and injected into another bacteria
by the bacteriophage.
Sort of like a bizarre, vaccine version of trading genetic material.
Fungi reproduce asexually by budding, fragmentation, and spores, but they can also reproduce sexually
when environmental conditions call for it.
If you thought some of those were weird, just imagine plants.
They can’t even really move!
This has led to some really neat innovations in the plant bedroom department, but we’ll
save those for our plant-themed month.
Thanks for watching this episode of Nature League.
There are a lot of different ways that life makes more life, and we’re excited to explore
this theme more in the coming weeks.
If you’d like to learn more about human sex, check out our sister channel “Sexplanations”
with Dr. Lindsey Doe at https://www.youtube.com/sexplanations.