- [Voiceover] What we're gonna talk about in this video
is what I consider one of the most fascinating subjects
in biology and that's the variation we see
from species to species in life histories
and lifespans and the rate of reproduction.
For example, we have three different species here.
On the left we have an African elephant,
and African elephant you might know,
can live a long time, especially out in the wild.
It can live many decades, even 40, 50, 60 years.
And their life history actually parallels human,
at least modern human, life history in a lot of ways.
The first 10 years of their life,
they are very dependent on their parents.
After that, they kind of enter into a bit
of an adolescence very similar to how humans do
where in theory, they could reproduce
but they don't tend to
and they are still somewhat dependent.
And then they move into a phase
when they do reproduce.
And they will reproduce on the order of
once every two to four years.
A female African elephant will reproduce.
Their gestation periods, the amount of time the baby
elephant will be in the mother's womb is on the order of,
it's actually longer than, for humans.
Humans you probably know is nine months,
for an African elephant it is 22 months.
And so because of that,
they can reproduce about once every 2 to 4 years.
Now, another example, and these are --
actually elephants and rabbits might not look
closely related to you, but they are actually
pretty closely related if you think about
the entire tree of life.
They are both mammals and actually everything
we're considering here are animals.
We're going to consider African elephants, rabbits
and we're going to consider salmon.
But what I'm talking about about applies to all life.
It applies to bacteria, it applies to trees,
there's a huge variation in their fecundity,
the rate at which they reproduce.
Let me write that word down.
Fecundity.
Fecundity, the rate at which they reproduce,
and also variation in their actual lifespan,
whether we're talking about a tree
or a bacteria, or a fish, or a mammal.
Just going from one mammal to another
let's go to a rabbit, and depending on which type
of rabbit you're talking about, but a rabbit could,
lifespan is in the single-digit years.
But unlike an elephant, an elephant,
the first 10, 15, 20 years of their life, they
aren't in that reproductive phase of their life.
A rabbit enters into that reproductive phase
of their life within several months,
within four, five months of birth.
And so, once they enter into that reproductive phase
and I'm showing the reproductive phase in magenta here,
they can reproduce a lot.
They have high fecundity, they have very high
reproductive rate.
Every time a female rabbit has a litter,
it can have many, many baby rabbits in it.
The numbers I found were one to 14, one to 14 rabbits.
And not only can they have one to 14 rabbits
every time they have a litter, but they can have
they can do this, on the order of once a month.
So every, every month.
So even though the lifespan of the female rabbit
depending on which type of rabbit you're talking about,
it might be three, four, five, six years,
depending on the type of rabbit you're talking about.
You can imagine, if they're producing let's say,
10 rabbits every month per year, they could produce
120 rabbits, or, if they can produce 10 rabbits per month
12 months a year, that's 120 rabbits a year
over several years.
And then you can imagine those rabbits,
very quickly, the female ones, if you assume roughly
half of them are female, that half
can very quickly get into that reproductive phase
and start reproducing at a similar rate.
So, on an individual level
a female rabbit has high fecundity,
and on a population level, that group of rabbits
will also have very, very high fecundity.
And then we can look at another example.
And this is the example of salmon
and there are many types of salmon
but the general way that salmon,
the general life cycle that salmon go through
is they are born then and they are usually born
up some stream and usually some water
where there isn't a strong current,
and then once the baby salmon are born,
and they can be born in groups of hundreds
or thousands, they make their way down that river
down that stream, into the ocean and then they have
many years of a growth phase in the ocean
where they get larger and larger.
They're not reproducing then
and then when they are ready to reproduce
they fight their way back up the same stream
that they were born in or the same river
that they were born in, they fight their way
back up to it they reproduce, and this is
both the males and females.
The males fertilize the females produce the eggs,
the males fertilize the eggs and then they die.
So they have one reproductive event.
So you have one reproductive then death
and then they kill, they just die.
People still understanding,
why exactly does this happen?
So one reproductive event, reproductive event
and then they die.
And there's actually a technical term for species
that do this.
Salmon isn't the only one where they have that one,
where they go out with you can kind of view it as
a big bang, where they have that one reproductive event
where they might have hundreds or even thousands of eggs,
but then they die.
This is called semel parity.
Let me write this down.
So this is called semel parity.
Semel comes from the Latin for once.
Parity comes from the Latin for to beget so,
to beget once.
You're reproducing once, and then in the case of
salmon, you are are dying.
And you might say okay, if that's semel parity
what would we call an elephant or rabbits,
for sure, and an elephant as well.
They can have multiple reproductive events.
Well, that is called itero parity.
Itero, itero parity.
You might've heard the word iterate
that means to repeat something
or to do something over and over again.
Itero is the root for, it means repeat.
So, itero parity, beget repeatedly.
And so that's what animals like elephants and,
for sure, rabbits are actually doing.
And what's fascinating about all of this
and it is a question that I have wondered
many, since I first realized when I was young
was, wow, why is there so much variation here?
Is, why has nature selected for or why have
these species found niches in which they can operate
in which it makes sense, where natural selection
has selected for these very different lifespans,
these very different reproduction rates
this variation in fecundity,
this, you know sometimes itero parity,
sometimes a semel parity.
And it is a bit of, it's not a mystery,
people are studying this and they have
good hypothesis, but we don't know for sure,
especially from species to species.
And a framework you could use
to think about it
is a species,
they're trying to optimize survival
and not even of the individual,
they're trying to optimize survival of
really their genetic information.
It's not like the species or the genes are
actively trying to do it, but natural selection
is doing that for them.
So let's call this box natural selection.
Natural selection,
and so, and what you have
coming out of this is the fittest,
fittest genes.
And when we talk about fittest
genes, were not talking about
somehow that some are better
than others, we're just saying for that environment,
the ones that seem, the genes that produce the traits
that are most suitable to survival
and most suitable towards reproduction
and then the inputs that are
going into this natural selection box
are things like availability of energy,
of food, of what I called free energy.
Availability, because it's not just obviously plants
can get that free energy from the sun.
Availability, availability of energy,
we can talk about the
predatory environment,
predatory, predatory environment.
We can talk about disease, disease.
Every moment that an organism is alive
it has to worry about these things.
It has to worry about finding food
or competing for food.
It has to worry about predators,
it has to worry about disease.
And once again the individual organism
is not sitting there, it's not necessary
that these salmon are like,
"Oh, I hope I don't catch a disease,"
or they might not even be stressed about the bears
that might try to grab them as they go upstream
but these are the factors that play into how
or what gets selected for I guess
is that the best way to phrase it.
And in terms from a species point of view
the various styles will these are things like
reproduction, like what is the species decide to do
given these constraints?
And so the various styles are fecundity,
actually, let me write this,
rate of reproduction, age of reproduction
and these are related, age of reproduction
things like lifespan and these are all related
in some way.
Lifespan, growth, growth, health.
And a species and an organism is
making trade-offs all the time.
You know, the salmon goes through that huge phase
where it's deciding to apply most of its energy
towards growth and survival, and then all of a sudden
it kicks into another gear, where it actually
uses a lot of energy that was stored up
to go upstream and goes into a reproductive phase
and then and then it dies.
And natural selection has, this has happened
arguably because that somehow helps
the salmon's DNA to spread more.
Maybe somehow it adds nutrients to the water
or you know, they put all that energy to go upstream
so that their offspring will have an easier time
going downstream.
But there's also other trade-offs.
You could have things that lay,
a salmon might, a female salmon might
lay thousands of eggs but very few of those
actually make it, make it through the full cycle.
The estimates that I've seen is that
out of those thousands of eggs that get laid
only about three make it back.
This was the example I saw for sockeye salmon,
on average only three of them make it back
for the next year.
So you have a huge amount of, I guess you could say,
attrition, while in the case of an elephant,
they invest more per offspring
and you have a much higher probability
that each of those offspring will survive.
So there's all sorts of interesting trade-offs
to think about when you think about life history,
lifecycle, lifespan and things like fecundity
and how organisms reproduce.