Organism life history and fecundity | Ecology | Khan Academy

We're going to 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 life spans and their rate of reproduction.

For example, we have three different species here. On the left, we have an African elephant. An African elephant, you might know, can live a long time, especially out in the wild. It could live many decades, even 40, 50, or 60 years. 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. Then, they move into a phase when they do reproduce.

They will reproduce on the order of once every two to four years. A female African elephant will reproduce. Their gestation period, the amount of time that the baby elephant will be in the mother's womb, is on the order—it's actually longer than for humans. Humans, you probably know, is nine months; for an African elephant, it is 22 months.

Because of that, they can reproduce about once every two to four years. Now, another example—and these are actually elephants and rabbits—might not look closely related to you, but they are actually still pretty closely related if you think about the entire tree of life. They are both mammals, and everything we're considering here are animals. We're going to consider the African elephant, rabbit, and we're going to consider salmon, but what I'm talking 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. The rate at which they reproduce and also variation in their actual lifespan, whether you’re talking about a tree, a bacteria, a fish, or a mammal.

Just going from one mammal to another, let's go to a rabbit. Depending on which type of rabbit you're talking about, a rabbit's lifespan is in the single-digit years. But unlike an elephant, the first 10, 15, or 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 or five months—of birth.

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 a 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 1 to 14. Not only can they have 1 to 14 rabbits every time they have a litter, but they can do this on the order of once a month.

So, every month—even though the lifespan of that female rabbit, depending on which type of rabbit you're talking about, might be three, four, five, or six years—you can imagine if they're producing, let's say, 10 rabbits every month, per year they could produce 120 rabbits. Or if they could produce 10 rabbits per month, 12 months a year, that's 120 rabbits a year over several years.

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 then start reproducing at a similar rate. So, on an individual level, a female rabbit has high fecundity, and then as a population level, that group of rabbits would also have very, very high fecundity.

Then we could look at another example, and this is the example of salmon. There are many types of salmon, but the general way that salmon—the general life cycle that salmon go through—is they are born. They are usually born up some stream and usually in water where there isn't a strong current. Once the baby salmon are born—and they could be born in groups of hundreds or thousands—they make their way down that river, down that stream into the ocean.

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, and they reproduce. This is both the males and the females.

The males fertilize the females, who produce the eggs, and then they die. So, they have one reproductive event. You have one reproductive event and then death, and then they just die. People are still understanding why exactly this happens.

So, one reproductive event, and then they die. There's actually a technical term for species that do this. The salmon isn’t the only one where they have that one—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 semelparity. Let me write this down: so this is called semelparity.

Semel comes from the Latin for once; parity comes from the Latin for to beget—so to beget once. In the case of salmon, you are dying. You might say, “Okay, if that's semelparity, what would we call an elephant, or rabbits?” Rabbits for sure, and elephants as well, could have multiple reproductive events. Well, that is called iteroparity.

You might have heard the word iterate—that means to repeat something or to do something over and over again. Itero is the root for it, and it means repeat. So, iteroparity—beget repeatedly—and so that’s what animals like elephants and, for sure, rabbits are actually doing.

What's fascinating about all of this is—and this is a question that I've wondered many, you know, since I first realized when I was young that, 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 iteroparity, sometimes semelparity.

It is a bit of—it’s not a mystery; people are studying this, and they have good hypotheses, but we don’t know for sure, especially from species to species. A framework you could use to think about it is a species trying to optimize survival, and not even of the individual; they’re trying to optimize survival of really their genetic information.

That's what it’s—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. What you have coming out of this is the fittest genes. When we talk about fitted genes, we’re 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 toward reproduction.

The inputs that are going into this natural selection box are things like availability of energy, of food—I'll call it free energy availability. It’s not just, obviously, plants can get that free energy from the sun.

Availability of energy—we could talk about the predatory environment, predatory environment. We could talk about 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.

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—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 what gets selected for, I guess, is the best way to phrase it.

In terms of, from a species point of view, the various dials—well, these are things like reproduction, like what is it—what does a species decide to do given these constraints?

The various dials are fecundity—actually, let me write this—rate of reproduction, age of reproduction (when these are related), age of reproduction, things like lifespan, and these are also all related in some way: lifespan, growth, health. An organism is making trade-offs all the time.

Do you know the salmon goes through that huge phase where it’s deciding to apply most of its energy towards growth and survival? Then, all of a sudden, it kicks into another gear, where it actually uses a lot of that energy that was stored up to go upstream, and it goes into a reproductive phase, and then it dies.

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 of that energy to go upstream so that their offspring will have an easier time going downstream.

But there are 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 through the full cycle. The estimates I’ve seen is 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 are all sorts of interesting trade-offs to think about when you think about life history, life cycle, lifespan, and things like fecundity and how organisms reproduce.