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Video transcript

- [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.
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