Life History - Online Tutor, Practice Problems & Exam Prep

Life history in organisms involves strategic allocation of limited energy, resources, and time, impacting traits like survivorship, fecundity, and growth. Survivorship refers to the proportion of individuals surviving to a certain age, while fecundity indicates reproductive capacity, often measured by viable offspring produced. A key trade-off exists between high survivorship and low fecundity, exemplified by fruit flies and African bush elephants. Additionally, reproductive strategies are categorized into semelparity, with one massive reproductive event, and iteroparity, involving multiple reproductive events throughout an organism's life.

Life History

This video, we're going to introduce life history. And so what's important for you to realize is that energy, resources, and time are all very limited. And so, living organisms throughout their entire lifetimes must strategically allocate them and make fitness trade-offs. This leads us to the term life history, which encompasses a wide variety of aspects and can be used to refer to essentially any individual trait, strategy, or trade-off impacting an organism's survivorship, fecundity, or developmental growth throughout that organism's entire lifetime. Survivorship, fecundity, and growth are just a few of the key aspects that are typically focused on when we discuss life history, but there are other aspects of life history as well.

And so, really, you can think of this term life history as the organism's entire life story. Now most of us know what we mean when we say growth, but what do we mean when we say survivorship and fecundity? Well, the term survivorship is not to be confused with the lifespan of an organism or how long the organism is expected to live. Instead, survivorship refers to the proportion of individuals in a population surviving to a given age, and is essentially the opposite of mortality. And so, mortality is the proportion of individuals dying at a given age.

Now the term, fecundity, simply refers to the capacity or ability for organisms to reproduce, and fecundity is often expressed as the average number of viable offspring produced per reproductive event or produced per lifetime. A big takeaway of this video is for you to realize that one of the most important life history trade-offs that all living organisms need to make is between survivorship and fecundity. An organism having high survivorship comes at the cost of having low fecundity and vice versa. For example, fruit flies have really high fecundity as a single female fruit fly can produce between 40 and 100 offspring in its lifetime. That's a lot of babies.

But, again, having high fecundity comes at the cost of having low survivorship, and so only a small fraction of all of those babies are expected to reach adulthood and live the vast majority of their lifespans. Often, but not always, low survivorship is correlated with having a shorter lifespan. And indeed, that's the case with these fruit flies as they're not expected to live more than about a month or so. On the other hand, these African bush elephants have very high survivorship in comparison, meaning that a large portion of their offspring are expected to survive to adulthood and live the majority of their expected lifespans. But again, having high survivorship comes at the cost of having low fecundity.

And so, these African bush elephants only produce about 4 to 6 offspring throughout their entire lifespans, which is way less than the 100 produced by these fruit flies. Again, high survivorship is often, but not always, associated with having longer lifespans, and indeed, that is the case with these African bush elephants as their lifespan is about 70 years or so. And so, really, all that we're saying here is what is seen here in this graph, which is that there is a trade-off between fecundity and survivorship, as you can see by this trend line here, where most living organisms represented by these yellow circles fall on this trend line. Organisms that have really high survivorship and really high fecundity are very rare, to say the least. This here concludes this video, and I'll see you all in our next one.

Life History Example 1

So here we have an example problem that asks, which of the following is least likely to occur in nature? And we've got these four potential answer options down below. Now to solve this problem, we'll need to recall from our last lesson video that most living organisms need to make a trade-off between their fecundity and their survivorship. And so having really high fecundity comes at the cost of having really low survivorship and vice versa. And so with that in mind, notice that answer option a says, an organism that can produce hundreds of offspring per year and live up to 80 years old.

Now, at first glance, this might look like a really tempting correct answer option. However, it's not actually correct, because although producing hundreds of offspring per year is associated with high fecundity, living up to 80 years old actually reveals nothing about the survivorship. Instead, it only reveals information about the lifespan. And so recall once again that there's not a trade-off between fecundity and lifespan. The trade-off is between fecundity and survivorship.

And so what we're saying is that answer option a is very possible to occur in nature or possible to occur in nature, and for that reason, we can cross it off since we're looking for the option that is least likely to occur in nature and there is another option that is less likely to occur. Now answer option b says, an organism that produces fewer than 10 offspring per year and can live up to 20 years old. And once again, we're comparing the fecundity of this organism to the lifespan of the organism, and there's no trade-off between the two. So option b is a possibility in nature and for that reason, we can cross it off. So now we're between either option c or option d, which are both finally comparing fecundity to survivorship.

And what you'll notice is that answer option d says, an organism that produces fewer than 5 offspring per year, which is a relatively low fecundity, where 90% of whom survive to adulthood, which is a really high survivorship. And that's exactly what we would expect to see in nature. So for that reason, we can eliminate answer option d, since again we're looking for the option that is least likely to occur. And so this leaves answer option c as the only option and it is the correct answer, which says, an organism that produces hundreds of offspring per year, which is again associated with high fecundity, where greater than 50% of whom survive to adulthood, and that is a high survivorship, and that is unexpected in nature. And so that is why answer option c is least likely to occur.

So we can go ahead and indicate that answer option c is the correct answer to this practice example, and I'll see you all in our next video.

Life History: Population Reproductive Strategies

This video, we're going to talk about another aspect of life history, which is population reproductive strategies. Different populations of species actually vary in how often they reproduce, leading to these two categories, semelparity and iteroparity. The word semelparity has the root semel in it, which means once, and the root parity, which means to produce. It's no surprise that organisms that exhibit the semelparity reproductive strategy have just a single or one-time massive reproductive event often with 100 or thousands of offspring, after which the organism typically dies. Notice that down below in the image on the left-hand side, we're showing you an example of an organism that exhibits semelparity, which is the century plant.

Notice that in all of these graphs, we have the lifetime of the organism on the x-axis and the number of offspring that the organism produces throughout its lifetime on the y-axis. You'll notice that the century plant lives most of its life without reproducing at all, and then some point towards the end of its lifetime, it will have a single and one-time massive reproductive event producing 100 or thousands of seeds, after which the century plant shortly dies right after. Iteroparity, on the other hand, has the root itero in it, which means to repeat, like the word iterate means to repeat. Organisms that exhibit the iteroparity reproductive strategy have repeated reproductive events or, in other words, multiple reproductive events throughout the organism's lifetime. Iteroparity can either be seasonal, meaning that the reproductive events only occur during distinct breeding seasons, or iteroparity can be continuous, meaning that the reproductive events can pretty much occur at any time.

Notice that for seasonal iteroparity we're showing you the American Robin, and notice that it has indeed multiple reproductive events throughout its lifetime, and those reproductive events are evenly spaced apart from one another because the reproduction occurs only during distinct breeding seasons, in this case only during the spring season if you will. You'll also notice that the number of offspring is much lower than the number of offspring produced by organisms that exhibit semelparity. Now, with continuous iteroparity, we're actually showing you this red howler, right here, this monkey, and what you'll notice is, again, it has multiple reproductive events, but they're not evenly spaced apart from one another. That means that the reproduction can occur pretty much at any time as soon as the organism becomes reproductively capable. This here concludes our lesson on population reproductive strategies, and I'll see you on our next video.

Life History Example 2

So here we have an example problem that wants us to complete the table down below by filling in the blanks, and notice that this table down below is comparing semelparity on the left-hand side to iteroparity on the right-hand side. And so, of course, recall from our last lesson video that organisms that exhibit the semelparity reproductive strategy will reproduce only once in their entire lifetime, producing many offspring, usually between 100 or 1000 offspring. And usually, most of the offspring will die before adulthood because those offspring are typically left to survive on their own and they don't get much parental care. And so an example of an organism that exhibits semelparity is this cicada that you can see here in this image. Now, on the other hand, organisms that exhibit the iteroparity reproductive strategy will reproduce repeatedly or multiple times throughout their lifetime, and they will typically produce fewer offspring at a time in comparison to organisms that exhibit semelparity.

And also, the offspring of organisms that exhibit iteroparity are usually more likely to survive to adulthood because those offspring may be cared for by their parents. Now, this parental care is certainly species-dependent, but, again, it tends to be the case for iteroparity more so than for semelparity. And so an example of an organism that exhibits iteroparity is this elephant that you can see here in this image. So that concludes this video, and I'll see you in our next one.

Which of the following is an example of semelparity?

• A. An agave plant which stores energy throughout life, then releases many seeds before dying.
• B. Fruit flies, who can lay hundreds of eggs per week once they reach maturity.
• C. A female labrador who has 3 puppies one year, then another 5 puppies the next year.
• D. None of the above.