From a fitness perspective, the best (that is, most "fit") reproductive scenario is one in which the organism begins to reproduce early and often, and all the offspring survive. But that's not the way it happens in nature, mainly because of the trade-offs between survival and reproduction. The resources required for survival are resources that aren't available for reproduction, and the resources necessary for reproduction aren't available for survival. So when and how often an organism reproduces, and how many offspring it produces, are the results of trade-offs that give that organism the best chance for sending its genes into the future.
You find various life histories in nature. Some organisms produce over and over again; some produce a few times throughout their lives; and some produce a single time and then die (perhaps the most extreme case of putting all your eggs in one basket!). Each species evolved in a way that allowed it to persist in its physical environment.
It's good to reproduce often, except when it's not
Organisms that reproduce several times — humans, birds, and numerous other creatures — are called iteroparous. Organisms that reproduce only once are called semelparous. Semelparity is the most extreme example of the trade-off between survival and reproduction. The salmon is one example of a semelparous organism. After swimming back to the spawning ground where it was born, a salmon devotes every last ounce of its energy to reproduction; then it dies.
Two classes of environmental conditions lead to selection favoring the "reproduce once and die" genes:
^ When getting a second chance to reproduce is pretty much impossible
^ When some particular condition of the environment makes the reproductive payoff for reproducing once and then dying much greater than the payoff for reproducing over several years
The following sections examine these scenarios.
One chance to make good
Reproducing just once before dying is the fittest thing to do when the odds of making it to the next reproductive season are very low.
After dodging all those bears and jumping all those waterfalls on the way to the spawning grounds, the friendly salmon is pretty lucky to have made it at all. If a gene appeared that made the salmon produce fewer offspring, with the expectation that it would swim back out to sea to feed again so it could reproduce more the following year, the fish would leave some descendents, but the odds of its leaving any more are fairly low. Instead, it might die as it tried to make it to the spawning grounds a second time. For this reason, this particular "produce less now so you can produce more later" gene isn't very likely to increase through time. In fact, from what scientists know about the risks facing the salmon, it makes sense that they don't see salmon with this strategy. Salmon are wonderful examples of the "reproduce once and die" strategy, but they aren't the only ones.
Lots of different organisms have a pretty low chance of making it from one reproductive season to the next. For many of these organisms, the culprit isn't anything nearly as picturesque as cascading waterfalls and hungry predators; it's the rather mundane reality of living in a seasonal world.
Imagine some tiny plant in the desert that's managed to germinate and grow during the wettest part of the year. If that plant is to have any chance of leaving descendents, it has to do it fast, while it still has enough water to survive. Because the adult plants can't make it to the hottest part of the year, they devote all their energy to reproduction, producing seeds that can withstand drought and soaring temperatures and that will germinate during the next rainy season.
Most semelparous organisms — those annual desert plants, for example — are relatively short lived. They gather the resources they need to reproduce and get on with it. Other very long-lived semelparous species could reproduce earlier but don't; they just hang out, continuing to grow and acquire resources, all the while taking the chance that they'll die before they get around to reproducing. You can see the potential down side of this strategy. So what's the advantage?
Although the details vary from case to case, the bottom line is that peculiarities in the ecology of a particular species result in a disproportionate benefit to having a big bang in reproduction that outweigh the risks. The risk is big, but the payoff is also big, big enough that the genes for hanging back and stocking up are favored.
One of the best-known examples of this pattern is the blue agave (Agave tequilana), the plant from which we distill tequila. The blue agave has a very high rate of year-to-year survival yet exhibits a life-history pattern of semel-parous reproduction. (Although most agaves behave in a similar fashion, iteroparous agaves exist as well, but they're rare.)
When the blue agave finally decides to reproduce, it reallocates all its energy to reproduction and then dies. The selective pressures on this agave are different in nature from the pressures on the small annual desert plants growing all around. The little plants have to reproduce once because they can't survive the heat of the summer. The agave, on the other hand, can survive just fine in the summer, but it still has a greater chance of getting its genes into the next generation if it devotes all its energy to reproducing just one time.
The reason has to do with a peculiarity of agave reproduction. The agave needs something that's in short supply: pollinators, which are animals that move pollen to and from other agaves so that the plant can make seeds. When an agave reproduces, it makes a flower stalk. These stalks can be huge, reaching 25 feet in height. The agave uses every last bit of energy it has to make the largest, most visible floral display possible — a good plan, given that pollinators preferentially go to the most visible spike. If your spike is only half as big as another, you get fewer than half as many pollinators.
An agave with a gene that produced the trait of making a smaller spike and surviving until next year to make another smaller spike eventually would be eliminated from the population. Although that particular agave would keep surviving, it wouldn't make as many seeds because it wouldn't be as attractive to the animals that pollinate agaves.
Early vs. later reproduction: Why wait?
An important component of an organism's life history is the way that the life span is divided between the pre-reproductive and the reproductive periods — or, in more common terms, between the time spent as a juvenile and the time spent as an adult. When scientists look at nature, they see a huge variation in the ages at which different organisms mature and become reproductive. Some organisms reproduce at very young ages; others wait until they are much older. Once again, organisms can do the same thing in many ways — in this case, producing offspring.
As I mention earlier in this chapter, you may think that the best strategy for promulgating genes would be reproducing early, making a lot of offspring, and doing all that for a very long time. But studies of the natural environment show that it's not possible to do everything well.
When it comes to reproduction and getting your genes into the next generation, it's not just how soon you can make the first offspring, but the total offspring you can produce throughout your lifetime. An organism that waits a little bit longer to get started but makes many more offspring than an organism that started earlier will have a greater effect on the genetic makeup of the next generation — that is, a higher proportion of organisms in the next generation will carry its genes.
The following list outlines the advantages and disadvantages of both strategies:
^ Reproducing early: The benefits of early reproduction seem obvious. Genes involved in early reproduction will make it into the next generation that much faster. All other things being equal, earlier reduction would always be better than later reproduction. But early reproduction creates problems, the main one being that producing early saps the energy an organism needs for survival and growth.
i Reproducing later: An organism that delays reproduction has more resources to devote to survival than an organism that reproduces early. In addition, large organisms are often much better at making offspring than smaller organisms; they can produce more or healthier offspring. So it seems that some types of organisms get a real payoff from devoting energy to growth early in life and waiting until it's more efficient to begin reproducing.
In comparing the two strategies, it certainly appears that the better reproductive strategy is to wait until the organism is large enough to make many more offspring. In turn, these offspring should overwhelm the descendents of the individuals that reproduce early, which are wasting precious energy to maximize their reproductive output. But one pesky problem occurs: During the time that an organism is devoting energy to growth rather than reproduction, it could die, leaving no descendents. So which strategy is better: early or later reproduction? The answer is that it depends.
Scientists' understanding of evolution by natural selection suggests that as the risk of mortality increases, organisms evolve to reproduce earlier. Genes connected with early reproduction are favored and are more likely to make it to the next generation. Organisms with genes that result in later reproduction have a good chance of dying before they reproduce; as a result, these genes are not favored.
ProVing the point with guppies eSiv^*^. David Resnick and co-workers set up an experiment in a natural environment tj/ to examine how altering the likelihood of surviving at different life stages could lead to evolutionary change in life-history parameters, such as when to start reproducing. Resnick conducted his experiments with naturally occurring guppies in a series of streams on the island of Trinidad. The goal of the experiment was to see whether changing the relative importance of mortality for juveniles and adults would lead to changes in the guppies' life history. Specifically, Resnick wanted to see whether guppies' reproductive patterns, such as age at maturity, would change if their risk of death changed.
The basic structure of the experiment was to figure out some way in nature to tinker with the survival chances of the fish. Resnick made use of a few things he knew about guppies in Trinidadian streams:
1 The major cause of mortality for guppies is getting eaten by two other types of fish. For simplicity's sake, I'll call these species the big predator and the little predator. Although both species eat guppies, they prefer different sizes: The little predator likes to eat little guppies, and the big predator likes to eat big guppies.
1 Different places in streams have different assortments of fish. Some guppies live in areas of a stream that have a large predator; other guppies live in areas that have a little predator; and some places in streams don't have any guppies. In other words, the distribution of fish is uneven. Some guppies live in a low-predation environment where the adult guppies are less threatened because only the little predator lives there. Other guppies live in a high-predation environment with a big predator that's very fond of eating adult guppies.
The reason for these variations is that in Trinidad, many streams run down hills steep enough to produce waterfalls. Because fish can't get up waterfalls easily, the waterfalls effectively keep them from moving from one place to the next.
i The guppies in the different habitats had different characteristics.
Resnick noticed that guppies living in the low-predation environments have different life-history characteristics from guppies living in high-predation environments:
• In the low-predation environment, guppies reproduced at a later age, when they were larger. They were able to invest energy in growth and survival that ultimately delayed reproduction — a successful strategy in the absence of a big predator. When these guppies made it to adult size, they had a reduced chance of being eaten.
• In the high-predation environments, the guppies reproduced earlier, when they were smaller. This strategy could be the result of predation pressure from the large fish. Delaying reproduction to a later date, when you're a bigger fish, isn't advantageous if you have a good likelihood of being eaten before you make any offspring. As the chance of being eaten increases, so does the selective pressure for early reproduction.
What Resnick saw was consistent with scientists' understanding of life-history evolution. As the chance of not making it to an older age increases, so does the advantage of reproducing earlier. Even better, not only were the results consistent with hypotheses about the evolution of reproductive life history, but this natural system was ready-made to test these ideas in nature.
Resnick found places in the streams that had little predators but neither big predators nor guppies. He proceeded to move guppies that had been coexisting with big predators into this new environment, where they had to contend with only a little predator. (To be sure that other fish wouldn't swim into his experimental areas, Resnick chose study locations in parts of the stream separated by waterfalls.)
Natural settings vs. labs
The advantage of conducting artificial selection experiments in the laboratory is that it's possible to control all the variables of the experiment. The experimenters can change just one thing, hold everything else constant, and then see what the result of changing that one thing was. The goal of any experiment is to change as few things as possible so that when it's time to analyze the results, a lot of confusing questions won't be asked, such as exactly what caused those results. Another advantage of the laboratory is that it is easy to repeat the experiment to make sure of getting the same result a second time. The disadvantage, of course, is that laboratory experiments are always open to criticism that whatever happened in the laboratory doesn't really happen in nature. (For an example of a laboratory experiment, refer to "Methuselah flies: The evolution of life span in the laboratory" earlier in this chapter.)
Experiments conducted in nature overcome this problem. Any effects that are measured are obviously the result of natural processes. For this reason, they are important for developing scientists' understanding of how evolution functions in the wild. It's exciting to see that over a relatively short period, natural selection can lead to changes in populations. But this advantage comes at a cost: Changing just one thing is rarely possible. Controlling for all the variables is impossible, so natural experiments will always be open to criticism that the results were caused not by whatever the experimenter was manipulating but by something else.
Which type of experiment is better? Actually, neither. Researchers need laboratory experiments in which they can alter just a few things at a time, but they also need experiments in the field, where they can make some manipulations, let nature take its course, and see what happens.
After moving the fish, he waited to see how the guppies would evolve. If the pattern of early reproduction he observed among guppies in the presence of the big predator was due to increased predation pressure, moving the guppies to the different environment might result in the evolution of later reproduction.
After 4 years in one experiment and 11 years in another, here's what he found: The populations that were moved from a high-predation environment evolved their life-history characteristics in the expected direction. That is, the fish devoted more energy early in life to growth and less to reproduction. As a result, they were larger when they first reproduced.
Determining whether the changes were heritable tt^HSil To determine whether the changes were heritable — that is, genetic and therefore capable of being passed from one generation to the next — and not the result of some environmental factor, such as different food or different water chemicals in the various streams, Resnick took back to his laboratory some guppies from the old population that was living with the large predatory fish and some guppies from the experimental population that had been living with just the little predators. In the lab, he grew the guppies separately for several generations in identical fish tanks, to eliminate any effects due to the environment rather than the fishes' genes.
What Resnick found was that the fish really were genetically different. The fish from the low-predation experiment really had become different genetically from the original population. They spent more energy growing and reproduced later than the original population. The fish had indeed evolved.
One fish, two fish, small fish, adieu fish: The evolution of overfishing
Understanding life-history evolution can help researchers understand some of the changes in major fisheries, where commercial fishing has made humans the major predator.
Different kinds of fish grow to different sizes, based on what's most advantageous in their environment: Tuna get really big; trout so big. A fish's life history involves growing for a while and then spending energy on reproduction. Natural selection favors variants that get the details right: Reproduce at too early or too late an age, and you won't be as successful as the fish that reproduces at just the right time. So the basic strategy is to grow, grow, grow until the point when it's better to start reproducing rather than growing some more. Bottom line: Fish in nature grow to some particular size because that's a good size to be as far as getting your genes into the next population.
Now along comes the fisherman, who sets out some fishing nets to catch fish. These nets aren't designed to catch just any fish, but bigger fish, and they're so effective that that they often cause a noticeable decline in the numbers of fish in the ocean. The fish numbers become so depleted that the fisheries stop or reduce their operations, with the intent of giving the fish stocks a chance to recover. Sounds all well and good, but guess what? The stocks don't recover as quickly as expected. Even after a lull, the big fish that the nets catch just aren't there.
Why not? What's happened is that a new selective force has been added to the environment. All of a sudden, being a fish of the size that gets caught in those nets is bad. What used to be a good size for getting your genes into the next-generation is now a good size for getting your genes grilled and covered with a nice lemon dill sauce.
Scientists and conservationists hypothesize that, in the past, before commercial fishing, genes for reaching adulthood at a smaller size hadn't been favored. To best survive a life in the ocean, fish evolved to grow to a sufficient size and then reproduce. Commercial fishing add a new selective pressure that makes it better to be a smaller fish. Their nets don't just physically remove the big fish from the population, but they also remove the genetic variants that result in larger fish, leaving only those that don't grow so big. Although being smaller may not be great as far as life in the ocean is concerned, it's the best thing going. Fish that don't grow big enough to get caught in the nets will pass
<SW£y more genes to the next generation. The result is a population of fish that just doesn't get so big. They reach adulthood at a smaller size and start devoting energy to reproduction. Just because we stop fishing doesn't mean that all the little fish that are left will grow up to be bigger fish; they've already grown up. And this is an explanation for why fisheries stocks don't rebound when we reduce the fishing pressure.
Evidence from natural fisheries suggests that the fish humans harvest have indeed shifted to earlier reproduction and smaller adult size. Northern cod populations, for example, exhibited a decline in the size at which females became reproductive before the collapse of the fishery.
To test the hypothesis that fish will evolve to be smaller if mortality at large sizes is increased, Matthew Walsh and coworkers conducted an experiment using laboratory populations of Atlantic silverside. They set up three different treatments and harvested fish differently in each one. From the control tanks they took a random sample of fish, from the other tanks they took large individuals — a protocol that mimics fishing. (To find out about the third treatment, read the sidebar "Fishin' for a small one.")
All the populations started out exactly the same — a bunch of wild fish in tanks. Over five generations, the fish populations responded to the new selective pressure. The results were exactly what the researchers had predicted. When large fish are more likely to be harvested from a population, the advantage of alleles that cause fish to grow large decreases. These alleles are less likely to make it into the next-generation while alleles that result in smaller size now have higher fitness. These changes will cause the fish to be smaller, and that's exactly what happened. Selection favoring small fish over large ones resulted in fish that just don't grow as big. These changes included differences in metabolic efficiency, foraging, and even the number of vertebrae.
Fishin' for a small one
Walsh and his coworkers also had a treatment from which they removed the smallest fish, which is not something any fisherman—commercial or otherwise—ever does when actually fishing for food, but it's a nice experiment anyway because the prediction is that removing the smaller individuals would cause the fish to spend less of their lives at the smaller size. Alleles that caused the fish to grow really fast would be favored because they'd be less likely to be removed from the tank by the experimenters. It's bad to be a small fish, and the only way to not be a small fish is to grow. The result of this treatment was bigger fish.
Figure 10-1 shows the result of Walsh's experiment. The fish on the right came from the population from which the largest fish were harvested. In the center are fish from the control tank. On the left are fish that came from the population from which small fish were harvested (see the sidebar "Fishin' for a small one for details on this part of the experiment).
The results of Walsh's experiments.
Photograph taken by Stephan Munch
The take-home message is two-fold:
1 Over-fishing is bad for all the reasons we used to think it was, plus it's bad because the study of the evolution of fish suggests that fish stocks won't bounce back to include a large individuals as fast as we'd like them to if we stop fishing. We used to think that if we stopped fishing, the little fish we hadn't caught would grow up to be big fish. Now we have the added worry that we've selected for fish that don't grow up to be big.
1 The remaining fish may not be as well adapted to their environment as they would have been had they not had to respond to fishing pressure. Fish that reach maturity at the smaller size are favored when there are lots of nets, but that may be the only thing good about a smaller size. The smaller size wasn't favored before fishing, and it may just not be a very good size with respect to doing all the things fish have to do to survive in the ocean.
Just as a huge variation exists in life span and reproductive timing among organisms, a huge variation exists in fecundity and in clutch size, the number of offspring an organism produces at any one time. Trade-offs come into play here as well.
For any given amount of resources devoted to reproduction, the pie can be divided in many ways. (For fun, call this division resource partitioning among offspring.) It comes down to the question of whether a species produces lots of little offspring or fewer bigger ones. Human clutch size, for example, tends to be one, despite rare cases of twins or triplets, and total lifetime fecundity rarely exceeds ten. But a sturgeon — the kind of fish from which we get caviar — can easily make 60,000 eggs at a time. That figure makes humans look pretty pathetic in comparison until you consider that the human pattern seems to be successful; an awful lot of humans are around, and our numbers just keep going up.
What explains the huge differences in fecundity, and why haven't sturgeon taken over the world? The number of offspring produced is important, but so is the probability that these offspring survive to reproduce themselves. Any gene that results in the production of a huge number of offspring, but none of which ended up making any offspring of its own, wouldn't last very long. The gene's frequency would increase for an instant, but by the time of the grandchild generation, it would be gone.
As always, organisms go about producing fit offspring in many ways, favoring different strategies in different environments. Specific conditions favor different clutch sizes. Parental care — or the lack thereof — also has an impact, as the following sections explain.
Not all organisms have a reproductive strategy that includes parental care. Sturgeons, for example, don't care for their young. After they release their eggs, they hit the road, leaving the eggs to fend for themselves. For these organisms, the question becomes how best to use the available resources for egg production to maximize the number of successful offspring. The organism can make a few large eggs or many small eggs. Whether big eggs or little eggs are best (or the most fit, in evolution-speak) depends on the environment the eggs will face.
At minimum, the egg needs to have sufficient resources to develop into a juvenile. Although an individual offspring's survival is increased if more than the minimum resources are provided, the parents' fitness depends on producing the maximum number of surviving offspring. Bottom line: How hostile or welcoming the environment is affects how much energy the parents invest in each individual offspring. Some environments result in a little extra provisioning of fewer eggs, but when the environment is hostile enough to make it unlikely for any particular offspring to reach maturity, the parents are better off spreading the risk among many eggs rather than investing a great deal of energy in each individual egg. The result? Smaller but more plentiful eggs.
This pattern occurs in sturgeons (60,000 eggs at a time, remember?) and in the plants commonly referred to as weeds, which tend to make large numbers of very small seeds that disperse through the environment. Where the seeds end up — whether in your freshly turned garden or in the middle of the highway — is anyone's guess. The point is they're scattered in the wind and have a low probability of survival.
Oak trees employ a different strategy. Although an individual tree makes lots of seeds, these seeds are quite large by plant standards. An oak tree doesn't make the maximum number of very small seeds; instead, it prepares the seeds — acorns — with a larger number of maternal resources. Acorns don't disperse very far from their mother plant. Instead, they tend to fall on the forest floor under a tree's canopy, and they need enough energy to sprout. With a little luck, they'll have a chance of getting big enough to gather enough light to survive. If the oak tree made just very small seeds, none would survive, because they would never have the energy to get big enough in the dark canopy of the oak forest. Thus, the environment facing the offspring of the oak tree favors genes for larger seeds, whereas the environment facing the weed seeds favors small seeds.
In the cases where natural selection favors organisms that provide parental care, it doesn't make any sense to produce more offspring than the parents can care for at any one time, because the parents continue to provide resources to the offspring after birth or hatching.
A fair number of studies have been conducted on why birds lay the number of eggs they do. The naturally occurring variation in the number of eggs produced gives scientists a sense of the optimal number for any particular species.
Ideally, the birds would produce exactly the number of offspring that they could feed well enough so that the offspring would survive and reproduce, passing the parents' genes into the future. What researchers find when they look at natural patterns, however, is a point in the number of eggs produced at which laying more eggs results in fewer surviving baby birds. The number of eggs that birds lay seems to cluster around this ideal number.
For an interesting case of how parental care can limit clutch size, think back to the emperor penguin. Penguin parents care for their single egg in a unique way: Dad balances the egg on his feet to keep it off the ice while Mom goes off to catch fish. Maybe the daddy penguin could balance a bigger egg or a smaller one, but it's not clear how he could balance two at the same time. Because no nest-building materials are available in Antarctica, an emperor penguin can't have more than one egg at a time.
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