Til Death Do Us Part The Evolution of Life Span

Evolution via natural selection acts to maximize fitness, and fitness is all about making sure that your genes are around in the future. That being the case, dying just doesn't seem like an especially good idea. After all, death doesn't seem to bode well for the "Get your genes to last forever" imperative of natural selection.

You may think that the simplest way to get your genes into the future is for you to exist into the future. After all, you've got all your genes; if you live into the future, your genes live into the future, too. Voilà — fitness without the rather depressing and (often) messy process of dying. But organisms don't get their genes into the future by living forever, even though they may live for a very long time. All this leaves evolutionary biologists, and not just older ones, puzzling over why things eventually (or not so eventually) die.

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Evolution has led to many types of life spans. Giant sequoias, for example, live for thousands of years, but most plants have much shorter life spans. Your pet guinea pig, with the best food and care, might live as long as 8 years, but humans live longer than that, and other animals live much longer than we do.

Why die? Trade-offs and risks

It's not hard to think of genes that definitely should increase in frequency. Imagine an animal that lived forever; reproduced early and often; and had huge numbers of offspring, all of which survived. Talk about fit! Those are some fantastic genes. But these genes don't occur in nature. Why not? Two reasons:

1 Trade-offs: Often, one thing happens only at the expense of another.

1 Risks: The longer you're around, the more likely it is that something bad will happen to you.

Farewell, sweet Harriet

Biologists were much saddened in 2006 by the passing of Harriet, age 176 (approximate). Before you run out to get whatever vitamins she must have been taking, I should mention that Harriet was a giant tortoise Charles Darwin collected from the Galapagos Islands.

Tortoise or not, Harriet showed us via her longevity that an animal can live to be 176 years old. The heart can beat that long, brain cells can think that long — it's biologically possible. But most animals (sadly, humans included) just don't bother.

150 Part lll: What Evolution Does

A common misconception is that dying is an adaptation to make room for younger, more vigorous individuals — the idea that organisms die for the good of the species. Although this theory may sound good at first, remember that selection acts most strongly at the level of individuals. In a population in which some individuals have a gene for graciously dying to make room for everybody else and other individuals don't, it wouldn't take long for the "die graciously" gene to go extinct.

Even if such a gene were good for the species as a whole, it would be bad for the individuals that had it because they'd be more likely to die, making them less likely to pass this gene on to the next generation. As a result, the gene would decrease in frequency over time and vanish from the population, along with any individuals who were dying for the good of the population. Not surprising, nature shows no evidence of a "die graciously" gene.

Trade-offs: Evolutionary cost-benefit analysis

The different life-history components involve trade-offs. Because an organism has only a finite number of resources available, it doesn't have the energy to do everything. Energy spent on reproducing, for example, is energy that can't be spent on surviving. Reproducing early and often may mean not having enough energy left to stay alive.

In this scenario, allocating lots of resource for reproducing even though it makes you die sooner would be adaptive if, by trading longer life for more offspring, the organism increases its ability to pass on its genes. The crucial point here is that the organism is getting more copies of its own genes into the next generation, not just making room for somebody else's.

Risky business

Living longer can be risky. The longer an organism lives, the greater the chance that it will become ill or get eaten by a predator. As the risk of death increases, so does the advantage of earlier reproduction, even if this early reproduction results in a shorter life span.

Think of the salmon, which jumps waterfalls and dodges bears to make it all the way upstream: She puts every last calorie into reproducing because the chance of making it up that river twice is too small to make reproducing fewer eggs the first time worth the risk. When the risk of death is higher, natural selection favors genes for earlier reproduction.

Methuselah flies: The evolution of life span in the laboratory

Laboratory experiments have shown that life-history traits — specifically, life span and metabolic trade-offs — do evolve as scientists expect. This section

Test tubes and teacups

Peter Brian Medawar was the first person to articulate the idea that allocating resources to survival instead of reproduction increased exposure to risk. Where'd he get this idea? By imagining a population of test tubes. Test tubes don't reproduce, but neither do they grow old and die. So why do you ever have to buy more test tubes? The answer, of course, is that test tubes experience accidental death: They get dropped or knocked over, shattering into pieces so that they're no longer usable.

You can say the same thing about teacups. Even though teacups have been made for thousands of years, you've probably had to buy some yourself, because they didn't all survive.

Medawar's key insight was recognizing that an organism whose strategy for making sure its genes were around in the future consisted of devoting all its energy to surviving instead of reproducing would eventually run out of luck. Even if it's possible to avoid aging, in the end the risk of death remains by means other than old age. You could get eaten, for example, or you could be crushed by a falling tree.

As an interesting aside, Medawar spent only part of his time thinking about evolutionary issues and shopping for test tubes. His primary research involved the immune system — work for which he received the Nobel Prize in Physiology or Medicine in 1960.

looks at one experiment conducted on fruit flies by evolutionary biologist Michael Rose and company. With their fruit flies, these scientists conducted two experiments that tested the following:

1 Whether aging could be postponed by strengthening the force of selection at later ages. If so, this result would provide evidence that a contributing factor of aging is that selection doesn't remove mutations that are only harmful late in life, after you're done reproducing. The mutations aren't neutral from an individual fly's health perspective, but they are selectively neutral because, by the time they rear their ugly heads, they've already been passed on to the next generation. An experiment that increases the strength of selection at later ages changes these mutations (assuming they exist) from neutral (because there isn't any selection at later ages) to deleterious (because the researchers have added selection at later ages).

i Whether metabolic trade-offs may be involved in life-history evolution.

If so, this result would show that the trade-offs are real. It makes sense that energy spent doing one thing can't be spent doing something else. The scientists hypothesized that spending all your energy on reproduction means you have less energy to devote to survival because of these trade-offs.

The art of testing evolutionary theories

Biologists often perform experiments to test evolutionary theories. People often wonder how this is possible, because they think that evolution is supposed to take a really long time. The answer is evolution can take a long time, but it doesn't have to. The trick is picking the right organisms — ones that live fast and die soon. Insects, for example, can be good experimental subjects, but elephants — not so good.

You need organisms with very short life spans so that you can squeeze in as many generations as possible. These organisms should also be small, so that it's easy to keep large numbers of them, and they should be easy to raise in a laboratory.

Fruit flies (often, the species Drosophila melan-ogaster) meet these criteria nicely.

Laboratory experiments replace natural selection with artificial selection, which means that the experimenter — not nature — decides what traits will increase an organism's chance of contributing to the next generation. This replacement is done for many generations, and the researchers track how the organisms change in response to the laboratory selection regime.

Humans have actually been using artificial selection for thousands of years. It's how we make different breeds of dogs, cows that give more milk, and roses that are more resistant to pests.

Expérimentât selection for increased fife span

To test the hypothesis that aging can be postponed by strengthening the force of selection at later ages, Rose set up 10 replicate populations of fruit flies in his laboratory. For each population, he allowed the flies to feed, mate, and lay eggs; then he transferred a sample of eggs to a new container with fresh food. These eggs hatched; the flies matured, mated, and laid eggs; and the process was repeated with each new generation of flies.

The flies Rose used had been living in the laboratory for 5 years before he started his experiment, and for those 5 years, the eggs used to start the next generation of flies had always been the ones produced on the 14th day after transfer. Females from this laboratory population lived on average about 33 days, and a fit fly was one that made lots of eggs at age 14 days. But that situation was about to change.

In 5 of his 10 populations, Rose changed nothing. New generations continued to be founded with eggs produced on the 14th day. These populations were the control populations. In the other 5 populations, Rose progressively increased the age at which the eggs used to start the next generation were collected; instead of gathering the eggs on the 14th day, Rose began collecting them on the 15th day, the 16th day, and so on. These populations were the ones experiencing Rose's artificial selection. All of a sudden, a fly wasn't very fit unless it could produce eggs at an older age; it didn't matter how many eggs a fly produced on Day 14, because none of those offspring were going to make it into the next generation.

After just 15 generations (15 transfers of eggs to new containers), Rose measured the life span of flies from the 10 populations and found that the flies from the populations selected for later reproduction lived an average of 20 percent longer than flies transferred every 14 days! By increasing the importance of events later in life, Rose had increased the strength of selection at later ages; as a result, the flies evolved to live longer.

4jtJABCi Selection experiments always compare a control group of organisms with an experimental group. The experimental group experiences the artificial selection regimen, and the control group doesn't. In all other ways, both sets of organisms are treated exactly the same; they are kept in the same lab environment, handled by the same people, and so on. This technique eliminates doubt that any interesting results are caused by the artificial selection, not by some random factor (such as how hot the lab was that summer). As a further precaution, the same experiment with the same control treatments are performed many times. In experimental science, unless something happens several times, it doesn't really happen at all.

Testing for metabolic trade-offs

With the two different groups of flies that he created in his life-span experiment, tjff Rose tested the theory that metabolic trade-offs are involved in life-history evolution. Rose and his co-workers set out to look for evidence of these trade-offs in their selected flies. Here's what they found:

^ The longer-lived flies had increased storage reserves of fats and carbohydrates compared with the control flies.

^ The longer-lived flies had lower fecundity (lifetime reproductive potential) and devoted less energy to the production of eggs than the control flies did.

Bottom line: Rose's experiment produced evidence of the predicted life-history trade-offs: The longer-lived flies spent more energy on living and less on reproducing.

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