14.11–14.13: Ecology influences the evolution of aging in a population.

Protected from predators by their quills, North American porcupines are extremely long-lived.
14.11: Things fall apart: what is aging and why does it occur?

Picture your grandparents or people in their seventies or eighties. Clearly they have aged. But what exactly does that mean? And does it differ from the way that people in their thirties or forties have aged? Sometimes aging involves sagging skin. Sometimes it involves weakened muscles and bones. Sometimes it involves senility.

The difficulty comes when we realize that no two people age in exactly the same way, yet without fail everyone ages (FIGURE 14-18). So we must retreat to a somewhat vague definition. For an individual, aging is the gradual breakdown of the body’s machinery. With this definition, we see how each individual can experience aging differently. But aging can be seen much more clearly by examining a population as a whole.

Figure 14.18: What does “aging” mean?

From a population perspective, aging emerges as a definitive and measurable feature: it is simply an increased risk of dying with increasing age, after reaching the age of maturity. For example, among humans, people are more likely to die between the ages of 70 and 71 than between 60 and 61. Similarly, they are more likely to die between the ages of 44 and 45 than between 34 and 35. The causes of death at these ages often differ, but the shuffle toward death becomes more and more dire, with no respite. This is the most useful definition of aging for ecologists, and it doesn’t apply only to humans. Aging can be measured and assessed for any species in the same way.

Jeanne Louise Calment lived for more than 122 years, the longest life span ever documented for a human. While this is spectacularly long relative to most organisms on earth (and is significantly longer than the average human life span in the United States, 78 years), it pales in comparison with some of the longest-lived organisms. Bristlecone pine trees are the longest-lived, surviving for thousands of years. The growth rings of one bristlecone pine showed that it was more than 7,000 years old!

Among animals, lobsters and quahogs (a type of clam) are the longest-lived invertebrates (up to 100 and 200 years, respectively). Long-lived vertebrates include striped bass, which can exceed 120 years, and tortoises, which can reach 150 years. Toward the other end of the spectrum, rodents may live just a couple of years, and fruit flies and many other insects generally live and die within the span of a few weeks. Interestingly, bird and bat species live 5–10 times longer than similarly sized rodents. These data prompt the questions: Why is there so much variation? And what controls how long the individuals of a species live? We explore those questions in the next section, but first we consider the more general question of why organisms age at all.

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“Things fall apart; the centre cannot hold;

Mere anarchy is loosed upon the world,

The blood-dimmed tide is loosed, and everywhere

The ceremony of innocence is drowned;

The best lack all conviction, while the worst

Are full of passionate intensity.”

— WILLIAM BUTLER YEATS, The Second Coming, 1919

Aging, in fact, is not a mystery. The question of why we age is one of the great solved problems in biology. Oddly, though, it may also be one of the best-kept secrets in science. Let’s investigate.

It is easier to state the evolutionary explanation for why we age than to understand it. But that’s where we start. The explanation is based on one fundamental fact: the force of natural selection lessens with advancing age. So what does this mean?

1. Imagine a mutation that causes a person carrying it to die at age 10. Will that person pass the mutation on to many offspring? Of course not. The carrier of that mutant allele will die before she gets a chance to pass it on. Alternative versions of the gene that don’t cause death will be the only ones that persist.

2. Now imagine a mutation that causes a person carrying it to die at the age of 150. Will that person pass it on? Yes! The carrier of this fatal allele will already have had children, passing on the mutation to them long before she even knows she carries it. In fact, she will no doubt die long before this mutation has the opportunity to exert its disastrous effect. The same thing happens if the mutation has its negative effect at age 100…or 70…or even 50 (though it may be the cause of death at these “younger old” ages).

Q

Question 14.6

Many genetic diseases kill old people, but almost none kill children. Why not?

Figure 14.19: Carrying a time bomb.

In the first example, we see that natural selection “weeds out” mutant alleles that cause sickness and death early in life. Those alleles never get passed on (FIGURE 14-19). No one inherits them, and they disappear from the population without fail. Individuals with the alternative versions of the gene pass on the “good” versions at a higher rate. Consequently, those “good” alleles end up as the only versions of the gene in the population. These are the genes that we inherited from our ancestors, and this is why very few people have genetic diseases that kill them in their teens. We sometimes think of natural selection as only acting positively, to increase or improve a trait—the length of a giraffe’s neck, for example. But natural selection can also select against a trait—in this case, disease.

Natural selection isn’t so effective with some other genes. The second example describes mutations that arise and only later in life make their carriers more likely to die. Examples include mutations that increase the risk of cancers, heart disease, or other ailments. Unfortunately, the carriers of these mutant alleles will already have reproduced and passed on the alleles before they have had their negative effect. In other words, it doesn’t matter whether you carry one of these mutants or an alternative allele that does not harm you, the number of offspring you produce—your reproductive output—is the same. Consequently, these mutants are never “cleaned” out of a population. The practical result is that we all have inherited many of these mutant alleles that have arisen over the past hundreds and thousands of generations.

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These mutant alleles that have their adverse health effects later in life are responsible for aging. The later in life they have their effect—and therefore the more likely it is that they are passed on to offspring—the more common they will be in the population. This is why there are many causes of death, and why it seems as if all of our bodily systems fall apart as we get older.

Q

Question 14.7

A cure for cancer may be discovered, but not for aging. Why the difference?

TAKE-HOME MESSAGE 14.11

Natural selection cannot weed out harmful alleles that do not diminish an individual’s reproductive output. Consequently, these mutant alleles accumulate in the genomes of individuals of nearly all species. This leads to the physiological breakdowns that we experience as we age.

Does natural selection act upon a mutation that causes death at age fifty? Explain.

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