Organisms often seem to undergo a gradual breakdown before death — a process referred to as aging. Because aging, like death, seems to be a bad idea, you may think that natural selection would favor individuals that age less. Unfortunately (for humans), it doesn't.
The process of aging turns out to be consistent with scientists' understanding of how evolution works. You may think natural selection would eliminate genes that make people age, but natural selection won't eliminate certain classes of genes that cause aging, either because the detrimental effects of these genes don't show up until later or because the gene that causes aging offers some benefit earlier:
1 Bad genes that act later in life: Selection acts less strongly on traits that are expressed late in life. Natural selection certainly won't favor any genes that cause the aches and pains of old age, but it can't select against them either. Long before the traits appear and can be noticed by selection, the genes that control them have already been passed to the next generation.
1 Genes that are bad later in life but good when you're younger: Natural selection favors genes that have beneficial effects when the organism is younger, even if these same genes are responsible for old age. The reason? Selection is stronger earlier in life. It doesn't matter what the genes do when you're old, because by then, you've already passed them to your children.
The two classes of genes are not mutually exclusive, and evidence exists that both types of genes are involved in the process of aging. Yippee.
If you're finding it a bit tricky to keep track of all these hypothetical classes of genes, try considering more specific examples. Conduct a thought experiment: Imagine a specific gene, and then try to figure out whether natural selection will favor it, select against it, or remain neutral. If you want to try to understand aging, for example, imagine the fate of a series of genes that does exactly the same thing, but at different times in the organism's life. In this way, you can get a better handle on how the frequencies of genes that act at different ages change in response to natural selection. Start with an easy one — say a gene the causes spontaneous combustion — and let your mind play with the ideas from there:
1 Imagine a gene that causes people to spontaneously combust at age 10. What happens to people with this gene? It's safe to say that they won't be making a lot of offspring, so extremely strong selection will be against this gene. Because this gene causes death before reproductive age, it will be eliminated from the population as fast as new mutations make it appear. In other words, the genetic lifeguard says, "You're out of the gene pool!"
1 How about a gene that makes people spontaneously combust at age 150? In this case, natural selection will never see the trait. None of us makes it to 150, so no one would ever go up in flames. This gene is completely neutral, and if it appeared in one of your children, you'd never even know it. Whether or not it increases in frequency depends only on whether your child has more or fewer children. Because the "combust at 150" gene has no effect on fitness, it's selectively neutral — just along for the ride.
1 How about spontaneous combustion at 60? This, you'd notice. Would it make you less fit? Not really. By the time you're 60, you've pretty much finished passing on your genes. This gene wouldn't help you reproduce more, but it wouldn't hurt you, either — unless you happen to be one of the occasional people who burst into flames. Hence, the "combust at 60" mutation is also selectively neutral.
1 How about a gene that made you spontaneously combust at age 60 but also made you much more likely to have lots of children? This gene increases your fitness because it increases your contribution to the next generation. Natural selection causes this gene to increase in frequency.
These last two categories of genes are the kinds of genes that would be responsible for the phenomena of aging. The negative effects of these genes occur only late in life, after they've already been passed to the next generation. In the last example, selection will cause the gene to increase in the population because it's advantageous early, even though it's really bad later.
Now step back from the example of spontaneous combustion, and think about some of the more real examples of things associated with aging. Nothing about your eyes failing or your knees getting creaky is at odds with the mechanism of evolution by natural selection. By the time those phenomena start to occur, you're probably finished reproducing. There may be better ways to build an eye or a knee, but natural selection won't favor those methods unless they have value earlier in life.
One last important note: Zero evidence exists that spontaneous human combustion really happens. It's in the same category as ESP and Roswell aliens. But just coincidentally, most of the people who are said to have spontaneously combusted were older than 60.
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