Genetics and lifespan

The New York Times is carrying an article by Gina Kolata that discusses research on genetics and longevity. She has quotes from several big figures in the field -- Vaupel, Finch, Christensen -- and it's a good article.

The main idea is that lifespan is weakly heritable:

Now, Dr. Christensen and his colleagues have analyzed the data. They restricted themselves to twins of the same sex, which obviated the problem that women tend to live longer than men. That left them with 10,251 pairs of same-sex twins, identical or fraternal. And that was enough for meaningful analyses even at the highest ages. "We were able to disentangle the genetic component," Dr. Christensen said.
But the genetic influence was much smaller than most people, even most scientists, had assumed. The researchers reported their findings in a recent paper published in Human Genetics. Identical twins were slightly closer in age when they died than were fraternal twins.
But, Dr. Christensen said, even with identical twins, "the vast majority die years apart."

Basically, the argument is that death is complicated -- lots of "random" events can cause it:

The likely reason is that life span is determined by such a complex mix of events that there is no accurate predicting for individuals. The factors include genetic predispositions, disease, nutrition, a woman's health during pregnancy, subtle injuries and accidents and simply chance events, like a randomly occurring mutation in a gene of a cell that ultimately leads to cancer.

Of course, it can't be only that. After all, stature (one of the strongly heritable traits used here as a contrast) is likewise determined by disease, nutrition, mother's health, subtle injuries, and so on. It is not only the complexity, but also the size of the random component. Essentially, what is left to kill people are all the things that genes can't avoid very easily. This includes stuff that is fundamentally rare (so strength of selection is slight) as well as stuff that feeds on complexity (with multiple subsystems that can break).

There is also an interesting bit about the heritability of deaths in given age intervals -- apparently there is no significant heritability for deaths under 60. Of course, that won't be true for societies where disease causes a significant number of deaths under 60. Sure in Western Europe where you have basically sporadic cancers, heart disease and accidents, the heritability will be zero. But throw in malaria and a few disease resistance alleles, like sickle-cell, and the story will be quite a bit different. So part of the story is that we have done so well eliminating some of the main causes of mortality that interact significantly with a person's genetics.

But as the article points out, the kinds of quantification that work well for classes of people don't work so well as applied to individuals -- even identical twins. And there is some confusion there, since "heritable" doesn't mean the same thing as "identical in monozygotic twins." If you're looking for articles on genetics to give out to students, this is a good one for raising points for discussion!