Paul Hughes writes, regaring Heinlein's "Howard Families":
> This brings up the question of how such selective breeding would be determined.
> How did they determine a persons longevity until they actually died? And once
> dead, how would you get them to breed without modern biotechnology? Unless of
> course this breeding program was started in the 20th century, and this story
> takes place several centuries hence.
Although this is not how it was done in the book, the PBS show last night on longevity suggests another way. They described selective breeding experiments in fruit flies where they were bred for the ability to reproduce late in life, rather than for long life per se. This was done by the simple expedient of keeping flies together who had hatched on the same day, and destroying eggs that were laid early in the flies lifetimes. The only eggs which are allowed to hatch are those laid by flies who can reproduce relatively late in their lives. Since the flies can only lay eggs until about 15 days this is pretty easy.
Doing this for ten years, 1000 generations, the flies now live twice as long as when they started. They can lay eggs until about 50 days old. Not only do the flies live longer, they are much hardier and can survive longer without water, or in ultra-hot environments, than control flies.
I think the show had mentioned, when discussing human centenarians, that a number of the women had had children late in life. This is generally a marker for slow aging; people who can have children late are more likely to be able to live long. So presumably Heinlein's fictional Howard Foundation would have been able to use this method to identify likely candidates, had they known about it.