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Monday, Aug. 7, 2000

NATURAL SELECTIONS

Long life versus reproductive success


Money can delay it, but nothing can prevent it. Aging is the one thing that will happen to us all, however healthy our genes or our bank accounts. But why does it happen? What does it mean?

Like most things in life, it comes down to a tradeoff: You can't have your cake and eat it too. Too much of one thing means less of another, and researchers have now confirmed that there is a tradeoff with humans and life span: The more children a woman has, the shorter her life span.

Aging is the fall in the chance of survival and reproductive potential as you get older. It happens for a number of reasons: Because "bad genes" (in scientific parlance, "deleterious mutations") turn on as we get old, and because expending energy on reproduction leaves less to be spent on repairing wear and tear as we get older.

Bad genes turn on as we get old because the strength of natural selection decreases with age. If things start to go wrong after the age of 50, that's just our tough luck, because by then reproduction has already occurred. Such late-acting errors are not as important to selection as those which affect our ability to pass on our genes.

Luckily, humans are a social lot, and look after each other even past reproductive age.

In the fruit fly Drosophila melanogaster, the model lab animal and one of the best understood organisms on the planet, there is evidence of tradeoffs between life span and reproduction in both males and females. More reproduction means a shorter life.

The converse is also true: Fruit flies artificially selected for longer life span are less fertile than normal "wild type" flies.

Fruit flies start reproducing at 12 days old, but giant tortoises wait for 30 years before they start reproduction. There's a clue to the cause of aging here: Sex (or, more precisely, reproduction) takes it out of you. But is the tradeoff theory as applicable to humans as it is to other animals?

According to new research published in the Journal of Evolutionary Biology, it is. F. Thomas and colleagues at the Centre d' Etudes sur le Polymorphisme des Micro-Organismes in Montpellier France, analyzed a huge data set detailing life spans and female reproduction in Homo sapiens from 153 countries.

They were following up work by a Dutch and an English scientist, Rudi Westendorp and Thomas Kirkwood, who looked at 1,200 years of historical data on the British aristocracy.

Westendorp and Kirkwood found that the number of children was small when women died at a very early age, and increased with the age of death, stayed about the same through the sixth, seventh and eighth decades of life, and was lower again for women who died at 80 or over. When they took into account only women who had reached menopause, who were aged 60 and over, they found that, like fruit flies, women who had more children had a shorter life span.

The French researchers found the same pattern, even when controlling statistically for factors which confuse the data. There are many factors which will influence human life span, proper nutrition and a mild climate being obvious ones. A recent U.N. report showed that the rich live longer and the poor die younger, a conclusion that many would accept intuitively.

The Human Development Report says that in the 30 countries rated as having the highest level of human development, life expectancy at birth is more than 75 years. In sub-Saharan Africa it is 48.9 years, falling to 39.1 years in Malawi and 37.9 years in Sierra Leone.

As the report makes clear, money has a strong influence on life span. But to see what else influences it, Thomas and coworkers statistically controlled for the effect of money. They also controlled for what they called the effects of history: ethnic differences between human populations. In their analysis they also standardized for other confounding factors such as religious differences, geographical differences and parasitological components.

When they'd controlled for all these things, they found that humans showed the pattern predicted by life history theory. Across those 153 countries they found the same pattern as that found in the British aristocracy and in fruit flies: a tradeoff between reproduction and longevity. Women who reproduced more had a shorter life span.

Such a pattern supports the prediction that heavy investments in reproduction divert resources away from the maintenance and upkeep of the body. As a result, in general, aging and an earlier death follow.

But it took two huge data sets -- the first from 1,200 years of data, the second from 153 countries -- to detect the link between reproduction and life span in humans. It is a confirmed link, but one which can be smoothed away by the influence of a good or bad environment.

The birthrate and the total number of births in Japan sank to record lows last year, accelerating the already widespread graying of the population, according to data recently released by the Health and Welfare Ministry.

The low birth rate in Japan and other contemporary populations masks the effects of the tradeoff between reproduction and longevity. But the effect may be stronger where the birthrate is higher. And the effect will be exacerbated where the birthrate is higher, because these countries are likely to be developing, and financially far poorer than us.

"Advances in the 21st century will be won by human struggle against divisive values," said the U.N. Human Development Report, "and against the opposition of entrenched economic and political interests."



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