Old ideas about aging gain new respect

The first new theory for the evolution of aging to come along in thirty years appeared in the March issue of Evolutionary Ecology Research. Current theories are based on the idea that aging could not logically evolve on its own, so it has come to us on the coattails of some other powerful incentive – most probably fertility. But these theories have run afoul of reality, in the form of recent experiments. The new theory, by Josh Mitteldorf of the University of Arizona, models aging as an adaptation in its own right. Mitteldorf presents a computer model in which aging is selected for its role in taming unsustainable population growth.

The evolution of aging has posed a conundrum to theorists for over a century. Evolutionary theory seeks to explain adaptations in terms of their effect on an individual’s fitness: An adaptive gene is one that enables an organism to leave more offspring, so that more copies of the gene are put in circulation. Since aging has only a negative effect on individual fitness, theorists have assumed that it can’t be an adaptation. The best-accepted current theories posit that aging is a side-effect of other genes that are adaptive. Aging is explained as part of a tradeoff, or devil’s bargain: The body is literally going for broke, trying to reproduce as fast and furiously as possible, and giving up its own life in the process.

But here is where the story gets interesting. The classic theory predicts that fertility and longevity should be locked on a see-saw. The only way to increase life span is to sacrifice fertility, and vice versa. But recent experiments have found no such link. In fact, it is easy to breed animals that live longer and bear more offspring. Stranger yet, there are whole families of genes that regulate life span, and these genes are as old as animal life, having been preserved intact by evolution. In other words, natural selection seems to be treating these genes as if they had a purpose, even though their only effect is to destroy the body.

Mitteldorf’s theory is a response to this dilemma. Aging is taken to be an adaptation, but one that benefits the community, not the individual. This is “group selection,” and most theorists have been skeptical that evolution can work this way. Group selection is too slow and inefficient, they say, to compete with individual selection.

But Mitteldorf claims that population crashes constitute an exception to this rule. Animal communities can’t afford to go consuming food and reproducing as fast as possible – they would end up starving their own children. This leads to a mechanism of group selection that is swift and ruthless.

Aging has evolved as one way to limit population growth. Mitteldorf calls it the “Demographic Theory of Aging.” It is the first theory to regard aging as an adaptation since August Weismann abandoned his ideas in the 1890’s; and it invokes a different mechanism from Weismann’s, one that is stronger and more efficient.

These evolutionary arguments are more than academic exercises, because evolutionary theory plays a role in guiding medical research. For decades, evolutionary theorists have warned that treatments to combat aging may be too difficult to develop, because so many of the body’s systems fail simultaneously in old age. But suppose these systems aren’t just wearing out – suppose that aging is really like an organized program of self-destruction. Then it may be controlled by just a small number of master genes, some of which have already been identified. To thwart the action of a particular gene pathway is something that pharmaceutical researchers have done in the past with great success. Blocking our aging genes may be a manageable task. In fact, some upstart pharmaceutical companies are already picking up on this idea.

http://mathforum.org/~josh/LogiSen-EER.pdf

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