Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [52]
As a graduate student in the 1950s, Williams had listened to his teachers explain that death existed for the good of the species. The old had to make way for the young, or else a species would become extinct. Williams thought that was nonsense. Instead, he considered how natural selection acting on individuals might create old age. Williams argued that it could be a side effect caused by genes that offered advantages in youth. As long as the advantages of these genes outweighed the disadvantages, they would become widespread. Cancer, declining stamina, deteriorating vision, and the other burdens of old age might all be the result of natural selection.
Williams argued that organisms face these sorts of evolutionary tradeoffs throughout their lifetime. How much energy should they invest in maturing before they start to have babies, for example, or how much energy should they invest in raising offspring before they search for another mate? Natural selection ought to find the balance between those demands. Williams speculated that animals could also keep track of how those factors change over their lifetime and adjust their behavior accordingly, like an investor deciding which stocks to keep or sell.
Over the past forty years, Williams’s theory has evolved into an experimental science of aging. Now scientists can predict which species will get old and why. In a 2005 study, to pick just one example from hundreds, scientists studied the sockeye salmon that return to Pick Creek, Alaska, each year. The salmon come back in July and August. Once the female salmon have mated, they select a spot to lay their eggs and dig a nest in the gravel bottom of the creek. After they lay their eggs, they cover them and guard them from other females that might want to take over the nest to lay their own eggs.
The salmon of Pick Creek face just the sort of trade-off Williams proposed. Once they leave the ocean to travel to their breeding grounds, they stop eating for good. They have only a fixed amount of energy to divvy up among the things they do before they die. The females have to put some of their energy into their developing reproductive system in order to make eggs. They can also put some of their energy into maintaining their bodies so that they will live long enough to fight off other salmon. It’s a zero-sum game.
The scientists predicted that salmon arriving at Pick Creek earlier in the season would live longer than the salmon that came later. A salmon that lays its eggs in July has weeks of battling ahead. If it puts all its energy into eggs and dies early, other salmon will take over its nest, and its genes won’t have a chance of getting into the next generation. If a late-arriving salmon invests its energy in long life, it’s wasting its effort, since it will still be alive when the rest of the salmon have died off. Late arrivers should invest in making extra eggs.
When the researchers compared early and late arrivers, they found their predictions met. The early arrivers survived on average for twenty-six days at Pick Creek, whereas the late arrivers survived only twelve days. The early arrivers put roughly an equal amount of energy into maintaining their bodies and protecting their eggs. The late arrivers put twice as much energy into protecting their eggs as into maintaining their bodies.
Williams’s predictions work not just for salmon but for fruit flies, vinegar worms, guppies, swans, humans, and many other species as well. But until recently experts on aging considered E. coli off limits. A trade-off between long life and reproduction seemed simply not to exist. E. coli did not have parents and children. An individual E. coli just duplicated its DNA and pulled itself apart into two new individuals. The parent became the children. Starvation might slow E. coli down, and chemical warfare and other assaults might kill the bacteria outright. But left to themselves with enough food, E. coli would reproduce forever, each new microbe as healthy as