Os profetas de Darwin

sexta-feira, julho 18, 2014

The New Science of Evolutionary Forecasting

Newly discovered patterns in evolution may help scientists make accurate short-term predictions.

Jonathan Losos

After comparing the DNA from different anole lizard species in the Caribbean, scientists found predictable patterns in their evolution.

July 17, 2014

Michael Lässig can be certain that if he steps out of his home in Cologne, Germany, on the night of Jan. 19, 2030 — assuming he’s still alive and the sky is clear — he will see a full moon.
Lässig’s confidence doesn’t come from psychic messages he’s receiving from the future. He knows the moon will be full because physics tells him so. “The whole of physics is about prediction, and we’ve gotten quite good at it,” said Lässig, a physicist at the University of Cologne. “When we know where the moon is today, we can tell where the moon is tomorrow. We can even tell where it will be in a thousand years.”
Early in his career, Lässig made predictions about quantum particles, but in the 1990s, he turned to biology, exploring how genes evolved. In his research, Lässig was looking back in time, reconstructing evolutionary history. Looking ahead to evolution’s future was not something that biologists bothered doing. It might be possible to predict the motion of the moon, but biology was so complex that trying to predict its evolution seemed a fool’s errand.
But lately, evolution is starting to look surprisingly predictable. Lässig believes that soon it may even be possible to make evolutionary forecasts. Scientists may not be able to predict what life will be like 100 million years from now, but they may be able to make short-term forecasts for the next few months or years. And if they’re making predictions about viruses or other health threats, they might be able to save some lives in the process.
“As we collect a few examples of predictability, it changes the whole goal of evolutionary biology,” Lässig said.
Replaying the Tape of Life
If you want to understand why evolutionary biologist have been so loathe to make predictions, read “Wonderful Life,” a 1989 book by the late paleontologist Stephen Jay Gould.
The book is ostensibly about the Cambrian explosion, a flurry of evolutionary innovation that took place more than 500 million years ago. The oldest known fossils of many of today’s major animal groups date to that time. Our own lineage, the vertebrates, first made an appearance in the Cambrian explosion, for example.
University of Cologne

Michael Lässig, a physicist at the University of Cologne, is studying the evolution of flu strains.

But Gould had a deeper question in mind as he wrote his book. If you knew everything about life on Earth half a billion years ago, could you predict that humans would eventually evolve?
Gould thought not. He even doubted that scientists could safely predict that any vertebrates would still be on the planet today. How could they, he argued, when life is constantly buffeted by random evolutionary gusts? Natural selection depends on unpredictable mutations, and once a species emerges, its fate can be influenced by all sorts of forces, from viral outbreaks to continental drift, volcanic eruptions and asteroid impacts. Our continued existence, Gould wrote, is the result of a thousand happy accidents.
To illustrate his argument, Gould had his readers imagine an experiment he called “replaying life’s tape.” “You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past,” he wrote. “Then let the tape run again and see if the repetition looks at all like the original.” Gould wagered that it wouldn’t.
Although Gould only offered it as a thought experiment, the notion of replaying the tape of life has endured. That’s because nature sometimes runs experiments that capture the spirit of his proposal.
Predictable Lizards
For an experiment to be predictable, it has to be repeatable. If the initial conditions are the same, the final conditions should also be the same. For example, a marble placed at the edge of a bowl and released will end up at the bottom of the bowl no matter how many times the action is repeated.
Biologists have found cases in which evolution has, in effect, run the same experiment several times over. And in some cases the results of those natural experiments have turned out very similar each time. In other words, evolution has been predictable.
One of the most striking cases of repeated evolution has occurred in the Caribbean. The islands there are home to a vast number of native species of anole lizards, which come in a staggering variety. The lizards live in the treetops, on forest floors and in open grassland. They come in a riot of colors and shapes. Some are blue, some are green and some are gray. Some are huge and bold while others are small and shy.
To understand how this diversity evolved, Jonathan Losos of Harvard University and his students gathered DNA from the animals. After they compared the genetic material from different species, the scientists drew an evolutionary tree, with a branch for every lizard species.
When immigrant lizards arrived on a new island, Losos found, their descendants could evolve into new species. It was as if the lizard tape of life was rewound to the same moment and then played again.
If Gould were right, the pattern of evolution on each island would look nothing like the pattern on the other islands. If evolution were more predictable, however, the lizards would tend to repeat the same patterns.
Losos and his students have found that evolution did sometimes veer off in odd directions. On Cuba, for example, a species of lizard adapted to spending a lot of time in the water. It dives for fish and can even sprint across the surface of a stream. You won’t find a fishing lizard on any other Caribbean island.
For the most part, though, lizard evolution followed predictable patterns. Each time lizards colonized an island, they evolved into many of the same forms. On each island, some lizards adapted to living high in trees, evolving pads on their feet for gripping surfaces, along with long legs and a stocky body. Other lizards adapted to life among the thin branches lower down on the trees, evolving short legs that help them hug their narrow perches. Still other lizards adapted to living in grass and shrubs, evolving long tails and slender trunks. On island after island, the same kinds of lizards have evolved.
“I think the tide is running against Gould,” Losos said. Other researchers are also finding cases in which evolution is repeating itself. When cichlid fish colonize lakes in Africa, for example, they diversify into the same range of forms again and again.
“But the question is: What’s the overall picture?” Losos asked. “Are we cherry-picking the examples that work against him, or are we going to find that most of life is deterministic? No one is going to say Gould is completely wrong. But they’re not going to say he’s completely right either.”

Read more here: Quanta Magazine