News: Would you walk by on the other side?
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Mon, 06 Aug 2007 21:18:41 GMT
Would you walk by on the other side?
Martin Nowak, director of the Program for Evolutionary Dynamics at
Harvard University, has a new take on evolution: it's all about
cooperation
By CARL ZIMMER
NY Times News Service, New York
Sunday, Aug 05, 2007, Page 19
http://www.taipeitimes.com/News/feat/archives/2007/08/05/2003372873
When Martin Nowak was in high school, his parents thought he would be
a nice boy and become a doctor. But when he left for the University of
Vienna, he abandoned medicine for something called biochemistry. As
far as his parents could tell, it had something to do with yeast and
fermenting. They became a little worried. When their son entered
graduate school, they became even more worried. He announced that he
was now studying games.
In the end, Nowak turned out all right. He is now the director of the
Program for Evolutionary Dynamics at Harvard. The games were actually
versatile mathematical models that Nowak could use to make important
discoveries in fields as varied as economics and cancer biology.
"Martin has a passion for taking informal ideas that people like me
find theoretically important and framing them as mathematical models,"
said Steven Pinker, a Harvard linguist who is collaborating with Nowak
to study the evolution of language. "He allows our intuitions about
what leads to what to be put to a test."
On the surface, Nowak's many projects may seem randomly scattered
across the sciences. But there is an underlying theme to his work. He
wants to understand one of the most puzzling yet fundamental features
of life: cooperation.
When biologists speak of cooperation, they speak more broadly than the
rest of us. Cooperation is what happens when someone or something gets
a benefit because someone or something else pays a cost. The benefit
can take many forms, like money or reproductive success. A friend
takes off work to pick you up from the hospital. A sterile worker bee
tends to eggs in a hive. Even the cells in the human body cooperate.
Rather than reproducing as fast as it can, each cell respects the
needs of the body, helping to form the heart, the lungs or other vital
organs. Even the genes in a genome cooperate, to bring an organism to
life.
In recent papers, Nowak has argued that cooperation is one of the
three basic principles of evolution. The other two are mutation and
selection. On their own, mutation and selection can transform a
species, giving rise to new traits like limbs and eyes. But
cooperation is essential for life to evolve to a new level of
organization. Single-celled protozoa had to cooperate to give rise to
the first multicellular animals. Humans had to cooperate for complex
societies to emerge.
"We see this principle everywhere in evolution where interesting
things are happening," Nowak said.
While cooperation may be central to evolution, however, it poses
questions that are not easy to answer. How can competing individuals
start to cooperate for the greater good? And how do they continue to
cooperate in the face of exploitation? To answer these questions,
Nowak plays games.
His games are the intellectual descendants of a puzzle known as the
Prisoner's Dilemma. Imagine two prisoners are separately offered the
same deal: if one of them testifies and the other doesn't talk, the
talker will go free and the holdout will go to jail for 10 years. If
both refuse to talk, the prosecutor will only be able to put them in
jail for six months. If each prisoner rats out the other, they will
both get five-year sentences. Not knowing what the other prisoner will
do, how should each one act?
The way the Prisoner's Dilemma pits cooperation against defection
distills an important feature of evolution. In any encounter between
two members of the same species, each one may cooperate or defect.
Certain species of bacteria, for example, spray out enzymes that break
down food, which all the bacteria can then suck up. It costs energy to
make these enzymes. If one of the microbes stops cooperating and does
not make the enzymes, it can still enjoy the meal. It can gain a
potential reproductive edge over bacteria that cooperate.
The Prisoner's Dilemma may be abstract, but that's why Nowak likes it.
It helps him understand fundamental rules of evolution, just as Isaac
Newton discovered that objects in motion tend to stay in motion.
"If you were obsessed with friction, you would have never discovered
this law," Nowak said. "In the same sense, I try to get rid of what is
inessential to find the essential. Truth is simple."
Nowak found his first clues to the origin of cooperation in graduate
school, collaborating with his PhD adviser, Karl Sigmund. They built a
version of the Prisoner's Dilemma that captured more of the essence of
how organisms behave and evolve.
In their game, an entire population of players enters a round-robin
competition. The players are paired up randomly, and each one chooses
whether to cooperate or defect. To make a choice, they can recall
their past experiences with other individual players. Some players
might use a strategy in which they had a 90-percent chance of
cooperating with a player with whom they have cooperated in the past.
The players get rewarded based on their choices. The most successful
players get to reproduce. Each new player had a small chance of
randomly mutating its strategy. If that strategy turned out to be more
successful, it could dominate the population, wiping out its
ancestors.
Nowak and Sigmund observed this tournament through millions of rounds.
Often the winners used a strategy that Nowak called, "win-stay,
lose-shift." If they did well in the previous round, they did the same
thing again. If they did not do so well, they shifted. Under some
conditions, this strategy caused cooperation to become common among
the players, despite the short-term payoff of defecting.
In order to study this new version of the Prisoner's Dilemma, Nowak
had to develop new mathematical tools. It turned out that these tools
also proved useful for studying cancer. Cancer and the Prisoner's
Dilemma may seem like apples and oranges, but Nowak sees an intimate
connection between the two. "Cancer is a breakdown of cooperation," he
said.
Mutations sometimes arise in cells that cause them to replicate
quickly, ignoring signals to stop. Some of their descendants acquire
new mutations, allowing them to become even more successful as cancer
cells. They evolve, in other words, into more successful defectors.
"Cancer is an evolution you don't want," Nowak said.
To study cancer, however, Nowak had to give his models some structure.
In the Prisoner's Dilemma, the players usually just bump into each
other randomly. In the human body, on the other hand, cells only
interact with cells in their neighborhood.
A striking example of these neighborhoods can be found in the
intestines, where the lining is organized into millions of tiny
pockets. A single stem cell at the bottom of a pocket divides, and its
daughter cells are pushed up the pocket walls. The cells that reach
the top get stripped away.
Nowak adapted a branch of mathematics known as graph theory, which
makes it possible to study networks, to analyze how cancer arises in
these local neighborhoods. "Our tissue is actually organized to delay
the onset of cancer," he said.
Pockets of intestinal cells, for example, can only hold a few cell
generations. That lowers the chances that any one will turn cancerous.
All the cells in each pocket are descended from a single stem cell, so
that there's no competition between lineages to take over the pocket.
As Nowak developed this neighborhood model, he realized it would help
him study human cooperation. "The reality is that I'm much more likely
to interact with my friends, and they're much more likely to interact
with their friends," Nowak said. "So it's more like a network."
In experiments conducted by other scientists with people and animals,
Nowak's mathematical models seem to fit. Reputation has a powerful
effect on how people play games. People who gain a reputation for not
cooperating tend to be shunned or punished by other players.
Cooperative players get rewarded.
"You help because you know it gives you a reputation of a helpful
person, who will be helped," Nowak said. "You also look at others and
help them according to whether they have helped."
--
Bob.
.
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