NYT: Scientists Square Off on Evolutionary Value of Helping Relatives
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http://www.nytimes.com/2010/08/31/science/31social.html
By CARL ZIMMER
Why are worker ants sterile? Why do birds sometimes help their parents
raise more chicks, instead of having chicks of their own? Why do
bacteria explode with toxins to kill rival colonies? In 1964, the
British biologist William Hamilton published a landmark paper to answer
these kinds of questions. Sometimes, he argued, helping your relatives
can spread your genes faster than having children of your own.
For the past 46 years, biologists have used Dr. Hamilton's theory to
make sense of how animal societies evolve. They've even applied it to
the evolution of our own species. But in the latest issue of the
journal Nature, a team of prominent evolutionary biologists at Harvard
try to demolish the theory.
The scientists argue that studies on animals since Dr. Hamilton's day
have failed to support it. The scientists write that a close look at
the underlying math reveals that Dr. Hamilton's theory is superfluous.
"It's precisely like an ancient epicycle in the solar system," said
Martin Nowak, a co-author of the paper with Edward O. Wilson and Corina
Tarnita. "The world is much simpler without it."
Other biologists are sharply divided about the paper. Some praise it
for challenging a concept that has outlived its usefulness. But others
dismiss it as fundamentally wrong.
"Things are just bouncing around right now like a box full of Ping-Pong
balls," said James Hunt, a biologist at North Carolina State
University.
Dr. Hamilton, who died in 2000, saw his theory as following logically
from what biologists already knew about natural selection. Some
individuals have more offspring than others, thanks to the particular
versions of genes they carry. But Dr. Hamilton argued that in order to
judge the reproductive success of an individual, scientists had to look
at the genes it shared with its relatives.
We inherit half of our genetic material from each parent, which means
that siblings have, on average, 50 percent of the same versions of
genes. We share a lower percentage with first cousins, second cousins
and so on. If we give enough help to relatives so they can survive and
have children, then they can pass on more copies of our own genes. Dr.
Hamilton called this new way of tallying reproductive success inclusive
fitness.
Each organism faces a trade-off between putting effort into raising its
own offspring or helping its relatives. If the benefits of helping a
relative outweigh the costs, Dr. Hamilton argued, altruism can evolve.
Dr. Hamilton believed that one of the things his theory could explain
was the presence of sterile females among ants, wasps, and some other
social insects. These species have peculiar genetics that cause females
to be more closely related to their sisters than to their brothers, or
even to their own offspring. In these situations, a female ant may be
able to spread more genes by helping to raise her queen mother's eggs
than trying to lay eggs of her own.
But as the years passed, Dr. Wilson's enthusiasm for the theory waned.
"It was getting tattered," he said. Many species with sterile females,
for example, do not have the strange genetics of ants and wasps. And
many species with the right genetics have not produced sterile females.
After reading a 2008 article in which Dr. Wilson aired his misgivings,
Dr. Nowak got in touch with him. Dr. Nowak and Dr. Tarnita were
studying the mathematical underpinnings of evolution. They wanted to
carry out a mathematical analysis of natural selection in general and
inclusive fitness in particular. Dr. Wilson joined them.
The scientists developed equations that described two different
behaviors in a population. One strategy might be selfish and the other
altruistic--leaving their nest after they hatch versus staying to
help rear young, for example. The scientists then calculated the
conditions in which one strategy or the other takes over the whole
population.
The researchers found that inclusive fitness theory worked only under
special conditions. All the effects that the animals had on each other
had to take place on a one-to-one basis. In the real world, individuals
may benefit from many other individuals as a group.
Standard natural selection, the scientists argue, explains everything
inclusive fitness theory was supposed to, without these special
conditions.
Dr. Nowak and his colleagues argue that their analysis should free
scientists to think of other ways that altruism and other kinds of
social behavior might evolve.
Thinking about why a worker would sacrifice her own offspring turns out
be the wrong perspective on the question, they argue. Instead, they
say, we should put ourselves in the queen's perspective. They offer a
mathematical model suggesting how natural selection could produce
offspring that stay at a queen's nest. If she produces daughters that
stay in the nest, she can spend more time laying eggs, rather than
hunting for food to feed her young.
"I think they've done a very thorough job," said Michael Doebeli of the
University of British Columbia. He has also grown skeptical about the
importance many colleagues have put on inclusive fitness in recent
years. "The people who swear by this method somehow think there's
something magic about it that explains everything," he said.
Dr. Hunt, who studies social wasps, does not find Dr. Hamilton's ideas
useful because it is nearly impossible to calculate the costs and
benefits of helping relatives.
"I have never felt that inclusive fitness has contributed to an
understanding of what's going on," he said. The Harvard team, Dr. Hunt
said, is "basically on target."
A number of scientists strongly disagree, though. "This paper, far from
showing shortcomings in inclusive fitness theory, shows the
shortcomings of the authors," said Frances Ratnieks of the University
of Sussex.
Dr. Ratnieks argues that the Harvard researchers cannot rule out
kinship as a driving force in social evolution because their model is
flawed. It does not include how closely related animals are.
It would be as if a team of researchers carried out a study on the
effects of diet and exercise on health. Their subjects get different
amounts of exercise but stay on the same diet. In the end, the
experiment might show that exercise makes people more healthy. But it
would not make any sense to also conclude that diet plays no role.
"If you don't vary something you cannot say how important it is," said
Dr. Ratnieks.
Andy Gardner, an evolutionary biologist at Oxford, said bluntly, "This
is a really terrible article." One problem Dr. Gardner points to is the
Harvard team's claim that the past 40 years of research on inclusive
fitness has yielded nothing but "hypothetical explanations."
"This claim is just patently wrong," Dr. Gardner said. He points to the
question of how many sons and daughters mothers produce among the many
insights inclusive fitness has brought.
In most species, the balance is 50-50. But there are exceptions. In
some ant species, for example, the ratio is around three daughters for
every son. That is because the sterile female workers invest more into
female larvae than males. Inclusive fitness theory predicts just this
situation, since the workers are more closely related to their sisters
than to their brothers.
Dr. Gardner and a number of other biologists have co-authored a reply
that they will be sending to Nature to challenge the new paper.
Dr. Hunt hopes to move the debate toward a resolution with a meeting he
is to run in October at the National Evolutionary Synthesis Center in
Durham, N.C. He will be bringing together scientists who build models
of all the potential factors that drive the evolution of societies,
from their kinship to their ecology. Ultimately, the scientists hope to
build a model that can take into account all of these factors at once.
"They're all stoked, and I am too," Dr. Hunt said.