Monday, November 24, 2008

Why do they?

Honeybees vexed Charles Darwin. It wasn't that he feared being stung; rather, he knew bees posed a challenge to his theory of natural selection. Darwin knew that worker honeybees forgo reproduction and put their energies instead into rearing the offspring of others. That behavior seems to contradict natural selection, which postulates that living creatures strive, above all else, to survive and reproduce. Why do the bees act like heroes in Victorian novels, such as Charles Dickens's A Tale of Two Cities, sacrificing themselves for the sake of others?

Since Darwin's time, evolutionary biologists have struggled to account for this dilemma, to explain why bees, ants, wasps, and termites will give up their own offspring to rear those of others. In recent decades many biologists have agreed on a single theory to explain the behavior. That theory, known as "kin selection," says that selfless insects aren't really selfless at all. Instead they benefit by aiding the survival of their close relatives, which share their genes. Now in a move that has caught evolutionary biologists by surprise, a world-renowned scientist is arguing for a new perspective on the problem. Edward O. Wilson, the father of sociobiology (the branch of biology that deals with social behavior) and once a champion of kin selection theory, argues for a new look at the problem.

If as a child you ever crouched down over an anthill, watching ants come and go, then you know the kind of insects Wilson loves. Wilson is a world authority on ants and has studied them intensively since he was a boy.

Ants, along with many termites, bees and wasps, are the kind of insects scientists call "eusocial," meaning they have distinct social classes: queen, soldier, drone, worker. In eusocial society, some members, like the queen, get to reproduce; others, like the workers, give up the chance to have offspring of their own, devoting their energies instead to helping the queen raise her young. For 150 years evolutionary biologists have struggled to answer how such selfless, or altruistic, workers evolved. Such insects seem to defy natural selection.

It should be pointed out that neither Wilson nor any other scientist in this debate is calling into question the theory of evolution itself. What is in doubt is how exactly evolution works in this instance. Scientists want to understand how and why eusocial insects evolved.

In the 1960s, two competing schools of thought arose on the problem. The first school said that natural selection is not operating on the individual ant or wasp or bee. What is happening is a sort of group selection, where the survival of the group as a whole prevails. The sterile insect is doing work that aids the whole group, therefore the group is more likely to survive. You can think of this theory, which is called "group selection," as the idea of taking one for the team.

A competing school of thought also arose around the same time. That school says that although the sterile worker has no offspring of its own, it is essentially helping to pass on genes by taking care of its close relatives. After all, siblings share on average half of their genes; cousins one-eighth. So natural selection is still at work, because sterile workers do get to pass on their genes, by ensuring the survival of their closely related kin. This theory is called "kin selection." You can think of it as the theory of looking out for your own kind.

It didn't take long for scientists to settle on one theory and discard the other. Group selection was soundly rejected in favor of kin selection. Decades earlier, Darwin himself seemed to come down on the side of kinship in Origin of Species when he wrote that the problem of insect altruism "disappears when it is remembered that selection may be applied to the family, as well as the individual, and may thus gain the desired end." In other words, the sterile worker still gets to pass on its genes, albeit via the offspring of its close relatives instead of its own offspring.

For many years, Wilson himself believed that eusocial evolution could be explained by kin selection. But now Wilson has changed his tune, and is saying that what matters is the colony evolving as a group.

To understand how and why eusocial behavior evolved, Wilson says we must look to new studies on social insects. In the past decade, scientists have made careful observations of insects from all over the world. They have amassed a mountain of data on Japanese bees, South American ants, Australian beetles, and many others. These studies detail how social insects live, how they cooperate, and how they communicate. Wilson says that we can find even more clues in the lifestyles of insects that are likely to evolve social characteristics, as well as in the lifestyles of those that never do.

Let's start with an insect that would never evolve into a social creature. Take, for instance, a wasp that flies solo, laying its eggs far and wide. Such an insect is an inherently solitary creature and likely to remain so. It simply doesn't have the making of a social creature, Wilson says.

But a nest-building wasp, even a solitary one, has the possibility of becoming social, particularly if it is flexible in its behavior. Consider the Japanese stem-nesting xylopine bee. Most of the time the mother bee nests alone, but once in a great while she will nest together with another bee. Such flip-flopping in behavior leaves the door open for cooperation to evolve.

Another trait that turns out to be important is how a mother feeds her young. Imagine a nest-building female wasp that seals her offspring inside the nest along with all the food the growing larva needs. Such an insect is unlikely to become social.

But now imagine a different wasp. This mother continually leaves and enters the nest, bringing food daily to her growing larva, called progressive feeding. While the mother is inside feeding her offspring, she must leave the nest entrance unsealed and unguarded. That leaves the nest vulnerable to predators, and for that reason progressive feeders are more likely to benefit by cooperation. Another adult can guard the nest while the mother is inside feeding her young.

Now imagine that the progressive feeding mother has multiple offspring, and they reach adulthood, not all at once, but in overlapping waves. Now the mother can enlist an older sibling to pull guard duty outside the nest while she is inside feeding the younger offspring. That type of cooperation is the first step toward eusociality.

Wilson says that traits like these—nest-building, flexible behavior, having offspring in waves—are pre-adaptations that favor an insect evolving eusocial behaviors. He sees group evolution at work, with the entire group leaping toward eusociality all at once. "One small step for a wasp, one giant leap for hymenoptera," Wilson told New Scientist. He outlines his ideas in a book titled The Superorganism, written with Bert Hölldobler of Arizona State University in Tempe.

As for kin selection theory, Wilson says kinship is important for starting down the eusocial path, as in the case of the sibling on guard duty. He maintains, however, that it is not the reason eusocial behavior develops. "Kin selection theory is not wrong; it is simply relatively ineffective," writes Wilson, claiming that the kin hypothesis doesn't help scientists make worthwhile predictions about insects.

For instance, if kin selection theory were true, Wilson claims, close relatives would be more altruistic than distant relatives, because kin theory says that altruistic insects work to pass on the genes of their close relatives. But he points out that unrelated colonies often thrive, and some colonies have ways of keeping relatedness low, such as bee colonies in which the females disperse when the nests are large and their brothers are present.

Wilson claims the kinship theory leads to other problems. If workers favored their own siblings more than distant relatives within the same colony, as kinship theory predicts, then the result would be infighting among subgroups, which would be bad for the colony as a whole.

David Queller at Rice University in Houston, Texas, still favors kin theory. "We are finding it useful in studying microbes," Queller told New Scientist. "It turns out that when they are altruistic they are related too. We weren't too surprised, because that is a prediction of kin selection theory." Andrew Bourke of the University of East Anglia in England also disagrees with Wilson. "I think he has rather underestimated what kin selection theory has achieved," Bourke told New Scientist.

Wilson is not surprised that not everyone agrees with him. "Of course they are going to remain critical for a while," he told New Scientist. "If those who defend the standard theory come back in lucid terms and show, point by point, why these hypotheses are already understood, I'll be glad to concede. But I haven't seen any evidence yet."

No comments: