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.
Since
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
To understand how and why eusocial behavior evolved,
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,
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.
As for kin selection theory,
For instance, if kin selection theory were true,
David Queller at
No comments:
Post a Comment