Bees, wasps, and ants live in highly structured colonies, with each individual performing particular tasks. Each colony, for example, contains a single reproducing female and many non-reproducing females that gather food, feed the young, and defend the nest but don't lay eggs of their own. So how does helping the group help (that is, increase the fitness of) the individuals, especially those that forgo reproduction themselves?
4jtJABBi You could say that all these insects are working toward the good of the colony (and every animated ant or bee film ever made offers this explanation), but you'd be wrong. Evolution rarely works at the group level; it is more likely to work at the individual level. Each ant, wasp, or bee is really working toward its own benefit — not the colony's.
In these species, non-reproductive females (the workers) increase their own fitness more if they help the queen reproduce than if they reproduce themselves. It's a numbers game. Of all the relationships (parent to offspring, sister to sister, sister to brother), females in these systems share more genes with their sisters than they do with either parent, their brothers, or their own offspring. More sisters mean more fitness. The following sections explain.
In Hymenoptera reproduction, fertilized eggs become females; unfertilized eggs become males. Because they result from fertilized eggs, females are diploid. A female, being diploid, gets half of its genetic material from each of its parents, just as humans do. What's different is that the daughters receive exactly the same genes from the father, because the males, having arisen from unfertilized eggs, have only a single copy of the genetic material to pass on to their children.
The upshot of this strange sex-determination mechanism is that females in this reproductive system are three times as closely related to their sisters as they are to their brothers, and mothers are more closely related to their sisters (three quarters) than they are to their offspring (one half). In terms of kin selection, being more related to your sisters than to your own offspring means that a female can increase her fitness more efficiently by helping her mother make another reproductive daughter than to make a daughter herself.
And guess what? That's exactly what happens in many Hymenoptera species. That beehive, anthill, or wasp's nest in your backyard usually contains one reproducing female and many non-reproducing daughters that take care of the nest and the babies but don't reproduce themselves.
Intrigue in the queen's court
The mere fact that non-reproductive workers aren't laying eggs on their own doesn't mean that they're not affecting the reproductive decisions of the hive. Consider bees as an example.
The queen bee controls which eggs get fertilized, using sperm she stored from her maiden mating flight. Because her sons and daughters each share half of her genes, she would just as soon make half of each. It's all the same to her. But regardless of the sex ratio of the eggs laid by the queen, the ratio of reproductive bees produced by a hive often has a three-to-one bias in favor of females. So if half of the Queen's offspring are males and half are females, how does the hive end up with many times more females than males?
The answer? Her daughters, the worker bees. For their own fitness (being three times more related to their sisters than to their brothers), they prefer that the queen make more daughters. And because the worker bees control the feeding of the larvae, they control how many fertilized eggs will become reproductive females and how many unfertilized eggs (males) will survive.
Genes that result in worker bees producing more sisters are selectively favored because the sisters are likely to have those genes. A gene that favors producing more brothers is not as likely to get passed to future generations, because the brother bee is not as likely to share that gene (or any particular gene) with the sister.
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