Biology
The Evolutionary Significance of Altruistic Behavior in Eusocial Insects
Quick fact
Worker honeybees are more genetically related to their sisters (75% on average) than they would be to their own daughters (50%). This unusual relatedness, due to haplodiploidy, makes helping the queen raise more sisters an evolutionarily better strategy than reproducing themselves.
Why this is interesting
In many ant colonies, worker ants never reproduce—they spend their whole lives building and defending a nest that belongs to the queen. Why would natural selection, which seems to favor genes that make more copies of themselves, ever allow such a selfless lifestyle?
Read the full explanation
Understanding The Evolutionary Significance of Altruistic Behavior in Eusocial Insects
At first glance, altruism seems like a paradox for evolution. If genes for self-sacrifice exist, shouldn't they quickly die out? The resolution lies in looking at genes, not individuals. The 'goal' of an organism is not to survive but to get its genes into the next generation. If an individual helps a relative reproduce, and that relative shares the same altruistic genes, those genes are still passed on—even though the helper never breeds. This idea is called inclusive fitness: the total genetic success an individual achieves by its own reproduction plus the success of its relatives, weighted by how closely related they are. For eusocial insects, the system is pushed to an extreme. In bees, ants, and wasps, males are haploid (they develop from unfertilized eggs and have one set of chromosomes), while females are diploid (two sets). This means workers, who are female, share 75% of their genes with full sisters—more than the 50% they'd share with their own offspring. So by raising the queen's new daughters, a worker actually passes on more of its own genes than by having its own daughters. This is a compelling, though not complete, explanation for why worker sterility evolves in these groups.
A deeper explanation
The mathematical rule behind altruism is Hamilton's rule: b c/r (or rb c), where c is the cost to the altruist, b is the benefit to the recipient, and r is the genetic relatedness between them. Altruism evolves when the benefit to the recipient, discounted by relatedness, exceeds the cost to the helper. In a honeybee colony, a worker's cost of not reproducing is high, but the benefit to the queen (who lays thousands of eggs) is enormous, and relatedness is high (r=0.75). Thus, the inequality holds. However, haplodiploidy alone does not fully explain eusociality; termites are diploid and still eusocial. More modern explanations also consider ecological factors, such as high benefits of group living (e.g., fortress defense) and low costs of helping (e.g., when reproduction is already constrained). Once altruistic helping is established, colonies can perfect reproductive division of labor, leading to physical castes. The evolutionary significance is profound: it shows that natural selection can produce self-sacrifice when genes are shared, and it highlights the importance of genetic relatedness in the evolution of cooperation.