As the trees flower with the return of spring for those in the northern hemisphere, pollinators return. The honey bee, an animal ubiquitous with pollination, flies between flowers, collecting pollen and nectar to bring back to their hives. While eight species exist, the European honey bee now has a global distribution, after being domesticated and distributed by humans.
But one aspect of these insects has stumped evolutionary biologists. All of those honey bees, spending their entire adult lives collecting food, protecting the hive, and feeding their young, are infertile. All of the larvae, which reside in the many combs of the nest, are their siblings, not their offspring. The queen of the hive, while not actually a ruler in any sense, is the mother of every worker.
This system is known as eusociality, and very few animals exhibit it. Biologists identify it by the following characteristics: rigid divisions of labor, collective care of young, multigenerational groups, and sterile workers. Most eusocial species exist within the order Hymenoptera, which the honey bees belong to. In fact, eusociality independently evolved between 8–11 times among the many wasps, ants, and bees which make up this order.
The reason why this perplexes biologists is the existence of reproductive division. Worker bees, despite not having any offspring of their own, will dedicate their lives to caring for the young of the queen. For a time, the existence of such a system appeared contradictory. Charles Darwin himself, in On the Origin of Species, called the existence of sterile workers a “special difficulty which at first appeared to me insuperable, and actually fatal to my theory.” The reason for this concern was that evolution functioned by individual organisms, being the fittest for survival, reproducing and therefore spreading their genes. Sterile workers do not reproduce and instead exhibit altruism by caring for the young of the fertile ants. This seemingly contradicts the idea of natural selection, as these workers do not reproduce, and should therefore not exist. Nonetheless, eusociality has existed since the Jurassic period over 150 million years ago.
However, for the gene-centered view of evolution, eusociality makes perfect sense. Look at a carpenter ant, for example. In this model, explained in Richard Dawkins’ book The Selfish Gene, evolution is understood through the success of individual alleles, or a specific form of a gene. Within the context of a bee colony, the workers are an extension of the phenotype — the physical appearance which the allele codes for. If the whole colony as a unit does better because of an allele, the queen will reproduce, and therefore spread the allele even more.
Another idea, kin selection, also explains the altruistic behavior of sterile workers. Proposed by evolutionary biologist W. D. Hamilton, this theory suggests that individuals will sacrifice their own reproductive success for that of their relatives, as they share genetics. He proposed this idea using Hymenoptera, in fact. Because of their unique sex-determining system known as haplodiploidy. When a queen lays an egg, it can either become male or female. When the egg is unfertilized, it only contains half of the mother’s genes, without any father. This produces a male, who therefore only shares genetics with his mother. If a male breeds with the queen, and produces fertilized eggs, the offspring will become female, and will share half of their genes with their mother, and half with their father. This system means that males only have half of the genetic information as their female counterparts.
Kin selection theory suggests that these sterile female workers work for the success of their mother, the queen, because they share genetics. By raising her sisters, the worker bee ensures the success of half of her own genes.
While this system does likely play a role in the persistence of eusociality, how kin selection plays a role in the evolution of eusociality remains a controversial topic. The first issue with the theory became apparent as biologists discovered eusociality in other animals. From the pistol shrimp Synalpheus regalis, to the naked mole rats of East Africa, to the many species of termites. None of these non-hymenopteran examples exhibited haplodiploidy.
An alternative theory suggests that eusociality evolves through the benefits of communal living. Many of the other, non eusocial species in hymenoptera substantiate this theory. The mason bee, Hoplitis Anthocopoides is solitary, meaning every female produces and cares for her own young. But these bees form aggregations, or shared nests. Though each bee only cares for their young, they all collectively defend the aggregation from threats. This could represent the first stage in the evolution of eusociality.
Somewhere along the line to eusociality, group selection becomes a factor. When this is the case, the community becomes the unit of selection instead of the individual. This is already the case for the eusocial species, where the whole colony, being related, shares traits. If the reproductive females die, the whole colony does, therefore causing group selection. But this may be the case for primitively eusocial species as well. A sweat bee, Halictus ligatus, does express some aspects of eusociality, such as division of labor and communal care of young, but does not have rigid, sterile castes. Instead, this species appears to decide the roles based on aggression and size. The founders of the colonies usually become the “queens” in this species, with developed ovaries and increased size to store energy. The helper females, by contrast, were far smaller, and had underdeveloped ovaries, but still appeared able to reproduce. These bees, while not fully eusocial, do have a division of reproduction and labor, suggesting they are intermediary between solitary and fully eusocial ones.
While the exact mechanism for eusocial evolution remains up for debate, this fascinating behavior still contributes greatly to our understanding of the natural world. The theories developed by researching these animals now appear relevant to the evolution of other, non eusocial ones. As we come to understand how this unique adaptation comes to be, we may spot other animals in developmental stages.