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Varying levels of social organization are observed within the animal kingdom, ranging from simple to highly complex. These systems can greatly enhance the survival and reproductive success of an individual or a population. Of these, eusociality is the highest level of social organization, involving divisions of labor based on social castes. However, this system is rarely observed in nature, as it requires individuals to help others in the population at great survival or reproductive costs to themselves1. When this behavior pattern, termed altruism, was originally observed in species like the honeybee, it caused early evolutionary biologists to reconsider their antiquated definition of evolutionary fitness. Original theories considered fitness only on an individual level. However, in light of eusocial systems, W. D. Hamilton first proposed a theory of inclusive fitness. This postulates that the measure of an individual’s fitness can include the reproductive fitness of family members, who also carry a proportion of that individual’s genes. In this way, it can make evolutionary sense for individuals to make personal sacrifices for the benefit of related individuals.
Hamilton’s theory is driven by the concept of relatedness, which is defined as the proportion of shared genes between two individuals. For example, 50% of an individual’s genes are shared with direct offspring, while only 25% are present in the following generation. Accordingly, investment in a closely-related individual confers greater fitness than investment in those less closely-related. The following equation describes this relationship:
r (relatedness) × B (benefit to the recipient) > C (cost to the altruist)
In this equation, “benefit to the recipient” (B) is equivalent to the increase in number of offspring produced by that individual while “cost to the altruist” (C) is equivalent to the decrease in the number of offspring produced by the altruist. In situations where the product of “r” times “B” is not greater than “C”, altruism is not favored by natural selection. However, as relatedness increases, so does the value of “r” times “B.” Thus, altruistic behavior becomes more likely when the altruist and recipient are more closely related.
Not every species that exhibits altruistic behavior is considered eusocial. Ultimately, eusociality is characterized by three main traits. First, the division of labor (including reproduction) into social castes leads some individuals to forego reproduction to enhance the reproductive success of family members. Other characteristics include overlapping generations, in which individuals of multiple generations live and work together, as well as cooperative brood care, in which non-parent individuals assist in the care of offspring.
One well-known example of eusociality is displayed by the honeybee. A typical beehive contains different groups of bees performing varied tasks. Only one reproductive female, the queen bee, repopulates the hive while other females act as workers. Reproductive males, called drones, are fairly uncommon. Honeybees belong to the order Hymenoptera along with ants and wasps2-3. This order contains the most eusocial species, but is not the only order with eusociality. Other groups that contain eusocial species include termites, marine snapping shrimp, and naked mole rats. Some have even proposed that humans may be eusocial. Though overlapping generations and cooperative brood care are generally present, humans appear to lack a separation between reproductive and non-reproductive groups. Though it is not a perfect fit, aspects of eusociality and altruism are frequently observed in humans and play a large role in our own social structure.
Though non-eusocial communities structures are much more commonly observed, eusociality confers evolutionary advantages to the species in which it is found. Two main hypotheses have been proposed to explain the evolution of eusociality: the ecological hypothesis and haplo-diploidy hypothesis.
The first, the ecological hypothesis, considers the many ecological drivers that may favor eusocial structures. These include the ability of eusocial communities to utilize common nest sites, facilitate group protection from predators, and reduce competition among individuals. The second theory, the haplo-diploidy hypothesis, takes into account the complex genetic structure of many eusocial groups, including bees and ants. In these groups, males commonly exhibit haploidy. This means that they carry only one copy of each chromosome, and thus contain half the genetic information of females. As a result, fathers pass 100% of their genes to their daughters while mothers, who are diploid, pass on 50%. Accordingly, when calculating the relatedness between sisters in a haplo-diploid species, 75% of genes are shared. This is significantly more than the 50% observed in diploid species. Increased relatedness among sisters may explain why workers in a colony are generally exclusively female and are able to work together for the good of the colony. This pattern is seen in honeybees which include haploid males and diploid females4. However, not all eusocial species exhibit haplo-diploidy. Both the ecological and the haplo-diploidy hypotheses likely play a role in the evolution of eusociality.