Eusociality is the extreme result of kin selection
Hamilton’s rule can be applied to explain eusociality: social groups that include nonreproductive members (that is, members that as individuals do not reproduce). The most obvious examples of eusociality occur among the Hymenoptera, an insect group that includes wasps, bees, and ants. In a honey bee colony, for example, the thousands of individuals in the colony are sterile females.
Media Clip 52.1 Social Shrimps
The key to understanding the evolution of eusociality in hymenopterans is their sex determination mechanism, haplodiploidy, in which diploid individuals are female and haploid individuals are male. The queen carries a lifetime supply of sperm obtained during her single mating flight, and she controls whether her eggs are fertilized or not. An unfertilized egg develops into a haploid male; a fertilized egg develops into a diploid female. The queen’s daughters share all of their father’s genes and, on average, half of their mother’s genes. As a result, the sterile female workers in the hive—all sisters—share, on average, 75 percent of their alleles (Figure 52.19). The basic unit of reproductive success in eusocial insects is the creation of new colonies around a new queen. If a new queen developed from a worker’s offspring, the worker would share fewer genes with the new queen than if the new queen developed from a sister.
Figure 52.19 Favoring Sisters over Daughters Female honey bees are diploid and males are haploid. Thus if a female worker bee were to reproduce, she would share approximately 50 percent of her genes with her daughters. However, she shares an average of 75 percent of her genes with her sisters. In terms of inclusive fitness, then, a sister is more valuable than a daughter for this species.
Haplodiploidy is not essential for the evolution of eusociality. Nearly all eusocial animals construct elaborate nests or burrow systems within which their offspring are reared. Such a structure represents an enormous investment of resources. Naked mole-rats are eusocial mammals that live in elaborate underground tunnel systems (Figure 52.20). A colony includes 70–80 individuals but only 1 reproductive female and a few reproductive males. The other colony members are sterile workers that dig and maintain the tunnels, guard against intruders, harvest food (tubers), and use their feces to feed the queen and her offspring. Individuals attempting to found new colonies have a high risk of failing or being captured by predators. When chances of individual reproductive success are practically zero, an individual can best maximize its inclusive fitness by staying with and helping maintain the colony.
Figure 52.20 A Eusocial Mammal Naked mole-rats live in a large colony with one reproductive female and a few reproductive males. Their home is an elaborate tunnel system excavated by the colony over time.