The concept of inclusive fitness has become the foundation of modern biology and may be one of the most important advances in evolutionary biology since Darwin introduced the theory of natural selection. If natural selection occurs at the individual level (i.e., individuals with higher reproductive success have the greatest genetic representation in future generations), how do cooperative breeding systems evolve? The apparent paradox of some individuals within a population foregoing the opportunity to breed can be explained if some individuals help their close relatives reproduce. By assisting close relatives, altruists still manage to pass on their genes indirectly. This behavior can arise when the genetic relatedness between an altruist and beneficiary (r) multiplied by the fitness gain to the beneficiary (b) minus the fitness cost to the altruist (c) is greater than zero; that is, (rb - c) > 0.
Consequently, the degree of relatedness between the altruist and the beneficiary determines the probability that they share a particular gene, and therefore, the probability that the altruistic gene will be passed on to the next generation. Sterile castes in social insects are an extreme case of quasi-altruism (worker females give up reproduction and raise their mother's offspring). This occurs most commonly in the Hymenoptera (bees, wasps, ants), and is probably the result of their haplodi-ploid chromosomal organization where males have single chromosome copies and develop parthenogenetically from unfertilized eggs and females have two copies of each chromosome. Because males are haploid, there is no meiosis and all daughters of a particular male have identical paternal genes. Nonreproductive females have a greater chance of passing on their own alleles via their reproductive sisters than they do by reproducing themselves, so zero direct fecundity for these individuals results in the highest indirect lifetime reproductive success.
Eusocial systems are an evolutionarily advanced level of colonial existence in which adult colonial members
(1) belong to two or more overlapping generations,
(2) care cooperatively for the offspring, and (3) are divided into either reproductive and nonreproductive (or less-reproductive) castes. The presence of castes is an integral requirement of eusociality, and it is most commonly found in social insects, especially ants, bees, wasps, and termites. As a reproductive strategy it is rarely observed in nature, but it appears to be an important mode of reproduction nonetheless. Although eusocial ants comprise only 2% of the c. 900 000 insect species known globally, this taxon comprises more than half of the global insect biomass. The strategy is obviously successful in evolutionary terms, so why then, despite this apparent success, has eusociality rarely evolved? It is generally thought that eusociality is the result of some extraordinary environmental circumstances that existed in the evolutionary past when a particular genome coding for group breeding and subordinate reproductive suppression was selected. Selection of this genome consequently led to a cooperative breeding system and was maintained not by the inability of individuals to breed, but because individuals are outcompeted by breeding conspecifics in well-integrated communal colonies.
Implicit in understanding fecundity patterns in eusocial systems is recognizing the factors that regulate body size and growth. The eusocial female that does all or most ofthe breeding is usually the biggest and most dominant. As such, in species of cooperative insects living in large groups, selection for increased fecundity has repeatedly resulted in the evolution of an increased body size for females. Whether this is also true of cooperative vertebrates is currently debatable; however, increased size of dominant breeding females has been documented in naked mole rats (Heterocephalus glaber) and meerkats (Suricata suricatta). Eusociality and the resultant reduction or eradication of fecundity in some individuals among diploid animals is rare and appears to occur only when (1) species live in burrows, (2) food is abundant, (3) parents care for offspring, and (4) mechanisms exist for mothers to manipulate other females, such as pheromones that inhibit their breeding.
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