Another way to phrase the above explanation for altruism is in terms of group selection: when groups contain genetically related individuals (there is between-group genetic
(a) Enforcement + inclusive fitness: colonies with a queen ra ra e
Asian paper wasp Saxon wasp
(b) Inclusive fitness: colonies without a queen
German wasp «
Asian paper wasp
Genetic relatedness among workers
Figure 4 Worker altruism is driven by a combination of inclusive-fitness effects and enforcement in social insect colonies. (a) Altruistic self-restraint due to enforcement. In colonies where the mother queen is alive, the workers can raise either the queen's or other workers' eggs. In species where relatedness among workers is high, they tend to raise the workers' eggs because they are highly related to them, but in species where relatedness among workers is low, like the honeybee, workers 'police' each others' eggs and remove them. This reduces the benefits to worker reproduction which, alongside indirect fitness benefits, promotes reproductive self-restraint. (b) Altruistic self-restraint due to inclusive-fitness effects. If the queen dies the workers compete to lay eggs. However, when relatedness is high, many show altruistic self-restraint and do not attempt to reproduce. Reproduced from Wenseleers T and Ratnieks FL (2006) Enforced altruism in insect societies. Nature 444: 50.
variance), selection can favor altruistic actions that invest in the group and increase its productivity. Importantly, and despite occasional misguided claims to the contrary, this logic is fully compatible with and complementary to inclusive-fitness theory: one can explain worker sterility by focusing on benefits to relatives (inclusive fitness), or the benefits at the colony level (group selection), but in the end both genetic relatedness and benefits are required for Hamilton's altruism (Figure 3 a). Like inclusive fitness, group-selection thinking can be traced back to Darwin (and also Spencer), and there were brief but explicit mathematical models by Haldane and Wright in the mid-twentieth century. However, it then got a bad name when Wynne-Edwards applied it uncritically to groups of unrelated individuals, such as large vertebrate populations, where individual-level selection will dominate and suppress altruism. It was correctly reformulated in the 1970s with the work of George Price, D. S. Wilson, and, once more, Hamilton. Price's work, specifically the Price equation, has since been central to the development of many branches of social evolution theory. This includes the development of cultural models of cooperation, where imitation within groups increases between-group variance and promotes the spread of cooperative traits through 'cultural group selection'. But there remains a point of departure between group selection and inclusive fitness when it comes to definitions.
In the group-selection framework, altruism has been defined as cooperative acts that lower reproductive share in the group. However, this can include actions that increase personal reproduction (Figure 3b), which is not altruism by Hamilton's definition. Consider, for example, a prairie dog (Figure 2a) that contributes to the tunnels in its town and suffers a 10% decrease in its reproduction relative to another group member. This can evolve through selfish benefits alone if the tunnels allow all town members to double their reproduction. This is illustrated by a simple extension of Hamilton's rule:
Indirect/kin benefit b
Direct /individual benefit
where n is group size, b is the group benefit of which each individual gets a share b/n, and c is the individual cost. The individual-benefit term contains relatedness of the actor to itself, rself = 1, and even with no relatives in the group (r = 0), tunneling can still evolve if there are feedback benefits to the actor. This type of behavior has been termed 'weak' altruism (Figure 3b) because it carries a personal (direct fitness) benefit, which distinguishes it from Hamilton's (strong) altruism, like that of sterile insect workers (Figure 3 a).
Because of the conceptual overlap, group-selected altruism includes all of the inclusive fitness examples above. Furthermore, feedback benefits of the sort that generate weak altruism must be common in many societies but are difficult to distinguish from inclusive fitness benefits. One example of weak altruism, however, is cooperative next founding by unrelated social insect queens. Here, co-investing in the colony can provide feedback fitness benefits when queens are later able to contribute to sexual offspring.
fitness) have been proposed to explain altruism-like behaviors. In the 1970s, Robert Trivers showed that helping can be selected when it increases the chance of return help, which he termed reciprocal altruism (tit for tat; Figure 3 c). A closely related idea is that of indirect reciprocity, whereby helping others improves reputation, which then increases the chance of being helped. More generally, feedback benefits to personal reproduction (direct fitness) are central to all manner of cooperative behaviors, including cooperation among genes and species, for example, plants provide nectar and insects pollinate in return:
individual flowers which had the largest glands or nectaries, and which excreted most nectar, would oftenest be visited by insects, and would be oftenest crossed; and so in the long-run would gain the upper hand. (Darwin, 1859)
A focus on direct fitness has led to a third general approach to modeling social evolution, called direct fitness or neighbor-modulated fitness theory, which again complements the inclusive-fitness and group-selection approaches. However, an action that evolves purely through direct-fitness feedbacks means increased personal reproduction and departs from Hamilton's altruism. Curiously, however, Hamilton started his original papers with a neighbor-modulated model (the fitness effect of others on the focal individual), before making a switch to inclusive fitness (the fitness effect of the focal individual on others) on which he based his rule.
Reciprocal altruism and indirect reciprocity are extremely important in human cooperation, but the requirement for recognition and memory of others means that they occur in relatively few other species. Potential examples include other primates and vampire bat blood-sharing, but inclusive fitness and group benefits also occur in these systems. More generally, however, cooperation that is selected due to direct-fitness feedback benefits is fundamental to social evolution, including between-species cooperation.
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