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5. KIN SELECTION

COOPERATION AND COMPETITION BETWEEN RELATIVES

S.A. West et al (University of Edinburgh, UK) discuss cooperation between relatives, the authors making the following points:

1) Kin selection theory provides a solution to the problem of altruism (1,2). The problem is, why should an individual forego reproduction, and instead help another to breed (e.g., in cooperatively breeding birds, in mammals such as meerkats, or in social insects such as ants, bees, wasps, and termites)? This can be explained relatively easily if they are helping their close relatives reproduce and hence are still passing on their genes to the next generation, albeit indirectly. A pleasingly simple and elegant way of quantifying this idea of kin selection is Hamilton's rule (1,2). This states that individuals will be selected to perform altruistic behaviors for the benefit of relatives when rb - c > 0, where c is the fitness cost to the altruist, b is the fitness benefit to the beneficiary, and r is their genetic relatedness.

2) More generally, kin selection theory and Hamilton's rule can be applied to any situation involving conflict or cooperation. For example, it has been suggested that in birds and mammals, individuals are more likely to warn close relatives about the approach of predators; or that higher relatedness between parasites within a host will lead to less intense competition for the host resources, and so lower damage (virulence) to the host (1-4). Furthermore, kin selection theory is fundamental to explaining conflicts of interest between relatives. For example, an individual passes on one-half of its genes to her own offspring, but on average shares only one-fourth of her genes with those in the offspring of a brother or sister (full sibling), and so an individual should value her own reproduction twice as much as her brother's or sister's. Consideration of such conflicts of interest have been particularly successful in explaining the social behavior of social insects, where reproductive interests of the queens and their workers can often differ (5).

3) The authors are concerned with the often-neglected fact that altruistic behavior toward relatives may at some later time lead to increased competition between relatives, reducing or even completely removing the net selective advantage of altruism (4, 7-16). Put simply, altruism toward a relative is less advantageous if their increased fitness comes at a cost to your other relatives.

4) In summary: Individuals are predicted to behave more altruistically and less competitively toward their relatives, because they share a relatively high proportion of their genes (e.g., one-half for siblings and one-eighth for cousins). Consequently, by helping a relative reproduce, an individual passes its genes to the next generation, increasing their Darwinian fitness. This idea, termed kin selection, has been applied to a wide range of phenomena in systems ranging from replicating molecules to humans. Nevertheless, competition between relatives can reduce, and even totally negate, the kin-selected benefits of altruism toward relatives. Recent theoretical work has clarified the processes and selective forces underlying this effect and has demonstrated the generality of the effect of competition between relatives.

References (abridged):

1. W. D. Hamilton, Am. Nat. 97, 354 (1963)

2. W. D. Hamilton, J. Theor. Biol. 7, 1 (1964)

3. W. D. Hamilton, Annu. Rev. Ecol. Syst. 3, 193 (1972)

4. S. A. Frank, Foundations of Social Evolution (Princeton Univ. Press, Princeton, NJ, 1998)

5. A. F. G. Bourke, N. R. Franks, Social Evolution in Ants (Princeton Univ. Press, Princeton, NJ, 1995)

Science 2002 296:72

Related Background:

TESTING HAMILTON'S RULE WITH COMPETITION BETWEEN RELATIVES

S.A. West et al (University of Edinburgh, UK) discuss Hamilton's rule, the authors making the following points:

1) Hamilton's rule(1,2) provides a tool for understanding a range of social interactions, including altruism, aggression, selfishness and spite. It states that altruism (or less aggression) is favored when (rb - c > 0), where (c) is the fitness cost to the altruist, (b) is the fitness benefit to the beneficiary and (r) is their genetic relatedness. For a given benefit and cost, the evolution of altruism therefore relies upon a sufficiently high relatedness between interacting individuals. Hamilton(2) originally suggested that a high relatedness could arise in two ways: (1) behaviour based upon direct kin recognition between individuals, or (2) limited dispersal (population viscosity).

2) However, the importance of limited dispersal in increasing the relatedness among interacting individuals and favoring altruism has been controversial(3). Hamilton's original suggestion has been contested because limited dispersal can also increase competition between neighboring relatives, which opposes the evolution of altruistic behaviour(3). Unfortunately, empirical tests of theory, that determine the relative importance of increases in both relatedness and competition between relatives, have been hindered because both factors are influenced by dispersal, and so their effects are usually confounded(4).

3) The variable form of mate competition and population structure across fig wasp species with wingless males offers an opportunity for disentangling the confounded effects of relatedness and competition between relatives in viscous populations. Fig wasps are species that develop within the fruit of fig trees, and include mutualistic pollinating species as well as parasitic non-pollinating species. In many species the males are wingless, and mate with the winged females before the females disperse. The level of aggression between these non-dispersing males varies enormously across species. At one extreme, males of some non-pollinating species are highly modified for combat with armored bodies and huge mandibles. These mandibles are used to tear soft tissue and sever body parts, including limbs, head and abdomen, and can result in extremely high mortality levels. At the other extreme, males of other non-pollinating and most pollinating species show no modifications for combat or aggression.

4) In summary: Hamilton's theory of kin selection suggests that individuals should show less aggression, and more altruism, towards closer kin. Recent theoretical work has, however, suggested that competition between relatives can counteract kin selection for altruism. Unfortunately, factors that tend to increase the average relatedness of interacting individuals --such as limited dispersal -- also tend to increase the amount of competition between relatives. Therefore, in most natural systems, the conflicting influences of increased competition and increased relatedness are confounded, limiting attempts to test theory. Fig wasp taxa exhibit varying levels of aggression among non-dispersing males that show a range of average relatedness levels. Thus, across species, the effects of relatedness and competition between relatives can be separated. The authors report that -- contrary to Hamilton's original prediction but in agreement with recent theory(5) -- the level of fighting between males shows no correlation with the estimated relatedness of interacting males, but is negatively correlated with future mating opportunities.

References (abridged):

1. Hamilton, W. D. The evolution of altruistic behavior. Am. Nat. 97, 354-356 (1963)

2. Hamilton, W. D. The genetical evolution of social behavior, I & II. J. Theor. Biol. 7, 1-52 (1964)

3. Grafen, A. in Behavioral Ecology: An Evolutionary Approach (eds Krebs, J. R. & Davies, N. B.) 62-84 (Blackwell Scientific, Oxford, 1984)

4. Murray, M. G. & Gerrard, R. J. Conflict in the neighborhood: models where close relatives are in direct competition. J. Theor. Biol. 111, 237-246 (1984)

5. Wilson, D. S., Pollock, G. B. & Dugatkin, L. A. Can altruism evolve in purely viscous populations. Evol. Ecol. 6, 331-341 (1992)

Nature 2001 409:510

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