Testing Kin Selection Theory

As well as being supported by a large body of mathematical theory, the parameters in Hamilton's rule are measurable and the theory is testable. However, it is also deceptively simple - in many cases this simple rule can hide a huge amount of complexity. For example, the term kin selection is used to refer to selection in two different situations: when the gene of interest is shared due to common ancestry alone or more broadly to any situation where the gene is shared. For example, in the latter case, the relatedness between two individuals who are known to carry the gene for an altruistic trait would be r = 1, regardless of kinship by co-ancestry.

Hamilton's rule clarifies the predictions of kin selection theory: traits will be more likely to spread if they maximize r and b and minimize c.

Maximizing Relatedness

There are two ways in which an appreciable relatedness between social partners can arise.

1. Kin discrimination. If, as is often the case, an animal is faced with a decision of who to help, potential beneficiaries may or may not be related. Kin discrimination refers to a process by which an altruist discriminates with respect to relatedness when deciding who to help. In long-tailed tits, in which case helpers have the choice of helping at several different nests in the territory, it has been shown that they preferentially provide help at the nests of relatives (see Figure 1).

2. Population viscosity. This refers to a population structure where dispersal is limited from a natal patch. By

Figure 1 The difference in the amount of help provided at the nests of relatives (94%) relative to nonrelatives (6%) in a cooperatively breeding bird, the long-tailed tit.

Figure 3 Each circle represents a species of cooperatively breeding bird or mammal. For each species, the extent of kin discrimination was measured as the effect of kinship on the likelihood or the amount of help given by nonbreeding helpers. The extent of kin discrimination was correlated with the benefit of providing help which was measured as the proportion of offspring surviving to 1 year. As predicted by kin selection theory, when helpers were more helpful, they were also more choosy about which individuals they helped.

Figure 2 Two populations of bacteria were maintained and the changes in frequency of an altruistic trait (production of a molecule involved in the scavenging of iron) was monitored across time. When there was high relatedness between cells the frequency of altruism increased, providing the first experimental support for the prediction that relatedness facilitates the evolution of altruism.

Figure 3 Each circle represents a species of cooperatively breeding bird or mammal. For each species, the extent of kin discrimination was measured as the effect of kinship on the likelihood or the amount of help given by nonbreeding helpers. The extent of kin discrimination was correlated with the benefit of providing help which was measured as the proportion of offspring surviving to 1 year. As predicted by kin selection theory, when helpers were more helpful, they were also more choosy about which individuals they helped.

chance, a potentially altruistic individual will be surrounded by relatives and so any altruistic act it performs will, by chance, benefit those who share the altruistic gene. This is the case in social insect colonies, which are typically founded by one or a few reproductive queens. Viscosity may account for the fact that there is no evidence for kin discrimination in eusocial insects. The costs of such a system are not worth the benefits as workers have not evolved in an environment where help is squandered on nonrelatives.

Although there is a wealth of evidence in support of kin selection theory, much of it is correlative. This is mainly because it is difficult to design experiments where relatedness and altruism can be manipulated. Recently, however, this has been made possible using microorganisms. In populations where relatedness between social interactants was higher, a higher level of cooperation was selected for (see Figure 2).

Maximizing Benefit

One field where a great deal of work has been done on the ability of kin selection theory to explain altruistic behavior is in the study of cooperative breeding in birds and mammals. In such species, nonbreeding helpers remain in the natal territory to help raise offspring in subsequent breeding seasons rather than dispersing to reproduce. The question often asked is: do helpers at the nest preferentially give help to more related individuals.? Cooperatively breeding species offer the opportunity to test this prediction of kin selection theory. In several species, such as the long-tailed tit and the Seychelles warbler, helpers were found to discriminate in favor of kin. However, in other species, such as the kookaburra and the meerkats, workers do not appear to discriminate in favor of kin. In such cases, other explanations for helping behavior are needed based on direct fitness benefits. However, it turns out that Hamilton's rule predicts this pattern. Further analyses have shown that the extent to which helpers discriminate depends on the amount of benefit provided by helping: if there is no benefit to help in terms of offspring raised then there is no incentive to discriminate (see Figure 3). This provides an across-species test of Hamilton's rule - when b is higher, preferential helping of relatives is more likely to be favored.

Minimizing Cost

There is less empirical support for the prediction that cost, in terms of direct fitness, is minimized for the simple reason that it is a difficult parameter to measure. Whereas relatedness can be estimated using pedigree or genetics and benefit can be measured by counting offspring, the cost resulting from competition with relatives is more difficult to quantify. However, kin selection theory has been supported by a study which measured the cost of helping in terms of direct fitness in the hairy-faced hover wasp. In the hairy-faced hover wasp, there is a single dominant breeding female, and helpers that provide aid to the dominant, form a queue to reproduce. The queue is based on age: when the dominant female dies, the second oldest female takes over the breeding position. Cost in this example is easily measured in terms of queue length. Helpers help less when the queue to reproduce is shorter: a shorter queue means more to lose in terms of direct fitness and so selection does not favor investment in helping.

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