Kin Selection - Mechanisms

Mechanisms

An altruistic case is one where the instigating individual suffers a fitness loss while the receiving individual benefits by a fitness gain. The sacrifice of one individual to help another is an example of altruism.

Hamilton (1964) outlined two ways in which kin selection altruism could be favoured:

The selective advantage which makes behaviour conditional in the right sense on the discrimination of factors which correlate with the relationship of the individual concerned is therefore obvious. It may be, for instance, that in respect of a certain social action performed towards neighbours indiscriminately, an individual is only just breaking even in terms of inclusive fitness. If he could learn to recognise those of his neighbours who really were close relatives and could devote his beneficial actions to them alone an advantage to inclusive fitness would at once appear. Thus a mutation causing such discriminatory behaviour itself benefits inclusive fitness and would be selected. In fact, the individual may not need to perform any discrimination so sophisticated as we suggest here; a difference in the generosity of his behaviour according to whether the situations evoking it were encountered near to, or far from, his own home might occasion an advantage of a similar kind." (1996, 51)

Kin Recognition: First, if individuals have the capacity to recognize kin (kin recognition) and to adjust their behaviour on the basis of kinship (kin discrimination), then the average relatedness of the recipients of altruism could be high enough for this to be favoured. Because of the facultative nature of this mechanism, it is generally regarded that kin recognition and discrimination are unimportant except among 'higher' forms of life (although there is some evidence for this mechanism among protozoa). Note also that kin recognition may be selected for inbreeding avoidance, and little evidence indicates that 'innate' kin recognition plays a role in mediating altruism. A thought experiment on the kin recognition/discrimination distinction is the hypothetical 'green beard', where a gene for social behaviour is imagined to also cause a distinctive phenotype that can be recognised by other carriers of the gene. Due to conflicting genetic similarity in the rest of the genome, there would be selection pressure for green-beard altruistic sacrifices to be suppressed, thus making common ancestry the only likely form of inclusive fitness (see Hamilton, below).

Viscous Populations: Secondly, even indiscriminate altruism may be favoured in so-called viscous populations, i.e. those characterized by low rates or short ranges of dispersal. Here, social partners are typically genealogically close kin, and so altruism may be able to flourish even in the absence of kin recognition and kin discrimination faculties — spatial proximity and circumstantial cues serve as a rudimentary form of discrimination. This suggests a rather general explanation for altruism. Directional selection will always favor those with higher rates of fecundity within a certain population. Social individuals can often ensure the survival of their own kin by participating in and following the rules of a group (assuming the implied faculties for group discrimination).

Hamilton later modified his thinking to suggest that an innate ability to recognise actual genetic relatedness would be unlikely to be the dominant mediating mechanism for kin altruism (see also kin recognition):

But once again, we do not expect anything describable as an innate kin recognition adaptation, used for social behaviour other than mating, for the reasons already given in the hypothetical case of the trees.(Hamilton 1987, 425)

Hamilton's later clarifications often go unnoticed, and because of the persistence of the long standing assumption that kin selection theory necessarily predicts that social cooperation requires innate powers of kin recognition, some theorists have tried to clarify the position in recent work:

In his original papers on inclusive fitness theory, Hamilton pointed out a sufficiently high relatedness to favour altruistic behaviours could accrue in two ways — kin discrimination or limited dispersal ( Hamilton, 1964, 1971,1972, 1975). There is a huge theoretical literature on the possible role of limited dispersal reviewed by Platt & Bever (2009) and West et al. (2002a), as well as experimental evolution tests of these models (Diggle et al., 2007; Griffin et al., 2004; Kümmerli et al., 2009 ). However, despite this, it is still sometimes claimed that kin selection requires kin discrimination (Oates & Wilson, 2001; Silk, 2002 ). Furthermore, a large number of authors appear to have implicitly or explicitly assumed that kin discrimination is the only mechanism by which altruistic behaviours can be directed towards relatives... here is a huge industry of papers reinventing limited dispersal as an explanation for cooperation. The mistakes in these areas seem to stem from the incorrect assumption that kin selection or indirect fitness benefits require kin discrimination (misconception 5), despite the fact that Hamilton pointed out the potential role of limited dispersal in his earliest papers on inclusive fitness theory (Hamilton, 1964; Hamilton, 1971; Hamilton, 1972; Hamilton, 1975). (West et al. 2010, p.243 and supplement)

The assumption that innate power of kin recognition must be present, and that cue-based mediation of social cooperation based on limited dispersal and shared developmental context are not sufficient, has obscured significant progress made in applying kin selection and inclusive fitness theory to a wide variety of species, including humans, on the basis of cue-based mediation of social bonding and social behaviours.