Everything about Kin Selection totally explained
From the time of antiquity field biologists have observed that some organisms tend to exhibit strategies that favor the reproductive success of their relatives, even at a cost to their own survival and/or reproduction. The classic example is a
eusocial insect colony, with sterile females acting as workers to assist their mother in the production of additional offspring. Many evolutionary biologists explain this by the theory of
kin selection.
The earliest expressions of the basic concepts were by
R.A. Fisher in 1930,
JBS Haldane in 1955, but it was
W. D. Hamilton who truly formalized the concept, in works published in 1963and - most importantly - in 1964, while the actual term "kin selection" may first have been coined by
John Maynard Smith (1964) when he wrote "These processes I'll call kin selection and
group selection respectively. Kin selection has been discussed by Haldane and by Hamilton. ... By kin selection I mean the evolution of characteristics which favour the survival of close relatives of the affected individual, by processes which don't require any discontinuities in the population breeding structure."
Kin selection refers to changes in [(gene)] frequency across generations that are driven at least in part by interactions between related individuals, and this forms much of the conceptual basis of the theory of
social evolution. Indeed, some cases of
evolution by
natural selection can only be understood by considering how biological relatives influence one another's
fitness. Under
natural selection, a gene encoding a
trait that enhances the fitness of each individual carrying it should increase in frequency within the
population; and conversely, a gene that lowers the individual fitness of its carriers should be eliminated. However, a gene that prompts behaviour which enhances the fitness of relatives but lowers that of the individual displaying the behavior, may nonetheless increase in frequency, because relatives often carry the same gene; this is the fundamental principle behind the theory of kin selection. According to the theory, the enhanced fitness of relatives can at times more than compensate for the fitness loss incurred by the individuals displaying the behaviour. As such, this is a special case of a more general model, called "
inclusive fitness" (in that inclusive fitness refers simply to gene copies in other individuals, without requiring that they be kin).
Hamilton's rule
Formally, such genes should increase in frequency when
»
where
» r = the genetical relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same
locus is identical by descent.
B = the additional reproductive benefit gained by the recipient of the altruistic act,
» C = the reproductive cost to the individual of performing the act.
This inequality is known as
Hamilton's rule after
W. D. Hamilton who published, in 1964, the first formal quantitative treatment of kin selection to deal with the
evolution of apparently altruistic acts. Altruistic acts are those that benefit the recipient but harm the actor. The phrase
Kin selection, however, was coined by
John Maynard Smith.
Originally, the definition for
relatedness (r) in Hamilton's rule was explicitly given as
Sewall Wright's
coefficient of relationship: the
probability that at a random
locus, the
alleles there will be
identical by descent (Hamilton 1963, American Naturalist, p. 355). Subsequent authors, including Hamilton, sometimes reformulate this with a
regression, which, unlike probabilities, can be negative, and so it's possible for individuals to be negatively related, which simply means that two individuals can be less genetically alike than two random ones on average (Hamilton 1970, Nature & Grafen 1985 Oxford Surveys in Evolutionary Biology). This has been invoked to explain the evolution of
spiteful behaviours. Spiteful behavior defines an act (or acts) that results in harm, or loss of fitness, to both the actor and the recipient.
In the 1930s
J.B.S. Haldane had full grasp of the basic quantities and considerations that play a role in kin selection. He famously said that, "I would lay down my life for two brothers or eight
cousins".
Kin altruism is the term for altruistic behaviour whose evolution is supposed to have been driven by kin selection.
Haldane's remark alluded to the fact that if an individual loses its life to save two siblings, four nephews, or eight cousins, it's a "fair deal" in evolutionary terms, as siblings are on average 50% identical by descent, nephews 25%, and cousins 12.5% (in a
diploid population that's randomly mating and previously
outbred). But Haldane also joked that he'd truly die only to save more than one identical set of twins or more than two full siblings.
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.
Firstly, 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's generally regarded that kin recognition and discrimination are unimportant except among 'higher' forms of life (although there's some evidence for this mechanism among
protozoa). A special case of the kin recognition/discrimination mechanism is the hypothetical '
green beard', where a gene for social behaviour also causes a distinctive phenotype that can be recognised by other carriers of the gene. Hamilton's discussion of greenbeard altruism serves as an illustration that relatedness is a matter of genetic similarity and that this similarity isn't necessarily caused by genealogical closeness (kinship).
Secondly, even indiscriminate altruism may be favoured in so-called viscous populations, for example 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. 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 their own kin by participating in, and following the rules of a group.
It should be noted that these mechanisms explain a relatively high r between interacting individuals. Absolute genetic similarity isn't a measure of r; rather, r shows the “excess” relatedness between an actor and a recipient compared with the relatedness between an actor and a random member of the population. Thus, in a clonal population with 100% genetic similarity, r = 0 (as strange as that may sound). This is because there can be no correlation between genetic similarity and interaction strengths if genetic similarity is constant. This is why it has often been observed that altruism can't be maintained in a population of randomly interacting individuals (see and references therein). In such a population, the correlation between genetic similarity and interaction strength is necessarily absent, thus r = 0 and rB < C for any C > 0. This is why mechanisms such as spatial structure and kin recognition are so important for the long-term stability of altruistic traits, and why measures such as "population-wide average r" are meaningless in the absence of such mechanisms.
Kin Selection in Evolutionary Psychology
Evolutionary psychologists have attempted to explain prosocial behavior through kin selection by stating that “behaviors that help a genetic relative are favored by natural selection.” Human beings have developed a tendency over time to frame and interpret their actions as an avenue to the survival of their genetic material, making kin selection not a completely altruistic form of prosocial behavior and is perhaps better described as a component of social exchange theory. This theory doesn't necessarily imply that people “compute” genetic benefit when helping others, but there's an indication that those who behave in such a way are more likely to pass on their genes to future generations.
Evolutionary psychology (abbreviated EP) is a theoretical approach to psychology that attempts to explain mental and psychological traits—such as memory, perception, or language—as adaptations, that is, as the functional products of natural selection or sexual selection.
Examples
Eusociality (true sociality) is used to describe social systems with three characteristics: one is an overlap in generations between parents and their offspring, two is cooperative brood care, and the third characteristics is specialized castes of nonreproductive individuals.
Social insects are an excellent example of organisms that display presumed kin selected traits. The workers of some species are sterile, a trait that wouldn't occur if individual selection was the only process at work. The relatedness coefficient
r is abnormally high between the worker sisters in a colony of Hymenoptera due to
haplodiploidy, and Hamilton's rule is presumed to be satisfied because the benefits in
fitness for the workers are believed to exceed the costs in terms of lost reproductive opportunity, though this has never been demonstrated empirically. There are competing hypotheses, as well, which may also explain the evolution of social behavior in such organisms (see
Eusociality).
Alarm calls in ground squirrels are another example. While they may alert others of the same species to danger, they draw attention to the caller and expose it to increased risk of predation. Paul Sherman, of Cornell University, studied the alarm calls of ground squirrels. He observed that they occurred most frequently when the caller had relatives nearby.In a similar study, John Hoogland was able to follow individual males through different stages of life. He found that the male prairie dogs modified their rate of calling when closer to kin. These behaviors show that self-sacrifice is directed towards close relatives and that there's an indirect fitness gain.
Alan Krakauer of University of California, Berkeley has studied kin selection in the courtship behavior of wild turkeys. Like a teenager helping her older sister prepare for prom night, a subordinate turkey may help his dominant brother put on an impressive team display that's only of direct benefit to the dominant member.
Recent studies provide evidence that even certain plants can recognize and respond to kinship ties. Using
sea rocket for her experiments, Susan Dudley at McMaster University in Canada compared the growth patterns of unrelated plants sharing a pot to plants from the same clone. She found that unrelated plants competed for soil nutrients by aggressive root growth. This didn't occur with sibling plants.
Further Information
Get more info on 'Kin Selection'.
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