Inclusive Fitness and Altruism

The theory of inclusive fitness describes the mechanism that enables the evolution of social behaviors. (Queller, 1992) The significance of the genetic transmission of relatives who are not direct descendents of an individual often explains the origin of altruistic actions. (Queller, 1992) Relatives share a number of common genes in proportion to the proximity of their familial relation. Natural selection will favor altruism if the behavior increases the probability of the reproductive success of the genes that encode the behavior, regardless of the carrier identity.

Classical theories of evolution designate every characteristic of an organism as a means and a consequence of competition between individuals. (Eberhard, 1975) This qualification discounts the persistence of behavior that benefits another individual at a net cost to the performer. (Eberhard, 1975) Fitness is the measure of reproductive success as the number of mature offspring generated by a discrete unit. (Eberhard, 1975) Altruism is defined as actions that increase the fitness of a recipient and are detrimental to the personal fitness of the performing individual. (Eberhard, 1975) William Donald Hamilton derived a model titled inclusive fitness in order to account for the evolutionary origin of social altruism. (Eberhard, 1975)

Inclusive fitness is an extension of the conventional evolutionary theory that explains behavior that may appear decrease the fitness of an individual. (Eberhard, 1975) Hamilton’s inclusive fitness is the summation of the personal fitness of the individual and the resulting fitness change of relatives attributed to the individual's social behavior. Inclusive fitness considers the effect of the individual on the gene pool of succeeding generations through both the production of the individual’s own offspring as well as the effect of the individual on the reproduction of others. (Eberhard, 1975) A social behavior that increases the inclusive fitness of the performer is favored by natural selection and may be expected to increase in frequency within a population. (Eberhard, 1975)

Hamilton's rule describes the balance between the cost and benefit of altruistic actions. (Queller, 1992) The benefit of the altruist is proportional to the benefit of the recipient combined with the percent of genotype in common between the two individuals. (Queller, 1992) The net beneficial gain over the investment is the inclusive fitness of an altruistic behavior. (Queller, 1992) The simple formula proposed by Hamilton has been altered in numerous situations in order to model various circumstances. However, the foundation all successive modifications has been this simple relationship. Hamilton’s rule is also often expressed as an inequality comparing the product of benefit and relation to the cost of an action. (Foster, Weseleers & Ratnieks, 2006) An altruistic behavior is productive when the benefit experienced by the altruist is greater than the investment. (Queller, 1992)

The presence of the altruism gene in beneficiaries compensates the cost of donation. (Queller, 1992) The net inclusive fitness is positive if the product of the degree of relation between two individuals and the benefit of the recipient is a greater value than the investment required from the altruistic individual. (Queller, 1992)

The ultimate minimum of relatedness between individuals that can increase inclusive fitness through altruism is a value greater than the average degree of relation of the population as a whole. (Eberhard, 1975) Selection favors altruism when this criterion is satisfied. (Eberhard, 1975) While the change of allele frequencies may be very slow for social individuals with little relation compared to the total population, the direction of change will be positive regardless of the rate as long as there is some increased correlation of like genetics. (Eberhard, 1975)

The fraction of the beneficiary’s genotype that is not common to both social individuals contains a random collection of alleles present in the population. (Eberhard, 1975) The benefit of the random unrelated alleles in the recipient does not diminish the relative increase in frequency of the recipient genes in common with the donor. (Eberhard, 1975) A random process may periodically benefit any single unit, but this effect will become statistically insignificant over a continued length of time.

The kinship component of inclusive fitness is the lifetime summation of the increase of fitness to relatives weighted by the degree of relationship to the altruist. (Eberhard, 1975) The inclusive fitness of an individual is the sum of the classical personal fitness and the kinship component. (Eberhard, 1975) In this model, all social behaviors can be classified as selfish contributions to personal fitness or altruistic contributions to the kinship component. (Eberhard, 1975) These categories are not exclusive as it may be common to find both personal and kin fitness augmented simultaneously in a single action. (Eberhard, 1975)

The inclusive fitness of a behavior is quantified by a ratio of benefit to cost. (Eberhard, 1975) The threshold value for altruism that is advantageous to the individual is calculated by dividing the relatedness of the donor to their own personal offspring by the relatedness of the donor to the offspring of the beneficiary. (Eberhard, 1975) This value represents the number of additional offspring the beneficiary must produce to compensate each offspring lost by the donor. (Eberhard, 1975) The inclusive fitness of an action of altruism must exceed this critical threshold in order be advantageous to an individual. (Eberhard, 1975) However, altruism below this threshold will still increase the frequency of common alleles as long as some correlation of genotype commonalty is present relative to the population average. This increased allele fitness is not distributed to the majority of the genome when the degree of relation is less than the individual advantage ratio.

Relatedness is a statistical average. (West, Pen & Griffin, 2002) Individuals can be close relatives and share very little common genetics due to the principles of meiotic chromosome segregation into gametes. (West et al, 2002) It is assumed that siblings will share approximately a quarter of their genome on average, but it is also possible that they may share effectively no genes in common. (West et al, 2002) The opposite is also true. Some siblings may share nearly the entire genome as is the case with identical twins.

Gametes only receive one of two chromosomes in each homologous pair present in the parent. The constitution of any specific gamete is a random combination of individual homologs that are assorted independently. The zygote resulting from fertilization will contain a collection of homologous pairs that contain one chromosome inherited from each parent. The parental contribution is randomly determined by the segregation events of meiosis. The most anticipated common genotype proportion is fifty percent per degree of relative separation. Other combinations are present in a frequency corresponding to the statistical probability of inheritance.

Altruistic interaction frequency is anticipated to be proportional to the degree of relation between individuals. (Eberhard, 1975) However, altruism is common in social groups of individuals with various degrees of relation indistinguishable to members of the population. (Eberhard, 1975) In this situation, kin selection theory predicts that altruistic behaviors will occur with a frequency proportional to the average degree of relation of all potential beneficiaries within the population. (Eberhard, 1975) The average relatedness of the population also indicates the probability of observing specific types of altruism. (Eberhard, 1975)

The interaction of the three components of inclusive fitness indicates the expected frequency and intensity of altruism. (Eberhard, 1975) An altruistic act may be performed when there is very little probability of genotypic commonality if the cost is low enough and the benefit of the recipient is very high. (Eberhard, 1975) An act of altruism may also be expected when little benefit is experienced by the beneficiary if there is a large degree of relationship or a sufficiently low investment required by the donor. (Eberhard, 1975) It is also likely that altruistic behaviors will occur when the expense of the altruist is high if both the relatedness and the benefit of the recipient are large. (Eberhard, 1975)

A correlation between reproductive inferiority and altruism is likely to evolve in many populations. (Eberhard, 1975) Individuals with a low probability of reproduction may increase their fitness by facilitating the reproductive success of relatives. (Eberhard, 1975) This reproduction rate differentiation may be the result of a combination various incidental or genetic influences. (Eberhard, 1975)

Ecological conditions frequently play an important role in determining the nature of appropriate altruistic behaviors. (Eberhard, 1975) The probability of altruism occurring is inversely proportional to the degree of competition between two individuals. (Eberhard, 1975) Competition is often a function of the carrying capacity of the environment as well as the population size and structure. (Eberhard, 1975) The degree of relation between social individuals becomes critical when each additional conspecific individual places significant stress on a population. (Eberhard, 1975)

The majority of social interactions are likely to be essentially competitive. (Eberhard, 1975) The degree of competition between two individuals is positively correlated with similarity. (Eberhard, 1975) The most severe competition can be expected to be experienced between relatives due to spatial proximity and dependence on similar resources. (Eberhard, 1975) However, competition is also influenced by inclusive fitness. (Eberhard, 1975) The various factors controlling the dynamics of competition will tend to evolve in the direction that increases the fitness of the determining genes. (Eberhard, 1975) Competitive behavior frequency will increase only if the result is positively correlated with reproductive success of the superior competitor. (Eberhard, 1975) Competitive behavior will not increase if this correlation is negative or absent. (Eberhard, 1975)

The principles of kin selection apply to both competition and altruism. (Eberhard, 1975) Competition is the reverse application of inclusive fitness relative to altruism. The result of competition is typically the reduction of the fitness of the inferior individual in order to increase the fitness of the superior competitor. Competitive behaviors are likely to decrease in frequency if the fitness detriment to the common genes of the inferior is greater than increase of fitness of the victor.

Individuals are more likely to behave altruistically toward relatives due to the high proportion of gene commonality. (West et al, 2002) Helping a relative reproduce increases the number of common genes passed into successive generations. (West et al, 2002) However, competition between relatives can reduce the benefits of altruism. (West et al, 2002)

Hamilton’s rule of kin selection can be applied to any situation involving cooperation and conflict. (West et al, 2002) Altruistic behavior toward relatives may often increase the competition between relatives in successive generations. (West et al, 2002) The inclusive fitness of altruism is inversely proportional to the increase of competition between descendants. (West et al, 2002) The extent of the benefit reduction due to competition is dependent on the natural history of the population. (West et al, 2002)

Limited dispersal of individuals increases the relatedness of social participants. (West et al, 2002) However, limited dispersal also increases the relatedness of potential competitors. (West et al, 2002) The increased relation of competitors opposes the increase of inclusive fitness due to altruistic interactions. (West et al, 2002) Ultimately, the inclusive fitness of a behavior is determined by the net effect on future generations. (West et al, 2002)

The time of dispersal affects the inclusive fitness of altruism. (West et al, 2002) The fitness of an altruistic behavior is greatest when it occurs before dispersal and then competition occurs after dispersal. (West et al, 2002) More generally, the inclusive fitness of altruism is maximized when the relatedness of individuals is increased locally when the altruism occurs, and competition occurs globally. (West et al, 2002) The global distribution of competition minimizes the severity experienced between relatives. (West et al, 2002)

Hamilton’s rule can be altered to account for the increased competition between relatives due to altruism. (West et al, 2002) Inclusion of competition into models of inclusive fitness alters the net benefit experienced by altruistic individuals. (West et al, 2002)

The decrease to inclusive fitness is essentially zero if the altruist is unrelated to the competitors of the beneficiary. (West et al, 2002) An altruistic act will not increase competition in this situation. (West et al, 2002) Resulting competition increases in proportion to the degree of relation between competitors and the beneficiary. (West et al, 2002)

Discrimination capacity facilitates the function of altruism in saturated environments. (Lehman & Perrin, 2002) The evolution of altruistic actions requires interacting individuals to be more related than the population on average. (Lehman et al, 2002) While neighbors are likely to be relatives, they are necessarily also likely to be competitors in dense populations. (Lehman et al, 2002) Mechanisms of kin discrimination enable individuals to select social partners based on indications of relation. (Lehman et al, 2002)

Discrimination based on proximity allows the evolution of some altruistic behavior in the absence of severe ecological strain. (Lehman et al, 2002) However, environmental conditions may often cause wide dispersal of relatives or competition between relatives. (Lehman et al, 2002) Associative learning enables the recognition of relatives based on phenotypic characteristics. (Lehman et al, 2002) Discrimination based on characteristics promotes altruism as the inclusive fitness of interactions is increased. (Lehman et al, 2002)

The principles of natural selection indicate that discrimination abilities should increase in frequency and accuracy. (Lehman et al, 2002) Phenotype matching is the principle method of kin recognition. (Lehman et al, 2002) The donor compares the phenotype of the potential partner to a template standard based on a series of uncorrelated traits. (Lehman et al, 2002) An optimal acceptance threshold minimizes errors of both incorrect participation with unrelated individuals as well as the rejection of relatives. (Lehman et al, 2002) Various conditions will determine the requirements of a favorable acceptance threshold. (Lehman et al, 2002) Generally, the specificity of an acceptance threshold will likely decrease as when there is an increase of average relatedness to all potential recipients, a decrease of investment cost or an increase in the benefit of the recipient. (Lehman et al, 2002)

Spatially based discrimination requires certain ecological situations in order to increase the frequency of altruistic behavior. (Lehman et al, 2002) Discrimination founded on regional proximity is only effective when altruism has little cost because resources are abundant and few migrants are present in a population. However, associative learning may favor altruism without any environmental preconditions. (Lehman et al, 2002) The optimal specificity of discrimination exists as a continuum correlated with the severity of ecological strain. (Lehman et al, 2002) The scarcity of resources increases the cost of altruistic investment as well as the benefit of migration. (Lehman et al, 2002) Migratory dispersal increases the necessary acceptance threshold of altruism. (Lehman et al, 2002)

Cue heritability is correlated with recognition capacity. (Lehman et al, 2002) Environmental conditions may alter the acceptance threshold through the influence of phenotypic characteristics. (Lehman et al, 2002) The most accurate kin discrimination methods are necessary when cue heritability is low and environmental influence is high. (Lehman et al, 2002) This combination requires a great threshold of similarity of kin relative to the population average in order to favor altruistic actions. (Lehman et al, 2002)

Kin selection theory has received some criticism that the behaviors attributed to kin selection often do not necessarily require inclusive fitness in order to account for observed frequencies. (Eberhard, 1975) However, there are often multiple factors that contribute to the fitness of any single allele. (Eberhard, 1975) Mary Jane West Eberhard provides the example of the farmer who saves the life of his brother. (Eberhard, 1975) The farmer has increased the frequency of both his genetic alleles in the population as well as secured his own personal fitness by increasing the productivity of the farm. (Eberhard, 1975)

Mutualism is another explanation of altruistic behavior. (Clutton-Brock, 2002) Mutualism is defined as reciprocal cooperation. (Eberhard, 1975) The benefit of a mutualistic association is experienced directly by the interacting individuals without the necessity of common genes. (Clutton-Brock, 2002) This model accounts for cooperation observed in unrelated individuals. (Clutton-Brock, 2002) Individuals may exchange beneficial acts in a reciprocal altruism scenario similar to a symbiosis. (Clutton-Brock, 2002)

The increase of fitness through mutualism requires strict controls on cheating. (Eberhard, 1975) Cheaters are individuals that experience the benefit of population investments without returning any reciprocal fitness increase to donators. This type of population fitness distribution likely only occurs in intelligent animals. (Eberhard, 1975) However, kin selection theories require no direct benefit to the individual and do not necessitate a mechanism of reciprocity. Although, some individuals, such as migrants, may experience the increased fitness while not possessing genes in common with the majority of the population.

Hamilton’s rule does not relate to the fitness of a single individual genotype. (Queller, 1992) Rather, the inclusive fitness benefit is experienced by the genes within a population. (Queller, 1992) The individual is only able to personally benefit from altruistic investment if there is a significant probability of reciprocity. Hamilton’s definition of altruism implies that there is no expectation of reciprocation between individuals. This type of nonreciprocal altruistic behavior is best accounted for in the consideration of the fitness of the genes that encode the behavior. The genes correlated with altruism experience an intrinsic increase of fitness assuming that the recipient shares the genes in common. The individual may not experience any direct benefit of the behavior, but the gene fitness is increased within the population.

A competing theory used to account for altruistic behavior is group selection. (Queller, 1992) All participants may contribute to some common good that is beneficial in a group selection situation. (Clutton-Brock, 2002) Group selection accounts for altruism by claiming that populations containing altruistic genes experience increased fitness compared to other populations that do not. The foundation of this hypothesis is the distribution of the success experienced by an altruistic individual throughout a population to increase the total fitness of the population to a greater value than may be anticipated if the success of each individual is considered distinct. However, the apparent difference in the two models is only an artifact of formulation. (Queller, 1992) The two approaches are alternative but equivalent methods of selection analysis defined by different terms of relative fitness.

The fitness of group members increases in proportion to the size of some cooperative populations. (Clutton-Brock, 2002) This group augmentation scenario offers an explanation accounting for many forms of cooperative behavior. (Clutton-Brock, 2002) The correlation between group size and fitness may create strong selection pressures favoring cooperative behavior. (Clutton-Brock, 2002)

The frameworks of selection at the level of the colony and kin are simply different methods of formalizing the same problem. (Foster et al, 2006) Kin selection models often produce results at the level of the colony. (Foster et al, 2006) Group section models often require differential degrees of relation within a population relative to other groups. (Foster et al, 2006)

The analytical perspective is the primary distinction between models of group and kin selection. Group selection theories require that an altruistic population contain a large frequency of altruistic alleles. Inclusive fitness theories account for the allele frequencies within a population. Group selection theories are only able to account for the frequency of altruistic populations within a metapopulation. While selection at the group or population level may occasionally affect the fitness of social genes, the allele must first become established at the population level. (Eberhard, 1975) The differentiation between the two theories is essentially the frame of reference in determination of relative fitness. However, the absolute fitness of the altruistic genes is the same regardless whether the basis of relative comparison is other individuals within a single population or other competing populations.

Hamilton’s definition of inclusive fitness considers primarily the donor of the relationship. (Eberhard, 1975) However, it is also possible that selection on recipient individuals may influence the probability of the altruistic behavior. (Eberhard, 1975) Selection may operate on other members of the population in order to facilitate or inhibit performance of the behavior. (Eberhard, 1975) It is possible that inclusive fitness may be increased in certain scenarios if the beneficiary refuses aid that comes at too great a cost to the altruist. (Eberhard, 1975)

An individual may often increase their fitness in coordination with others who do not share the benefit. (Clutton-Brock, 2002) The consequence of the interaction may be neutral or negative for many involved in some situations. (Clutton-Brock, 2002) Coercion often accounts for the participation of individuals that do not benefit from social behaviors. (Clutton-Brock, 2002)

Inclusive fitness does not predict the evolution of altruism without the component of common genetics. (Foster et al, 2006) Manipulations often contribute to cooperative behaviors, but this type of interaction is not altruistic. (Foster et al, 2006) The coercion of cooperation is more accurately described as a neutral symbiosis or parasitism if the acting individual is the only beneficiary. (Foster et al, 2006)

The multiple forces of causality in phenotypic expression introduce factors that may affect fitness but are not correlated with the genetic trait. (Queller, 1992) The application of Hamilton’s rule is most general when fitness is attributed to genes rather than phenotypes. (Queller, 1992) Consideration of the inclusive fitness of a gene allows the model to partition other effects on fitness. (Queller, 1992) This enables quantification of multiple factors contributing to the fitness of the individual phenotype including and in addition to the inclusive fitness of altruistic genes. (Queller, 1992)

Kin selection is the theory, based on Hamilton’s ideas, that refers to the subcategory of natural selection in which genetic allele frequencies change due to the differential reproductive rate of relatives within a population. (Eberhard, 1975) The central premise of kin selection is that the degree that an altruistic behavior increases the fitness of an individual is correlated to the proportion of genes identical by descent shared between relatives. (Eberhard, 1975) The definition of inclusive fitness as the application of Hamilton’s rule to altruistic genes results in the description of a special case of general selection models. (Queller, 1992) In this method, altruism is accounted for as the increased gene reproduction probability in relatives. (Queller, 1992) Increasing the reproductive success of genes identical by descent increases the frequency of the individual’s own alleles. (Eberhard, 1975)

Literature Cited

Clutton-Brock, T. (2002) Breeding Together: Kin Selection and Mutualism in Cooperative Vertebrates. Science, 296(72), 69-72.

Eberhard, M. (1975) The Evolution of Social Behavior by Kin Selection. The Quarterly Review of Biology, 50(1), 1-33.

Foster, K. Wenseleers, T. & Ratnieks, F. (2006) Kin Selection is the Key to Altruism. Trends in Ecology and Evolution, 21(2), 57-60.

Lehman, L. & Perrin, N. (2002) Altruism, Dispersal, and Phenotype-Matching Kin Recognition. The American Naturalist, 159(5), 451-468.

Queller, D. (1992) A General Model for Kin Selection. Evolution, 46(2), 376-380.

West, S. Pen, I. & Griffin, A (2002) Cooperation and Competition Between Relatives. Science, 296(72), 72-75.