Kin Selection, Altruism, and Inclusive Fitness Explained

Introduction to Kin Selection and Altruism

The organization and functioning of human and animal societies has long been the subject of intense investigation by natural scientists, sociologists, and geneticists. Darwin, who laid the foundation for the modern theory of evolution, suggested kin selection as an explanation for the existence of sterile females — the worker caste — in social insects such as ants, bees, and termites. Later, W. D. Hamilton mathematically established the Theory of Kin Selection as a mechanism for the evolution of such apparently altruistic sterile castes.

Altruism refers to the actions of an individual that aid in increasing the survival and reproduction of another individual while sacrificing its own survival and reproduction. Kin altruism is the technical term used to define altruistic behavior that is theoretically explained by kin selection. Though kin altruism would appear to act counter to natural selection — the driving force of the evolution of species — Hamilton proposed that kin selection is just another form of natural selection. Certain traits evolve because they are passed on by relatives (kin) of individuals who express those traits.

Altruism is a significant trait that is thought to have evolved through kin selection. The best example of kin selection is seen in social Hymenoptera such as ants, wasps, and bees, where sterile workers allow their sister, the queen of the colony, to carry on all reproduction. In these insects, fertilized eggs (diploid) develop into females, which carry genes derived from both parents, while unfertilized eggs (haploid), having only the maternal complement of genes, develop into males. This form of sex determination, known as haplodiploidy, leads to a high coefficient of relatedness and, according to Hamilton, predisposes these insects to the evolution of altruism through kin selection.

In such advanced social systems — known as eusociality — members of the colony sacrifice reproductive opportunities for the common good of the colony. Hamilton's simple and elegant theory offers a scientifically acceptable explanation of hymenopteran eusociality; however, it does not fully explain the evolution of eusociality in other groups, such as termites, where haplodiploidy is not the mechanism of sex determination. A possible explanation for the occurrence of eusociality in such groups could be the genetic relatedness that arises from inbreeding.

Hamilton's Rule and Inclusive Fitness

Kin selection may be used to explain the evolution of human societies as well as the social structures in insects such as ants, wasps, bees, and termites. Altruism is a genetic trait, which may or may not be expressed by the individuals that carry it. If an altruistic individual helps another individual to reproduce and the recipient of such help is genetically related to the altruist, the recipient is likely to carry the allele for altruism and to reproduce it. Thus, the frequency of occurrence of the allele for altruism can be enormously increased even though the altruist does not directly reproduce.

Altruism evolves not because of increased survival and reproduction of the altruist — which in fact has reduced reproduction — but rather because it is reproduced by the kin of the altruist, who carry but do not express the genes for altruism. Like any other genetic trait, altruism will evolve only if it is passed on from generation to generation in a proportion greater than alternative alleles for non-altruism.

Hamilton described the conditions under which an allele for altruism will have a higher frequency of occurrence — and therefore evolve — in a formula: br – c > 0, where b stands for "benefit" and refers to the enhanced reproductive benefit gained by the recipient of altruism; r refers to the probability that the aided individual carries the same gene for altruism; and c stands for "cost" to the altruist in terms of the number of offspring the altruist could have produced had it not acted altruistically. Hamilton's formula leads to the concept that, in a randomly mating outbreeding diploid population, an individual should sacrifice itself to save "two siblings, four nephews, or eight cousins," since siblings share 50% of an individual's genes, nephews 25%, and cousins 12.5%.

Altruistic individuals, with their impaired or reduced reproduction, cannot be directly responsible for the evolution of the alleles for altruism through the typical process of natural selection. An alternate mechanism — called inclusive fitness — refers to the degree to which a trait is passed from generation to generation. A trait may be passed on to the next generation directly by individuals who express the trait, or indirectly by individuals who carry the trait but do not express it. In the latter case, those individuals are helped by altruistic individuals and consequently produce more offspring.

Hamilton's formula, br > c, explains when altruism will have higher inclusive fitness than non-altruism. According to the formula, the number of offspring that a recipient of altruism produces (b) will increase when the probability (r) that the recipient also carries the allele for altruism is higher. Thus, the more offspring the recipient has, the greater will be the occurrence of the allele in subsequent generations. The degree to which non-altruism is passed on is given by c. If br is greater than c, it indicates that the allele for altruism is passed on more frequently than the allele for non-altruism, and therefore altruism has higher inclusive fitness, causing it to evolve.

Haplodiploidy, Eusociality, and Genetic Relatedness

Hamilton's formula may be used to predict or explain situations in which altruism evolves. Factors that tend to increase b or r will support the evolution of altruism, while factors that make c larger will not promote it. In nature, when food or nesting sites become scarce, c would be low and b — the benefit to individuals who receive help and reproduce more — would be high. Ecological factors can also affect r. If individuals who interact with each other are not related, r will be low and altruism will not evolve. The genetic system of a species — for example, haplodiploidy — can also affect r, as is true in social insects such as ants, wasps, and bees, which have very high levels of altruism.

When determining relatedness, the half inherited from the mother and the half inherited from the father must be considered. In a diploid species where both male and female are diploid, about half the alleles — that is, one quarter of the total alleles — that sisters inherit from the mother will be the same, and likewise about one quarter of the alleles inherited from the father will also be identical. The total relatedness is obtained by adding the proportion of identical alleles received from the mother to the proportion received from the father: 1/4 + 1/4 = 1/2.

In haplodiploid species, the inheritance from the mother is the same as in diploid species, since the mother is diploid. However, on the father's side there is a difference: sperms are produced by mitosis and therefore carry the same number of chromosomes as other somatic cells. This means that all of the alleles received from the father are identical between sisters. This increases the proportion of the whole genome that is shared between sisters. The total relatedness (r) in haplodiploid species is therefore: 1/4 (from the mother) + 1/2 (from the father) = 3/4.

The fact that full sisters in haplodiploid species share a higher relatedness of 3/4 explains why sterile castes (female workers) that help sisters (the queen) have evolved in social Hymenoptera. Altruistic behaviors such as alarm calling in squirrels and nest-helping in scrub jays — in which animals appear to cooperate despite apparent disadvantage to themselves — are not eliminated by natural selection as might otherwise be expected. This is explained by the fact that the recipient of the altruistic act is a relative of the donor. Relatedness is expressed as a coefficient, r, defined as the percentage of genes that two individuals share by common descent.

A simple example illustrates the coefficient of relatedness in a diploid system. The offspring (F1) inherits half of the genome from a particular parent and therefore has a coefficient of relatedness of 0.5. One generation further, the grandoffspring (F2) has half the genome of F1, or one quarter of the genome of its grandparent; thus the grandparent and grandoffspring have a coefficient of relatedness, r, of 0.25. In general, r = 0.5ⁿ, where n is the number of generational links. Relatedness and altruism together constitute the concept of inclusive fitness.