Kin Selection Term Paper

  • Length: 11 pages
  • Subject: Genetics
  • Type: Term Paper
  • Paper: #11768218

Excerpt from Term Paper :

Kin Selection

The organization and functioning of human and animal societies has long been the subject of intense investigations by natural scientists, sociologists and geneticists. Darwin, who laid the foundation for modern theory of evolution, suggested 'kin selection' as an explanation for the existence of sterile females, the worker caste, in social insects like 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 aids 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 behaviour 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 the 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 the traits. (Kin selection and kin altruism)

Altruism is a significant trait that is supposed to have evolved through kin selection. The best example of kin selection is seen in social hymenoptera such as ants, wasps and bees, where the sterile workers allows their sister, the queen of the colony, to carry on all reproduction. In these insects, fertilized eggs (diploid) develop into females, which carry the genes derived from both parents, and, 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 haplodiploidy predisposes these insects to the evolution of altruism through kin selection.

In such advanced forms of social systems, eusociality, members of the colony sacrifice reproductive opportunities for the "common good of the colony." However, Hamilton's simple and elegant theory that offers adequate scientifically acceptability explanation of hymenopteran eusociality, it does not 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 eusociability in such groups could be genetic relatedness that may arise from inbreeding. (Kin altruism and eusociability)

Kin selection may be used to explain the evolution of human societies as well as the social structures in insects such 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 reproduces it. Thus the frequency of occurrence of the allele for altruism can be enormously increased even though the altruist does not reproduce it. Altruism evolves not because of increased survival and reproduction of the altruist, which has reduced reproduction, but rather because it is reproduced by the kin of the altruist, which have 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 proportion greater than alternative alleles for non-altruism. Hamilton has described the conditions under which an allele for altruism will have higher frequency of occurrence and, therefore, evolve. He states these conditions in a formula: br - c > 0 or b x r > c where b, stands for 'benefit," and refers to the enhanced reproductive benefit gained by recipient of altruism; r refers to the chance that the aided individual carries the same gene for altruism; c stands for "cost," to the altruist in terms of the number of offspring the altruist could have had if it had not been an altruist. Hamilton's formula leads to the concept that in a randomly mating and out breeding diploid population, an individual should sacrifice itself in order to save" two siblings, four nephews or eight cousins" since siblings share 50% of the individual's genes, nephews 25% and cousins 12.5%.

It is evident that 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" which 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, the individuals are helped by altruistic individuals and consequently produce more offspring.

Hamilton's Formula, bxr >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 chance (r) that the recipient also has the allele for altruism is higher. Thus, the more offspring the recipient has, the greater will be the occurrence of the allele in the offspring. The degree to which non-altruism is passed on is given by c in the formula. If br is greater than c, it indicates that the allele for altruism is passed on more than the allele for non-altruism and therefore altruism has higher inclusive fitness, causing it to evolve. Thus the formula, br-c>0, indicates when altruism will evolve. (Inclusive fitness)

Hamilton's formula may be used to predict or explain situations in which altruism evolves. Factors, which tend to increase b or r, will support the evolution of altruism and factors that would make c larger will not promote the evolution of altruism. In nature, when food or nesting sites become scarce, c would be low and b, 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, then r will be low and altruism will not evolve are related. Genetic system of a species, for example haplodiploidy can also affect r. This is true in social insects like 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 is considered. In a species where both male and female are haploid, about half the alleles, that is one quarter of the alleles, the sisters inherit from the mother will be the same.

Likewise, about half of the alleles, that is a quarter of the alleles, the sisters will inherit from the father will also be the same. The total relatedness is obtained by adding the proportion of identical alleles received from the mother to the proportion of identical alleles 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. But on the father's side, there is a difference because the sperms are produced by mitosis and therefore sperms have the same number of chromosomes as in other somatic cells. So half the alleles received from the father are the same between sisters. This increases the proportion of the whole genome that is same. The total relatedness, r, in this case will be: 1/4 (from the mother) + 1/2 (from the father) = ae. (Evolution of altruism)

The fact that full sisters in haplodiploid species have higher relatedness explains why sterile castes (female workers) that help sisters (queen bee) have evolved in social hymenopterans. Altruistic behavior such as alarm calling in squirrels, helpers at the nest in scrub jays etc. In which animals appear to cooperate in spite of disadvantage to themselves is not eliminated by natural selection as could be expected. This deviation may be explained by the fact that the recipient of the altruistic act is a relative of the donor. Relatedness is expressed as a coefficient, r, and is defined as the percentage of genes that the two individuals share by common descent. A simple example will be to calculate the coefficient of relatedness between a parent and its offspring in diploid system. The offspring (F1) inherits half of the genome from the particular parent and will have a coefficient of relatedness of 0.5. One generation down, the grandoffspring (F2), will have half of the genome of F1 or quarter of the genome of its grand parent. Thus, a grandparent and grandoffspring have a coefficient of relatedness, r, of 0.25. In general, r=0.5n where n is the number of generation links. Relatedness and altruism form the concept of inclusive fitness.

The eu-social system of termites, which is similar to ant societies in many ways, could have been derived through kin selection provided the individuals practiced brother-sister matings or mother-son matings. Genes become identical across the colony, by descent through inbreeding. In sib-sib mating, in contrast to haplodiploidy, the probability of identity of descent through repeated inbreeding can lead to values that rival or exceed the 0.75 of haplodiploid hymenoptera. If…

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