Biology 301 - Inclusive Fitness

Ecology and Evolution: Biology 301 -- Wethey
Department of Biological Sciences
University of South Carolina

Inclusive Fitness

Altruism

Natural selection generally occurs at the individual level - the individual with the highest reproductive success has the greatest genetic representation in future generations.
How then can altruistic traits (like giving up reproduction and caring for relatives' offspring) evolve?
Since relatives share genes by common descent, an altruistic gene in an individual that gives up reproduction, may be passed on by the relative who is aided. This is kin selection. The degree of relatedness between relatives determines the chance that the relative has a particular gene, and therefore the chance that the altruistic gene will be passed on.
Relatedness (r):

Altruistic traits can spread by kin selection under the following conditions according to evolutionary theorist JBS Haldane: If the altruist sacrifices C offspring and the recipient gains B offspring as a result of the altruism, the altruistic gene will spread if

rB - C > 0

and r is the relatedness between the altruist and the recipient. Haldane claimed that he would give up his life for 2 brothers or eight cousins, based on this calculation.

Evolution of Sterile Castes in haplodiploid Insects

Sterile castes in social insects are an extreme case of altruism. Worker females give up reproduction, and raise their mother's offspring. This occurs in bees, wasps, and ants.
WD Hamilton explained the evolution of altruism in bees, wasps and ants on the basis of their genetic system, which is haplo-diploid. Males are haploid and develop from unfertilized eggs. Females are diploid and develop from fertilized eggs. Because males are haploid, there is no meiosis and all daughters of a particular male have identical genes from their father.
Relatedness in haplodiploid social insects is complicated (Stiling Table 4.3):

Females are more closely related to their sisters (r=0.75) than to their own offspring (daughers or sons r=0.5). Thus by giving up reproduction, they have a greater chance of passing on their own alleles via reproductive sisters than they do by reproducing themselves.

Evolution of Sterile Castes in Diploid mammals and insects

Sterile workers also exist in diploid organisms including termites and naked mole rats. RD Alexander proposed that this kind of sociality could evolve in in diploid organisms because females are related both to daughters and sisters by 0.5 so raising sibs or daughters is equally advantageous. He argued that the following conditions would favor sociality:

species living in burrows
abundant food supply
parents care for offspring
mechanisms exist for mothers to manipulate others
heroism is possible as individuals sacrifice themselves for the queen

Naked mole rats show these characteristics

Group Living: Selfish herds

WD Hamilton proposed that group living in flocks, herds and schools is advantageous to the selfish individual within the group. In a circular herd, only the individuals at the periphery are at risk from predators, so even if every member of the group is selfish, the individual risk is less in the group than alone.
Large groups are more likely to detect predators than small groups or individuals. This provides an advantage to the individuals in the group.
Individual fluid dynamic advantages (geese and fish use vortices from members of a group to produce lift or reduce drag).
Individual foraging efficiency can be greater in a group where food is patchy - there is a greater chance of exploiting patchy prey if many individuals are searching, and individuals follow one another to prey.