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.
siblings: r = 0.5
children to grandparents: r = 0.25
first cousins: r = 0.125
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.
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
daughter to father: r= 1.0
daughter or son to mother: r=0.5
sister to sister: r=0.75
sister to brother: r=0.25
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
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