Fitness is normally defined as the number of offspring an individual will
produce.
Natural selection works to maximize fitness, because traits can only be selected
for if they are passed down to progeny. Natural
selection therefore seems to favor selfish
actions
that promote one's own fitness over the fitness of other members of one's
species. This seeming truth runs into difficulties, however, when faced with
actions of social altruism that are quite prevalent in animal society. For
example, in eusocial colonies, some individuals forgo reproduction
altogether. How can such an action be explained in light of what we know about
natural selection and evolution?
The answer comes when we consider an individual's inclusive fitness, which
is the sum of an individual's direct fitness, the number of offspring
produced, and indirect fitness, the number of relatives (nieces and nephews)
produced multiplied by the degree of relatedness of those individuals.
Relatedness
The degree of relatedness (r) is the
probability of being identical by descent through a common ancestor. As we
shall see, this value is dependent on the genetic nature of the species
involved. The degree of relatedness between diploid animals will defer than
that for haplodiploid animals.
Relatedness Among Diploid Animals
The parent to offspring relationship is r=.5 because each parent has half its
genes in the offspring. The offspring to parent relationship is also r=.5
because half the offspring's genes come from each parent. Full siblings are
related by r=.5 and half siblings are related by r=.25. shows how to calculate relatedness.
Figure %: Calculated relatedness among individuals
There may be several paths of genetic transmission from one individual to
another. The r's between each pair of individuals are multiplied along a path,
and paths are summed to yield the total degree of relatedness. If the lineage
of any path is uncertain, we multiply the uncertain path by the probability of
guessing the correct lineage. For instance, a daughter's degree of relatedness
to one of four men who all are equally likely to be her father is
r=(.5)(1/4)=.125.
Relatedness Among Haploiddiploid Animals
Let's now examine the case for haplodiploid organisms, such as ants, in
which females
inherit a set of genes from both parents, but males are haploid, resulting from
unfertilized eggs, and receive only half of their mother's genes.
Figure % Calculated relatedness among haplodiploid individuals
As you can see in , the mother's relationship to her
daughter, and the daughter's relationship to her mother, are still r=.5. A
mother's relationship to her son is r=.5 because he received half of her genes,
however the son's relationship to the mother is 1 because all of his genes are
from his mother. A daughter's relationship to her father is r=.5 because half
of her genes are from her father, but a father's relationship to his daughter is
r=1 because she has all his genes since he only has one copy. There is no
father/son relationship because males result from unfertilized eggs.
Kin Altruism
Now that we have included indirect fitness in our definition of inclusive
fitness, we can see that individuals can derive some benefit by helping to raise
their siblings or other relatives: through these siblings and relatives the
genes of the helping individual are passed on. This is really not "altruism" in
the true sense of the word because the individual does indirectly benefit. When
and who should an individual help? The answer lies in the relative costs and
benefits of the aid. Danger or giving up your own chance to reproduce are costs
(C) of altruism. The benefit (B) is the aided individual's reproductive
success. Therefore, individuals should only act altruistically when the
indirect benefit is greater than the cost, or mathematically, when B > C.
