Encyclopedia of Evolution.pdf - Online Reading Center

altruism
Three Processes That Can Lead to the Evolution
of Altruism
Term Meaning
Kin selection Individual sacrifices for close
genetic relatives
Reciprocal altruism Individual sacrifices for another
individual that is likely to
reciprocate in the future
Indirect reciprocity Individual gains social status by
being conspicuously altruistic
services and take risks for what appears to be the good of the
population.
Like many rodents, Belding’s ground squirrels of the
Sierra Nevada Mountains of California have alarm calls that
alert the entire group to the presence of predators. The rodent
that sounds the alarm may or may not put itself in greater
danger of predation by doing so. Animals that sound an
alarm against predatory birds such as hawks actually reduce
their own risk; alarm calls against hawks are therefore not
altruism. An animal that sounds an alarm against predatory
mammals such as cougars, however, puts itself at greater risk
of being killed by that predator; alarm calls against cougars
are therefore altruism.
The problem with explaining altruism relates to evolutionary
fitness. natural selection will eliminate any tendency
to perform altruistic acts that result in a net loss of
fitness (successful transmission of genes into the next generation).
This is because the costs of altruism, unless they
are very minor, will deplete the resources that an individual
would use to produce or provision its own offspring and may
very well put the sacrificial altruist at risk of danger or death.
Evolutionary scientists must find individual benefits that
result from altruism; altruism cannot rely on a group benefit
(see group selection). The benefits, furthermore, must outweigh
the costs of altruism. At least three processes by which
altruism can evolve have been suggested: kin selection, reciprocal
altruism, and indirect reciprocity (see table).
Kin Selection
Evolutionary biologist W. D. Hamilton explained that the
total fitness of an individual includes not only those genes
passed on by the individual itself, through the successful production
of offspring, but also by the offspring produced by its
genetic relatives. He called this inclusive fitness.
Any two relatives share what Hamilton called a coefficient
of relatedness. Consider two half-sibs, such as two children
with the same mother but two different fathers. They
share half their parents, and from the shared parent there
is a 50 percent chance that they will inherit the same allele
(see Mendelian genetics). Their coefficient of relatedness
is ½ × ½ = ¼. Since full sibs share both parents, they
share ¼ + ¼ of their alleles, resulting in a coefficient of ½.
This is a matter of probability; in actuality, the two sibs may
share more, or less, than half of their alleles. On the average,
though, they will share one half of their alleles. Coefficients
of relatedness are reciprocal, which means they are equal in
both directions of comparison. An animal shares half of its
genes with its offspring (coefficient of relatedness = ½), as
well as with its parents. An animal shares one-fourth of its
genes with its nephews and nieces (coefficient of relatedness
= ¼), as well as with its aunts and uncles. An animal shares
one-eighth of its genes with its cousins. (Such a measure of
relatedness is not the same as a measure of DNA similarity;
see DNA [evidence for evolution]). An animal can
get its genes into the next generation half as efficiently by
devoting itself to its siblings and enabling them to reproduce,
as it would to produce its own offspring; devoting itself to
its cousins would be one-eighth as efficient. As early 20thcentury
biologist J. B. S. Haldane said, “I would die for two
brothers or ten cousins” (see Haldane, J. B. S.). A good
mathematician, Haldane said 10 rather than eight just to be
on the safe side.
Self-sacrifice, even to the point of death, can be favored
by natural selection, so long as it benefits the transmission of
genes through one’s relatives. Natural selection through inclusive
fitness is appropriately called kin selection. Kin selection
is also sometimes called nepotism, borrowing a term from
human interactions. Hamilton’s rule indicates that kin selection
can favor altruism if Br > C, in which B is the benefit,
C is the cost, and r is the coefficient of relatedness, all measured
in terms of fitness, the number of offspring. Altruism
may evolve if the benefit is large, the cost is low, and/or the
altruists and their recipients are close relatives.
According to this reasoning, an individual animal should
discriminate as to which other individual animals receive the
benefits of its altruism. But can an animal distinguish between
different degrees of relatedness? Species with intelligence and
complex social behavior can learn the identities of different
relatives. Mice can distinguish full from half-sibs on the
basis of body chemistry, particularly the major histocompatibility
complex proteins that function in the immune system.
Research by zoologist Paul Sherman shows that the Belding’s
ground squirrels mentioned above issue alarm calls against
cougars (true altruism) more often when close relatives are
present than when more distant relatives are present.
In some cases, fledgling birds expend their efforts defending
and providing food to the nests of other birds rather than
starting their own. This altruism can be explained by the fact
that the best territories have already been taken, and the fledgling
would be unlikely to establish a nesting territory that
would allow successful reproduction. The young bird does the
next best thing: it assists other birds in their reproduction. In
almost all cases, it is the close relatives that benefit from the
altruism of these birds, as one would expect from kin selection.
Kin selection has been particularly successful at explaining
the evolution of altruism in social insects, particularly
ants, bees, and wasps (order Hymenoptera) and termites
(order Isoptera). Social insects live in large colonies in which
the reproduction is carried out by one or a few dominant
individuals. Hymenopterans have a type of sexual reproduction
known as haplodiploidy, in which females have pairs of