and Kin Selection
If, as Darwin suggested, animals
should behave selfishly and strive to
produce as many offspring as possible,
why do some animals help others at
some risk to themselves? Why do some
individuals show utmost cooperation
with members of their social group
and even forego breeding themselves?
Why do some individuals appear to
sacrifice themselves so that other members
of their group can survive? Until
the mid 1960s, scientists had trouble
explaining in Darwinian terms how
such altruistic behavior
in a population.
Most instances of altruistic behaviors
were explained using a groupselection
argument. Group selectionists
suggested that animals that helped
others or that failed to mate did so for
the benefit of the other members of the
group. Therefore, such behaviors produce
increased survivorship of groups
whose members behaved altruistically.
According to proponents of this argument,
selection occurs at the level of
the group, not at the level of the individual
as Darwin suggested. However,
the group-selection argument as originally
proposed by V. C. Wynne-
Edwards in 1962 has been rejected by
the vast majority of behavioral ecologists
for a number of reasons.
For example, if in a social group
there were randomly distributed genes
for a risky altruistic behavior, such as
giving calls to warn others of predators,
those lacking such genes would
flourish. They would be warned with
no risk to themselves; their chances of
reproduction would be greater and, in
time, the “selfish” alleles would eliminate
the altruistic ones from the
group’s gene pool.
In 1964, W. D. Hamilton, based
largely on his studies of insects, proposed
a new way to explain altruistic
behavior by modifying Darwin’s original
concept of fitness. He reasoned
that fitness is measured not just by the
number of offspring produced but by
the increase or decrease in particular
alleles in the gene pool. Thus, an individual
may act altruistically, even at
great risk, if it helps increase representation
of its alleles in the gene pool.
Alleles are shared by all relatives,
including parents and offspring, brothers
and sisters, cousins and other relations.
Alleles that influence altruistic
behavior among relatives would persist
in future generations. Since the most
closely related animals share the most
genes by common descent, we expect
that altruistic behavior would be most
common among closely related individuals.
Thus, if everything else were
equal, brothers who on average share
half their alleles would be more likely
to aid one another than they would a
cousin who shares on average only
25% of their alleles. Hamilton’s hypothesis
based on this genetic explanation
for altruism and cooperation is called kin selection
. Essentially, kin selection
is the selection of genes by individuals assisting the survival and reproduction
of individuals who possess the same
genes by common descent.
Hamilton’s hypothesis revolutionized
evolutionary and behavioral biology.
The main criterion of Darwinian
fitness is the relative number of an
individual’s alleles that are passed to
future generations. Hamilton, however,
developed the concept of inclusive
which is the relative number
of an individual’s alleles that are
passed on to future generations either
as a result of an individual’s own
reproductive success or that of related
Thus, kin selection and
inclusive fitness may be able to explain
many altruistic behaviors that have
perplexed biologists for many years.
|Figure 38-20 Haplodiploidy in honey bees, showing degrees of relatedness
of a female worker bee (labeled SELF)
to individuals she might raise.
In honey bees, as in other haplodiploid animals, diploid females develop
fertilized eggs,and males develop from unfertilized eggs.
Each daughter of a male gets all his
genes (purple bar) and full sisters receive an identical one half of their genome from the same father.
Open bars represent other, unrelated alleles. Because full sisters also
share half the genesthey receive
from their common mother (yellow bar), the relatedness of SELF to a full sister is 0.75,the average of
0.5 and 1.0.
(In a diploid-diploid system as in humans,the relatedness of siblings
is 0.5 because both
paternally and maternallyinherited genes have a
50% chance of being present in a sibling.)Note that
female workers to a brother is only 0.25, because brothers are fatherless.
A good example of altruism and
kin selection in nature is the remarkable
cooperation and coordination
among euscocial insects such as ants,
bees, and wasps. Through haplodiploidy
, where males are
haploid and females are diploid, sisters
are genetically related on average by
75% rather than 50% (Figure 38-20).
Sisters are more closely related to each
other than to their own daughters!
Therefore, they cooperate with other
members of their social group, forego
breeding themselves and aid the queen
to produce more sisters who are more
closely related (75% related) than
potential offspring (50% related).
|Figure 38-21 Belding’s ground squirrel,
Spermophilus beldingi,gives an alarm call
to warn of the
approach of a predator.
This risky behaviorendangers the callers
more than noncallers
Female Belding’s ground squirrels,
found in the High Sierra of California,
give alarm calls when a predator
approaches (Figure 38-21). Alarm calling
warns other members of the social
group and is risky to the alarm caller.
However, the benefits to alarm calling
outweigh the risks because alarm callers
are warning related
alarm-calling behavior, even if it is risky,
may be favored by selection if it
increases inclusive fitness of the caller.
Kinship theory suggests that animals
may evolve an ability to recognize
categories of relatives so that cooperation
or aid-giving behavior will
be directed more efficiently toward
relatives. Although kin recognition
behavior was discussed by Hamilton,
little was known about it until almost
20 years after he wrote his seminal
papers. Through a number of experimental
studies we now know that a
variety of species can discriminate
between kin and non-kin. Such species
are found among invertebrates, including
isopods and insects, and vertebrates,
including fishes, frog and toad
tadpoles, birds, squirrels, and monkeys.
Some species can even discriminate
between full siblings and half-siblings
and between cousins and unrelated
individuals. Thus, some species have a
finely tuned ability to identify relatives
of various degrees of relatedness. The
cues used in kin recognition vary from
species to species. Birds use vocalizations
whereas many other species use