Biology Reference
In-Depth Information
BOX 11.1
CLASSIFYING SOCIAL BEHAVIOURS
Bill Hamilton (1964) showed how social behaviours can be defined according to
their fitness consequences for the actor and recipient. A behaviour is social if it
has fitness consequences for both the individual that performs that behaviour
(the actor) and another individual (the recipient). Social behaviours are
classified according to whether the consequences they entail for the actor
and recipient are beneficial (increase fitness) or costly (decrease fitness)
(Table B11.1.1). A behaviour which is beneficial to the actor and costly to the
recipient (
+
/
−
) is selfish, a behaviour which is beneficial to both the actor and
the recipient (
) is mutually beneficial, a behaviour which is costly to the actor
and beneficial to the recipient (
+
/
+
) is altruistic, and a behaviour which is costly
to both the actor and the recipient (
−
/
+
) is spiteful (Hamilton, 1964). Selfish and
mutually beneficial behaviours can be explained from the perspective of
individuals maximizing their reproductive success. Altruistic and spiteful
behaviours can only be explained by also taking account of the indirect
consequences of the behaviour.
−
/
−
Table B11.1.1
Classification of social behaviours
Effect on actor
Effect on recipient
+
−
+
Mutually beneficial
Selfish
−
Altruistic
Spiteful
Kin selection and inclusive fitness
The most familiar example of an individual giving aid to another is, of course, parental
care. We are not surprised to see a parent bird hard at work feeding its offspring because
natural selection favours individuals who maximize their genetic contribution to future
generations. The young will have copies of their parent's genes, so parental care is one
way in which parents can increase their genetic contribution to the next generation.
We can quantify the probability that a copy of a particular gene in a parent is present
in one of its offspring. In diploid species, when an egg and a sperm fuse to form a zygote,
each parent contributes exactly 50% of its genes to the offspring. Therefore, the
probability that a parent and an offspring will share a copy of a particular gene because
the offspring obtained it from that parent is 0.5. This likelihood that individuals share a
gene that is identical by descent (inherited from the same ancestor) provides a measure
of the
coefficient of relatedness,
often denoted by
r
.
Now offspring are not the only relatives to share copies of the same genes identical
by descent. Again we can calculate the probability that a copy of a gene in one
The coefficient of
relatedness
measures the
genetic similarity
of two individuals
relative to the
population at
large
