Biology Reference
In-Depth Information
The main point is that there is nothing particularly special about offspring as kin; if
we saw a bird helping to feed a younger brother or sister this could also be favoured by
selection as a means of passing on copies of genes to future generations. To put this
more generally, a gene can increase its transmission to the next generation either by
increasing the reproductive success of the individual in which it is in, or by increasing
the reproductive success of other individuals who carry copies of that gene. These
different routes for passing a copy of a gene to the next generation are termed direct and
indirect respectively.
Hamilton showed that when these indirect effects are taken into account, natural
selection on genes will lead to individuals behaving in a way that maximizes their
inclusive
fitness
rather than their own or direct reproductive success. Inclusive fitness is defined as
the sum of direct and indirect fitness, where
direct fitness
is defined as the component of
fitness gained from producing offspring, and
indirect fitness
is defined as the component
of fitness gained from aiding related individual, both descendant and non-descendant
(Box 11.4). Maynard Smith (1964) coined the term
kin selection
to describe the process
by which characteristics are favoured due to their effects on relatives.
A gene's eye
view: direct
and indirect
transmission
Inclusive fitness is
the sum of direct
and indirect
fitness
Kin selection:
effects on
relatives
Hamilton's rule
The conditions under which an altruistic act will spread due to kin selection are as
follows (Hamilton, 1963, 1964). Imagine an interaction between an altruist (or actor)
and a recipient in which the costs and benefits of the interaction can be assessed in
terms of survival chances of the actor and recipient. The actor could be helping the
recipient in any way, such as giving an alarm call, or feeding it. If the actor suffers cost
C (through, for example, making itself more visible to a predator, or losing a food item)
and the recipient gains a benefit
B
as a result of the altruistic act, then the gene causing
the actor to behave altruistically will increase in frequency if:
B
1
r
>
or alternatively, if
BC
−
>
0
,
r
C
where
r
is the coefficient of relatedness of the actor to the recipient. This pleasingly
simple result is known as 'Hamilton's rule' (Charnov, 1977). Put into words, altruistic
cooperation can be favoured if the benefits to the recipient (
B
), weighted by the genetic
relatedness of the recipient to the actor (
r
), outweigh the costs to the actor (
C
).
An intuitive understanding of Hamilton's rule can be seen as follows. As an extreme
example of altruism imagine a gene that programs an individual to die in order to save
the lives of relatives. One copy of the gene will be lost from the population in the death
of the altruist, but the gene will still increase in frequency in the gene pool if, on average,
the altruistic act saves the lives of more than two brothers or sisters (
r
Hamilton's rule
predicts when
altruistic acts will
be favoured by
selection
=
0.5), more than
four nieces or nephews (
r
0.125). Having made
these calculations on the back of an envelope in a pub one evening, J.B.S. Haldane
announced that he would be prepared to lay down his life for the sake of two brothers or
eight cousins!
=
0.25) or more than eight cousins (
r
=
