Biology Reference
In-Depth Information
It is often useful to measure the costs and benefits in terms of offspring lost and gained,
in which case we use the following form of Hamilton's rule:
r
B
donor to own offspring
donor to recipient's offspring
>
r
C
Two examples will help to make this clear. Imagine an individual has a choice between
rearing its own offspring and helping its mother to produce offspring. The individual's
own offspring and its mother's offspring, assuming they are full siblings, both have
r
1. Therefore, helping will be favoured by
kin selection if by your help your mother produces more extra offspring than you have
'sacrificed' through providing help (i.e. through forgoing the chance to produce your
own offspring). If the individual was faced with the alternative of rearing its own
offspring or helping its sister to produce offspring, then the expression becomes
B
/
C
=
0.5, so the expression above becomes
B
/
C
>
2
(0.5/0.25). In this case helping would evolve only if it resulted in two or more extra
offspring produced by the sister for every one offspring lost by the donor. The above
examples stress that kin selection isn't just about genetical relatedness (
r
), it is also
about the ecological factors that determine the cost (
C
) and benefit (
B
) of behaviours.
>
Examples of altruism between relatives
The social insects provide good examples of extreme altruism. Worker bees have barbed
stings and attack predators which approach their nests. In the act of stinging the
predator the barbs of the sting become embedded in the victim and the worker bee dies
as a result. The evolution of such suicidal behaviour poses a problem until we discover
that the beneficiaries of the altruistic act are, in fact, close relatives of the worker.
Workers are altruistic in another way because they rarely reproduce themselves, but
instead help others in the nest to produce offspring. Darwin regarded this observation as
potentially fatal to his theory of natural selection. How can such altruism evolve? The
theory of kin selection immediately suggests a possible answer to this problem: the
sterile workers usually help their mother (the queen) to produce offspring, and so she is
the one who passes the altruistic genes onto future generations (Chapter 13).
Not all acts of altruism are as extreme as suicide or sterility. We now consider two
examples where the costs to the altruist are smaller, but where it is still likely that kin
selection has been a major force in driving the evolution of the behaviour.
Extreme altruism:
suicide and
sterility in the
social insects
Cooperation and alarm calls in ground squirrels and prairie dogs
Paul Sherman made an extensive study of Belding's ground squirrel,
Urocitellus beldingi
,
a diurnal social rodent which inhabits the subalpine meadows of the far western United
States (see photographs in Figs. 11.5 and 11.6). This species hibernates during the
winter, emerging above ground in May. Soon after emergence the females become
sexually receptive and mate. After mating the males wander off and leave the females to
rear their young alone. A female establishes a territory surrounding its nest burrow and
produces a single litter of three to six young per year. The pups first emerge above
Less extreme
altruism - alarm
calls
