Biology Reference
In-Depth Information
Andy Gardner and colleagues (2004) argued that the production and action of such
bacteriocins fulfils all three of the conditions required for spite. Firstly, the production of
bacteriocins incurs a large direct cost, so cannot be explained as a selfish behaviour.
Indeed, in some species, cell death is required to release the bacteriocins into the
environment, and so it represents another sterile trait. Secondly, the negative
consequences of bacteriocins are directed towards relatively unrelated cells. The reason
for this is that there is genetic linkage between the bacteriocin gene and the immunity
gene for that bacteriocin, such that close relatives will have the capacity to both produce
and be immune to a particular bacteriocin, or neither. Consequently, the bacteriocin
will only harm non-relatives. Thirdly, the benefits of killing non-relatives go to close
relatives. This is because the clonal growth of bacteria means that that when multiple
clones grow and compete in an area cells can be near both close relatives (clone mates,
r
Bacteria produce
chemicals that kill
non-relatives, to
reduce the
competition
experienced by
relatives
=
1) and non-relatives (
r
=
0).
Summary
Altruism is acting to increase the number of offspring that another individual produces
at a cost to one's own chances of survival or reproduction. Extreme examples of altruism
include the sterile workers of social insects and the stalk cells of slime moulds. Hamilton
showed that altruism could be explained by kin selection. The idea here is that an
individual can increase its genetic representation in future generations by helping close
relatives, who share copies of its genes. The conditions for the spread of an altruistic
behaviour are given by Hamilton's rule,
rB-C
>0. Examples of traits that may be
explained by kin selection include alarm calling in ground squirrels and cooperative
courtship in wild turkeys.
Kin selection requires a sufficiently high relatedness between interacting individuals.
One way to obtain this is kin discrimination, where individuals are able to assess
relatedness and then preferentially help closer relatives. An extreme form of genetic
discrimination is greenbeard genes, which lead to both recognition and helping, and
hence the gene only helping other individuals that have that specific gene. However,
whilst examples exist, such as in the fire ant, this mechanism is unlikely to be of general
importance. The more common form of discrimination is to use kinship as an indicator
of shared genes. Such kin discrimination can involve genetic cues of kinship (e.g. slime
moulds), environmental cues of kinship (e.g. long-tailed tits, chapter 12) or both (e.g.
Belding's ground squirrels).
The alternative way to obtain a high relatedness is limited dispersal. Limited dispersal
would keep relatives together, and hence indiscriminate altruism could be favoured
towards neighbours because they would be likely to be relatives. An example of this is
the production of iron scavenging siderophore molecules in bacteria.
Kin selection theory also predicts that individuals should behave less selfishly when
interacting with closer relatives. This can explain why salamanders are less likely to
cannibalize closer relatives.
Finally, kin selection theory has a darker side, predicting that spiteful behaviours can
evolve if they are preferentially directed at non-relatives. Spite appears to be rare, but
examples include the sterile soldiers in polyembryonic parasitoid wasps and chemical
warfare (bacteriocins) in bacteria.
