Biology Reference
In-Depth Information
bearded
bearded
actor
or
non-
bearded
Fig. 11.3
Greenbeard genes cause the actor to either provide help to other individuals
who have the beard or harm individuals who do not. Photograph of the fire ant,
Solenopsis invicta,
shows workers executing a queen who does not carry the
b
allele.
From Gardner and West (2010). Photo © Ken Ross.
Laurent Keller and Ken Ross (1998) discovered an example of a greenbeard gene in
the fire ant,
Solenopsis invicta
. In this species, nests contain multiple queens, with new
queens being recruited into nests after they have mated. The green beard gene is the
Gp
-9 locus. Workers with the
b
allele at the
GP
-9 locus use odour to determine whether
prospective queens also carry this allele - and decapitate them if they do not. This
benefits the queens with the
b
allele, who are recruited and do not have to compete for
resources in the nest with females who do not carry the
b
allele.
However, such greenbeards are unlikely to be common or generally important. One
problem is that they would need to be complex, doing three things - signal, recognition
and direct cooperation. It is hard enough to imagine a gene that completely encodes for
one behaviour, let alone three! Another problem is that they could be easily invaded by
'falsebeards' that displayed the beard, without also performing the altruistic behaviour.
Consistent with this, only a very small number of greenbeards have been discovered.
Most examples of greenbeards are in microbes, possibly because they have a relatively
simpler link between genotype and phenotype, which could prevent a decoupling of
beard and social trait, and hence make it harder for falsebeards to arise.
Fire ants have a
greenbeard gene,
causing workers
to eliminate
queens that do
not have this
gene
Greenbeard genes
are unlikely to be
common, because
they can be
outcompeted by
falsebeards
Direct genetic kin discrimination and armpits
Given the problem with directly recognizing a copy of it itself, the next best thing is to
use kinship as a probabilistic means of sharing genes (Table 11.1). One way to
recognize kin would be on the basis of direct genetic cues that can be recognized
phenotypically. For example, if some aspect of scent was genetically determined, then
more closely related individuals would smell more similar. In this case, kin
discrimination could occur via rules such as 'be altruistic to neighbours who smell
similar to you'. Richard Dawkins rather evocatively termed such kin discrimination
the 'armpit effect'.
Recognizing kin
directly on the
basis of genetic
similarity
