Biology Reference
In-Depth Information
larval development. In
Lasius
, for example, whether a larva develops into a queen or a
worker appears to depend on factors such as nutrition, temperature and age of the
queen who laid the egg. In honeybees the queen can suppress the development of new
queens by chemical signals which prevent the workers from feeding larvae the special
diet ('Royal Jelly') needed to make them grow into queens.
Caste
differentiation
is usually
non-genetic
The economics of eusociality
Before discussing the factors that could have favoured the evolution of eusociality, it is
useful to remind ourselves how the problem can be phrased in terms of kin selection
theory, with Hamilton's rule, as described in Chapter 11. Hamilton's rule states that an
altruistic behaviour can be favoured if
rB − C
> 0, where
C
is the cost to the actor,
B
is
the benefit to the recipient and
r
is the genetic relatedness of the actor to the recipient.
This idea that altruistic behaviours can be selected if they are directed at other
individuals with the altruistic genes is the only plausible current explanation for
altruism. Consequently, if we want to explain eusociality, we must determine what
factors would have led to both a sufficiently high relatedness (
r
) between interacting
individuals, and a sufficiently high benefit/cost ratio (
B/C
) of helping to raise the
offspring of other individuals rather than breeding independently. In the next three
sections we will consider factors which could have influenced
r, B
and
C
.
Kin selection
theory tells us
how eusociality
could have
evolved …
… but not what
factors led to a
high enough
r
and
B
/
C
ratio
The pathway to eusociality
The two possible pathways along which the evolution of sterile castes could have
proceeded are considered in this section. The hypotheses described refer to evolutionary
history and, therefore, cannot be tested by direct experiments. In fact, both pathways
involve intermediates that are observed in present-day 'primitively eusocial'
hymenoptera, so the two hypotheses attempt to generalize from present patterns to
evolutionary history.
The first possible route to eusociality is via offspring remaining at their natal nest
after reaching maturity, and then helping their mother rather than breeding
independently. This is termed the subsocial route and intermediates can be observed in
some 'primitively eusocial' species, such as amongst the
Polistes
paper wasps and the
Stenogastrinae hover wasps. The second possible route is via multiple females coming
together to found a nest, termed the parasocial route. In the most primitively social case
each female would lay her own eggs and rear her own young, but this could then evolve
to a situation where one female gained dominance and the others became her workers.
Shared nest founding is also observed in several primitively eusocial species, such as
allodapine bees and other paper wasps. In termites the two routes would also involve
helping fathers or brothers respectively.
How can we test between these two alternate hypotheses? To do this it is necessary to
use the comparative approach and look at the distribution of different types of social
groups, maping them onto phylogenies. When this is done, all the evidence points
towards eusociality having evolved via the subsocial route. In the Hymenoptera, there
is no documented example where parasocial breeding is ancestral to the evolution of
The subsocial
route: staying at
home to help
your mother
The parasocial
route: sharing a
nest with sisters
