Biology Reference
In-Depth Information
obligate sterile castes (Bourke & Franks, 1995; Boomsma, 2007; Hughes
et al
., 2008).
Furthermore, examining the cases where both types of nest occur in the same species,
the helpers in subsocial groups tend to be more altruistic than the helpers in parasocial
groups, in terms of giving up greater amounts of their own reproductions (Reeve &
Keller, 1995). In the termites, both workers and soldiers are specialized juveniles, as
expected from the subsocial route, and there is no evidence that reproductives move
between colonies in a way that would allow parasocial breeding. Furthermore, the
parasocial route is also inconsistent with the existing support for a key role of monogamy
in the evolution of eusociality, which we shall discuss in greater detail later.
Eusociality
evolved by the
subsocial route
The haplodiploidy hypothesis
The masters of eusociality are clearly the Hymenoptera. Despite the fact that they
constitute only approximately 6% of all insect species, eusociality has evolved more
times in the Hymenoptera than in any other taxa (Crozier, 2008). Bill Hamilton (1964,
1972) was the first to suggest that this might be because the Hymenoptera have a
genetic predisposition to the evolution of sterile castes. The special feature is
haplodiploidy:
males develop from unfertilized eggs and are haploid, while females develop from
normally fertilized eggs and are, therefore, diploid.
A haploid male forms gametes without meiosis, so that every one of his sperm is
genetically identical. This means that each of his daughters receives an identical set of
genes to make up half her total diploid genome. With a diploid father, a female would
stand a 50% chance of sharing any particular one of his genes with her sisters, but with
a haploid father she is certain to share all of them (assuming the mother mated only
once). The other half of a female hymenopteran's genes come from her diploid mother,
so she has a 50% chance of sharing one of her mother's genes with a sister. If we now
think about the total degree of relatedness between sisters we come to a remarkable
conclusion. Half their genome is always identical, and the other half has a 50% chance
of being shared, so the total relatedness is 0.5
Haplodiploidy:
a special feature
of Hymenoptera
…
0.75. In other words,
because of haplodiploidy, full sisters are more closely related to one another than are
parents and offspring in a normal diploid species. Hymenopteran queens are diploid and
are, therefore, related to their sons and daughters by the usual 0.5 (Box 13.1,
Table 13.1). A sterile female worker can, therefore, make a greater genetic profit by
rearing a reproductive sister than she could if she suddenly became fertile and produced
a daughter!
The potential consequences of this for the evolution of altruism can be illustrated
with Hamilton's rule. If we measure the costs and benefits in terms of offspring lost and
gained, this leads to the following form of Hamilton's rule:
+
(0.5
×
0.5)
=
… results in
unusual patterns
of relatedness …
r
B
Cr
donor to own offspring
>
donor to recipient' soffspring
If we then compare the relative advantage to a worker of either producing a daughter
(
r
daughter
0.75) then Hamilton's rule would be
satisfied as long as
B
/
C
> 2/3. Put into words, this means that a worker would make a
genetic profit if, by helping her mother to reproduce, she could raise slightly more than
=
0.5) or helping raise a sister (
r
sister
=
