Biology Reference
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Haplodiploidy can
even predispose
groups against
the evolution of
eusociality
helping to pay is
B/C
>
= 2
. This is twice the value for a diploid species, showing
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that as worker control of the sex ratio spreads through a population, the resultant
female bias in the population can select against helping in haplodiploids! This is because
it makes sons more valuable than a female-biased mixture of sisters and brothers. The
consequences of haplodiploidy for the evolution of eusociality are unresolved, and this
remains a contentious area.
Let us summarize the complicated ups and ups downs of the haplodiploid
hypothesis. Hamilton's (1964, 1972) haplodiploid hypothesis was that haplodiploidy
would lead to the relatedness to siblings being greater than that to offspring, and so
would lead to a genetic predisposition that favoured the evolution of eusociality.
However, in the 1970s it was realized that things were not so simple, and that an
increased relatedness to sisters would be exactly cancelled by a decreased relatedness
to brothers. Biased sex ratios cannot rescue the haplodiploidy hypothesis, unless
there are also 'split sex ratios', but even they are unlikely to have been hugely
important, and may even have hindered the evolution of eusociality. So, overall, the
haplodiploidy hypothesis may have been a bit of a red herring. Instead, we must turn
to aspects of mating system and ecology to explain why eusociality has arisen
multiply in the Hymenoptera. It is also important to not confuse the haplodiploidy
hypothesis with kin selection (e.g. Wilson & Hölldobler, 2005). The kin selection
explanation for eusociality does not rely on the haplodiploidy hypothesis, which was
just a suggestion for how to make the evolution of eusociality especially easy in
haplodiploids.
A red herring?
The monogamy hypothesis
Numerous authors have suggested that monogamy could have been important in the
evolution of eusociality. The reason for this is that if queens mate multiply, then this
reduces the relatedness between their offspring (Box 13.2; Fig. 13.4), hence lowering
the
r
term in Hamilton's rule, making it harder to satisfy. However, it was Koos Boomsma
(2007, 2009) who realized monogamy couldn't just help, but was actually crucial! In
particular, Boomsma argued a role for strict lifetime monogamy, in which females only
mate with one male in their entire life.
Monogamy leads to a potential worker being equally related (
r
0.5) to her own
offspring and to the offspring of her mother (siblings). In this case, any small efficiency
benefit for rearing siblings over their own offspring (
B
/
C
> 1) will favour cooperation
that could eventually evolve towards eusociality if the benefit persists uninterrupted
over many generations (Fig. 13.5). This holds for both haplodiploids and diploids. In
contrast, even a low probability of multiple mating means that potential workers would
be more related to their own offspring. In this case, costly helping would require a
significant efficiency advantage to rearing siblings over own offspring (
B/C
=
Kin selection
theory predicts
that lifetime
monogamy can
greatly aid the
evolution of
eusociality
1; Fig.
13.5). Until group living is established, allowing the evolution of specialized cooperative
behaviour and division of labour, the ratio
B/C
cannot be expected to greatly exceed
one. For example, feeding a sibling is unlikely to be hugely more beneficial than feeding
an offspring by the same amount. Consequently, in the absence of strict monogamy the
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