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| Solitary bee (Megachile) and social bee (Apis) |
Among the insect order Hymenoptera there are many species that live
independently, but there are also many others who
live together in societies. Social life has
evolved several times, both among ants (all of which are social) and between
bees and wasps, many of which live alone. This fact should be explained: why is
social life so prevalent among these insects, and how could it have evolved? In
other words, did it provide any evolutionary advantage? In which way?
In insect societies, most
individuals renounce reproduction and dedicate their lives to care for the
queen (the only member of the society who lays eggs) and for their brothers and
sisters, while they are larvae. Normally there are at least two separate castes:
those who are sexually active (male and female) and those who are not (usually
asexual females). The differences between active and neutral females come from the
type of food they receive during their larval stage.
The problem can be stated thus: Why do neutral females renounce reproduction? The
theory of evolution is based on the survival of those individuals who are more capable
of reproduction. How can certain individuals be induced to delegate their
reproductive capacity in some of their brothers, as Hymenoptera do?
We have an additional fact: Hymenoptera are haplodiploid,
which means that some individuals (the males) are haploid, because they are
born of unfertilized eggs. Females, however, are born from fertilized eggs and
are diploid. Does this affect the problem of the origin of life in society?
Many biologists think that it does,
that this provides a plausible explanation of the mystery. As the males are
haploid, they have a single genetic endowment which (possible random mutations aside)
is transmitted in full to all their children. The active females, however,
being diploid, transmit to their children only half their genes. In other
words: a wasp mother has half her genes in common with any of her children, whereas
two sisters, daughters of the same parents, share 75% of their genes.While those
from the father are identical, those from the mother only have 50% chance to
match. Therefore, from the point of view of the genes, it is more convenient
for a female to forgo reproduction and focus on helping her mother to lay more of
her sisters, who share with her 75% of their genes, than reproduce herself,
because her own children would only ensure the survival of one half of her genes.
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| Coptotermes and African termite nest |
The explanation looks very
reasonable, and would be more so but for an impressive counter-example: insects
of a different order (the Isoptera,
related to cockroaches) also build societies, even more spectacular than those
of the Hymenoptera. Indeed, termites
build nests, sometimes huge, where millions of individuals live, more numerous
than the inhabitants of any anthill, wasp nest or hive. It happens that the Isoptera are all diploid. Therefore, the
above explanation cannot be applied to them. How did Isopteran societies emerge?
What evolutionary advantage could they get by living together? Just now, no one
knows.
Manuel
Alfonseca
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