Thursday, December 10, 2015

How insect societies arose

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.

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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