How eusociality is created

What is the origin of the phenomenon that individuals incapable of reproduction survive in an ant or bee colony??

When charles darwin developed his theory on the origin of species, ant and bee colonies were initially a thorn in his side. In the first edition of the "origin of species" he even admitted this: an individual that cannot reproduce reduces its fitness to zero in darwin’s model and was therefore no longer allowed to play a role in evolution. Of course, the naturalist could not miss the insect states – so he resorted to the amption that close-knit family ties could play a role. In addition, the advantages of a common food procurement and defense were obvious for him, too.

The phenomenon that some animal species go so far in their division of labor as to leave sexual reproduction to a few specimens is called eusociality. Depending on how broadly one understands the term, not only some insects belong to the eusocial creatures: even naked mole rats (see naked mole rats: africa’s wild wights) live in a community ruled by a queen.

How eusociality develops

Eusocial naked mole rats. Photo: zdf

Not a habit of minorities

If one allows in the definition of eusociality also a non-randomly distributed distribution of reproduction, some researchers even count humans among the eusocial species, although here reproduction is not reserved for the queens. But even if you choose the harsher definition, it is not a habit of minorities. The biomass of all ants in the world represents about half of the biomass of insects as a whole and is larger than that of all terrestrial vertebrates (except humans) combined.

But how can eusociality develop?? The standard explanatory model is based on what is known as kin selection. It is in the interest of each individual to pass on as many of its genes as possible to the next generation. However, in ants, among others, males and females are genetically differently equipped.

In the cells of the males the chromosome set is only single, the males are haploid. Females, on the other hand, have a double set of chromosomes, they are diploid. Any female specimen therefore shares with all its sisters the genes of the father completely. Additionally, it has a 50 percent chance of sharing the mother’s genes as well. The sisters are thus more closely related to each other than a female would be to her children (who all carry the complete set of genes of the male). In the sense of gene transmission, it would therefore make more sense for any female to help raise her sisters than to reproduce herself.

Doubts about the "idea of total fitness

However, this explanation has a problem – not all proven eusocial species are also haplo-diploid – for example the termites, shrimps of the genus synalpheus or also the hubsche naked mole rats. In addition, many indications have been found in the meantime that not a high degree of relatedness, but a preferably coarse genetic variability increases the survival probability related to the entire state.

These artifacts are explained by three biologists in the current ie of the science magazine nature now with the fact that possibly a basic amption of the kin selection is not correct: namely, the idea of overall fitness, which supposedly governs evolution and has also led to the concept of the "selfish gene".

According to this, the influence of a certain action of an individual on fitness in the evolutionary sense is calculated as the sum of the fitness of the specimen itself plus the resulting increase in fitness of related individuals, multiplied by the degree of relatedness. In their paper, the researchers now show that this construct, which has long been considered plausible, is probably rather unsuitable. Or rather, it is unnecessary: unlike the theory of natural selection, the idea of overall fitness works only in special cases.

And even in those cases, the scientists show, it does not simplify the consideration, but only complicates it. Finally, the researchers also succeed in showing mathematically how eusociality can evolve completely without aming kin selection or overall fitness – in several stages that can be so observed in various species in nature as well.

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