Evolution of eusociality
This 'true sociality' in animals, in which sterile individuals work to further the reproductive success of others, is found in termites, ambrosia beetles, gall-dwelling aphids, thrips, marine sponge-dwelling shrimp (Synalpheus regalis), naked mole-rats (Heterocephalus glaber), and many genera in the insect order Hymenoptera.
[1] The fact that eusociality has evolved so often in the Hymenoptera (between 8 and 11 times[2]), but remains rare throughout the rest of the animal kingdom, has made its evolution a topic of debate among evolutionary biologists.
Eusocial organisms at first appear to behave in stark contrast with simple interpretations of Darwinian evolution: passing on one's genes to the next generation, or fitness, is a central idea in evolutionary biology.
Eusociality can be characterized by four main criteria: overlapping generations, cooperative brood care, philopatry, and reproductive altruism.
In The Origin of Species, he described the existence of sterile worker castes in the social insects as "the one special difficulty, which at first appeared to me insuperable and actually fatal to my whole theory".
[1][5] This insight led to inclusive fitness and kin selection becoming important theories during the 20th century to help explain eusociality.
[3] William D. Hamilton proposed that eusociality arose in social Hymenoptera by kin selection because of their interesting genetic sex determination trait of haplodiploidy.
Moreover, males share only 25% of their sisters' genes, and, in cases of equal sex ratios, females are related to their siblings on average by 0.5 which is no better than raising their own offspring.
If a queen is lifetime-strictly monogamous - in other words, she mates with only one individual during her entire life - her progeny will be equally related to their siblings and to their own offspring (r=0.5 in both cases - this is an average of sisters [0.75] and brothers [0.25]).
Thus, natural selection will favor cooperation in any situation where it is more efficient to raise siblings than offspring, and this could start paving a path towards eusociality.
[9][10] In many monogamous animals, an individual's death prompts its partner to look for a new mate, which would affect relatedness and hinder the evolution of eusociality: workers would be much more related to their offspring than their siblings.
With each molt, termites lose the lining of their hindgut and the subsequent bacteria and protozoa that colonize their guts for cellulose digestion.
[1] These pre-conditions led to the two lifestyle characteristics that are observed in all eusocial species: nest building and extensive parental care.
[15][16] In the case of termites, as recent phylogenetic studies have showed them being closely related to wood-eating cockroaches (genus Cryptocercus),[17] which are subsocial and engage in biparental care, their eusocialty may have evolved from subsocialty.
This means there is a high cost to dispersing (individual may not find another source before it starves), and these resources must be defended for the group to survive.
[22][23] Genes determining worker behavior and division of labor have been found in regions of the Apis genome with the highest rates of recombination and molecular evolution.
[22] These mechanisms are likely important to the evolution of eusociality because high recombination rates are associated with the creation of novel genes, upon which natural selection can act.
[23] Another hypothesis is that the lower overall genetic diversity as eusociality levels increase throughout the family Apidaeis due to a decreased exposure to parasites and pathogens.
