What is it for?

What is haplodiploidy for? While there have been a lot of theories relating eusociality and haplodiploidy, both in science and in popular culture, there hasn't been much conclusive evidence yet that can explain why haplodiploidy can be beneficial to diploid worker individuals when they cannot reproduce.

In a study of one possible explanation(Avila and Fromhage 2014), a model involving imaginary eusocial allele A was used to determine whether diploid, non-reproductive worker individuals are compensated for their low life reproductive success with synergy. Synergy is defined as a colony of a given size experiences benefit that is more than proportional to its size. This condition has two main causes: the fortress defense mechanism and life insurance mechanism. The former describes individuals in a population experiencing increased benefit from protecting a shared resource and the latter describes individuals in a population experiencing an increased benefit as the survival rate of offspring with dead parents increases.

The true cause of eusociality, as identified in this experiment, was mother-daughter asymmetry, where mothers can provide an increased benefit to others per individual than daughters. It was not other workers that provided fitness to workers, but non-worker females- typically the foundress queen of the population.