How does haplodiploidy change relatedness and predicted helping, and why do non-social haplodiploid species weaken this explanation?

Why haplodiploidy makes sisters unusually close relatives

In the usual diploid system, a female shares about half her genes with a full sister and half with her own daughter. Haplodiploidy changes that arithmetic. A male has only one set of chromosomes, so all his sperm are identical, and every daughter receives the same paternal genes. Full sisters therefore share all of their father's contribution and, on average, half of their mother's, which can push their relatedness above the level they would share with offspring [1][2]. This relatedness asymmetry is the core of the classic idea that haplodiploidy could favor altruism: a female may gain more inclusive fitness by helping rear a sister than by rearing a daughter [2].

The effect is not automatic or fixed. It depends on the parents' mating arrangements, especially how many males a queen mates with. If a queen mates with several males, daughters fathered by different males are only half-sisters, and the relatedness advantage shrinks [4]. So haplodiploidy creates the possibility of unusually high sister relatedness, but paternity patterns decide how much of that possibility is realized in a given species [4].

What the relatedness shift predicts about helping

The prediction is straightforward in principle: where haplodiploidy raises relatedness between sisters, the inclusive fitness payoff of helping should rise, making reproductive altruism easier to evolve [1][2]. The argument does not require an active sex-ratio bias or deliberate kin discrimination, which makes it an appealing explanation for how helping could get started [1]. In social insects, this logic has been used to explain why daughters might stay and help rather than leave to reproduce [1][2].

Yet the same body of work stresses that relatedness is only one input. Termites, which are not haplodiploid, still evolved elaborate social systems, and the perceived importance of the haplodiploidy hypothesis faded as it became clear that relatedness alone does not account for social evolution [3]. In termites, reduced relatedness can lower the inclusive fitness benefits of helping, showing that the payoff of helping is sensitive to the actual family structure rather than to chromosome system alone [3].

Why nonsocial haplodiploid species weaken the explanation

If haplodiploidy reliably made helping pay, then haplodiploid species should tend to be social. Many are not. Nonsocial haplodiploid insects, including an ancient nonsocial sawfly ancestor of social insects, show that the relatedness asymmetry by itself is not sufficient to drive altruism [2][5]. These species have the same chromosome system and the same potential sister-sister relatedness, but they do not show the helping behavior the hypothesis would predict [2].

That gap points to other requirements. The contrast between the eusocial Hymenoptera and nonsocial relatives is a central reason researchers look beyond haplodiploidy, because the chromosome system cannot explain why some lineages became social and others did not [4]. Ecology, mating patterns, and the specific costs and benefits of staying versus leaving appear to be part of the answer, and the haplodiploidy hypothesis is best treated as one contributing factor rather than a complete explanation [2][3][4].

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