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Why is haplodiploidy not the only explanation for eusocial behavior in Hymenoptera?

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Bumblebee pollination — Hymenoptera An endangered rusty patched bumblebee (*Bombus affinis*) browsing on a wild bergamot flower.

Haplodiploidy is considered a predisposing factor rather than the exclusive cause of eusocial behavior in Hymenoptera because several biological realities dilute its genetic influence.

While the system theoretically increases relatedness among sisters to 75 percent, this effect is frequently undermined in nature:

Multiple Mating: Queens often mate with multiple males, which introduces sperm from different sources and reduces the average genetic relatedness among sisters.1
Multiple Foundresses: In some species, colonies are established by multiple queens, meaning workers may be raising nieces rather than sisters, which alters the inclusive fitness calculations.1

Consequently, the most widely accepted explanation for eusociality is a broader application of kin selection combined with significant ecological constraints on independent reproduction. Factors such as high predation rates, a shortage of suitable nest sites, and limited breeding seasons make it difficult for individuals to survive or reproduce on their own. Under these conditions, staying within the natal group to assist in rearing kin becomes a more viable strategy for maximizing inclusive fitness than attempting to breed independently.12