A 60-year-old theory about ants, bees and wasps may be wrong

A massive insect study has challenged a 60-year-old theory about the evolution of social living in ants, bees, and wasps.

For decades, scientists have debated whether an unusual genetic system shared by these insects helped drive the evolution of their complex societies. However, a new analysis from Arizona State University suggests that genetics alone may not be the key to understanding this phenomenon.

The study, published in *Current Biology*, found that eusociality - a form of social organization where reproduction is divided between queens and workers - does seem to arise more often among insects with a haplodiploid genetic system. However, nearly all of this pattern can be traced to one particular group: the aculeate Hymenoptera, which includes stinging wasps, bees, and ants.

When insects are considered more broadly, haplodiploidy alone does not reliably predict whether eusociality will evolve. This suggests that the repeated emergence of complex insect societies may depend more on biological traits specific to certain evolutionary groups than on the way chromosomes are inherited.

The researchers used phylogenetic comparative methods to estimate how frequently eusociality evolved among insects with different genetic systems. They found that the apparent connection between haplodiploidy and eusociality is largely the result of evolutionary history within particular lineages.

This shifts attention toward other characteristics that may have helped ants, bees, and wasps repeatedly evolve highly cooperative colonies. Features such as stingers, specialized nesting behaviors, and other biological traits may have created conditions that made cooperation more advantageous and helped sophisticated insect societies emerge again and again.

The research represents one of the most comprehensive empirical tests yet of a foundational idea in evolutionary biology. It also highlights the value of using large comparative datasets to revisit theories that have remained influential for decades.

## Why Eusocial Insects Are So Unusual

Eusociality is considered one of the most advanced forms of social organization in the natural world. Species that live this way form colonies containing overlapping generations, cooperate in raising young, and divide reproductive responsibilities. Only one individual or a small number of individuals reproduce, while the others serve as workers.

All ants are eusocial, along with honey bees and some wasps. By contrast, eusociality has appeared only rarely in other insect groups, including termites, thrips, aphids, and a small number of beetles.

For many years, scientists have focused on haplodiploidy as a possible explanation for why this lifestyle evolved so often in ants, bees, and wasps. Under this genetic system, females develop from fertilized eggs and possess two sets of chromosomes, while males develop from unfertilized eggs and have only one set.

This arrangement can make sisters more closely related to one another than they are to their own offspring. Evolutionary theory therefore proposed that females might sometimes gain a greater evolutionary advantage by helping raise their sisters than by producing offspring of their own.

## Testing a 60 Year Old Evolutionary Idea

To put the hypothesis to a large-scale test, the researchers assembled information on social behavior and genetics from tens of thousands of insect species. They then placed those characteristics onto two of the largest available species-level insect family trees.

Using phylogenetic comparative methods, the researchers estimated how frequently eusociality evolved among insects with different genetic systems. Initially, the findings appeared to support the traditional explanation. Eusociality seemed to emerge more frequently among haplodiploid insects.

A deeper examination, however, produced a very different picture. Nearly the entire statistical signal was coming from a single branch of the insect family tree. When the researchers formally tested the hypothesis, they found that there is no real association between the genetic determination system and eusociality.

It has more to do with environmental factors and the life-history traits of insects. After accounting for the unusual evolutionary history of the aculeate Hymenoptera, the researchers found that haplodiploid insects outside that group developed eusociality at rates similar to those seen in diploid insects.

## Nearly 69,000 Species Reveal a Different Pattern

The findings also help bring together recent research that has raised doubts about how important haplodiploidy really is to the evolution of complex insect societies.

By examining nearly 69,000 insect species representing a wide range of insect diversity, the Arizona State University team found that the apparent connection between haplodiploidy and eusociality is largely the result of evolutionary history within particular lineages. It does not appear to represent a universal biological rule.

That shifts attention toward other characteristics that may have helped ants, bees, and wasps repeatedly evolve highly cooperative colonies. Features such as stingers, specialized nesting behaviors, and other biological traits may have created conditions that made cooperation more advantageous and helped sophisticated insect societies emerge again and again.

The research represents one of the most comprehensive empirical tests yet of a foundational idea in evolutionary biology. It also highlights the value of using large comparative datasets to revisit theories that have remained influential for decades.