For more than six decades, evolutionary biologists have debated whether an unusual genetic system shared by ants, bees and wasps helped drive one of nature’s most sophisticated forms of social organization: eusociality. In eusocial colonies, reproduction is divided between queens and workers, creating highly cooperative societies with specialized roles.
New research from Arizona State University now suggests that genetics alone may not explain how these complex societies evolved.
Published in Current Biology, the study found that eusociality does seem to arise more often among insects with a haplodiploid genetic system. However, nearly all of that 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.
The results suggest 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.
“People have been discussing this hypothesis for about 60 years,” said Sachin Suresh, a PhD student in the School of Life Sciences at Arizona State University and lead author of the study. “There were many theoretical predictions, but formal comparative tests across insects have been surprisingly rare.”
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.
The idea became highly influential in evolutionary biology and eventually appeared widely in biology textbooks. Despite its importance, however, the hypothesis had rarely been evaluated using large empirical datasets spanning a broad range of insects.
Testing a 60 Year Old Evolutionary Idea
To put the hypothesis to a large scale test, Suresh and senior author Timothy Linksvayer 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 we formally tested it, we found there is no real association between the genetic determination system and eusociality,” Suresh said. “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.
