The researchers found that turtle ants have evolved towards more specialization and vice versa.
Tree turtle ants stand out among others by the appearance of their soldiers - or rather, unusual heads. Some of them have heads similar to sewer covers: this allows them to block the entrances to the anthill. Other soldier ants have heads that are close to a rectangle, and insects can use them to create something like a living wall.
Such a variety of forms shows how species evolve, occupying increasingly narrow ecological niches. However, an article published in the Proceedings of the National Academy of Sciences argues that evolution does not always move towards more specialization - sometimes it turns a species into “generalists”.
"It's generally accepted that once a species specializes, it gets stuck in a very narrow niche," says co-author Daniel Cronauer, an expert in social evolution and behavior. "But tortoiseshell ants are an interesting case of a dynamic evolutionary trajectory with a lot of multidirectional shifts."
In these soldier ants, there is an almost fourfold difference in head size between the largest and smallest insects. “To make this easier to imagine, I often say that the smallest species [of the soldier tortoise ant] can sit comfortably on the largest head,” explains lead author Scott Powell. This size depends on the types of passages in the nest that the insect must guard. At the same time, tortoise ants do not make tunnels on their own: they use the moves made by tree beetles.
The ratio between the heads of the soldiers can provide a uniquely clear picture of the natural selection among these creatures. To investigate the evolutionary pathways that led to the emergence of different forms, the researchers grouped 89 species of tortoise ants based on their head type. The researchers then studied the evolutionary relationships between these species using previously collected genetic data.
If evolution were a one-way road, then the first turtle ants, which appeared about 45 million years ago, would surely lack soldiers. They then had to evolve from non-specialized to specialized ones, with different head shapes.
However, scientists have shown that this was not the case. The common ancestors of modern turtle ants were already specialized in a certain way and had a square head. Subsequently, they formed a huge variety of species - both those with a wide variety of head shapes among the individual soldiers, and those who have no soldiers at all in the social structure. In some cases, more specialized species changed the direction of their evolution, turning from specialized species to more "wide-profile" ones.
According to the researchers, this work not only demonstrates an example of reverse evolution, but also shows how amazingly flexible nature can be, adapting the shape of an organism to the context of the environment. “The space evolution plays in is actually a little wider than previously thought,” sums up Cronauer.