Two populations of the same species can become separated by natural barriers and evolve into different species over time. That is nothing new. But a study with beetles by biologists from our Institute proves that different behaviour – learned in the larval stage – can also cause populations to become isolated. ‘Learned behaviour can be an important first step in the formation of new species’, says biologist Frederik Hendrickx (RBINS), ‘even before there have been any genetic changes.’
Evolutionary biologists examine how new species come into existence. To study this, a key point is what is called the biological species concept; does mating between two groups produce fertile offspring? If not, they are different species. For example, a mountain range or a river can form a barrier between two populations of the same species. As these populations evolve independently from each other, they diverge genetically and become ultimately too different to be called the same species.
But can a new species arise when there is no such extrinsic barrier? Three researchers of our Institute found that learned behavioural differences can induce the separation of two populations from the same species. Behavioural differences are in this sense the first step in species formation, even before any genetic changes take place.
The biologists noted that in a beetle’s development, change in behaviour is triggered very early by its environment. It was not a coincidence that they chose beetles to study: insects are a great model organism for the study of speciation processes, because generations follow each other very quickly and you can easily breed them in large numbers.
The swamp beetle Pogonus chalceus lives in saltwater marshes in the coastal town of Guérande, Western France. One population lives in areas that are connected to the Atlantic Ocean and are subjected to tidal floods. During these frequent but short inundations the beetles do not fly away but stay underwater for a couple of hours, while capturing an air bubble under their wing cases. Under water, the beetles are protected against natural predators.
Only ten meters away you find the same species, but in a seasonal marsh that is inundated for a couple of months at a time each year. There, the beetles behave differently: when the water comes, they move away to escape these long inundations. They have longer wings than their counterparts that live on the rhythm of the tides.
Behaviour Comes First
Evolutionary biologists Steven Van Belleghem, Katrien De Wolf and Frederik Hendrickx (RBINS) noticed that this difference in behaviour is not genetically determined, but ‘learned’ by the beetles in their early larval phase. They subjected some larvae to simulated tides. When subjected to these tides, they tended to remain submerged rather than attempting to escape as an adult beetle. Conversely, beetles that had never experienced tidal flooding as a larva were faster to flee.
A remarkable discovery is that it does not matter if the larvae descent from a submerged- or a flee-type beetle, it is the environment that influences the behaviour. The difference between the two beetle populations from the same species causes the groups to stay in their own habitat, to which they are best adapted. The beetles that are used to being submerged for a couple of hours, would suffocate if the marsh was flooded for months at a time. The other beetles, that do not last long under water, would constantly attempt to avoid being submerged during the tides. Hendrickx: “That is why the populations do not mix and it is possible for them to build-up genetic differences. The genes that are responsible for wing length are already different in these populations. After a while, two separate species are formed. Genetic changes appear after changes in behaviour. Genes appear to be the followers, not the leaders in driving evolution.”
The research paper appeared in Evolution.