Nature is inhabited by an incredible number of plant and animal species, and in particular there is a tremendous amount of plant-feeding insects. Evolutionary biologists have been trying to explain the origins of this diversity for over 200 years, which is challenging since the actual diversification process is seldom directly observed. At the same time it is important to understand the origin of species diversity, for instance to be able to stay ahead of the pest insects competing with us for food.

Butterfly on a flower
Comma butterfly, an example of a butterfly capable of feeding on several quite different plant species. Photo by Niklas Janz.

Biologists Peter Raven and Paul Ehrlich presented a possible explanation in 1964, with their hypothesis of “co-evolution”. Their very influential hypothesis suggests that the diversity of plants and the insects feeding on them is at least in part the result of an “arms race” where plants at some times in evolution come up with new defences against insects, allowing them to diversify. However, sooner or later an insect will “crack” also these new defences and be able to then colonize this new plant group, in turn diversifying upon this untapped resource (the radiation scenario). More recently, an alternative explanation was proposed which states that insects can keep abilities to use host plants that they used in their evolutionary past, and this variability allows them to explore and exploit the environment in new ways when conditions change (the variability scenario). Such latent abilities allow insects to rapidly adjust to such changes and result in a cyclic pattern of expansions and contractions in the number of hosts actually used at a given time. These oscillations in host repertoire have been suggested to work like a diversification engine by repeatedly breaking up widespread species – using several host plants – into smaller specialized populations.

For the first time, the relative contributions of these two scenarios to insect diversification have now been evaluated, by investigating the network properties they predict. Network analysis is an ecological method to map and analyze the intricate patterns of interactions among, for example, insects and plants. By a novel application of this method in conjunction with phylogenetic analysis, the researchers have been able to begin quantifying how these two scenarios played out over ancient evolutionary time. The results show that variability in host use has been the major driver of diversification, but that radiations in the form of episodes of rapid speciation following major host shifts have also contributed. Indeed, the authors suggest that both scenarios may actually be part of the same general process, in that variability in host use is what sets the stage for the host shifts that can drive episodic rapid diversification onto new hosts.

Niklas Janz, senior author on the manuscript now published in Nature Communications, comments: “It is very satisfying to be able to contribute to resolving such a long-standing question in evolutionary ecology, concerning why there are so many plant-feeding insects around. However, there are also more practical implications following from that variability in host use is as an important ‘engine of speciation’. This means that plant-feeding insects often are not as specialized on a particular host plant as they may seem to be, rather they have some ability to feed on alternative hosts, including those that were used by their ancestors in their evolutionary past. This can be used as a starting point in predicting for instance which pest species are likely to invade new habitats – or which species are likely to survive –  when the climate changes, by shifting to alternative hosts that they already can feed on.”

Mariana Pires Braga, first author, comments further: “Indeed, we believe our ideas hold true for species associations well beyond insects and plants, including humans and the organisms that cause diseases. After all, what is often called ‘emerging diseases’ is just that; it’s a pathogen that has shifted to humans as an alternative host.”