The sorrel bush and the dandelion tree: how do species form on islands?

On Tuesday October 13 two Naturalis researchers – Ornela Dehayem Nanwou and Ryan Brewer – defended their PhD theses at Groningen University. They worked together on a long-standing mystery in island biology: why do some plant groups on islands explode into dozens of unique species, while others barely diversify at all?

Woodiness

When plants colonise remote islands, something remarkable tends to happen. Over generations, soft, small herbs evolve into woody shrubs and trees. This phenomenon is known as 'insular woodiness'. This insular woodiness is generally found in plant groups that produce many different species. On the Canary Islands – the most famous location for this – you will find shrubby daisies, dandelion trees, woody nettles and more. For decades, evolutionary biologists wondered: does becoming woody actively help plants form new species faster and avoid extinction, or do species simply transition towards woodiness frequently on islands?

Modelling work

“To test this hypothesis, we need both comprehensive genomic and trait datasets”, the researchers explain. “We also needed a model, because the data alone cannot tell us whether the prevalence of woody species in species-rich lineages is driven by higher speciation rates, lower extinction rates, or higher transition rates towards woodiness.” 

This required teamwork. Ryan generated datasets using a combination of natural history collections and fresh material from the field, using the facilities at Naturalis. This involved sequencing 353 genes for about 2,000 species, including 800 species from the Canary Islands and their closest relatives from the mainland, in order to estimate the colonization time of the Canary Islands for each plant group. 

Ryan: “Evolution is not as simple as one trait affecting diversification, so we generated a trait dataset of several traits that, according to the literature, have a significant effect on diversification. We used the Canary Islands as a study system, as it is one of the best-studied archipelagos globally, and plants from the islands have been studied in great detail, meaning we can also use published trait data.”

“The Canaries also have one of the most species-rich plant communities of any archipelago, with many radiations (groups with many species) and many colonisations that did not diversify. We fed our genomic data, along with a subset of the trait data, to our model”, Ornela says.

Age isn't everything

Ornela developed the mathematical model, a so-called inference model called TRAISIE. By combining Ryan's huge genomic dataset with Ornela’s model, the duo uncovered an important insight into island biodiversity: traits can significantly affect rates of evolution. Woody and long-lived plants undergo faster speciation and have lower extinction rates than short-lived, non-woody herbs. Further analysis of the data also reveals age isn’t everything: older plant families do not necessarily have more species.

More information

• Thesis Ryan Brewer: Trait-dependent diversification and biogeography of Canary Islands angiosperms.
• Thesis Ornela Dehayem Nanwou: Diversification in isolation – Traits, sampling, and inference in island biogeography.

Text: Naturalis Biodiversity Center
Images: Frederic Lens, Naturalis Biodiversity Center