The Canary Islands possess a rich diversity of plant life with a high degree of endemicity. Many plant groups in the islands appear to be monophyletic lineages resulting from a single colonization event followed by speciation and spread throughout the isles. Several different models have been proposed to explain how exactly this occurs.
One involves an east-to-west migration from Africa in which there is only a single species per island, or subsequent intra-island speciation, such that a given species’ closest relatives are on the same island. Another model involves inter-island colonization of conserved ecological niches, with independent lineages colonizing a given habitat throughout the island chain. In this scenario, a species’ closest relatives are on different islands. A third model, known as the “surfing syngameon hypothesis,” rejects a single colonization event in favour of multiple events masked by complex hybridization, introgression, and extinctions.
In a recent study published in Annals of Botany, Rafael G. Albaladejo and colleagues use phylogenetic reconstruction to consider which of the possible diversification scenarios best explains the patterns seen in the genus Helianthemum. The authors used genotyping-by-sequencing data to analyse the biogeographical history and ecological niche conservatism of 15 species from section Helianthemum that are restricted to the Canary Islands. Sampling for the sequencing included nearly the entirety of every known population for each of the species.
Phylogenetic reconstruction strongly supported Canarian Helianthemum as a monophyletic group, and recovered five distinct lineages within the section. None of the five lineages are restricted to a single island, suggesting a more complex evolutionary history with recurrent inter-island dispersal events. Analyses suggest that Helianthemum arrived in the Canaries in the early Pleistocene, circa 1.82 million years ago, from a Mediterranean ancestor.
As is the case with many Canarian plant groups, Tenerife was colonized first, likely due to its large size and variety of available habitats. From there, colonization progressed among the islands in a mostly westward direction. The diversification of Canarian Helianthemum seems to be largely built around climatic niche conservatism, a pattern underscored by the fact that only two closely-related species inhabit the same island. “Data in hand, we cannot confirm that ancient hybridization and introgression events fostered the successful colonization of certain insular regions, as predicted by the surfing syngameon hypothesis,” write the authors. “Thus, future population-level studies are still required to assess the real impact of natural hybridization and introgression in the evolutionary history of the Canarian Helianthemum.”