Scaly Tree Ferns Have Slow and Steady Diversification

How does diversity in species happen? One idea is that there are spurts of diversification, but this is not always the case among diverse groups of organisms. Oriane Loiseau and colleagues investigated the tree fern family Cyatheaceae. 

The Cyatheaceae is a family of over six hundred tropical and subtropical ferns, including the tallest ferns on the planet. In comparison to other fern families in the order Cyatheales, it is hyper-diverse. Its nearest competitor, the Dicksoniaceae has just 30-40 species. The Cyatheaceae diverged from the Dicksoniaceae around 145 million years ago, after which more and more Cyatheaceae species evolved.

Loiseau and colleagues have combined molecular data with fossil data to create a new phylogenetic tree of the Cyatheaceae. They date the origin of the group back to the Jurassic. But what resulted in the current diversity? The authors compare two approaches. One is that a slow and steady accumulation of species occurred. The other is that specific lineages drove diversification.

The scientists took data from nearly half of the extant Cyatheaceae species to help clarify the relationships between the different ferns. “We found support for four main clades within Cyatheaceae, which are here referred to as the genera Alsophila, Cyathea, Gymnosphaera and Sphaeropteris,” write Loiseau and colleagues. “We also recovered a monophyletic clade for the Neotropical species of Alsophila, which were split between two different clades in a previous phylogeny with lower sampling…”

The increased sampling, and using multiple methods that lead to similar conclusions gives the authors confidence in their results, even though they differ from some previous studies. “Results from our diversification analyses using different methods are coherent, and consistently indicate that the rate of diversification in Cyatheaceae was low and underwent very little variation throughout their long evolutionary history. This finding is consistent with a near-stable sampled diversity in the fossil record of Cyatheales, but contrasts strongly with previous reports of dramatic heterogeneity in the rates of diversification among Cyatheaceae. Previous studies were based on just about 10 % of the extant species and we believe that the 5-fold increase in taxon sampling in our analysis and advances in molecular dating methods are the basis of the divergent conclusions regarding the tempo of diversification in the group.“

“Cyatheaceae provide a compelling example of tropics as a museum of biodiversity and illustrate that rapid morphological differentiation, key innovations, and niche divergence are not a prerequisite for a clade to thrive for hundreds of millions of years.”