Taxonomic richness and phylogenetic richness
Updated: Jul 31, 2018
We had a new paper come out recently in the Journal of Systematics and Evolution.
In Australia there are 2655 genera of land plants (angiosperms, gymnosperms, ferns, liverworts, hornworts and mosses). That is an impressive number for its size and the fact that Australian botanists have an accurate count of its generic diversity (thanks Australian Plant Census).
My collaborator Andrew Thornhill added the non-angiosperms genera to his earlier phylogeny to create a phylogenetic tree with 2362 or nearly 90% the known genera. We are pretty sure this is the first generic level phylogeny of land plants for any country or continent. With this thorough sampling I thought it would be fun to compare taxonomic richness to phylogenetic richness. What we learned was surprising.
The ferns comprise only 4.7% (111) of the genera in Australia but 14% of the phylogenetic tree (the evolutionary history) is unique to the ferns. In fact there would need to be 311 fern genera in the tree, without adding any branches lengths, for there to be a one-to-one ratio of generic richness and phylogenetic richness. The data for the gymnosperms, liverworts, hornworts and ferns was similarly skewed. See table below.
The story was the opposite for the angiosperm groups studied. All groups, except the Caryophyllales, had much more generic richness than phylogenetic richness. In the paper we discuss various reason for this skewness and conclude that cultural reasons, such as taxonomic biases, are more important than methodological and biological phenomena in explaining these discrepancies.
We also point out that it is getting much easier for scientists to describe diversity in phylogenetic terms in addition of taxonomic terms. We have phylogenies, let’s use them!