The trouble with grevilleas

Sometimes in taxonomy there are good solutions and not-so-good solutions to taxonomic conundrums. And sometimes it just takes a while to reach a good solution.


One such conundrum arose when careful molecular phylogenetic studies showed that the well-known and diverse Australian genus Hakea is nested inside the even better-known and even more diverse genus Grevillea. With this result, the trouble with grevilleas began.

The spectacular Western Australian Hakea laurina. Source: https://www.needpix.com/photo/download/1117638/hakea-laurina-flora-nature-free-pictures-free-photos-free-images-royalty-free

Hakea and Grevillea were both named and described around the turn of the 19th Century, the former by the German botanists Heinrich Schrader and Johann Wendland in 1797, the latter by English botanists Joseph Knight, Richard Salisbury and Robert Brown in 1809. (In a curious case of history repeating itself, the publication of Grevillea was highly controversial at the time; we'll return to this later.)


Since that time, many species have been discovered and named in both genera, so that today there are nearly 150 species in Hakea and more than 350 in Grevillea. Grevillea is the third largest genus in Australia, after Acacia and Eucalyptus. New species are regularly discovered, especially in Grevillea.

The equally spectacular Grevillea 'Robyn Gordon', a favourite in cultivation. Source: https://www.flickr.com/photos/nhq9801/9207617544 CC-BY

Throughout their history, the differences between species of Hakea and those of Grevillea have been pretty clear. Hakea species all have thick, woody follicles (fruits) that persist on the plant for some time, protecting the seeds until a fire pops them open, while almost all species of Grevillea have thin-valved, leathery follicles that open without fire and are then shed after the seeds fall.


While this difference worked well for botanists for centuries, modern taxonomy has a more rigorous standard for recognising and naming genera and other taxa. More emphasis is now placed on evolutionary history and the evolutionary structure of life rather than on simple similarities and differences. In this modern view, a genus should comprise all and only the descendants of an original species. This makes sense - most people would expect that all species in Grevillea should be evolutionarily more closely related to each other than they are to species in Hakea.


Another way to say this is that genera in modern taxonomy should be whole branches of the tree of life, not partial branches that include some descendants but not others.


With the powerful insights offered by DNA sequencing and phylogenetics, Hakea and Grevillea have become problematic as genera. It turns out that Grevillea is a large branch on the tree of life, and Hakea is a sub-branch within it. That is, hakeas are simply grevilleas that have developed thicker follicles. (One other genus, the small New Guinea rainforest genus Finschia, is also a sub-branch of Grevillea.)


When this result was first confirmed in 2015, a number of possible taxonomic solutions were available to fix the problem. One solution would be to simply merge Hakea (and Finschia) into Grevillea, so that only the whole branch containing all species in all three genera is recognised as a genus.


If this were the case, the name Hakea would have priority (since it was published some 12 years before Grevillea, and a century before Finschia), so all Grevillea and Finschia species would need to be given new names in Hakea. An alternative solution would be to prune new genera out of Grevillea, ensuring that each genus comprises a whole sub-branch of the Hakea+Grevillea+Finschia branch. This would allow Hakea and Finschia to be retained, but would dismember the larger genus Grevillea.