Updated: Jan 14, 2022
Australia and its region (including New Zealand, Papua New Guinea, New Caledonia and the Western Pacific) has an unusually large number of evolutionarily remarkable lineages and species. Think of the monotremes (the echidnas and platypus), the phylogenetically sister taxon to all other mammals of the world, or New Zealand's tuatara, the sister to all the world's lizards and snakes. Even more striking, the New Caledonian endemic shrub Amborella trichopoda is the single closest relative to all 250,000 flowering plants in the rest of the world.
There is no good term for these cases, where a very small, geographically restricted lineage is sister to a very large, geographically widespread one. Sometimes the small lineage is described as ancestral or primitive, but of course no species alive today is ancestral to any other, or necessarily more primitive than any other: there may have been just as much evolutionary change along the small lineage as along the large one since each diverged from their common ancestor. Sometimes they are called 'basal lineages', but of course each is basal to the other.
A new term probably needs to be coined for these cases, and we in the Australasian region should probably coin it, as we appear to be a global epicentre for these small, evolutionarily important lineages. Anisophyly is a good candidate term, from the Greek for 'unequal branch'.
One such anisophyletic lineage is the Darwin giant termite, Mastotermes darwiniensis. One of the most destructive termites in Darwin – and there are a lot of destructive termites in Darwin – Mastotermes darwiniensis is the sole living species in its genus Mastotermes, which is the sole extant genus in its family Mastotermitidae, which is the sister lineage to all the other termites in the world (of which there are around 3000 species).
Calling Mastotermes darwiniensis the Darwin giant termite is slightly misleading, as it's widespread in the monsoon tropics of Australia roughly north of the Tropic of Capricorn. Calling it a giant termite isn't misleading at all, as this is a whopper. Workers and soldiers are more than 10 mm long, while the winged alates (the sexual individuals in ants and termites) have a wingspan of 50 mm! These are serious termites. Soldiers will draw blood from an incautious finger poked too close.
Modern taxonomy and biosystematics has shown that termites are blind, colonial, highly specialised cockroaches. Originally classified separately, they are now included with cockroaches in the insect Order Blattodea. In some ways it is perfectly appropriate to describe Mastotermes as primitive, as it has a number of morphological characters that are more cockroach-like than any other termite. In fact, Mastotermes look a bit like a cockroach body joined to a termite head. To entomologists, their wings are also distinctly cockroach-like in both shape and venation, and like cockroaches the queens lay eggs in bundles rather than singly like other termites.
Mastotermes darwiniensis is the sole living species in its family. Seven extinct genera in the Mastotermitidae have been described from fossils, as have more than a dozen extinct species of Mastotermes. And these tell an equally fascinating story: these fossils have been found all over the world, from Europe to Mexico and from China to Brazil. The most recent are from the Pliocene, only a few million years ago. So Australia is the only place where the Mastotermitidae have survived. Something has caused their extinction elsewhere, and Australia has been a refuge. This is a common pattern across Australia's anisophyletic lineages, and no-one currently has any idea why.
Whatever the reason, Australia and its immediate neighbours are rich in anisophyletic lineages, and these are crucial for understanding the evolution of life on Earth. That means we are living in one of the most remarkable of countries and regions. It also means that taxonomy and biosystematics – documenting biodiversity and understanding its evolution – in Australia, Papua New Guinea, New Zealand, New Caledonia and the Western Pacific, is globally important. If we can understand the survival of anisophyletic lineages such Mastotermes darwiniensis here and their extinction elsewhere, we are likely to have an important clue to a big evolutionary puzzle.
And one more thing. Not only is the Darwin giant termite remarkable in its own right, it also has one of the world's most bizarre creatures living in its gut. If you think the termite is cool, wait till you read about Mixotricha paradoxa!