Australasian treefrogs: islanders on a continental scale (sixty million years of evolution packed into a single genus)
(writing in progress)
Australasian treefrogs (i.e. Litoria1) have diversified widely in body shapes and sizes. They occupy such a wide range of niches that their morphological and ecological counterparts on other continents belong to many different families of frogs. No one genus, correctly defined, should encompass such extreme variety. For this reason, herpetologists should divide the oversize genus of Australasian treefrogs into several smaller genera.
The Australasian treefrog genus Litoria is one of the most2 diversified genera of vertebrates on Earth and deserves to be retained by taxonomists as a single group. This is because these frogs epitomise a particularly significant pattern in evolution. Despite the clear evolutionary convergence of the adults of various species of Australasian treefrogs with pond frogs, grass frogs, canopy frogs, tree-hollow frogs, reed frogs, torrent frogs, rock frogs, water-skipping frogs, bullfrogs and ant-eating frogs on other continents, the reproductive biology of this group of frogs remains surprisingly consistent. Although the post-metamorphic forms of Australasian treefrogs have diversified, their breeding behaviour and the developmental pattern from egg through tadpole to the point of metamorphosis have changed minimally for 60 million years. This conservatism of an ancestral mode of reproduction can be explained by the fact that pressures from predators3 have remained limited in Australasia, compared with larger and more connected landmasses. Whether Australasian treefrogs originated in Gondwana4 or reached Australasia from farther afield, their conservative reproductive modes continue to reflect Australia’s biogeographical and ecological status as the world’s largest island. Indeed, their insular radiation – in which body form has proved more plastic than the pattern of growth and development – is analogous with that of certain Australasian mammals. In monotremes5, for example, the two surviving lineages are classified in a single order [Will some readers perhaps retort: “Orders are different to genera. One could expect animals that look different to be in the same order; but animals within a genus should look similar”? We may need to craft a footnote response to such a query, or should we perhaps put it in the comment stream? I too am not really happy with this example. It would be better to use another lineage, such as Galapagos finches or Hawaiian honeycreepers. I don’t know if those lineages do show reproductive conservatism but if they do they would be better examples? Yes, I think we should research this. Should we ask Mohammed? Or do you think that would just cause us grief in terms of checking his work? Let’s ask him to do it and see how he goes…?]because both lay eggs despite being otherwise differentiated6. Similarly, in Australasian treefrogs, all species have ‘living fossil’ tadpoles despite having diversified adults. Because Australasian treefrogs are far more speciose than monotremes, they epitomise an ‘island radiation’. We therefore recommend that, far from being subdivided, their genus should be extended to include the closely related burrowing frogs7 and big-eyed tree frogs8. Such inclusion would reflect the common denominator of pre-metamorphic stages of development of Australasian treefrogs despite the diversification of their adult stages.
1 Litoria comprises at least 150 species in the family Pelodryadidae, occurring in Australia, New Guinea, the Moluccas, Timor and Sumba, and the Solomon Islands. Although Litoria is usually placed in the Hylidae and recent molecular analyses have indicated an origin in Central America, this raises the problem of how a northern family could possibly have reached Australia across the oceans. What is equally likely is that Litoria represents a purely Australasian family of Gondwanan origin, and that its extreme similarity to Hylidae is the result of convergent evolution.
2 One genus rivalling Litoria is the lizard genus Varanus (Varanidae), which is far more variable than Litoria in body size but is also far more widespread, spanning three continents.
3 Although of continental size, Australia in some ways resembles islands with their limited predation and consequent preservation of ancient modes of reproduction.
4 i.e. present in Australia since the formation of this continent
5 which, like Litoria, are restricted to Australasia
6 Order Monotremata
7 Pelodryadidae: Cyclorana, the members of which are partly convergent with various spadefoot toads (e.g. Pelobatidae, Scaphiopodidae, Pyxicephalidae) on other continents
8 Pelodryadidae: Nyctimystes, a genus occurring mainly in New Guinea
(writing in progress)
Comments
Add a Comment