With all the charisma that animals and plants bring to the study of macroevolution, it’s easy to forget the underlying principle behind it: genetic isolation. The biological species concept, first formulated by Ernst Mayr, basically states that isolation is the source of all new species. This isolation can be behavioural (as seen with the cichlids living in ancient African lakes, where the fish have radiated to several species due to simple differences in habitat and feeding choice), but the better documented type is geographical isolation, which leads to allopatric speciation.
If you look at this diagram, you will see what this means: a population gets split by some boundary, forming two distinct populations that cannot interbreed (because they can’t reach other). Their gene pools are therefore separate and they are left free to evolve independently of each other; over evolutionary time, they will accumulate enough genetic (and eventually morphological) differences to become incompatible. Note that I’m simplifying the process of speciation here – in reality, the line between species is so fuzzy that even asking a seemingly simple question like “what is a species?” can lead to an hours-long discussion (or a pages-long blog post. *gulp*.)
The nature of this barrier depends on what kind of organism you’re dealing with. For elephants, a mountain chain is a realistic barrier; for beetles, a road is enough. Rainforest insects are often endemic on single trees. The time scale however is not so variable: allopatric speciation takes a long time, even in ‘fast-evolving’ organisms. A river flood may be disastrous for the affected insect populations, but the area can always get recolonised once the event is over. A permanent change in the river’s course is a different story, as it then cuts right through a population’s territory, thereby splitting it (well, for insects and other invertebrates at least). You can imagine what human deforestation and urbanisation does to ecosystems – it even has its own name, habitat fragmentation.
This is where we come to this new paper. The same barrier principle also applies at the largest levels: the geological and climate change scales. The authors studied the phylogeny and phylogeography of the spiny frogs of the Paini tribe, a monophyletic clade endemic to Asia – see the picture above for the collection sites (I’m sure you know how to match colours. The arrowheads underneath indicate maximum and minimum heights at which they were found).
Imagine a population of frogs, happily living ~27 million years ago (Ma) in the Indochina region. The area was wet and humid – the continued uplift of the Himalayas in the former 25+ million years had really done wonders with the climate, creating a monsoon area perfect for frogs; frogs have a low tolerance of other types of regions (too easy to dehydrate and shrivel up), so their dispersal ability on land is quite limited by the prevailing climatic conditions.
On the diagram above, we are at Node 3 and Picture A. The large black arrow shows the direction of the tectonic plate movements. The Paini tribe had already been undergoing a radiation because of the favourable climate, and the divergence happened because of some geographic split – it’s not hard to imagine something like that happening in an active tectonic zone (remember, the Himalayas are forming and the Tibetan Plateau is rising); as the authors note, this major split is still evident today, with Nanorana species found in western China at higher ground, while the Quasipaa species are found in lower Indochinese territories (see the first picture).
The already-mentioned tectonics of the region continued (in fact, they’re still happening right now), leading to more habitat fragmentation and more divergences – see the yellow/red split in the Nanorana and B in the picture above. The arrows again show tectonic movements, this time indicating that Indochina was now getting pushed out, and the Tibetan Plateau is rising. This led to the separation of the major subgenera in the Nanorana, the final genetic cut-off happening at ~19 Ma. Similar patterns are observed in the further subdivisions – always changes in habitats leading to migrations and separate colonisations.
The authors also note some morphological uniqueness in the Nanorana, given their highland habitats on the Tibetan Plateau. Notably, they have a reduced tympanum, the sound-producing organ, as a possible adaptation to low oxygen levels – the tympanum needs quite a lot of energy to use, and energy is best conserved up there.
As for the Quasipaa, they are almost exclusively Indochinese, the main split within them happening between 24 and 18 Ma, with the formation of a mountain range (see B in the picture above). Again, same thing as with Nanorana: tectonics leading to changing habitats, etc.
The reason I wanted to blog this research wasn’t about the frogs – I couldn’t care less about them – but about the potential of interdisciplinary work. Those who know me know that I’m not above taking cheap shots at anything outside my own fields of study, but I jest (except with philosophy). Here we have the fusion of geology, phylogenetics and molecular clocks (which is how the divergence times were dated), which has helped elucidate the evolutionary history of this endemic frog tribe. This is not the first time such research has been done, but it is a relatively new way of working and holds a lot of promise. The rewards also go both ways: potentially unpreserved sections of time in the geological record are abundant, especially for terrestrial areas. Research such as this give insights into what the palaeoclimate and regional conditions were like, even in the absence of outcrops and fossils.
It also stresses what I feel is missing from many popular (and even scientific) accounts of the history of life on Earth: organisms may be simply living on the surface of the Earth, but that surface is constantly shuffling about, breaking apart at rifts or crumpling together at mountain chains – and the biosphere is not a passive observer to all this (nor is it a cause, no matter how heavy the dinos were), but it is directly affected by all of these processes. That should always be kept in mind.
Che, J., Zhou, W., Hu, J., Yan, F., Papenfuss, T., Wake, D., & Zhang, Y. (2010). Spiny frogs (Paini) illuminate the history of the Himalayan region and Southeast Asia Proceedings of the National Academy of Sciences, 107 (31), 13765-13770 DOI: 10.1073/pnas.1008415107