Why are there so many lizards in Australia? The ancient climate holds a clue
- Written by The Conversation
If you travel around Australia, you will find an incredible diversity of lizards.
The three-toed snake-tooth skink (Saiphos reticulatus), for example, is a peculiarly long and stumpy legged reptile that burrows in rainforest and is covered in a brilliant orange and black-banded pattern. Alpine water skinks (Eulamprus kosciuskoi) are incredibly cold-tolerant and mottled with black and greenish yellow, like mossy rocks in mountain streams. Prickly forest skinks (Concinnia queenslandiae) are delightfully chunky-headed, spiky, armoured rainforest gems.
These lizards are all members of Australia’s largest evolutionarily related group of vertebrate animals, known as the Sphenomorphini. Their ancestors arrived in Australia some 28 million years ago, likely crossing land bridges and rafting across islands from Southeast Asia during glacial periods when sea levels were lower.
In a new paper, colleagues and I describe the most complete and detailed evolutionary tree of this group to date. This helps us to understand why there is such a mountain of species diversity within this group. A crucial clue is in the climate.
Building the evolutionary tree
Previous estimates in Sphenomorphini lizards concluded there were about 270 species in the group.
For our new study, we gathered more than 5,000 genetic identifiers to build a “species tree” of the entire group that reveals a total of at least 314 member species.
Our evolutionary tree shows most modern Sphenomorphini genera in Australia seem to appear in a six-million-year burst.
The timing of this burst is telling. It coincides with the Early Miocene – a climatically tumultuous period roughly 23 million to 16 million years ago, marked by the expansion of Antarctica’s ice sheets.
Australia, which by then had broken off from the southern supercontinent of Gondwana, saw a significant reduction in rainfall. As rainforest declined, the continent became more arid.
This suggests climatic changes may have driven the diversification of Sphenomorphini, with new species forming in response to the changing conditions.
This raises the question: how exactly did the new species form?
While several processes are known to drive the evolution of new species (such as sexual selection and competition), two major forces appear to be crucial to the story of the Sphenomorphini.
One is known as “allopatric speciation”. This is when a new species forms by the simple physical splitting of a population. Over millions of years, each population accumulates enough mutations via simple chance that if they were ever to meet again they would be too different to interbreed.
The second major force is known as “ecological divergence”. This is when populations of a single species develop niche traits in response to different environmental conditions. The populations now have differing selective pressures. Eventually, they stop mating with each other and enough different mutations accumulate to create an entirely new species.
The exact role of each of these forces is still unclear and will be the focus of our future research.
Heeding the warnings
Lizards are a massive component of the storied history of life on this planet. Now that we are slowly unravelling the mysteries of their evolution, we should perhaps heed the warnings.
In the Sphenomorphini, the details seem to paint a picture of arrival, climatic change likely accompanied by extinction and diversification, and for some, persistence in the face of a changing environment.
But bear in mind, the climate shifts that upended the Australian rainforest domination and led the Sphenomorphini to generate such diversity were incredibly slow.
Much changed in the 12 million years between the so-called early Miocene and middle Miocene climatic events. Yet global temperatures only declined around 2°C–3°C.
An equivalent degree of warming in only a mere few centuries would likely be catastrophic for these remarkable creatures – along with so much other life on Earth.
