Molecular systematics of the diverse lizard genus Lerista: Adaptive radiation and the evolution of limblessness
A project undertaken at The Department of Environmental Biology, The University of Adelaide, and supervised by M Lee and M Hutchinson
The aim of the study is to investigate how lizards evolved into snakes. The "missing links" of that transition are long dead, but a similar process is happening before our very eyes, in many groups of lizards.
The ideal lizards in which to study this transformation are native to Australia, 80 or so species of small lizards belonging to the genus Lerista. These lizards all share the habit of wiggling into loose soil and sand to hide, to shelter from extreme weather or to hunt for hidden prey. Rather than snake-like, their sinuous bodies may be better thought of as eel-like, except that they 'swim' through sand instead of water.
These sand-swimmers are extraordinarily variable in their external appearance. There are species of Lerista with a 'normal' small lizard body with well-developed, five-toed limbs as well as two completely limbless species that superficially resemble tiny snakes. And in between are species with all the intermediate stages of limb and toe development.
The joint University of Adelaide - SA Museum project, funded through the Hermon Slade Foundation, will focus on the evolutionary relationships among the species of Lerista. To do this, the researchers will unravel the DNA sequences of a selected set of genes across all of the species, to reveal the Lerista family tree.
One result of knowing the evolutionary tree will be to discover if all species of Lerista having similar limb and body development are closely related. For instance, over a dozen species have almost totally lost their legs, retaining only tiny hind limbs with just two toes. Has this arrangement evolved several times, or just once? How long did it take? Answers will tell us just how "easy" it is to evolve a snake-like body. A second group of researchers collaborating in the USA will look at the early embryology of these lizards, looking at the developmental switches that control leg and toe development.
These lizards provide a good answer to a question often raised by people trying to understand the gradual evolution caused by natural selection - what good is "half an adaptation"? We have no trouble understanding the usefulness of four strong limbs and it is equally obvious how successful snakes are - but what good is it when an animal is neither one nor the other? Can a half-lizard, half-snake with tiny legs make a living? In the case of Lerista the answer is "Yes". Animals with exactly this body form exist, and seem to get along just fine.