Wednesday, 3 April 2019

Three-toed Skink Is an Evolutionary Intermediate

Which came first the lizard or the egg? - The University of Sydney

Three-toed skink, Saiphos equalis.
Today from the the reptile world, we have a very nice example of evolution in progress, or at least in a state of dynamic equilibrium between two characteristics, each of which could be advantageous in different circumstances.

This example is an Australian skink which appears to be so finely balanced between egg-laying (oviparous) and live-young bearing (viviparous), that one individual has been observed doing both in the same pregnancy. Several weeks after laying a batch of three eggs, an individual three-toed skink, Saiphos equalis, was seen to give birth to a live young.

Examples of individuals reptiles of the same species being either oviparous or viviparous (bimodally reproductive) are rare but by no means unknown, however this is the first observed example of the same individual being able to be both. The three-toed skink is native to Australia's east coast. In the northern highlands of New South Wales it normally gives birth to live young but in the Sydney area, it lays eggs.

This was seen by researchers from the University of Sydney, NSW, were studying the genetics of these skinks. They published their findings in Biology Letters today.

Biologically, the interesting thing here is that this example suggests there is an intermediate stage in the evolution of viviparity in lizards. Contrary to what we are told by creationists, the evolution of viviparity in lizards involves a loss of complexity in that there is a loss of the ability to produce an egg shell and a loss of the necessary glands in the oviduct to producing it.

There are at least 150 examples of vertebrates evolving viviparity but examples of reversion to oviparity are rare. Dollo's Law of irreversibility predicts that complex traits, once lost, either cannot re-evolve or will do so in a different way. This suggests that there is an 'evolutionary valley' from which a viviparous species can only escape with difficulty. However, the existence of both in the same individual suggests there is an intermediate stage in the evolution of viviparity were reversal is possible.

In fact, what may have evolved in this species is facultative viviparity, where the ability to switch according to the situation is an evolutionary advantage in itself, to the extent that switching one way or the other permanently would be a disadvantage. Evolutionary pressures acting both for and against viviparity across this species range seem to have resulted in a species in which flexibility wins over specialisation.

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  1. In case you missed it, Rosa. Have a look at:

    1. Thanks, Helmer. I wasn't aware of that site. Nice to see someone reads my blog posts. :-)


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