Jacky Dragon Offers Evolution Proof

Jan. 22, 2008 -- An Australian research team has provided what they say is the "Holy Grail" of evolutionary biology -- proof a 30-year-old theory about reptile sex and survival is right.

The findings explain the evolutionary advantage of a reptile's sex being determined by temperature.

The University of Sydney's Rick Shine and his former student Daniel Warner, now of Iowa State University, report that the temperature at which a reptile egg is incubated not only determines sex but optimizes the number of offspring in future generations.

Their findings, published online this week in the journal Nature, provide the first "unequivocal" demonstration that incubation temperatures affect the reproductive success of males and females.

In mammals and birds sex is determined by genotype at fertilization resulting in roughly equal numbers of sons and daughters. However in many reptiles and some fish, sex is determined after the egg is laid and is dependent on the environment, most commonly the temperature.

Shine, from the School of Biological Sciences, says in some species of reptiles only males or females will be born at certain temperatures.

He says about 30 years ago biologists Ric Charnov, now of the University of New Mexico, and Jim Bull, of the University of Texas, suggested this "environmental sex determination" was not just a quirk of nature.

Rather they believed males or females incubated at certain temperatures had an evolutionary advantage -- specifically, that they would have an optimized number of offspring.

But proving this theory was another matter and has since been the "Holy Grail" for evolutionary biologists, Shine says.One challenge in proving the ideas of Charnov and Bull was finding a species with a short enough life span so it was practical to measure the entire number of offspring it had over its life.

Most species with environmental sex determination have life spans of more than 60 years and reach sexual maturation late.

But the short-lived Jacky dragon (Amphibolurus muricatus), a common species of lizard found on the east coast of Australia, helped overcome this challenge. The Jacky dragon produces offspring within one year of hatching and lives no longer than four years.