New discoveries that show evolution in action are causing some scientists to say that the first scientific supermodel has arrived.
Biology is normally carried out within isolated specializations. Ecologists study one organism, molecular biologists another, while evolutionary biologists look over hundreds without probing too deeply into any particular one.
But one tiny little fish, the threespine stickleback, proves that a combination of genetics, molecular biology, developmental biology and population studies, can bring insight into the fundamental question of how evolution occurs in nature.
“The sticklebacks are a shining example of what can happen when you put all of these fields together,” said Dolph Schluter, a zoologist at the University of British Columbia. “It produces a paper that all of biology can appreciate,” he said.
Recently, Schluter, in collaboration with David Kingsley, from Stanford University School of Medicine, and other colleagues, found that a single gene seems to control changes in the armor of sticklebacks in the wild.
Threespine sticklebacks, Gasterosteus aculeatus, are five-cm-long fish that show a great deal of morphological diversity. In the ocean, you’ll find them with upwards of 36 bony plates, which run along their sides and are thought to serve as armor against predators. In freshwater lakes, these armor plates have been lost, likely due to a lack of predators.
The loss of bony armor in sticklebacks is a prime example of evolution in action. “It’s rather like a military decision, to be either heavily armored and slow, or to be lightly armored and fast,” said Kingsley in a press statement. “Now, in countless lakes and streams around the world these low-armored types have evolved over and over again. It’s one of the oldest and most characteristic differences between stickleback forms. It’s a dramatic change: a row of 35 armor plates turning into a small handful of plates – or even no plates at all.”
To better understand how this natural evolution works, Schluter, Kingsley, and their colleagues, crossbred heavily armored marine fish with those containing no armor at all. The offspring of this cross were then used to look for genes controlling the production of armor by a technique called chromosomal walking. Surprisingly, the authors found a single gene, called Ectodysplasin (Eda), which appears to control major changes in stickleback armor. The results of this work are published in the March 25, 2005, issue of Science.
A mutation in the Eda gene controls hair and teeth development in humans, being linked to the disorder ectodermal dysplasia. “This gene also turns out to be responsible for a fairly conspicuous change in evolution in nature,” said Schluter.
The discovery that Eda appears to control the adaptation of stickleback armor sheds light on an old debate about how evolution occurs in nature. Scientists have struggled to discover if evolution occurs by many small changes in numerous genes or significant changes in just a few genes. The findings of Schluter and his colleagues appear to support the latter.
Another big question about how genetics influences natural adaptation is how much evolution relies on new mutations versus variation already present in ancestral populations, said Schluter. The team was able to show that mutations in Eda predate that loss of armor in freshwater sticklebacks, revealing that new mutations may not be required to power some forms of evolution. “In this case, for this one trait [stickleback armor], it appears variation is ancestral,” said Schluter.
The discovery of Eda’s role could not have been made without the merging of distinct specializations, including gene isolation in Kingsley’s lab and the knowledge of developmental evolution in Schluter’s lab. “When I read the paper, I was just wowed by the comprehensive nature of it all,” said Greg Gibson, from North Carolina State University, who wrote an article in Science that accompanies the research paper.
“[Schluter’s paper] elevates the stickleback to the status of supermodel for the study of developmental evolution,” writes Gibson, in his accompanying paper.
“They have really put together a huge amount of research and shown the way forward,” said Gibson.
Dolph Schluter’s Webpage
Colosimo PF, et al. “Widespread parallel evolution in sticklebacks by repeated fixation of Ectodysplasin alleles,” Science, 307(5717):1928-33.
Gibson G. “Evolution. The synthesis and evolution of a supermodel,” Science, 307(5717):1890-1.