Researchers have discovered that genetically tinkering with fish can cause the opposite effect of evolution the survival of the least fit and the eventual extinction of a species. It would take just one genetically-modified fish to wipe out local populations of that species if it escaped into the wild.

William Muir and Richard Howard of Purdue University call GM genes "Trojan genes," because "This resembles the Trojan horse," says Muir. "It gets into the population looking like something good and it ends up destroying the population."

Biologists in the U.S. and the U.K. are currently engineering salmon so that they contain the human growth hor'one gene hGH, which increases their growth rate and size. Muir and Howard don't think this is a good idea, so they tested it by putting hGH in embryos of a Japanese medaka fish, which is a common aquarium fish that is used in research. They found the GM fish became sexually mature faster and produced more eggs. Also, the GM males attracted 4 times as many mates, probably because they were bigger. This means that the hGH gene will spread quickly through most fish populations.

But here's the twist: only two-thirds of the GM fish survived to reproductive age, compared with wild medakas. So the spread of the growth hormone gene could make populations unable to reproduce and lead to extinction.

The researchers used a computer model to find out what would happen if 60 GM fish were placed in a population of 60,000 wild fish and discovered that the fish population became extinct within 40 generations. A single GM fish could have the same effect, although it would take longer.

"You have the very strange situation where the least fit individual in the population is getting all the matings this is the reverse of Darwin's model," says Muir. "Sexual selection drives the gene into the population and the reduced viability drives the population to extinction."

Professor David Penman, a fish geneticist at the University of Stirling in the U.K., has discovered that some GM fish modified with growth hormone have reduced sperm production. "If large males tend to mate with large females, this would often result in matings between GM fish," he says. But if the larger GM males have less sperm, there would be fewer offspring produced, which would eventually end the species.

The creation of GM corn has wiped out the natural varieties in Mexico and will probably do the same thing to corn crops in the rest of the world but at least we will still have corn. But adding growth hormones to a fish species can mean there may eventually be no more of that kind of fish left to eat.

Originally published at Whitley Strieber's Unknown Country.