Researchers at Rice University have created a mathematical model that helps build the argument that evolution doesn't proceed solely through breeding and genetic mutations. Rather, organisms also swap large sections of DNA.
The study, published in the January edition of Physics Today, sheds additional light on one of the enduring mysteries of evolution, namely its seemingly accelerating rate of change. Fossil records indicate that single-cell life forms emerged 3.5 billion years ago, and then it took 2.5 billion years for multicell organisms to appear. Animals, plants and birds then took only 1 billion years to develop.
A principal factor in this acceleration is horizontal gene transfer (HGT), according to Michael Deem, the John W. Cox Professor in Biochemical and Genetic Engineering and a professor of physics and astronomy at Rice. In HGT, an organism will give (or exchange) large chunks of its own genetic material to another in a process that can be described as a naturally occurring gene graft.
Proteins produced by the native genetic code of an organism allow the organism to accept and graft the new genes into its own genome.
"It is like having a lot of genetic mutations at once. Enzymes allow DNA to be excised from one species to us," Deem said. "Bacterial geneticists have worked on HGT for 15 to 20 years, but not many of the other evolutionary biologists (have)."
Many HGT insertions, like most genetic mutations, will have no impact on the development of a species, and some transfers will be deleterious, he said. A small fraction of the transfers, however, will lead to sudden, beneficial changes in an organism's genome.
Some studies have stated that the adaptive immune system in humans and vertebrates resulted from an HGT insertion 400 million years ago.
"It led to a dramatic change for invertebrates," he said. "With HGT, a whole population becomes resistant at once."
The mathematical model created by Deem and visiting professor Jeong-Man Park simulates how beneficial HGT insertions propagate across a species.