January 30, 2007 -- It's a mystery why the speed and complexity of evolution appear to
increase with time. For example, the fossil record indicates that
single-celled life first appeared about 3.5 billion years ago, and it
then took about 2.5 billion more years for multi-cellular life to
evolve. That leaves just a billion years or so for the evolution of the
diverse menagerie of plants, mammals, insects, birds and other species
that populate the earth.
New
studies by Rice University scientists suggest a possible answer; the
speed of evolution has increased over time because bacteria and viruses
constantly exchange transposable chunks of DNA between species, thus
making it possible for life forms to evolve faster than they would if
they relied only on sexual selection or random genetic mutations.
"We have developed the first exact solution of a mathematical model of
evolution that accounts for this cross-species genetic exchange," said
Michael Deem, the John W. Cox Professor in Biochemical and Genetic
Engineering and professor of physics and astronomy.
The research appears in the Jan. 29 issue of Physical Review Letters.
Past
mathematical models of evolution have focused largely on how
populations respond to point mutations - random changes in single
nucleotides on the DNA chain, or genome. A few theories have focused on
recombination - the process that occurs in sexual selection when the
genetic sequences of parents are recombined.
Horizontal gene
transfer (HGT) is a cross-species form of genetic transfer. It occurs
when the DNA from one species is introduced into another. The idea was
ridiculed when first proposed more than 50 years ago, but the advent of
drug-resistant bacteria and subsequent discoveries, including the
identification of a specialized protein that bacteria use to swap
genes, has led to wide acceptance in recent years.
"We know that
the majority of the DNA in the genomes of some animal and plant species
- including humans, mice, wheat and corn - came from HGT insertions,"
Deem said. "For example, we can trace the development of the adaptive
immune system in humans and other jointed vertebrates to an HGT
insertion about 400 million years ago."
The new mathematical
model developed by Deem and visiting professor Jeong-Man Park attempts
to find out how HGT changes the overall dynamics of evolution. In
comparison to existing models that account for only point mutations or
sexual recombination, Deem and Park's model shows how HGT increases the
rate of evolution by propagating favorable mutations across populations.
Deem
described the importance of horizontal gene transfer in the work in a
January 2007 cover story in the Physics Today, showing how HGT
compliments the modular nature of genetic information, making it
feasible to swap whole sets of genetic code - like the genes that allow
bacteria to defeat antibiotics.
"Life clearly evolved to store
genetic information in a modular form, and to accept useful modules of
genetic information from other species," Deem said.
Source : Rice University
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