Title: Re: paleonet Here we go again!
A gneiss commentary today:
January 30,
2004
Carl
Zimmer
http://www.corante.com/loom/archives/001668.html
No SARS in Georgia
- Posted by Carl at
2:33 PM
Charles Darwin had no
great hope of witnessing natural selection at work in his own time. He
assumed that it would operate as slowly and imperceptibly as the water
that eroded cliffs and canyons. He would have been delighted to
discover that he was actually wrong on this count. By the mid-1900s,
scientists were running selection experiments in laboratories and
beginning to document the effects of natural selection in the wild,
such as the rise of insects that were resistant to pesticides. Still,
the work has been slow and painstaking. Peter and Rosemary Grant of
Princeton have done some of the best work on natural selection in the
wild, documenting its effect on Darwin's finches on the Galapagos
island. (Changes in climate lead to changes in the food supply which
in turn changes in the beaks.) The Grants have dedicated 30 years of
research to the evolutionary fate of this small group of birds. The
slowdown comes in part from the months or years that animals need to
reproduce, generating the new mutations and rearrangements of DNA that
natural selection needs in order to operate. So what would serve as a
better case study?
A virus.
Viruses can replicate madly even in a single sick person, and in some
cases they can spread across the planet in months. Added benefits
include their high mutation rate--which means that they undergo
natural selection even faster--as well as their tiny genomes, which
makes it far easier to pinpoint the changes that occur during
evolution.
Virologists have been studying the evolution of a number of viruses in
recent years--flu, dengue, HIV, and so on. And when SARS broke out,
they were ready. A team of researchers from China and the University
of Chicago have studied the virus from early in the outbreak in late
2002 to its final hushing down in February 2003. They have painted a
remarkable portrait of natural selection sculpting a virus for a new
host.
The early strains were most like the strains in civet cats, which seem
to be where at least part of the human virus came from. The virus did
a bad job of infecting humans at that point, in large part because its
machinery for invading cells was a bad match for our biology. But then
new variants emerged. They tended to lose DNA from one particular
gene. In addition, the researchers discovered some 299 individual
sites where one nucleotide was substitute for another. The researchers
showed that these substitutions altered the proteins made by the SARS
virus more often than would be expected by chance--a sign that natural
selection was at work. As the virus changed, it became far better at
infecting humans, to the point that a single person might infect
hundreds of new hosts. At this point, the mutations that emerged in
the virus were far less likely than chance to alter the SARS proteins.
This is a sign of purifying selection at work-- most mutants that
strayed even slightly from the new winning recipe were outcompeted. As
a result, by the end of the outbreak, the exuberant bush-like growth
of the SARS tree has dwindled to a few successful branches. And all of
this evolutionary change took place within just three months. This is
more than just an awesome glimpse at evolution in the wild (the wild
being our own bodies). It's insights like these that will help
scientists make vaccines to control SARS.
You'd think that breakthroughs
like these would get people fired up about the promise of evolutionary
biology. But apparently the state school superintendent in Georgia
would prefer that his students remain in the dark. I guess there must
be no SARS in Georgia.