[asa] Lab speciation

The following post appeared on TWeb a few minutes ago. It is a
fascinating story (IMHO)

Burgy

An upcoming PNAS paper describes an amazing experiment carried out
over a 20 year span that provides great insight into evolutionary
processes like speciation

From Carl Zimmer at The Loom

The experiment was launched by MSU biologist Richard Lenski. I wrote
about Lenski's work last year in the New York Times, and in more
detail my new book Microcosm. Lenski started off with a single
microbe. It divided a few times into identical clones, from which
Lenski started 12 colonies. He kept each of these 12 lines in its own
flask. Each day he and his colleagues provided the bacteria with a
little glucose, which was gobbled up by the afternoon. The next
morning, the scientists took a small sample from each flask and put it
in a new one with fresh glucose. And on and on and on, for 20 years
and running.
What happened next was amazing. After 33,000 generations, one of the
lines evolved a completely new way to eat. Investigation showed that a
series of random mutations allowed one line of the E coli to feed on
citrates, a compound that normal E coli can't touch.

After 33,127 generations Lenski and his students noticed something
strange in one of the colonies. The flask started to turn cloudy. This
happens sometimes when contaminating bacteria slip into a flask and
start feeding on a compound in the broth known as citrate. Citrate is
made up of carbon, hydrogen, and oxygen; it's essentially the same as
the citric acid that makes lemons tart. Our own cells produce citrate
in the long chain of chemical reactions that lets us draw energy from
food. Many species of bacteria can eat citrate, but in an oxygen-rich
environment like Lenski's lab, E. coli can't. The problem is that the
bacteria can't pull the molecule in through their membranes. In fact,
their failure has long been one of the defining hallmarks of E. coli
as a species.

If citrate-eating bacteria invade the flasks, however, they can feast
on the abundant citrate, and their exploding population turns the
flask cloudy. This has only happened rarely in Lenski's experiment,
and when it does, he and his colleagues throw out the flask and start
the line again from its most recently frozen ancestors.

But in one remarkable case, however, they discovered that a flask had
turned cloudy without any contamination. It was E. coli chowing down
on the citrate. The researchers found that when they put the bacteria
in pure citrate, the microbes could thrive on it as their sole source
of carbon.

In nature, there have been a few reports of E. coli that can feed on
citrate. But these oddballs all acquired a ring of DNA called a
plasmid from some other species of bacteria. Lenski selected a strain
of E. coli for his experiments that doesn't have any plasmids, there
were no other bacteria in the experiment, and the evolved bacteria
remain plasmid-free. So the only explanation was that this one line of
E. coli had evolved the ability to eat citrate on its own.
The neat part is that Lenski and his team were freezing and saving
samples of the strains every 500 generations, so they could go back
and examine the exact series of mutations that led to this entirely
new feature. What they found was a process exactly as predicted by the
Avida AI model. A few mutations occurred which actually were mildly
harmful but did not outright kill the strain. Then a few more
mutations occurred that combined with the previously harmful ones to
provide a huge net positive benefit.

Blount took on the job of figuring out what happened. He first tried
to figure out when it happened. He went back through the ancestral
stocks to see if they included any citrate-eaters. For the first
31,000 generations, he could find none. Then, in generation 31,500,
they made up 0.5% of the population. Their population rose to 19% in
the next 1000 generations, but then they nearly vanished at generation
33,000. But in the next 120 generations or so, the citrate-eaters went
berserk, coming to dominate the population.

This rise and fall and rise suggests that the evolution of
citrate-eating was not a one-mutation affair. The first mutation (or
mutations) allowed the bacteria to eat citrate, but they were
outcompeted by some glucose-eating mutants that still had the upper
hand. Only after they mutated further did their citrate-eating become
a recipe for success.
No matter how the cretos spin this, it is a laboratory example of the
evolution of a completely new feature, and by most definitions (there
is some disagreement) a new species.

Read the whole article here

http://scienceblogs.com/loom/2008/06..._evolution.php

This new evidence should give pause to the most rabid evolution doubters.

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Received on Wed, 4 Jun 2008 13:12:00 -0600