Focus Magazine

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REAL-TIME
EVOLUTION
Over 21 years,
Lenski has cultivated
more than 40,000
generations
of E coli
E COLI
GENERATION
all 12. The longer Lenski let the experiment ﬂ ask was cloudy. Hajela assumed the ﬂ ask
1988
0 Blount discovered that the bacteria
run, the more they evolved, and the more had been contaminated somehow. So she
Richard Lenski
were now taking in the citrate and eating
sets up12 lines
questions occurred to him and his students threw it away and thawed out some of the of E coli
1989
it. They were drawing energy from the
that could be answered with his bacteria. most recently frozen bacteria from that bonds between its atoms and using some
So they kept moving the bacteria to new line. Within three weeks, the same line of the atoms to build new molecules. One
ﬂ asks every day, building up the frozen had turned cloudy again. That couldn’t be 1990 of the hallmarks of E coli as a species is
fossil record. a coincidence.
5000
All 12 lines have
an inability to eat citrate when oxygen is
Today, after 45,000 generations, the Lenski also thought it was a false alarm, already evolved present. The ‘citrate eaters’ no longer had
bacteria grow over 75 per cent faster than but when the clouds returned to the ﬂ ask,
1991 to grow more
quickly than
to starve when their supply of glucose ran
they did at the beginning of the experiment. he enlisted his postdoctoral researcher
their ancestors
out. Instead, they could just start feeding
The rate at which they’re improving has Christina Borland and, later, his graduate on a new food.
1992
gradually dropped, but they’re still getting student Zachary Blount to ﬁ gure out what 10,000 Blount returned to the frozen fossil
better. “I believe they’d keep improving was going on. Borland quickly determined record to ﬁ gure out when the citrate eaters
for thousands of years,” says Lenski. The there was no contamination. The bacteria
1993
ﬁ rst emerged. The ﬁ rst bacteria with any
evolution of the 12 lines has also continued exploding in the ﬂ ask were the descendants ability to eat citrate appeared after 31,000
in roughly parallel ways: all 12 have also of the original strain of E coli, but a new generations, but before 31,500 generations
gotten about twice as big as their ancestors, kind, one that was doing something E coli is
1994
(about seven years ago). Over the next 2000
for reasons Lenski has yet to ﬁ gure out. not supposed to do. generations, they acquired new mutations
The broth Lenski rears the bacteria in is that vastly improved their ability to exploit
1995
E coli evolves a standard recipe developed decades ago to
15,000
citrate, leading to their population boom.
But the lines are not all the same and some let bacteria thrive in labs. E coli needs trace
have revealed new things. One day in amounts of iron to survive, but it can’t draw 1996 Creationist controversy
2003, his lab manager, Neerja Hajela, was in free iron atoms. The broth contains a In 2006, Lenski was elected as a member
performing the morning ritual of drawing molecule called citrate (the compound that of the US National Academy of Sciences for
a few drops of the ﬂ uid containing the makes lemons tart) that can bind iron, and
1997
20,000
his work on microbial evolution. Each new
bacteria from each ﬂ ask and adding them to in that form E coli can absorb it. The citrate member gets to publish an inaugural paper
a new one. She noticed something odd: one doesn’t enter the microbe, however. in the academy’s journal, so Lenski decided
1998
to unveil his citrate eaters. The paper,
published in the summer of 2008, was a
1999 clear example of how it’s now possible to
observe evolution in our own lifetime.
Creationists were not pleased. Some
2000
even demanded that Lenski surrender
his bacteria to them, hoping to prove his
2001
31,000
results were bogus. Instead, Lenski wrote to
In one of the 12
explain why their attacks were unfounded.
lines, the ﬁ rst His letter was reprinted on many of the
E coli bacteria
2002
with the ability
biggest science sites online.
to eat citrate
When Lenski ﬁ rst launched his
appears
experiment, it was difﬁ cult to ﬁ nd the new
2003
mutations in his bacteria. Now it costs just
a few hundred dollars to read a microbe’s
genome (its complete DNA sequence).
2004
35,000
Blount and postdoctoral researcher Jeff
Barrick are sifting through the genomes of
After 45,000
2005
generations,
the citrate eaters, comparing their DNA to
the bacteria
pinpoint the mutations that have allowed
in all the lines
grow over 75 them to make the transition to a citrate
2006
per cent faster
and are about
diet. Like other scientists who have not
twice the size yet completed their research, they won’t
of their original
2007 ancestors – and
disclose the details until they’ve been
they are still
vetted by reviewers and published. But they
evolving
were willing to say that they’ve now ﬁ gured
Richard Lenski, witness to the evolution of E coli, and possibly the emergence of a new species
2008
45,000
out at least part of the genetic story. It 3
www.bbcfocusmagazine.com February 2009 39
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