2
How did you decide to make a career
in microbial evolution?
I got interested in biology as an under-
grad at Oberlin College, and I was espe-
cially fascinated by ecology because there
were so many unanswered questions. So I
went to grad school at the University of
North Carolina to study ecology. I began
to see the deep connections between ecol-
ogy and evolution. Many ecologists have
lifelong interests in particular organ-
isms—birds, snakes, butterflies, or what-
ever. But I didn’t have any special skills
in that respect; I was more interested in
the general questions. I remembered
the elegance of the genetics experiments
with bacteria that I had learned about as
an undergraduate. So I decided that, for
my postdoctoral work, I should find a
lab where I could learn how to work with
microbes. I found a superb mentor, Bruce
Levin, who was interested in evolution.
Why did you perform your long-term
experimental evolution study with
Escherichia coli? What makes it different
from previous studies of evolution?
I started the experiment to ask one main
question: How repeatable is evolution?
Mutations occur at random, but popula-
tions become more fit over time if some
of the mutants survive and reproduce bet-
ter than their ancestors—that’s natural
selection. In essence, I wanted to know
how many different ways there were
for the bacteria to adapt to a particular
environment.
I set up 12 populations, all started from
the same E. coli strain, and each one in an
identical flask containing a medium where
glucose is the source of energy. Every day,
someone takes 1% of the volume from each
flask and transfers it to a new flask with
fresh medium. The bacteria grow and,
after some hours, deplete the glucose,
so it’s a “feast or famine” existence. The
dilution and regrowth allows about seven
bacterial generations per day. I started the
experiment in 1988, and the bacteria have
now been evolving for well over 50,000
generations. So many interesting things
have happened that I’ve kept it going all
these years. In fact, I hope the experiment
will continue even after I’m gone.
This project differs from most research
on evolution because we’re watching evo-
lution in action. Most evolutionary biolo-
gists study fossils or use the comparative
approach—that is, quantifying similari-
ties and differences in phenotypes and
genomes of living organisms—in order
to infer the characteristics of organ-
isms that lived in the past. In this E. coli
experiment, we can observe changes as
the generations go by, and we can directly
2
Richard Lenski
Evolution in the Lab
Richard Lenski, an evolutionary biologist, has taught for over 20 years as the
John Hannah Distinguished Professor at Michigan State University. Since
1988, Lenski and his students have been tracking phenotypic and genetic
changes in 12 initially identical populations of bacteria. Their report of E. coli
bacteria evolving a new trait in the laboratory earned headlines from the New
York Times and other media around the world. Lenski cofounded BEACON, the
National Science Foundation’s Center for the Study of Evolution in Action.
A N I N T E R V I E W W I T H :
Richard Lenski, Hannah Distinguished
Professor, Michigan State University.
COUR
TES
Y OF RICHARD LENSKI
COUR
TES
Y
OF RICHARD LENSKI
One E. coli population evolved the novel capacity to consume citrate for energy (clouded flask).
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AN EVOLVING SCIENCE, THIRD EDTION
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compare the evolving bacteria with their
ancestors. We’ve stored the ancestral
strain and samples from every 500 genera-
tions in a freezer, and with E. coli we can
revive the frozen cells. It’s like bringing
fossils back to life.
Over the 25 years of this experi-
ment, the research has involved dozens
of dedicated people. I have an excellent
technician, Neerja Hajela, who either
does the transfers herself or makes sure
someone else does them. Mike Travisano
was the first student to base his disserta-
tion research on this experiment, and
he’s now a professor at the University of
Minnesota.
What results have you obtained?
Have any results surprised you?
One result is that the average fitness in
each population increases over time. We
measure fitness by competing bacteria
from different generations against the
common ancestor. This result is not sur-
prising, since the environment has been
constant over time, but it’s a concrete
demonstration of adaptation by natural
selection, the same process that Darwin
discovered.
Another finding is that evolution can
be quite repeatable; that is, we’ve seen
many examples of parallel changes in the
replicate lines. For example, all 12 popu-
lations evolved to produce larger indi-
vidual cells than the ancestor produced.
And when we look at their mutations, we
see many cases in which some or even all
of the lines have mutations in the same
genes.
The most dramatic change we’ve
seen happened in only one population.
Glucose was the source of energy for
the bacteria, but there’s been another
resource—citrate—in the medium all
along. E. coli cells can’t use the citrate,
however, because they’re unable to take
up citrate in the presence of oxygen. In
fact, the inability to grow on citrate is a
key feature of E. coli as a species. But
after about 30,000 generations, a mutant
in one population discovered there was
something else to eat besides the glu-
cose. At first I thought we had a contami-
nant—some other species—in this flask,
but genetic analyses showed it really was
a descendant of the E. coli strain used
to start the experiment. So here’s a case
where one population evolved to be very
different from all the other populations.
Zack Blount, a postdoc in the lab, is ana-
lyzing the mutations that allow the bac-
teria to grow on citrate. Caroline Turner,
a grad student, is studying how this new
ability changes the ecological interac-
tions between different genotypes in the
population.
What new technologies have made
it possible to take full advantage
of your study?
When I started this experiment in 1988,
I couldn’t imagine the amazing tech-
nologies that would come along and
allow us to analyze the evolution that
has taken place. The ability to sequence
entire genomes is the most important
advance. By sequencing the genomes of
evolved bacteria and comparing them to
the ancestor’s genome, we’re finding the
mutations that led to improved fitness
and other phenotypic changes.
Does experimental evolution have
industrial applications?
Yes, it does. Humans can apply evolu-
tion for practical purposes, just like we
use other natural processes, such as grav-
ity and the action of water, to do work via
mill wheels and hydroelectric plants. In
fact, the selective breeding and domesti-
cation of farm animals, crop plants, and
even microbes (like baker’s yeast) show
that our ancestors employed evolution
for practical purposes long before the
mechanisms of evolution were under-
stood. More recently, scientists have been
pursuing genetic engineering to modify
microbes for new purposes, like biofuels.
Experimental evolution—where scientists
construct environments that select for
organisms with the desired properties—
offers a valuable complement to genetic
engineering.
How does your family relate
to your work?
I sometimes joke that I have two fami-
lies: my biological family with my wife
and kids, and my lab family, with all the
students and postdocs who’ve been a part
of it over the years. As much as I love my
work—and I can’t imagine a better job
than being a biology professor—there’s
always more research to be done. So
I’m grateful that my wife and kids have
been supportive of my work. Now I have
a granddaughter, and she reminds me
just how fortunate I am to see another
generation in the great evolutionary
tree of life.
PART 1
The Microbial Cell
3
1.2
0.2
0.4
0.6
0.8
1.0
Time (generations)
Cell volume (10
—15
liter)
0
2,000 4,000 6,000 8,000 10,000
Cell size increased in all 12 evolving
populations of bacteria.
Source:
Modified from Richard Lenski and Michael
Travisano. Dynamics of adaptation and diversi-
fication: a 10,000-generation experiment with
bacterial populations. 1994. PNAS
91:6808.
For further details on Lenski’s ex-
perimental evolution of E. coli, see
Chapter 17 Origins and Evolutions.
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