Name_______________________________________
Mr. Hobbs
BACTERIAL EVOLUTION
H.W.
Adapted from “Hitting the Redo Button on Evolution”
by Tina Hesman Saey
People have always wondered why living things work the way they do. Charles Darwin, an
English scientist who lived in the 1800s, was very curious about the shape of organisms, and did
lots of experiments to find some answers. He came up with an explanation for evolution, which
helps explain where all of the living things on earth came from.
A big part of Darwin’s evolution theory is a process called natural selection — you might
have heard it called “survival of the fittest.” Natural selection says that plants, animals and other
organisms that have the best adaptations for their environment will survive and reproduce more. If
the environment changes, then the organisms that can change their adaptations quickly will
survive better than others. Darwin knew that organisms can’t just change all of a sudden. It takes
a long time for multicellular organisms to reproduce and make babies that might have different
adaptations. What Darwin did not predict was how fast evolution can happen with organisms that
can reproduce every 20 minutes.
Richard Lenski at Michigan State University in East Lansing is doing experiments with
bacteria to find out if it is possible to actually see Darwin’s theory of natural selection happen in a
short amount of time. In 1988, Lenski lined up 12 flasks at the start of his experiment. In each flask
he placed identical bacteria, known as Escherichia coli (or E. coli for short), and fed all of the
bacteria the same food. The bacteria’s food is a liquid that contains vitamins, nutrients, and two
kinds of sugar called glucose and citrate (pronounced “SIT-rayt”). E. coli bacteria naturally eat
glucose, but they do not eat citrate.
We E. coli bacteria live inside of the
intestines of humans and other warmblooded animals, and we help you
multicellular creatures to digest your
food by breaking down big sugar
molecules into smaller chemicals you
can use.
Every once in a while, some E. coli
change into a dangerous form and
cause food poisoning, but don’t blame
us for those “bad apples”! We’re inside
of you right now, helping you out!
Just like each human is different, all bacteria are different from each other in little ways that
we can’t see right away. Bacteria that are better at using glucose grow faster. They also
reproduce faster than bacteria that don’t digest glucose as well. Each day, Lenski randomly takes
some of the bacteria and puts them in a new flask containing the same liquid. The remaining
bacteria are frozen, which is like hitting the pause button. The frozen bacteria stay alive, but they
go dormant and they stop reproducing until they are warmed up again. Scientists can go back
later and start the experiment again with the paused, frozen bacteria. By now, the bacteria have
reproduced more than 45,000 times!
These 12 flasks contain separate
populations of E. coli bacteria, all
evolved from a single ancestor.
Only the bacteria in flask A-3 (third
from left in the back row) evolved
the ability to eat citrate.
By using a dozen different flasks, Lenski was basically setting up 12 different recordings of
the same game, pushing “play” on all 12 at the same time and watching to see if all the games
would end the same. For thousands of generations, all of the bacteria in all of the flasks grew and
reproduced pretty much the same. All of the bacteria grew bigger than their ancestors and got a
little better at eating glucose.
But then something different happened. After about 30,000 generations, bacteria in one of
the flasks suddenly started to eat the glucose AND the citrate in the mixture.
“They’ve been eating the main course for thousands of generations,” Lenski says. “They
didn’t realize that there was a dessert tray around the corner.”
Doesn’t that make you wonder what humans will be eating 30,000 generations from today?
http://www.sciencenewsforkids.org/articles/20090211/Note2.asp
From Science News for Kids Feb. 11, 2009.
Name____________________________________
Date_____________________
“Bacterial Evolution”HW
Mr. Hobbs
Directions:
STEP 1: Read the article “Bacterial Evolution” and use it to answer the questions
below.
Your answers should be IN
COMPLETE SENTENCES.
***NOTE*** Leave these pages stapled together and turn them in together.
1. Who is the main scientist discussed in this article and where does he work?
______________________________________________________________________________
______________________________________________________________________________
2. Find a sentence that best summarizes Charles Darwin’s theory of Natural Selection and copy
that sentence below.
______________________________________________________________________________
______________________________________________________________________________
3. Where do E. coli bacteria normally live and how do they help humans?
______________________________________________________________________________
______________________________________________________________________________
4. What is the most important event that this article reports?
______________________________________________________________________________
______________________________________________________________________________
5. Which of the following best describes the author’s purpose in writing this article?
a. To persuade the reader that bacteria are dangerous because they evolve too quickly.
b. To teach the reader step-by-step instructions on how to conduct experiments with bacteria.
c. To inform the reader of a discovery about how quickly bacteria can evolve.
d. To entertain the reader with a story about a scientist and his experiments.