Lenski’s Long-Term Evolution Experiment: E. Coli

As you know, the biggest question about the theory of evolution to most people is that it's unobservable.
Lenski's Long-Term Evolution Experiment

Richard Lenski - Photo credit: G. L. Konuth, Michigan State University

But of course, scientifically, evolution is not “unobservable”. On the contrary, evolution can be observed every moment. The only problem is that what do people understand by the word “observation” is instantaneous and great observations. We want to see a living thing evolve into another species in a short time, just as the ball falls towards the ground when we drop it.

However, such a point of view is extremely wrong. Because every law in the Universe doesn’t have to work at the same speed and time. For example, you can easily observe gravity by dropping a ball but the same gravity is a force that keeps the nucleus of the stars in balance. As the star continues its life and the amount of matter in it decreases, the balance of gravity changes. As a result, the star eventually collapses inward and by explodes with the supernova explosion it may turn into another star, neutron star, or a black hole.

Although we can instantaneously observe the gravity that causes the ball to fall when the ball is dropped, it’s not so easy to instantly observe the changes in the star’s life. This doesn’t mean that gravitation is dysfunctional in the evolution and life of stars. This only shows that different laws may have different speeds and effects in various sizes and scales. In a similar way, there is no way to suddenly observe the evolution, the movement of the continents, the climate change, and all the enormous effects of them. However, If you look at it carefully, it’s possible to see the short-term effects of such slow and long processes. Even to increase their speed with cleverly designed experiments is possible and as a result the easier observations can be made.

The best way of seeing our evolution with our eyes is to repeat the evolution under laboratory conditions by artificial methods. The experiment we will talk about took almost 30 years of Richard Lenski’s life. And it’s an experiment that proves that evolution can be observed.

Who is Richard Lenski?

Richard Lenski is an American evolutionary biologist born on August 13, 1956. His father, Gerhard Lenski, is a renowned sociologist known for his sociology of religion, social inequality and eco-evolutionary social theory. Richard Lenski graduated from Oberlin College and received his doctorate from the University of North Carolina. In 1996 he won the prestigious science award MacArthur Fellowship, and in 2006 he entitled to enter the United States National Academy of Sciences. On February 17, 2010, he founded the NSF Science and Technology Center.

E. Coli Bacteria and Reasons for Use in Experiments

Escherichia coli (E. coli) bacteria are rod-shaped bacterial cells and it can breathe without oxygen. They reproduce asexually. They are among the most popular types of bacteria in biology experiments. They normally live in the intestines of warm-blooded animals (e.g. us). In fact, most E. coli bacteria are harmless, but some can lead to food poisoning. The harmless ones help to produce K2 vitamin in our body. Some E. coli bacteria may even prevent some other harmful bacteria from making us sick!

There are several reasons for using E. coli bacteria in the experiment. First, the speed of the division of these bacteria is about 20 minutes under good conditions. So these bacteria are divided once every 20 minutes when the conditions are appropriate. In other words, E. coli bacteria move forward 1 generation every 20 minutes in the evolutionary process. A human who lives 80 years can see no more than 2 or 3 generations in their lifetime. On the other hand, over 80 years E. coli bacteria can jump over 2 million generations under good conditions.

E. coli cells

E. coli cells – shutterstock

What does this mean? Change in evolutionary biology is directly related to generation. The faster an organism is moving towards the next generation, the more likely it will evolve in a certain time and differentiate from their ancestors. Therefore viruses that reproduce rapidly can evolve extremely fast and can overcome our vaccines. On the other hand, elephants, humans and other large animals reproduce much slower thus they evolve much more slowly. The number of generations to pass to observe a physical change is roughly estimated 1000. This number may vary considerably by species. But it’s quite difficult to look the same as ancestor after 1000 generations. We humans need 30,000 years for 1,000 generations, but E. coli bacteria need only around 330 hours, (i.e. 2 weeks). These numbers are of course of estimated, and most of the time need much longer time. For example, the last speciation which caused the emergence of our species and Neanderthals occurred 300,000 years ago. But remember! Evolution is not the emergence of one species from another. The 1000 generations here we are talking about are estimated time which is necessary for the observation of physical change that is enough to cause speciation. While it’s easier to observe the physical changes in a complex species, it’s quite limited in a simple species. So these numbers, though not precisely, would still be useful to compare the rate of evolution between species and to understand why E. coli bacteria are used in the experiment.

Another reason to use of E. coli lineage in the experiment is that these organisms reproduce not by bacterial conjugation but asexually. In this way, the effects of genetic processes such as crossing-over caused by sexual reproduction or additional selection pressures such as Sexual Selection can be eliminated altogether. The experiment examines evolution only through Natural Selection under the influence of mutations.

In addition, these bacteria can be easily frozen. Fossil record can be obtained thanks to frozen bacteria. Because the experiment lasted for decades and tens of thousands of generations, these fossil recordings have great importance for obtaining retrospective information. Just as the fossils give us very critical information about the past.

Considering all these reasons, E. coli bacteria are appropriate to observe evolutionary changes. Now let’s examine the experiment:

Richard Lenski’s Long-Term Evolution Experiment

The purposes of the experiment are to observe how living things change under mutations and selection pressures, to show how the speed of evolution changes depending on time and to see if the changes in living things in the same environment can be repeated.

The experiment began on February 24, 1988. Lenski allowed them to reproduce continuously by taking these 12 different groups of bacteria. He kept each group at a minimal growth rate. On each new day, he took 1% of the growing population into a new container. Every time this process is done, the bacterial lineages have moved forward to 6.64 generations on average.

Don’t be surprised! Because, if you remember, good conditions are required for 1 generation in every 20 minutes. But Lenski provided the worst, but still livable conditions (“minimal conditions”) to create survival pressures on these creatures. For this reason, the rate of reproduction of the creatures has decreased significantly. In other words, the E. coli used by Lenski were slower in the above information because there was not plenty of food in the environment.

Lenski, as we have already explained, should have “fossil evidence” to be able to examine the evolutionary past. For this, in every 500 generations (i.e. 75 days) he froze several members from the last generation with glycerol in cryoprotectant. And thus he succeeded in obtaining an intermittent but continuous record, just as in the fossils. Bacteria that are stored in cryoprotectants and glycerol can be defrosted when needed, which is a good opportunity to examine the changes that occur in bacteria.

Lenski constantly followed the mean fitness of the bacteria and did additional experiments on the specimens he received from the generations but never touched the main generation and followed and recorded as an evolutionary process.

Richard Lenski

Richard Lenski – Photo credit: G. L. Konuth, Michigan State University

Experiment Results

By February 2010, bacteria has moved 50,000 generations forward. In October 2012, there were 56,000 generations, and in April 2014, 60,000 generations, and they continue to divide! This number is very important in terms of mathematical evolution. Because this amount statistically means enough time for each nucleotide in the E. coli genome can pass the mutations multiple times. Even in the first 30,000 generations, each of the twelve lineages mutated billions of times.

The experiment is ongoing for 28 years! During this period, many scientific revolutions have taken place, the internet has entered our lives, televisions, radios, cars have developed. At the same time, there was another very interesting event: Evolution was once again observed in the laboratory!