- These are bacteria growing into
increasingly concentrated antibiotics. The bacteria stop growing when they hit the first antibiotic strip, but then a mutant appears
capable of surviving in the antibiotic. Then another mutation occurs and now the bacteria can survive
10 times the concentration, then a hundred times, and finally, after just
11 days of evolution, these bacteria can survive antibiotics a thousand times stronger than what would have
killed them at the start. You're watching evolution in action. This video was sponsored by Bounty, reminding you that a hygienic
way to clean up messes and spills is with paper towel. Now, bacteria are everywhere, especially in damp places like dishcloths. So I'm gonna do a little experiment where I add some fluorescent
powder to this dishcloth to represent bacteria without telling anyone in my household. And then I'm gonna come back later to see where all of this powder ends up. But for now, I'm off to Michigan to see the world's longest-running
evolution experiment. Let's go. (plane engine roaring) (bright upbeat music) - This is Richard Lenski. He started the experiment 33 years ago, and along with a team of colleagues, he has kept it going even
on weekends ever since. In this lab are 12 flasks
of live E. coli bacteria. They are the lucky few,
the ones that have survived over three decades of evolution in a lab. - So there are 12 long-term lines. - [Derek] In 1988, a
single common ancestor spawned these 12 separate populations. And ever since that day, they have been growing and
dividing independently. So how have they evolved? There are other long running
evolution experiments like since 1896, scientists at the University of Illinois have selectively bred corn, but they get only one generation per year, whereas these bacteria go through six or seven generations a day. So after 33 years, the bacteria in these flasks
are generation 74,500. If those were human generations, it would represent 1.5 million
years of hominid evolution. - What we do to start the experiment is we dilute a large
population of bacteria onto a Petri dish and each
individual cell makes a colony. And so then we take a
little bit from one colony to start one population, a little bit from another colony to start another population, but effectively they all
started from individual cells. And that's important because it means if we see the same thing happening
in replicate populations, it's not because they started out with the same genetic variants and natural selection just fished out the same thing over and over. You actually had to have
independent mutations that would give rise to
these competitive advantages, whatever it might be that
would produce the repeatability across the 12 replicate populations. - [Derek] The lab
environment is very different from the one the bacteria are used to. It's much simpler, there are no other organisms present, they're kept at 37 degrees Celsius, and they live in the same solution, a mixture including glucose,
potassium phosphate, citrate, and a few other things. Their only carbon source is
glucose, which is limited. - Above all else, consuming that glucose and
converting it to offspring, replicating as fast as possible has been essentially what
we seem to be selecting for. - [Derek] Every day in each flask the bacteria divide six or seven times, which increases their
numbers a hundred fold. Normally we think of generations as being limited by the
time that has elapsed, but in the case of these bacteria, they're limited by the
resources available to them. If they had 10 times the solution, they would increase their
numbers an additional 10 times. Then almost every day
for the past 33 years this transfer process has taken place. 0.1 milliliters or 1% of
the solution from each flask is transferred to a sterile new flask containing the same solution. It essentially dilutes the
bacteria a hundred fold. This gives them the space and resources they need to grow and divide again, increasing their
population a hundred times before this same process
is repeated the next day. And then the day after that
and the day after that, even on weekends, this process has been going
on for over three decades. What happens to bacteria
that are not transferred to the new flasks, the 99%? - That's the autoclave room. - [Derek] What happens
in the autoclave room? - Every day 99% of the
E. Coli meet their demise in this horrible room. - [Derek] Is this like
a bacterial crematorium? - Yeah. Yeah, exactly. - [Derek] You can imagine if
the scientists didn't do this, but instead gave all the bacteria a hundred times more solution
to grow on every day, well, the experiment would
soon become unmanageable. On day two, you would need
a cubic meter of solution, but by day 13 the experiment would be 10 times the volume of earth. And by day 42, the experiment would fill up
the entire observable universe. To me it seems like the
whole idea of genetics is for them to stay constant and for mutations to be rare. - Yes. - Yet in your experiment
it seems they're not rare. - So we estimate that in our bacteria only about one out of maybe a hundred or one out of a thousand cells will have even a single mutation. So that's not very much. By contrast, in humans it's
estimated that each one of our offspring has perhaps
10, 20, 50 new mutations. So the bacteria are
extremely conservative, but they're also billions of
them and even a tiny flask. And so if you have a chance
in a thousand of mutating and you have a billion individuals, then every day in one of these flasks we might have a million new mutations. So that's a lot of variation on which natural selection can act. Maybe half of those mutations
have no effect whatsoever on the bacteria's ability to grow and thrive in that particular environment. They might be mutations that would matter out in the great outdoors, but this was a very simple environment, maybe those genes aren't even expressed. Another half of the mutations
speaking very, very roughly might actually be deleterious mutations. They make the bacteria
an inferior competitor, but there's maybe out of
those million mutations that occur every day, maybe there's 10, maybe there's a hundred, maybe there's a thousand of them that actually change something in the cell that gives the bacteria
a competitive advantage over their progenitors. And those then grow over
the course of that day. Then every day 99% of the
population is eliminated. It's lucky 1% prevails, and if those lucky 1%
include one of the guys from the previous day that
was growing at 10% faster than the other guys, it has a higher
probability of contributing to that next flask, the next day's flask and in the fresh medium, and it will continue to
grow faster at a 10% clip, and that compounds by
an exponential process. So the mutations, y'know, are
really rare when they first occur and many of them are lost, but once they get common if they have that competitive advantage, they'll just sweep through the population. - [Derek] But how do you
know that the bacteria actually have a competitive advantage? I mean, how can you tell that
they're getting better suited to their environment? Well, this is where one
of the unique properties of bacteria come in. They can be frozen for
long periods of time and then revived. - And so they're stored in
suspended animation here. So these are the racks that
contain the frozen samples of the bacteria from
the various generations. - [Derek] Every 500
generations, roughly 75 days, they freeze a sample of each population. By freezing previous generations, Lenski and his team have
a frozen fossil record. - So our samples from over 30 years ago remain perfectly viable. And so that gives us an ability to do what I like to call time travel. We can literally compare
organisms that lived at different points in time. So we can compete bacteria from generation 70,000
against their ancestor. - [Derek] That's right. The way they measure
fitness is by competing the current generation of bacteria against older generations. It's like a strange bacteria fight club. - Hey! - [Derek] They thaw
out the old generation, mix it into a flask with
the current generation, and then plate out a
sample of the solution to see the relative abundances of the two populations at the start. Then they incubate the flask for a day, and then plate them out again. And the point is to compare
their relative growth rates, which generation was better
able to utilize the glucose and divide faster? - Well, how the heck do you
tell the evolve bacterium from its ancestor? Do they wave little flags at you and say, "Hey, I'm the evolved guy." And of course they don't. But what we have is this color marker embedded in the experiment. So six of our populations on
a certain kind of agar plate make red colonies and six
of them make white colonies. And we have one version of the ancestor that makes red colonies and
one that makes white colonies. We can compete one of the
red evolved populations against the white ancestor, or one of the white evolved populations against the red ancestor
and we can distinguish them. - [Derek] In this case, it's clear that the evolved red population outcompeted their white ancestor. Now, to determine the winner, all of the colonies are counted by hand. - So what was the earliest
sort of big findings from the experiment? - The first thing we found, not that there was any doubt about it, but it's one of the most
direct demonstrations of Darwinian adaptation
by natural selection, you can imagine. Yes, they're getting to be
better competitors over time. It's a common observation in
other evolution experiments that evolution in a new
environment gets off to a rip-roaring start and then tends to slow down over time. And so we repeated that observation and I imagined that the longterm lines would actually sort of
flat line at some point. And I actually thought about
stopping the experiment, but I got wise advice from colleagues and from my wife, Madeline,
let's keep it going. And so I agreed to that. - [Derek] And it's good
thing he did because in 2003, the bacteria started doing
something remarkable. - One of the 12 lineages
suddenly began to consume a second carbon source, citrate, which had been present in our medium throughout the experiment. It's in the medium is what's
called a chelating agent to bind metals in the medium. But E. coli going back to
its original definition as a species is incapable of that. But one day we found one of
our flasks had more turbidity. I thought we probably had
a contaminant in there. Some bacterium had gotten in there that could eat the citrate and, therefore, had raised the turbidity. We went back into the freezer
and restarted evolution. We also started checking those bacteria to see whether they really were E. coli. Yep, they were E. Coli. Were the really E. coli that had come from the ancestral strain? Yep. So we started doing genetics on it. It was very clear that one
of our bacteria lineages had essentially I like to
say sort of woken up one day, eaten the glucose, and unlike any of the other lineages discovered that there was
this nice lemony dessert, and they'd begun consuming that and getting a second source
of carbon and energy. Zack was interested in the question of why did it take so long to evolve this and has only one population
evolve that ability? He went into the freezer and he picked bacterial
individuals or clones from that lineage that
eventually evolved that ability. And then he tried to
evolve that ability again starting from different points. So in a sense, it's almost like, well, it's like rewinding the tape and starting let's go back to
the minute five of the movie. Let's go back to a minute 10 of the movie, minute 20 of the movie and see if the result changes
depending on when we did it, because this citrate phenotype there were essentially
two competing explanations for why it was so difficult to evolve. One was that it was just
a really rare mutation. It wasn't like one of these
just change one letter. It was something where
maybe you had to flip a certain segment of DNA and you had to have
exactly this break point and exactly that break point. And that was the only way to do it. So it was a rare event, but it could have happened
at any point in time. The alternative hypothesis is that, well, what happened was a series of events that made something perfectly
ordinary become possible that wasn't possible at the beginning because a mutation would
only have this effect once other aspects of
the organism had changed. To make a long story short, it turns out it's such a
difficult trait to evolve because both of those hypotheses are true. - [Derek] The experiment uncovered other surprising findings. Like instead of the bacteria getting more numerous over time, they actually decreased in number, but each individual bacterium got larger. Six of the 12 populations
evolved hypermutability, mutation rates a hundred times
higher than their ancestors, but these populations
subsequently acquired additional mutations that brought the mutation rate back down. I mean, it's advantageous to be able to evolve faster than others, but if the mutation rate is too high, then offspring have too
many deleterious mutations. But maybe the most
surprising finding of all is what didn't happen. - This view I had that
they were flat lining turned out to be quite false. I had sort of imagined a very
simple mathematical model. You can create something
called a rectangular hyperbola, I guess, which has an initial high slope and then reaches an asymptote, but they're equally simple models. There's a model that also
has just two parameters called a power law model
that says things slow down, but it doesn't have an upper bound. It says, just keep going
for time immemorial, and things will just keep going faster, but at a slower and slower
rate of further improvement. And it turned out that
model actually fits our data better than that original
model I had imagined. And not only does it fit it better, okay, you say statistics,
science, fitting curves, it actually predicts the future. And that's what's really cool
because the original model, if you give it say just 5,000 generations worth of our fitness data and ask it to predict into the future, it says the asymptote is here. But then when we get more data, no! The bacteria are up here. They've passed that asymptote. Whereas this power law model, which says things are slowing down, but never reaching an asymptote? We give it just 1/10 of
our data from the past and it projects very
accurately out to 50,000 and even 60,000 generations
when we last looked. It predicts sort of the future course of the evolutionary trajectory. And to me, that's kind of profound and it's sort of changed the
way I look at this experiment, and even a little bit how
I look at life on earth. I mean, life on earth
doesn't stop evolving. We know that. And we know that, but we think that's, oh, that's because
they're asteroid impacts. That's because of human impacts. That's because they're viruses that are attacking their hosts and that the co-evolution
is causing evolution is causing evolution to never stop. The world is always changing. So of course, evolution never stops. And that is 100% true. But what our experiment suggests is that even in the absence
of environmental change, there are so many opportunities of smaller and smaller magnitude to
continue to make progress that, in fact, progress
probably would never stop even in a constant environment. To me, it's one of the reasons to keep this experiment going. Does this model continue
to predict the unfolding of the future fitness trajectory? - Okay. Now the conclusion
of the experiment where I look for that fluorescent powder using this UV torch. So let's hit the lights. (laughs) Whoa. There is a lot of
fluorescent powder around here. Obviously there's a lot in the sink, but also here on the tap, it looks like someone wiped
down the handle and the faucet. Oh, check out the dishwasher. Yeah, and on the handle. This is a great way of visualizing how dishcloths can spread
bacteria around the house. What? (laughs) It looks like finger marks. As a dad, I encounter lots of big messes and sticky hands and slimy faces. So when there is a mess or spill in my kitchen I choose to clean it up with Bounty So I wanna thank Bounty
for sponsoring this video, and I wanna thank you for watching.
alma we made it
🍆
I'll cry if he ever films on campus