[Music]
Natural selection,
which drives evolution is a basic
and easy to comprehend process.
For natural selection to take place,
we have to meet only three conditions:
Variability,
Selection and Heritability.
The first condition is that
the population is variable,
because in a population where all
individuals are identical,
selection has nothing to work on.
I like to run marathons
and an important part of each race is
that there are faster and slower runners.
If all runners were the same, this
would make a very boring race.
The second condition is selection.
Circumstances under which individuals
can express their differences
to determine the most successful ones.
If all runners sit in a coffee shop,
it will be impossible to
know who is the fastest.
And like a marathon,
success in nature is ultimately
measured by one single criterion:
Who will have more offspring?
While there is only a single
measure for success,
there are endless modes of selection.
Who is the fastest runner to
escape predators or catch prey?
Who has the best camouflage, the best
sense of hearing, seeing or smelling?
Which plant is the tallest
to utilize the sun?
Or is best adapted to dry climate?
All these endless criteria make
nature incredibly rich.
If we would, for example, perform
a singing contest among our runners,
I assume that the fastest runner
won't necessarily be the best singer.
The third condition is heritability,
the capacity to transfer the trait
to the next generations.
In nature, success is measured
by the number of offspring,
and in most cases,
over many generations.
It is therefore, essential that the trait
that facilitates this success is heritable.
This is not trivial as there are traits
that are not inherited.
Children of the champion racers don't
necessarily run fast.
Charles Darwin formulated the
rules of natural selection,
and while he realized that traits
must be inherited,
he had no clue how inheritance works.
You are however much better off,
as you are shortly going to learn
the mechanisms and rules of heredity.
Michal will now perform an experiment
to demonstrate natural selection
and its three requisites:
Variability, Selection and Heritability.
The experiment will further show
that natural selection is not restricted
to complex creatures
and works also in simple
organisms, like bacteria.
We've got here a mixed population
of two identical strains of bacteria
that differ only by two characteristics:
Resistance to antibiotics
and expression of a protein
that causes them to stain blue.
The antibiotic-resistant
bacteria, stained blue,
while the non-resistant ones are white.
We use the blue color to distinguish between
the resistant and non-resistant ones.
It is difficult to see
in the liquid culture
and therefore, we are
preparing a solid medium.
We boil liquid medium with agarose,
which forms a gel when it cools down.
Michal is plating bacteria on the dish
and incubates it overnight at 37°C.
At this temperature, each
bacterium divides many times
and forms a visible colony.
Now we can observe the differences
between the white and the blue bacteria.
This is the first requisite of
natural selection, variability.
Now we will move to the second,
creating conditions under which
some individuals will
proliferate more than others.
We'll plate the bacteria
on dishes with antibiotics
and incubate them again
overnight at 37°C.
As you can see,
we have now only blue colonies.
Under the selection regime we applied
only the antibiotic-resistant
bacteria proliferated.
In many cases selection is not
that drastic
and doesn't lead to immediate extinction
of the less successful individuals.
In these cases, the ratio of the successful
individuals rises only gradually.
There are, however,
cases of rapid extinction,
like that of the dinosaurs
and the ammonites.
The third requirement is
that the more successful individuals,
in our case the resistant ones,
can transfer the selective
advantage to the next generation.
This is exactly what happens here,
as we've already seen,
that each colony originates from
a single bacterium
that underwent multiple divisions
and transferred the antibiotic
resistance to its offspring.
I said, that not all the
selections are as drastic
as that, that lead to the
extinction of the dinosaurs.
Let us use the bacteria we've got here,
to demonstrate such a gradual selection.
We'll start an antibiotic-free liquid
culture of resistant blue bacteria
and grow it over several days.
We'll then plate them out
on antibiotic-free culture plates.
As you can see here,
after only two days,
most of the colonies are white
and after a week, the blue
bacteria went almost extinct.
Why? What happened here?
The white bacteria originated from
rare events of loss of the genes
that expressed the
resistance in blue proteins.
Expressing these proteins
requires resources
that slightly reduce the
growth rate of bacteria
that expressed them when compared
to non-expressing bacteria.
Under conditions that make expressing
these proteins redundant,
lack of antibiotics in the medium,
non-expressing bacteria
that originated from these
rare events of gene loss,
have an advantage
that enables them to ultimately
take over the entire population.
