Populations evolve every day. For example,
populations of disease-causing organisms such
as multi-drug resistant tuberculosis bacteria
evolve. The Siberian prison in the PBS documentary
“Why Does Evolution Matter Now,” is ground
zero for the evolution of TB.
By taking antibiotics, we change that environment
of our body, which may kill some bacteria.
Bacteria that can survive in the presence
of the antibiotic have an advantage over those
that are killed. The increase of multi-drug
resistant TB bacteria in the body illustrates
Darwin’s theory of evolution by natural
selection.
The essential questions we wish to answer
are: How is antibiotic resistance selected
for in humans with a bacterial infection,
what is natural selection, and what is Darwin's
Model? This module will explore how TB bacteria
can survive and increase in some patients.
This is the power of his theory: it can be
used to explain the evolution of organisms
that Darwin did not even know about, like
bacteria.
In the following slides, we will ask a question
about the current problem of multi-drug resistant
TB at the top, and at the bottom we will show
how that question can be answered by Darwin’s
theory. So let’s begin. How fast can TB
bacteria reproduce? A single bacterium can
reproduce over a million times in a human
lifetime. This means that each bacterium can
become one million bacteria. Another way of
thinking about this is to say that the number
of bacteria can double about every twenty
minutes, when the environment allows. Darwin
didn’t study bacteria, but he studied other
populations. In his theory he stated that
populations can reproduce exponentially. How
long can bacteria survive in a patient? TB
is a chronic disease and many people can harbor
a TB infection for many years. A patient would
not be able to survive if the bacteria continued
to reproduce exponentially. Darwin also observed
that successful populations grow, but eventually
their numbers stabilize. In his theory, Darwin
stated a second fact: Populations are stable.
So what are the limits to what the human host
provides for the bacteria? Eventually a sick
patient cannot support the bacteria that have
multiplied in their lungs. The warm, moist,
and nutrient-rich environment becomes depleted.
When the population size exceeds what the
environment can support, the patient will
die. Unless the bacteria move to a new individual,
that is to say, the bacteria find a new environment,
they will become extinct. How can the population
continue to exist? How are the tuberculosis
bacteria able to spread? Tuberculosis bacteria
spread by traveling through the air when infected
individuals cough or sneeze. In a similar
way, Darwin stated in his theory that resources
are limited. Are all of the bacterial cells
in a population the same? If not, how are
they different? Although they may look the
same under the microscope, the bacterial cells
are not identical in their genetics. Their
fast rate of reproduction means that they
have many chances to mutate. So there may
be many genetic differences among the individuals
in a bacterial population. In his theory,
Darwin stated that individuals are unique.
He did not understand genetics, but he knew
that there was variability within a population
of the same species. Are the differences among
bacterial cells inherited? Yes, because those
differences are found in their genome. Bacteria
reproduce by means of binary fission, so they
pass on all their genes to the offspring.
Unlike sexually reproducing organisms who
only contribute half of their genes to each
of their offspring. So once a bacterium has
a mutation that makes it resistant to the
antibiotic, all their offspring will be resistant.
Thus, Darwin stated that individual variation
can be inherited, and we now know this variation
is found in the genes of the organism. Why
do some bacterial cells survive in a human
host, and others do not? When a patient takes
an antibiotic, why doesn’t the antibiotic
kill all of the bacteria?. Darwin’s first
inference, was that there is a struggle for
existence among individuals. Some of the bacteria
have a mutation that allows them to survive
in the presence of the antibiotic, and some
do not. The mutation occurred randomly, but
some mutated bacteria (unlucky for the human
host) were able to proliferate despite the
presence of the antibiotic. Remember that
an inference is a conclusion arrived at by
reasoning about the evidence. How does the
number of offspring of these mutated bacteria
affect the population of bacteria in the body?
The bacteria that did not have the mutation
have all died. The bacteria with the mutation
will have more offspring, since they were
able to survive. Darwin inferred, that there
would be differential survival among individuals
in a population. How does the bacterial population
change over time? If the mutations were not
present in the population the patient would
now be well. But instead, the entire population
became resistant to all of the antibiotics
that were given to the patient. So now the
bacterial population has changed. This population
change is what Darwin referred to as Evolution.
Here is a summary of how Darwin developed
his explanatory model of evolution, put together
in graphic form by Ernst Mayr. The first three
facts, populations can reproduce exponentially,
populations are stable and resources are limited,
led Darwin to conclude that there must be
a struggle for existence. If organisms struggle
to survive, some have mutations, which allow
them to be better adapted to their environment.
They will pass the ability to survive in the
presence of the antibiotic on to their offspring.
And their offspring will have the same mutation. So some individuals will survive
and some won’t; they will have differential
survival. The environment determines who lives
and who dies, thus nature controls who is
naturally selected. Darwin’s theory of natural
selection is supported by a great deal of
evidence from organisms he did not know about
or study. It is a powerful unifying theory
in biology.
