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.