Hey there, and welcome to another MP3 tutorial. This is Eric Simon and I’ll be guiding you through today’s topic: natural selection. Natural selection is relatively easy to comprehend, and yet it is largely responsible for the amazing diversity and complexity of life on this planet. First, let’s take a look at a well-documented example of natural selection in action, infection by HIV. Then let’s dissect the essential elements of the process. Finally, we’ll wrap up with some other examples of natural selection. Sound good? Let’s get started.
Let’s begin by discussing the tragic course of an HIV infection and the role that natural selection plays. Stay with me on this. It will take a bit of time to summarize how HIV works, but this will be time well spent.
HIV is a slow-acting virus that eventually devastates the immune system. When HIV first invades a host, it is able to reproduce rapidly. This is because it takes a while for the immune system to muster a response to the new invader. About six to eight weeks after a person is first infected with HIV, they may experience flu-like symptoms due to the high number of viruses in their bloodstream. Normally, the immune system starts to get the upper hand at this point and the symptoms go away. If this person’s blood is sampled, the virus is still present but the person appears to have normal health. Then much later—sometimes several years later—symptoms return. This means that the immune system has been overwhelmed and the disease has progressed to full-blown AIDS, a condition in which a devastated immune system can no longer fight off infection. Unless aggressively treated, death is inevitable. Why does HIV appear to go dormant only to reemerge with lethal results?
During the course of an infection, a deadly race is being run. On one side is the virus’s ability to adapt; on the other, the immune system’s ability to adapt. Each time a virus reproduces, random mistakes are made in the viral genes. This happens in all organisms when genomes are copied. These mistakes are called mutations. In most organisms, the rate of mutation is low and usually the mistakes in the sequence are quickly repaired. HIV is different. HIV is a retrovirus, which means that its genome is stored as RNA instead of DNA. It also means one of the first steps in the process of HIV infection is the conversion of the RNA genome into DNA, a process called reverse transcription. This process, which never takes place in human cells, is very prone to errors, much more so than normal DNA replication. In fact, during the course of a 24-hour period, it is estimated that the billions of HIV viral particles produced in an infected human have mutated every possible base position in the HIV genome at least a thousand times, a very high rate of mutation. The consequence is that many, if not most, of the newly produced viruses simply do not survive attack from the host’s immune system. So then, why would such a sloppy process end up so deadly? You’d think that this would lead to the host defeating the virus and not the other way around.
But consider this: some of the virus particles do survive, and these survivors produce offspring. These offspring carry the new genetic message and can be further refined to counter the host’s immune system. Over the course of an infection, the HIV genetic instructions become increasingly fine-tuned to the host’s immune system. If this process progresses sufficiently, the person will develop AIDS and the immune system will not be able to fight off other pathogens. HIV thus has the potential to evolve rapidly. The result is that HIV may counter the host’s immune system, and it will win the race unless treatment is administered.
With regard to our discussion of natural selection, there are two key points to this story: 1. The high mutation rate of HIV’s genetic material creates a great deal of genetic variation in the virus population. This variability affects the ability of the virus to survive the environment of the immune system. Since it is genetic information, this variation is passed between generations of viruses. 2. HIV produces billions of viral particles in a day. Most do not survive but a few do.
Putting together these two points, you can see that those viruses with resistance to the immune system can pass these characteristics on to the next generation of HIV. This in turn leads to a viral population more resistant to the immune system. This is natural selection at work.
More than 150 years ago, Charles Darwin proposed the theory of natural selection as a mechanism to explain the diversity of life on this planet. He was a meticulous observer of nature. He based his theory on two important observations: 1. He noted that populations of all sorts of organisms produce many more offspring than needed for replacement. If all these offspring survived to reproduce, a population would quickly outgrow its environment. 2. He noted that populations of organisms have a lot of variation and he proposed that this variation is inherited.
From these two observations, Darwin made a simple, yet profound, inference: The tendency of organisms to over-reproduce leads to a struggle for existence. Not all organisms survive and reproduce. Those organisms that do tend to have traits that contribute to their success, and these traits may then be passed on to their offspring. Over time, populations of organisms adapt to their environment. Darwin termed this insight natural selection because the natural environment provided the selection mechanism. Natural selection is a filtering process—the natural variation in a population is filtered through different rates of survival. Natural selection is also a refining process—it can only work on the available variations. Natural selection fine-tunes a population of organisms for survival in an ever-changing environment.
Back to HIV: We talked about the virus, but what about the human population? When HIV first appeared, the fatality rate seemed to be 100%, but today we know that some folks are long-term survivors. What do they have that others don’t? Well, just like other populations, there is genetic diversity in the human population. When HIV appeared, the natural environment for humans changed. Some people have random genetic mutations that make it more difficult for HIV to attack immune cells. Genetic analyses of long-term HIV survivors indicate that many of these people have a variation that makes them better suited to the new environment, one with HIV in it.
I hope this tutor session clarified this important topic for you. Please refer to your lecture notes and book for further information and more detailed discussion.
And, as always, good luck with your studies!