Chapter 10 : Natural selection.

10.1 AIDS and HIV.

AIDS or Acquired Immune Deficiency Syndrome is a disease of the immune system. The immune system normally protects you from infection. AIDS kills cells in the immune system, so that people are left defenseless against fungi and other infections that their bodies could normally keep at bay.

AIDS is caused by infection with HIV (Human Immunodeficiency Virus). This virus is transmitted sexually, through blood (for example unsterilized needles) and from mother to baby. HIV consists of RNA and protein, and once it infects a human cell it uses that cell to manufacture many more viruses. Fig 10.1. Usually the early symptoms of HIV infection resemble flu. The immune system recognises proteins called antigens on the surface of HIV and cells attack the virus by producing antibodies in the blood (Fig 10.2). Levels of the virus then drop for up to 10 years. Finally the virus levels rise again and this leads to AIDS and finally death. Fig 10.3.

10.2 Natural selection.

Charles Darwin was the first to describe natural selection in detail. This was based on four observations:

Individuals within populations vary. This has been shown in a wide variety of animals and plants, from wolves to sunflowers (Fig 10.4).
Some of this variation is inherited. It was only long after Darwin that Gregor Mendel was able to explain this though his understanding of genetics (Chapter 6).
Populations produce more offspring than can survive. One oak tree can produce thousands of acorns. A frog can lay hundreds of eggs at a time. Obviously most of these new individuals do not survive long - in the wild mortality rates are usually very high.
Survival is not random. Individuals who are born with the the best adaptations to the local environment will tend to survive, and pass on those useful genes to the next generation. Adaptations could include a larger beak (Fig 10.7) or brighter colored flowers to attract pollinators (Fig 10.8).

The result of this process is natural selection (nature selects which individuals will survive and breed). Any useful gene will tend to increase in the population from one generation to the next, and any harmful gene will slowly be lost. So the population gradually evolves to become better suited to its environment.

Testing natural selection

Artificial selection means that humans select which traits they want in domestic animals and then breed them for those traits. Fig 10.9. Most of the over 800 breeds of dog have been produced in the last few hundred years.

Natural selection in the lab has shown that populations of flies can rapidly evolve to a new environment, for example by being able to break down alcohol. Fig 10.10.

Natural selection in the wild has been shown in Galapagos finches (Fig 10.7) as well as many other organisms including moths, snakes and lizards. In 2009, studies have shown evolution in bacteria, and demonstrated one species splitting into two new ones in butterflies, beetles and birds.

Subtleties of Natural Selection
Natural selection does not change the genes of one individual, it just changes the frequency of an allele in the population. For example some flies are born with genes to break down alcohol, other flies are not. In an environment where flies are exposed to alcohol the ones that can break it down have an advantage, so produce more offspring. Fig 10.11.

Natural selection does not produce perfect organisms, or even "better" ones. A gene may be an advantage in one environment but a disadvantage in another (see the moths example above).

Natural selection makes species adapted to the current conditions, not necessarily the conditions in the future. There are several distinct types of selection:

Directional selection causes the traits of the population to change in one particular direction, for example towards darker colored flowers. Fig 10.14.
Stabilizing selection keeps the traits the same, because individuals with intermediate traits do best.
Diversifying selection means that two or more different traits are an advantage, so the population tends to split into two or more groups. Fig 10.14.

10.3 Natural selection and HIV.

HIV fits Darwin's ideas of natural selection.

Viruses in the blood stream vary.
The variation is inherited.
More viruses are produced than can survive (the immune system kills many).
Virus survival is not random. Some viruses are slightly different from others, and manage to evade the immune system.

When HIV infects a person the virus mutates, or changes genetically, to produce slight variations. The immune system has to continually battle these new types and after several years of this "arms race" the virus typically overwhelms the immune system.

10.4 Preventing HIV.

The first drug developed to fight HIV was AZT which blocked one of the enzymes that the virus contains. Although AZT treatment helped for a few months, patients taking AZT eventually still came down with AIDS, because the virus evolved to become resistant to the medication. Fig 10.16.

Now patients infected with HIV are given a three-drug cocktail, because it is very hard for HIV to simultaneously evolve resistance to three diffferent drugs. The 3-drug cocktail, or combination therapy, also slows the evolution of HIV. Fig 10.17. This allows patients to survive for several years, although it is not a cure for HIV. It also is expensive and requires patients to take up to 15 pills a day at regular intervals. HIV Prevention link

Last edited October 2011, by David Byres, dbyres@fscj.edu