Natural Selection Image Map

How Do We Know There is Natural Selection?

One way that we find out about natural selection is to see how humans change animals by selectively breeding them to increase the numbers of individuals that have certain traits. For example, if a farmer keeps the calves from his cows that give the most milk, eventually his herd will have more high-producing cows in it.

  1. Humans perform natural selection in selective breeding practices for crops, farm animals, and pets.
  2. Humans perform natural selection unintentionally when they alter the environment or introduce plants and animals into new niches.
  3. We can see niches and can see that niches and their occupants change - even in our lifetime. See some examples in the Populations unit.

Repeated use of antibiotics, for example, imposes a selection force. Those members of a species that just happen to have genes that impart resistance to antibiotic have a selective advantage. When confronted with antibiotic the susceptible bacteria die but a few will survive because they just happen to have genes that protect them. When the survivors reproduce, the genes that made them resistant proliferate in the population. Eventually, the antibiotic does not work anymore because all of the bacteria are resistant to it.  

A similar process has been observed to be occurring today with insecticides.

Radioactive Dating

Some chemical elements are radioactive. They decay at a constant rate , giving off energy (like beta particles or gamma rays) and turning into a "daughter" compound that is slightly different and does not decay. Such is the case with carbon.  Some carbon atoms are radioactive and decay at a fixed rate to become stable nitrogen. Living things and materials made from living things, such as bowls and decorations carved from wood, contain atoms of carbon. If you measure how much of this radioactive carbon is decaying now, you can use the known rate of decay to calculate how much radioactive carbon was incorporated at the time the living thing died.

Willard Libby had worked with the team making the nuclear bomb during World War II, so he was an expert in nuclear and atomic chemistry. After the war, he became very interested in peaceful applications of atomic science. He and two students were the first to measure the "half-life" of radiocarbon. The half-life is the time it takes for half the radiocarbon in a sample of bone or shell or any carbon sample to disappear. Libby calculated that it takes 5,568 years for half the radiocarbon to decay. After twice that time (about 11,000 years), another half of that remaining amount will have disappeared. After another 5,568 years, again another half will have disappeared. You can work out that after about 50,000 years, all the radiocarbon will have disappeared. Therefore, radiocarbon dating is not able to date anything older than about 50,000 years. 

So how do you measure age, of a fossil, for example? Radioactive carbon decays gives off a "beta particle," which can be detected and counted. Thus, you measure how many beta particles are being given off. Back-calculating using the known half-life rate of decay lets you determine the age of specimen being tested. A dead animal or plant cannot acquire new radioactive carbon. We know that the method works for relatively young objects because results agree with ages of things that have a historical record.  Radiocarbon dating is reasonably accurate over a period of about 50,000 years. For older materials, other radioactive methods have to be used.

For more details on the method, click here. 

To illustrate why carbon dating is an important tool, let us consider what it tells us about human origins. Distinctly human skulls and bones have been found that have been dated at about 1 million years old and human-like fossils and their tools have been dated at 2 million years old. One recent find of five skulls is dated at 7 million years. These were clearly human-like skulls (not ape), but they had small brain capacity (we know they were from adults because the skull bones were fused).  Canyon-forming rivers have carved out the earth sediments and exposed primitive human skulls. The sediments have been dated with ages similar to those of the skulls. See sites on Africa's Olduvai Gorge. 


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