How Can You Tell Evolution Is Happening?

Recognize that a change in gene frequency (the number of organisms that have a gene) in a population over the course of years is an important aspect of biological evolution.

Gene Frequency

A gene, or a characteristic that is part of your DNA, determines your traits. Genes have all of the information that makes you who you are, including hair color, skin color, eye color, height, and which hand you use to write. Since some characteristics are advantageous in certain environments, what color and texture your hair is, the color of your skin and more all depend on the environment where your ancestors were.

Two very happy people. With different skin tones.

For example, people who come from southern Africa typically have darker skin and darker hair than Europeans or northern Africans. There is a good reason for this. When sunlight hits human skin, our bodies use that energy to make vitamin D. Vitamin D is necessary for us to have strong muscles and fight disease, but we can also get too much vitamin D. An excess of vitamin D can cause your kidneys to fail and eventually result in death where there is a lot of sunlight! So, in areas where there is not a lot of sunlight, it pays to have light skin and absorb more sunlight and therefore make more vitamin D. In areas with a lot of sunlight, it pays off to have dark skin in order to not absorb as much sunlight and avoid an excess of vitamin D. Of course, there are many other reasons that we have a certain skin tone. More importantly, our skin tone is not just controlled by several different genes, it’s also affected by our environment!

Gene frequency refers to the amount of times in a population that a certain gene happens. From our example above, we would expect that the genes which cause a darker skin tone in humans have a higher frequency in Africa than in Europe. It is much easier to talk about evolution when you can talk about the differences in gene frequencies between different populations. When talking about human populations, the differences between an African and a European are actually very small – even though they are very noticeable! If you choose two random Americans that have similar skin color, facial features, height and weight they will still have many genetic differences. In fact, they will have as many genetic differences as between a light-skinned European and a dark-skinned African! As humans, we are all very complex and have millions of small differences, some more noticeable than others.

1. What is a gene?
2. Define gene frequency.
3. List at least four genetic characteristics.
Put it together
4. Explain why there is a difference in skin tone between Europeans and Africans.
5. Give an example of a gene that has different frequencies in different populations (meaning, a trait that looks different).
Think about it
6. Skin tone is one of many human characteristics. Choose a different human characteristic. Create a timeline with at least five events as to what will happen to this characteristic in American people over the next 500 years. Then, write three sentences:
a) What is your prediction of what will happen with this characteristic over the next 500 years?
b) Why do you think that this will happen?
c) Give a summary of your timeline.
7. Name two chemicals that are released by volcanoes.
8. Which are the advantaged offspring out of any population of offspring?
9. Is evolution happening right now?

Sickle-Cell Anemia


Hereditary Something that is given by parents to offspring
Prevalent Common
Immunity Ability to completely fight off disease
Resistance Ability to fight off some amount of disease
Spleen An organ in the body that filters blood


A gene known as HbS was the center of a medical and evolutionary detective story that began in the middle 1940s in Africa. Doctors noticed that patients who had sickle cell anemia, a serious hereditary blood disease, were more likely to survive malaria, a disease which kills some 1.2 million people every year. What was puzzling was why sickle cell anemia was so prevalent in some African populations.

How could a “bad” gene — the mutation that causes the sometimes deadly sickle cell disease — also be helpful? On the other hand, if it didn’t provide some survival advantage, why had the sickle gene stayed in the population at such a high frequency?

The sickle cell mutation is a like an error in the DNA code of the gene that tells the body how to make a form of hemoglobin (Hb), the thing that carries oxygen in our blood. Every person has two copies of the hemoglobin gene, one from their mother and one from their father. Usually, both genes make a normal hemoglobin protein. When someone inherits two mutant copies of the hemoglobin gene, the mutant form of the hemoglobin protein causes the red blood cells to lose oxygen and warp into a sickle shape during periods of high activity, like running (see picture on the left). These sickled cells become stuck in small veins and other blood vessels, causing a “crisis” of pain, fever, swelling, and tissue damage that can lead to death. This is sickle cell anemia.

(Outside) Normal red blood cells; (Inside) Sickled red blood cells

But it takes two copies of the mutant gene, one from each parent, to give someone the full-blown disease. Many people have just one copy, the other being normal. Those who carry the sickle cell trait (have only one copy) do not suffer nearly as severely from the disease.

Researchers found that the sickle cell gene is especially prevalent in areas of Africa hard-hit by malaria. In some regions, as much as 40 percent of the population carries at least one HbS gene.

It turns out that, in these areas, HbS carriers have been naturally selected, because the trait gives some resistance to malaria. Their red blood cells, containing some mutant hemoglobin, tend to sickle when they are infected by malaria. Those infected cells flow through the spleen, which removes them from the blood because of their sickle shape — and malaria is eliminated along with them.

Scientists believe the sickle cell gene appeared and disappeared in the population several times, but became permanent after a particularly bad form of malaria jumped from animals to humans in Asia, the Middle East, and Africa.

In areas where the sickle cell gene is common, the immunity that it gives has become a selective advantage. Unfortunately, it is also a disadvantage because the chances of being born with sickle cell anemia are relatively high.

For parents who each carry the sickle cell trait, the chance that their child will also have the trait — and be immune to malaria — is 50 percent. There is a 25 percent chance that the child will have neither sickle cell anemia nor the trait which gives immunity to malaria. Finally, the chances that their child will have two copies of the gene, and therefore sickle cell anemia, is also 25 percent. This situation is an example of an evolutionary “trade-off.”


  1. In the photograph, what is pictured in the darker area?
  2. What do sickle cells cause in the people who have them?
  3. How do you get sickle cell anemia?
  4. What effect does the sickle cell gene cause in people who only have one copy of the gene?
  5. Why is the sickle cell gene still around if its effects can be so harmful?
  6. What are the odds that the child of parents who each carry one normal gene and one sickle cell mutation gene will have sickle cell anemia?
  7. What are the odds that a child of two carrier parents will also be a carrier and, thus, be protected from malaria?
Sickle-Cell Anemia
  1. Choose a partner. In your pair, get two pennies.
  2. One coin represents the genes of the mother and the other coin represents the genes of the father. Each parent has one normal gene (H) and one mutated gene (h). If you flip heads, then it is a normal gene (H), tails it is a mutated gene (h).
  3. Study the genes and traits below:
    Gene Trait Result
    HH No mutation Normal
    Hh One normal gene, one mutated gene Protection from malaria
    hh Two mutated genes Sickle cell anemia
  4. Flip your coins 20 times and record your results below. After you have flipped, fill in the traits that result. The first row is an example:
    Gene from mother(Coin from first person) Gene from father(Coin from second person) Gene for offspring Trait
    H h Hh Protection from malaria
  5. Analyze your data by counting up the number of times (frequency) each trait occurs and then total up the frequencies to get the percentage of the time that it shows up:
    Trait Frequency Percentage (Frequency / Total) x 100
    Protection from malaria
    Sickle cell anemia
  6. Get the entire class’ data and fill in the following chart:
    Trait Frequency Percentage (Frequency / Total) x 100
    Protection from malaria
    Sickle cell anemia
  7. What advantage is there to having the mutated gene (h)? Hint: look at the individuals who have only one mutated gene (h).
  8. If having only one copy of the mutation did not provide protection from malaria, why would the mutated gene (h) not last very long in the population?
  9. If malaria was eliminated in Africa, predict what would happen to the presence of sickle cell anemia. Is this considered evolution? Why or why not?
Allele Frequency

The word frequency refers to the number of times something happens. Sometimes frequency is easy to figure out. For example, the frequency of the Summer Olympics is once every four years. With allele frequency, or the frequency of a particular genotype in a population, the task is more complex.

A dominant trait expressed in a population can be the result of either a heterozygous or homozygous genotype. Suppose you are doing research on a rare but serious medical condition. You know that the condition is caused by a single recessive gene. You want to know how many people carry this gene. Is it possible that this condition might eventually disappear from the population?

The Hardy-Weinberg principle can be used to predict the frequencies of certain genotypes if you know the frequency of other genotypes. The medical condition being studied is controlled by two alleles S and s. Because only one gene is involved, inheritance follows the rule of simple dominance.

  • The dominant allele is S, and the recessive allele is s.
  • Only homozygous recessive individuals show symptoms.
  • The population you are studying has 10,000 individuals.
  • There are 36 individuals affected by the condition.

Based on this information, use the Hardy-Weinberg equations to answer the questions.

Analyze and Conclude

  1. Write the Hardy-Weinberg equations in symbols and in words. What value represents the entire population?
  2. In the population you are studying, which allele is represented by p in the Hardy-Weinberg equation? Which allele is represented by q?
  3. What are the frequencies of the dominant S and recessive s alleles in the population? Hint: To find q2, divide the number of affected individuals by the total population.
  4. What are the frequencies of the SS, Ss, and ss genotypes? Show your calculations.
  5. What percentage of individuals in the population are likely to be carrying the s allele, whether or not they know it? Show your calculation.

Build Science Skills

You need more resources to study the effects of the medical condition caused by the recessive allele. Before you apply for grant money to continue your work, you need to know whether the recessive allele is likely to disappear from the population. What percentage of the population carries the s allele but does not show any symptoms? How many individuals does this number represent? Do you think the s allele is likely to disappear from the population?

Modeling Natural Selection

A mutation may offer an organism an advantage for a given environment. For a mutation to be considered an adaptation, it must increase an organism’s fitness. Fitness refers to the ability of an organism to survive and reproduce. An organism with increased fitness has a good chance of passing on the trait. Over time, the trait becomes more common in a population.


  • 75 blue plastic chips
  • 75 red plastic chips


In this activity, your desktop will represent an environment that goes through a change in climate. The blue chips represent a species that can only reproduce in temperatures between 10°C and 20°C. These are normal breeding temperatures. The red chips represent members of the same species with a mutation. The mutation allows them to reproduce in temperatures above 20°C as well as in the normal temperature range. These organisms are especially tolerant of heat.

As you do the activity, use the data table to record the number of red chips, blue chips, and total chips for each year.

  1. Scatter ten red chips and ten blue chips on your desk.
  2. In Year 1, the temperatures during the breeding season stay between 10°C and 20°C. Add a red chip or a blue chip for each pair of chips of the same color that are able to breed at these temperatures. Remove one red chip and one blue chip to allow for a death within each of the populations. Record the number of red chips, blue chips, and the total chips for Year 1.
  3. During Years 2–4, the climate changes. During the breeding season, the temperatures stay above 20°C. Use this information and the rules described in Step 2 to fill in the table for Years 2–4.
  4. During Year 5, the temperatures return to between 10°C and 20°C during the breeding season. Following the rules of reproduction in Step 2, record the population data for Year 5.


Year 1

Year 2

Year 3

Year 4

Year 5







Analyze and Conclude

  1. Make a line graph to compare the growth of the red and blue populations. Remember to title your graph and label both x- and y-axes. Either label the lines or include a key.
  2. Describe the general trend in temperature over the five-year period. How did this trend affect the size of each population?
  3. How many offspring can one breeding pair produce in a year? How do you know?
  4. How does the percentage of organisms with the mutation change from Year 1 to Year 5? Show your work.
  5. Assume that the temperatures stay within the normal range over the next five years. What would the populations look like at the end of ten years? Explain your thinking.

Build Science Skills

The model of population growth used in this activity is a very simple one. Identify two ways that the model oversimplifies the interactions of members of this species.