In this lesson, students investigate the phenomena of natural selection and how it is one way that evolution occurs. They extend this focus by exploring how the ability of an organism to survive changes when its environment changes.
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A Medical Mystery
In the late 1940s, a scientist named Tony Allison traveled with a group of other scientists to Kenya, a country in Africa. As they traveled around, Tony took blood samples of people from the different tribes who lived in the area.
Allison noticed something surprising in the blood samples. He observed that the sickle cell trait was much more common in certain populations of people than in other populations. Remember that sickle cell disease is caused by a mutation of one of the genes that codes for hemoglobin. This mutation causes some blood cells to become sticky, rigid, and shaped like crescent moons, rather than flexible and doughnut-shaped.
Scientists were puzzled because sickle cell disease is such a deadly disease. It generally kills its victims young, before they can reproduce and pass along the mutated gene to offspring. This should have resulted in the mutation for sickle cell disease dying out because it wasn’t getting passed on to future generations.
Adaptations and Survival
This has to do with the theory of natural selection. Natural selection states that organisms well fitted to their environments will have offspring and pass on useful adaptations, while those with harmful traits will die off. An adaptation is a trait that helps an organism survive in its environment. In other words, those traits that are beneficial are most likely to get passed along to future generations, increasing in frequency in the population. In contrast, harmful traits are likely to decline in frequency because organisms with harmful traits are less likely to survive and reproduce. Over time, this can lead to the development of new species.
Sickle Cell and Malaria
Allison’s data showed that there were far more people who had sickle cell anemia on the coast and near lakes. He realized that the sickle cell gene was present in hot, humid areas. These areas were also prime breeding grounds for insects carrying malaria. Malaria is a potentially deadly disease that kills more than a million people every year. It does this by invading red blood cells.
A Recessive Trait
Allison knew that sickle-cell disease was recessive. This means that in order to have the disease, a person must inherit the mutated sickle cell gene from both of their parents. Remember that individuals carry two alleles for each trait, one from each parent. Alleles are forms of the same gene that have small differences in the sequence of DNA bases. These alleles can be dominant or recessive. A dominant allele shows its effect even if the organism just has one copy of it.
For example, the allele for brown eyes is dominant. A person just needs to inherit one allele for brown eyes from one of their parents to have brown eyes. In contrast, recessive alleles only show their effect if the organism has two copies of it. You would need to inherit recessive alleles from both parents to have grey, green hazel, or blue eyes.
This is why a person has to inherit the sickle cell mutation from both parents in order to get the disease. Someone who has one sickle cell allele and one healthy allele is called a carrier, but they will not have the disease.
Solving the Sickle Cell Mystery
Allison wondered whether the sickle cell gene got passed on because those people with just one copy of the mutated gene had an advantage over people without any copies—they were protected from malaria.
Scientists now know that the sickle cell mutation produces an enzyme that protects against malaria. Anyone who inherits just one copy is protected against malaria and does not inherit the disease. Those people are more likely to survive and reproduce. Those people who inherit both copies have sickle cell anemia, which can be lethal. Those people who don’t inherit any copy of the mutated gene will not get sickle cell disease, but they will also not be protected against malaria.
The survival of the sickle cell gene is an example of evolution at work. In places where malaria is highly prevalent, those people who carry just one copy of the sickle cell gene have a strong evolutionary advantage. They aren’t affected by sickle cell anemia and they are protected from malaria.
This answered scientists’ question about why the sickle cell gene hasn’t died out. Having the mutation protects carriers of the gene in places where malaria is most common, helping them survive malaria.
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