Francesca Tomasi received her B.A. from the University of Chicago and is now a microbiologist.
Natural selection is the phenomenon by which organisms that are better adapted to their immediate environment tend to survive and reproduce more than individuals ill-fit to their surroundings. The point is that advantageous genes get passed on to offspring in order to increase their survivability, and disadvantageous genes are eventually flushed out of the gene pool. If you have a trait that helps you overcome a challenge today, does natural selection care if it kills you later? Not necessarily.
Sickle cell disease is a type of genetic blood disorder where red blood cells are malformed. This comes from an abnormality in the oxygen-carrying protein called hemoglobin found in red blood cells, which consequently form a rigid, sickle shape compared with healthy, round blood cells. Sickle cell disease is very painful and can cause a slew of health problems including anemia, bacterial infections, and stroke. When treated, the life expectancy for a sickle cell patient is around 50 years. Despite its heritable nature and, thus, susceptibility to natural selection, genes for sickle cell disease remain widespread in certain warm, mosquito-rich regions. This is because sickle cell also carries the baffling ability to protect its carrier from malaria. Malaria is a mosquito-borne disease caused by a parasite called Plasmodium that causes fever, chills, and flu-like illness. Left untreated, malaria can cause serious complications and even death. To this day, almost 200 million people contract malaria every year, and about 500,000 people die each year. Most cases and complications occur in children across Africa; incidentally, where sickle cell genes are still quite predominant compared to the rest of the world. An essential part of the Plasmodium lifecycle is residence in (infection of) red blood cells. As a result, it makes sense that abnormal blood cells no longer offer an environment conducive to a successful Plasmodium life cycle. Hence, a chronic illness that would otherwise very likely have been removed from the human gene pool by natural selection has remained within it. In regions where the more immediate threat of malaria cast a shadow over the more chronic symptoms associated with sickle cell disease, natural selection has favored keeping the “anti-malaria gene” for at least a while longer.
Our body’s preventive fight against malaria does not stop with sickle cell disease. Even more obscure is a vegetable-associated condition: fava beans are the namesake for an enzyme deficiency syndrome known as Favism. People with favism get quite sick upon consuming fava beans. When these individuals experience severe fatigue, nausea, and abdominal pain upon ingesting fava beans, on a molecular level an acute hemolytic anemia is wreaking havoc in their bloodstream. Red blood cells are rapidly broken down, faster than the bone marrow can replace them. As a result, the body's oxygen supply plummets. It turns out that sufferers of favism lack an enzyme called Glucose-6-Phosphate Dehydrogenase, abbreviated G6PD, which normally protects red blood cells against damage from some of the byproducts of metabolism. Fava beans have historically been cultivated in North Africa, the Mediterranean, the Middle East, and South Asia. Favism is prevalent in those regions. So is malaria. Inhospitable red blood cells during fava cultivation, which coincides with warmer, mosquito-rich times of year? No malaria.
The above two conditions are characterized as disorders because they can give rise to complications in their carriers over time, yet they carry a physiological benefit that keeps carriers out of trouble in malaria-ridden places. As a result, natural selection has acted in their favor; clearly, the trade-off between hemolytic or sickle cell anemia and protection from malaria infection led to increased survivability of local populations.
The next anti-malarial weapon is more of a nuisance than a sleeping health threat. If you are of Mediterranean descent, you have probably noticed that the hair on your arms and legs is thicker than most. In fact, the densest hair coincides geographically with malaria-ridden regions. The exception lies in extremely hot regions like parts of Africa, where other anti-malarial traits have been more conducive to the local climate. Thicker hair makes it more difficult for mosquitoes to land on our skin and bite us. No mosquito bite? No malaria.
Over time, as we adapted to different environments and slowly filled every pocket of the world, our genomes have retained some genes and discarded others in patterns consistent with the most immediate environmental threats we faced. Malaria is a serious infectious disease that has been with humans for thousands of years. As a result, it only makes sense that it has shaped our genomes as we acquired traits that happened to confer protection from the parasite.