The evolution of disease

Some diseases are caused by a person’s genetics. In many cases diseases can be predicted by looking at DNA for specific genes that predispose someone to a disease. Having said that DNA is also connected to evolution, in that genomes that confer an advantage to a person will increase the probability of that person reproducing. Over time well adapted and healthy genomes will become prominent in a species.

            Why is it that so many diseases can be passed genetically when the theory of evolution claims that only the fittest will survive? Would that not mean that genetic based diseases should have died out long before the modern day? In truth it is much more complicated than that and there are many factors at play. Genetic diseases could always occur in a later generation through mutation in reproductive cells, being passed in that way. Or a series of other advantages will allow someone to still be fit enough to pass along a disease. Some diseases hide as recessive traits, only appearing when two carriers of the recessive trait mate.

            Then there are some diseases that are relics of a different time. Some diseases are still prevalent because they conferred an advantage in the past, and even with the significant side effects of a disease the advantage proved invaluable. Once this advantage was selected for it became prevalent in certain populations over time. Fast forward generations later and people still have these diseases in different circumstances that no longer provide a benefit, yet medical science has advanced enough that even without the advantage the disease will still be a permanent part of the population.

            One interesting example is Type 1 diabetes. People born with Type 1 diabetes have an autoimmune response that prevents them from producing enough insulin. Without insulin the person will have unusually high amounts of glucose in the blood. It has been found that Type 1 diabetes is most prevalent in northern Europeans, occurring at a frequency that is more common than random chance would suggest possible, showing signs of having been selected for in that area in the past. What advantage would that give? As it turns out high amounts of glucose in blood will depress the freezing point, increasing the tolerance for cold in humans. In the past minor ice ages and unseasonably cold weather would kill those unprepared for it, selecting those with Type 1 diabetes because of their tolerance for the cold. Trading a healthy amount of insulin for this advantage proved invaluable in the past and modern northern Europeans still suffer today as a result.

This leaves an interesting new avenue for understanding the epidemiology of disease. People with sickle cell trait have a higher resistance to malaria because of their malformed hemoglobin. Iron deficiency improves the chances of surviving infection by Yersinia Pestis, the bacteria that causes the plague, better known as the Black Death.

Finding how many diseases conferred an advantage in the past would also lead to better understanding of what diseases may give advantages in the modern day. A more holistic view of disease could also lead to new treatment ideas as well.

Want to learn more? I have an article about the prevalence of Type 1 diabetes listed below. A well written book on the subject of advantageous diseases that I have read in the past is also below. The book is much more interesting than any textbook and I highly recommend it for some casual reading.

For further reading please see:

The sweet thing about Type 1 diabetes: A cryoprotective evolutionary adaptation

By: S. Moalema, K.B. Storeyb, M.E. Percyc, M.C. Perose, D.P. Perl

http://www.sciencedirect.com.ezproxy.shsu.edu/science/article/pii/S030698770500054X

A more in depth novel on the subject:

Survival of the sickest: A medical maverick discovers why we need disease

By: Sharon Moelem