On the surface, the life we see around us appears to be perfectly in tune with the surrounding environment: parasites that have subverted then co-opted complicated life-history patterns of their host, moths that have evolved camouflage to virtually disappear in their habitat, and highly efficient thermoregulatory apparatus to keep mammals cool in desert climates.
Superficially, evolution seems to act as a highly efficient process that always generates the most advantageous traits that allow organisms to survive and reproduce. But by digging a bit deeper, examples of absurd, overly complicated, and redundant biological behaviors and morphologies pepper the tree of life. These absurdities include the mammalian respiratory system which relies on the presence of carbon dioxide and not the absence of oxygen for regulation; the human vas deferens which opts for a long detour rather than an efficient one-way street from the testis to the penis; and the textbook example of the recurrent laryngeal nerve that connects the brain to the larynx which in giraffes travels a total of about five meters, all the way down its neck, and loops around the aortic arch—a waste of nerve cells that reduces signal strength. It is clear that evolution is imperfect and not created by an intelligent designer. For example, an intelligently designed window might have double-paned and tempered glass, multiple swivel and opening options, and a vacuum chamber. In contrast, an evolutionarily designed window might just be a hole in the wall: whatever gets the job done. A critical aspect of evolutionary thought is that evolution has no drive toward perfection; it is blind and often settles on ‘just good enough.' The biological imperfections that arise from evolution’s blind eye can have dramatic consequences for the health of a species.
Effective Population Size and Disease
Effective population size (Ne) is a notoriously esoteric and slippery concept in biology, but it is an important one to understand in relation to disease. The main difficulty with Ne is that there are many ways to describe it; for our purposes, Ne will simply mean the evolutionary potential of a population which is determined by the evolutionary history of that population. Imagine a genetically distinct population of 100 individuals. This population has a very fast DNA mutation rate so that all individuals have unique genetic information (each individual has differences in their DNA). Blood samples for each of the 100 individuals in this population were collected, combined, and then sent to space. Aliens discovered the blood and sequenced the DNA letters in the combined blood and determined that there were about 100 individuals in that population. They therefore determined the Ne to be ~100, about the same as the actual population size. This is because each individual has their own unique DNA code. Now imagine a population of 100 asexual and clonal individuals with a negligible mutation rate. Asexual clones all have the same DNA. If the blood samples of these 100 genetically identical individuals were combined and sent into space the aliens would conclude that there was only ~1 individual in that population, or an Ne of 1, about 99 less than the actual population size. Because the samples were combined and identical in this latter example, it appears that all the DNA only came from one individual. Turning to the current state of humans, if blood samples were taken of the approximately 7.8 billion humans on the planet, the aliens would surmise that there were only about 10,000 humans, or an Ne of 10,000—an enormous difference to the actual population size. This is due to a very fast expansion of the human species during its evolutionary history and not enough time for mutations in the DNA to arise to differentiate individuals.
The extraordinarily low Ne of humans has dramatic consequences for the health of the species because selection can be very effective when Ne is high relative to population size. This is because in a population with a lot of variation, selection occurs among different individuals that have different DNA. But if all individuals have the same DNA, there is no differences to select among. With a homogenous population like humans, there is little standing genetic variation on which selection can act. Having a large allelic diversity for genes like HsB (which confers sickle cell anemia, SSA) is important in populations that are near sources of the malaria parasite because having different versions of HsB confers resistance to malaria without the burden of SSA. This is also why some diseases like cancer can be so difficult to treat. Imagine the millions of cells that can comprise a tumor to be a population. Cancer can dysregulate the cell cycle (how cells make more of themselves) and result in tumors that are a genetically diverse population of cells caused by many independent mutations in the tumor cells. The Ne of the tumor can skyrocket because new genetic variants are created very quickly. This makes treatment a challenge because selection (in the form of chemotherapy or radiation treatment) can be extremely efficient in eliminating non-resistant cells and thus giving resistant cells a proliferative advantage. (See my infographic.)
Effective population size, and related concepts, have generated a framework for disease treatment called evolutionary medicine. Evolutionary medicine has become a powerful tool to inform treatment strategies for a variety of ailments. Evolutionary medicine is also important in that it can be useful as a teaching tool about the truth and nuances of evolution. Modern human health and medicine has been shaped by evolution and many of the medicines, procedures, and treatments that exist today can directly link their effectiveness to evolution because they were implemented using evolutionary principals. Educating the public about evolution can be met with hesitation or outright disbelief despite mountains of evidence to the contrary. Making visible the power and importance of evolutionary medicine to the public may be an effective method of encouraging acceptance of evolution compared with traditional methods.
For more on the topic of evolutionary medicine, check out my interview with Christopher Stipp, Associate Professor of Biology at the University of Iowa.