I hear this question come up a lot. The subtext is that, because humans have mastered our environment, we are no longer subject to the same pressures of natural selection that we once were.
First, let’s review what evolution is. Evolution is a change in the frequency of alleles or traits in a population over time. Natural selection is the mode of evolution where the change is based on environmental pressures that cause individuals with certain traits to reproduce more than individuals without those traits. Natural selection does not have a goal, or a more or less advanced state. Natural selection improves a population in that it increases the frequency of traits that cause the members of the population to leave more surviving offspring. It does not improve a population by necessarily making the individuals smarter, faster, stronger, or more complex. A cheetah is not more evolved than a sloth because it is faster. Both were designed by natural selection to do what they do.
Infectious disease was a huge problem for our ancestors. Good thing we cured them all. (In case there was any doubt, that last sentence was full of sarcasm.) While it is true that medical advances have greatly reduced the burden of infectious disease in the west, they are still a big problem in most of the world. Malaria, tuberculosis, HIV/AIDS, respiratory infections, and diarrheal disease each kill over a million people a year worldwide, mostly in developing nations.
Even in the medically-advanced United States, tens of thousands of people die every year from influenza alone. We may not have schistosomiasis or malaria, but we still have people dying of infectious disease in large numbers. If there exist any alleles in our population that confer resistance to diseases that we face, however slight, then we are evolving. If for example, people with a particular allele have even a 0.1% increased chance of surviving influenza, then that trait will increase in frequency in subsequent generations.
If diseases are not killing us randomly, then those diseases are causing us to evolve. If we cure those diseases, then the cure will also cause us to evolve. If a disease is killing people who do not have a particular trait, then the frequency of that particular trait will increase. If we cure the disease, then the frequency of that particular trait will begin to decrease again. Both the increase and decrease of the trait frequency are evolution. I think this is where a lot of people get hung up — because we are not “improving” we must not be evolving.
Our evolution is not limited to the effects of infectious disease. If you look at population growth in the world, it is not the same everywhere. Traits that occur at higher frequencies in the parts of the world with higher fertility rates will get passed on at a higher frequency. This is also evolution.
Evolution takes place over the course of many generations. Individual human observers will therefore have a difficult time observing human evolution. It may not look like we are evolving, but we certainly are.
Have a topic you want me to cover in a future post? Let me know in the comments or on twitter @CGEppig.