(Vaccines are a key factor of modern human life expectancy. Picture from Wikipedia )
Human longevity is overall increasing all around the world, thanks partly to better nutrition, health care and widespread vaccination. This lead to the suggestion that our modern means put a stop to human evolution, as it allows people who would have died in natural conditions to live a healthy life. However, one must remember that natural selection is relevant through reproduction and not healthiness or longevity only.
Modern medicine “put a halt to evolution”. To understand the claim that our species Homo sapiens does not evolve anymore, we must first understand the base of this claim. Without any doubts, modern medicine has done miracles. Never before human population has been so healthy. To compare, in our pre-industrial societies child mortality could be as high as 40%, while nowadays it is less than 5% worldwide. During Roman era and medieval England, a child surviving until teenage years had a life expectancy of about 30 more years (Cokayne 2003; Lancaster 1990). Currently, human longevity is as high as 71 years for the global population, the double compared to pre-industrial times. What can explain this huge difference?
Causes of deaths are not lacking, but our modern medicine got rid of plenty of them. Back then, even a tiny cut could prove fatal; there was no efficient remedy against diseases; populations were at the mercy of famines that could kill millions. Fortunately for us, some key innovations such as the discovery of microorganisms (by Hook and Leeuwenhoek, 1665), vaccines (by Jenner ca 1760 although some other forms existed before), antibiotics (by Flemmings in 1928 although it was somewhat known before) and intensive agriculture in the 1800’ increased human longevity and healthiness. Our modern medicine has answers and solutions to plenty of health problems that were incurable two centuries ago. Have you ever seen a case of smallpox, once a common infection that could killed up to 35% of infected people and took the life of millions of people? Most probably not, as this disease has been eradicated in the 70’ thanks to widespread vaccination. Were you wounded while working in your garden? No worries of death either: you are probably vaccinated against tetanus, and you can easily disinfect your wound with antiseptic spray found at the local store. These are examples of common affections modern medicine has solutions for. In other words, human population is not as sensible anymore to harmful agents and diseases as it used to be. Many medical conditions that would have killed a century ago are now curable. Did that stop natural selection in humans, aka the “elimination of the weakest and survival of the strongest”?
It has been suggested so, on the ground that as mentioned before, our modern means saved the life of millions of individuals who would have died otherwise. The “weakest” are not naturally eliminated anymore and survive along the “strongest”, blending up altogether. However, thinking that we are not evolving anymore because of this is missing a key concept of evolution: that evolution does not depend on survival but on how many offspring you leave.
So, first of all: what is natural selection? Simply put, natural selection is a term grouping several processes by which traits giving better survival are spread to the population. The idea behind is that if an individual possesses a characteristic allowing to cope better in its environment, compared to others, then this individual will survive longer, mate more and the characteristic will invade the population – as the offspring themselves cope better, reproduce more etc. A school example is given with the peppered moth Biston betularia. This butterfly, usually resting on birch during the day, possesses a black and a white morph. Some individuals are black, others white. In a pristine environment, the trunk of birches are white: while white individuals are not visible when resting, birds eat the conspicuous black ones. Here, the naturally selected trait giving advantage is the white color. At the beginning of the industrial era, pollution turned the white birch trunks into black. In this environment, the black individuals are selected as birds eat the white ones. A school case of selection in humans is given by the sleeping sickness or drepanocytosis. It is a genetic disease in which blood cells have abnormal forms, leading to severe anemia. Researchers investigating this disease found that drepanocytosis was common in area of malaria, while being an otherwise very rare condition. Later on, it was shown that people having the mutant gene were more resistant to malaria, even if as a side effect some individuals experienced drepanocytosis. In other words, the local population evolved resistance to malaria as the mutant gene providing resistance was selected.
Diseases are a big source of natural selection and evolution: people that cannot survive the illness disappear of the population, leaving only individuals who can resist it. However, natural selection alone is just a part of evolutionary processes. In the definition I gave above, I mentioned a key point that makes a big difference: “natural selection is a term grouping several processes by which traits giving better survival are spread to the population”. Having an improved health alone is not enough, because longevity does not make babies. If you live long but do not reproduce, your genetic material will never be transmitted: you are an evolutionary dead end. Instead, it is ultimately traits giving reproductive benefits that will be selected for, through processes of natural and sexual selection. A school case of sexual selection it is given by the peafowls Pavo cristatus. Peafowl males have this huge, colorful tail that on one hand confers a high vulnerability to predation, but is nevertheless selected because it attracts females: even if males may die because of their disproportionate tail, they leave more offspring than individuals who don’t have impressive tail. Again, this applies to humans and explains for example why females usually have larger amount of fat on their breast and hips than males, as males are usually attracted by such features in females.
Thus, even if human health and longevity increased, we are still evolving as it is traits giving reproductive advantages that are selected. Our modern medicine only altered some factors of evolution. But we are still sensible to other selective factors such as mate choice, as certain physical features are usually preferred in mates such as larger shoulders in men. You can have a short live and be very attractive; or a long life and be very unattractive: at the end what matters in evolution is not how long you live but how many babies you make. Spread love, for evolution’s sake!