Humankind has undergone a dramatic shift over the past few centuries. A combination of industrialised farming and modern medicine has allowed us to completely overcome our biology. We exist in record populations, live for longer than ever, and, most crucially are able to survive a wide range of situations that would have previously killed us. We are a social species. We naturally collaborate and help each other, and don’t stand by while others suffer. As a result of this, we no longer live under the law of ‘survival of the fittest’ – we exist as ‘survival of everyone’.
Evolution needs two things in order to work: genetic variation and selective pressure. Genetic variation is inevitable. It will always exist amongst living populations. Even if you had a theoretical population of exact clones, variation would arise over generations due to DNA copying errors, recombination and genetic drift.
Selective pressure, however, is not inevitable. It fluctuates wildly, and is totally dependent on situation. Some species live under heavy selection pressure, such as infection bacteria. The use of antibiotics kill bacteria by the billion, and so any genetic variation that resists antibiotics will confer a survival advantage. As a result, bacteria are evolving drug resistance very rapidly. Other species experience much lighter selection pressure. The ancestors of dodos lost the ability to fly as they lived on an island with no predators, and there was no pressure to keep flying. Unfortunately for the dodo, selection pressure (in the form of human predators) very suddenly became so strong that the species went extinct before it could adapt.
Selection pressure can drive a species in many different directions. I’ll illustrate this using a simple example. If being tall is an advantage, the species will get taller. If being small is an advantage, the species will get smaller. If there is an advantage for being tall and small, two distinct populations will arise; one tall and the other small, which over time may become two separate species. If there is an advantage to being middle height, but no advantage to being tall or small, a species will remain at that height. In a sense, natural selection acts to ‘trim’ or ‘prune’ a species into a form that is better suited to its environment.
Modern humans are experiencing a great lifting of selection pressure. Our environment is nowhere near as lethal as it has been in our past. As a result, we are genetically drifting in every direction. We are becoming more varied as a species. This is not a problem while the going is still good, but over centuries, doctors may find that their patients are so varied that a drug that works on one may not work on another. We may end up more vulnerable to epidemics, or our metabolisms might be less efficient, or our cognitive abilities could wane. The reality is that we don’t know what will happen to us, because this has never happened before. We are living a great experiment.