We all learned about how evolution led to homo sapiens and how through being more competitive, humans displaced other, similar but inferior, species. I often wondered how natural selection will manifest itself in the future -- who will replace us, humans? Is evolution per se dead now that we are intelligent and can influence our adaptability through our actions? Or will it take some other, more nuanced, form? I think the answer is closely related to cancer.
People think of cancer as a kind of defect that affects healthy cells. But from a microscopic point of view, where the survival of cells rather than large organisms is relevant, there is an alternative explanation. At that level, cancer is a higher stage of evolution of cells, since usually those cells have a higher chance of survival (they don't get recycled by the body, they take over other cells, they are much harder to eliminate than healthy cells). The existence of cancer is an outcome of natural selection of cells – evolutionarily, each cell undergoes mutations and those cells that happen to have superior features survive in a harsh natural environment. Cancer cells are regular cells that have mutated and gained features that allowed them to push past their harsh natural environment, i.e. a boundary defined by the human body. In a way, a human being dies of cancer when the cancer cells are too powerful to be confined in the body, that is, when cancer succeeds. So on a microscopic scale, we see a kind of natural selection of cancer cells over normal cells. The fact that they end up killing the body is a tragic side effect of the cancer being so successful.
Now let's look at this at a macro level. We live in an age where our bodies are bombarded by perturbations of many sorts -- electromagnetic waves permeating us, radioactivity and toxins (such as those in highly processed food) that we -- willingly or unwillingly – absorb. These perturbations increase the frequency with which our cells mutate, and so, unsurprisingly, the incidence of cancer is much higher now than it ever was in the past. These perturbations, of course, are a natural consequence of progress – a consequence of our intelligence. Intelligence, therefore, is a double edged sword: on one hand it's our greatest weapon, allowing us to reign over all other species, but on the other hand, it's our weak spot, crippling us by allowing parts of us to be naturally selected against the body -- and since evolution is a fundamental law of the Universe (since it's nothing really than a statistical phenomenon), it's likely we will not be able to fight it.
It's possible we have reached the end of the evolutionary path for species: once a species reaches the Age of Intelligence, it dooms itself. This instability brought about by the decay may ultimately cause us to lose our position as the Earth's most supreme beings; smaller, less intelligent creatures, less susceptible to toxins and disease, prevail. From our point of view this would seem like a regression, but evolution doesn't have a design in mind, of course.
It's also possible that evolution is circular – the species that survives us will not be an organism but a class of rapidly-mutating cells. Those cells will not need hosts as they will be so powerful no host will be able to harness them. Such rapidly-mutating cells could start the path anew, giving the lineage of species an unheard of momentum that encourages great diversity. And who is to say that the same hasn't happened before.
It's also possible that, being intelligent creatures as we are, we'll preempt our dark fate and change our lifestyles. A culture of "natural living" may become popular, a minimalistic (though not primitive) living aiming to perturb Nature the least. A culture of men realizing the fragility of life and striving to limit the rate at which they increase entropy in the Universe.