Even though Darwin didn't use it, the phrase “survival of the fittest” is one that everyone who knows about evolution has heard many times. In that phrase, what does the word “fittest” really mean exactly?
The typical suggestion is that it means the largest or the fastest or the smartest or some combination of those things. Surely those characteristics are often important in determining the “fittest”. But there are many other things that contribute to that as well.
The author Jared Diamond provided the best example of how other characteristics can make an organism the “fittest”.
Genetic studies have shown that domesticated dogs are descended from wolves. If you compare the average dog to the average wolf you will see that the wolf is larger, smarter, faster and quicker than the average dog. Our intuition would tell us, therefore, that wolves are “fitter” and should, therefore, be a much more dominant sub-species than are dogs.
Of course things are quite different than that. Dogs flourish all over the world. There are a huge number of different breeds. In contrast, wolves have numbers which are much smaller and there are even many places on Earth where wolf populations have diminished so much that they are an endangered species.
How can that be?
It turns out that dogs have developed a characteristic that is not present in wolves. They have developed a personality that allows them to form, in Dr. Diamond’s words, “a sort of symbiotic relationship with the most dangerous species on Earth – us”. That singular ability to relate to humans trumps all of the physical characteristics which would tend to make us believe that wolves should predominate.
Of course if humans were to vanish, the situation would change quickly. Books have been written speculating on what would happen if humans suddenly vanished. All of them seem to agree that one thing that is likely to happen is the abrupt disappearance of dogs. In that environment – one without humans – the physical and other advantages enjoyed by wolves would indeed allow them to become dominant.
But, as long as humans are present on Earth, dogs are the “fitter”.
One point that should be made is that in the very unlikely event that humans were to suddenly vanish, that would count, from an evolutionary perspective, as an environmental change. That statement probably startles some people. Most people think of the “environment” as the weather: how hot it is, how wet it is, etc. But from an evolutionary perspective, anything that changes those things that determine what is “fittest” counts as an environmental change. Surely hotter or colder, wetter or drier, weather affects that. But other things do so as well.
Such things include the presence or absence of human beings.
Therefore, ”fittest”, in regard to evolution, simply means having genetic characteristics that allow some individuals in a population and their descendents to survive at a higher rate than others that don’t have those genetic characteristics. Often the characteristics that make a population of organisms fitter are obvious. Many other times, as is the case with dogs vs. wolves, those “fitter” characteristics are more subtle and not so easy to see.